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Title: Plutons and plate motions: Mid-Cretaceous Farallon-North America plate kinematics inferred
from structures in the Jackass Lakes pluton-host rock system, central Sierra Nevada, CA
Article Type: Research Paper
Keywords: magma emplacement; transpression; shear zones; Sierra Nevada batholith; plutons;
magmatic foliations
Corresponding Author: Dr. Aaron S. Yoshinobu, PhD
Corresponding Author's Institution: Texas Tech University
First Author: Ryan J Krueger, M.S.
Order of Authors: Ryan J Krueger, M.S.; Aaron S. Yoshinobu, PhD
Abstract: Magmatic fabrics (foliations/lineations) within the Jackass Lakes granodiorite pluton and
structures within the Sing Peak pendant and related xenoliths, central Sierra Nevada, CA, record
regional deformation that was contemporaneous with magma chamber construction. Host rocks
consist of deformed metavolcanic rocks of the mid-Cretaceous Minarets Caldera sequence and minor
metasedimentary rocks. Structures in the Sing Peak pendant include: a) a penetrative, subvertical,
north-northwest-striking foliation; b) folded syn-magmatic dikes, and c) a dextral-oblique, reversesense ductile shear zone with a shallow/moderate plunging (~30-45º) north-northwest-trending
lineation. The shear zone may be traced into the Jackass Lakes pluton along the northern margins of
the pendant and involves both subsolidus but predominantly hypersolidus fabric development.
Structures within the Jackass Lakes pluton include a well-developed north-northwest-striking, steeply
west dipping magmatic foliation and a moderately north plunging (~40º) lineation. The magmatic
structures are continuous across compositional zones within the Jackass Lakes pluton and parallel to
the elongated axis of mafic enclaves observed within the pluton. Abundant xenoliths of metavolcanic
rocks occur throughout the pluton. Metamorphic foliations observed within the xenoliths are subparallel to those in the host rocks and the magmatic foliations in the pluton. Some xenoliths contain
folded syn-magmatic granodioritic dikes with axial planes that are sub-parallel to the metamorphic
foliation in the xenoliths, host rocks and magmatic foliations in the pluton. We propose that the
formation of magmatic and metamorphic structures as well as syn-magmatic folding of dikes occurred
during chamber construction and most likely due to regional deformation in a magmatic-to-plastic
shear zone with dextral-oblique shearing at ca. 98 Ma. Comparison with published middle Cretaceous
plate motion vectors indicates that dextral shearing may have initiated around 100 Ma, consistent with
our observations. Therefore, the Jackass Lakes pluton - host rock system represents the earliest
documented regional dextral transpression in the Cretaceous central Sierra Nevada batholith.
Cover Letter
11 June 2012
TO: Editors, Tectonophysics
FROM: Aaron Yoshinobu and Ryan Krueger, Geosciences, Texas Tech
University.
RE: manuscript submittal
Please find enclosed a manuscript titled, “Plutons and plate motions…”. This
manuscript uses new geologic mapping and structural analysis to evaluate the
nature and significant of a newly-recognized ductile shear zone in the central
Sierra Nevada batholith. This work is significant in that it documents a magmaticcrystal-plastic shear zone that was operative during pluton assembly during the
middle Cretaceous and records dextral-oblique shearing. Coupled with published
plate motion vectors for the Cretaceous, this work extends the history of dextral
transpression for the Sierra Nevada continental magmatic arc back to ~100 Ma.
A number of figures are in color for the web version. These will be black and
white for a printed version if accepted.
Thank you for considering this manuscript.
Sincerely yours,
Aaron Yoshinobu, Corresponding Author
Assoc. Professor
Department of Geosciences
Texas Tech University
Lubbock, TX 79409-1053
USA
email: aaron.yoshinobu@ttu.edu
*Abstract
Click here to download Abstract: Krueger-Yoshinobu-ABSTRACT.docx
Magmatic fabrics (foliations/lineations) within the Jackass Lakes granodiorite pluton and
structures within the Sing Peak pendant and related xenoliths, central Sierra Nevada, CA,
record regional deformation that was contemporaneous with magma chamber
construction. Host rocks consist of deformed metavolcanic rocks of the mid-Cretaceous
Minarets Caldera sequence and minor metasedimentary rocks. Structures in the Sing
Peak pendant include: a) a penetrative, subvertical, north-northwest-striking foliation; b)
folded syn-magmatic dikes, and c) a dextral-oblique, reverse-sense ductile shear zone
with a shallow/moderate plunging (~30-45º) north-northwest-trending lineation. The
shear zone may be traced into the Jackass Lakes pluton along the northern margins of the
pendant and involves both subsolidus but predominantly hypersolidus fabric development.
Structures within the Jackass Lakes pluton include a well-developed north-northweststriking, steeply west dipping magmatic foliation and a moderately north plunging (~40º)
lineation. The magmatic structures are continuous across compositional zones within the
Jackass Lakes pluton and parallel to the elongated axis of mafic enclaves observed within
the pluton. Abundant xenoliths of metavolcanic rocks occur throughout the pluton.
Metamorphic foliations observed within the xenoliths are sub-parallel to those in the host
rocks and the magmatic foliations in the pluton. Some xenoliths contain folded synmagmatic granodioritic dikes with axial planes that are sub-parallel to the metamorphic
foliation in the xenoliths, host rocks and magmatic foliations in the pluton. We propose
that the formation of magmatic and metamorphic structures as well as syn-magmatic
folding of dikes occurred during chamber construction and most likely due to regional
deformation in a magmatic-to-plastic shear zone with dextral-oblique shearing at ca. 98
Ma. Comparison with published middle Cretaceous plate motion vectors indicates that
dextral shearing may have initiated around 100 Ma, consistent with our observations.
Therefore, the Jackass Lakes pluton – host rock system represents the earliest
documented regional dextral transpression in the Cretaceous central Sierra Nevada
batholith.
*Highlights (for review)
Highlights to:
Plutons and plate motions: Mid-Cretaceous Farallon-North America plate
kinematics inferred from structures in the Jackass Lakes pluton-host rock system,
central Sierra Nevada, CA
Ryan J. Krueger1, Aaron S. Yoshinobu2
Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, USA
1
Now at ExxonMobil Exploration Company, 233 Benmar Dr., Houston, TX 77060, email:
ryan.j.krueger@exxonmobil.com, T: 281-654-6827, F: 281-654--7879;
2
corresponding author, email: aaron.yoshinobu@ttu.edu, T: 806-742-4025, F: 806-742-0100
Highlights:
• Geologic and structural mapping was completed in the Jackass Lakes granodiorite
pluton.
• Hypersolidus and subsolidus fabrics define consistent regional trends.
• Fabrics formed during 97-98 Ma. construction of the pluton.
• Fabrics define a dextral transpressional shear zone.
• Dextral-transpression is related to olique convergence of the Farallon plate with North
America.
*Manuscript
Click here to download Manuscript: Krueger-Yoshinobu-submitted.docx
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Click here to view linked References
Krueger and Yoshinobu, Plutons and Plate Motions… submitted to Tectonophysics, 2012
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Plutons and plate motions: Mid-Cretaceous Farallon-North America plate
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kinematics inferred from structures in the Jackass Lakes pluton-host rock system,
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central Sierra Nevada, CA
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Ryan J. Krueger1, Aaron S. Yoshinobu2
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Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, USA
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1
Now at ExxonMobil Exploration Company, 233 Benmar Dr., Houston, TX 77060, email:
ryan.j.krueger@exxonmobil.com, T: 281-654-6827, F: 281-654--7879;
2
corresponding author, email: aaron.yoshinobu@ttu.edu, T: 806-742-4025, F: 806-742-0100
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key words: magma emplacement, transpression, shear zones, Sierra Nevada batholith,
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plutons, arcs
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ABSTRACT
Magmatic fabrics (foliations/lineations) within the Jackass Lakes granodiorite
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pluton and structures within the Sing Peak pendant and related xenoliths, central Sierra
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Nevada, CA, record regional deformation that was contemporaneous with magma
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chamber construction. Host rocks consist of deformed metavolcanic rocks of the mid-
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Cretaceous Minarets Caldera sequence and minor metasedimentary rocks. Structures in
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the Sing Peak pendant include: a) a penetrative, subvertical, north-northwest-striking
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foliation; b) folded syn-magmatic dikes, and c) a dextral-oblique, reverse-sense ductile
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shear zone with a shallow/moderate plunging (~30-45º) north-northwest-trending
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lineation. The shear zone may be traced into the Jackass Lakes pluton along the northern
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margins of the pendant and involves both subsolidus but predominantly hypersolidus
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fabric development. Structures within the Jackass Lakes pluton include a well-developed
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north-northwest-striking, steeply west dipping magmatic foliation and a moderately north
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plunging (~40º) lineation. The magmatic structures are continuous across compositional
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zones within the Jackass Lakes pluton and parallel to the elongated axis of mafic enclaves
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observed within the pluton. Abundant xenoliths of metavolcanic rocks occur throughout
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the pluton. Metamorphic foliations observed within the xenoliths are sub-parallel to
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those in the host rocks and the magmatic foliations in the pluton. Some xenoliths contain
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folded syn-magmatic granodioritic dikes with axial planes that are sub-parallel to the
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metamorphic foliation in the xenoliths, host rocks and magmatic foliations in the pluton.
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We propose that the formation of magmatic and metamorphic structures as well as syn-
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magmatic folding of dikes occurred during chamber construction and most likely due to
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regional deformation in a magmatic-to-crystal plastic shear zone with dextral-oblique
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shearing at ca. 98 Ma. Comparison with published middle Cretaceous plate motion
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vectors indicates that dextral shearing may have initiated around 100 Ma, consistent with
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our observations. Therefore, the Jackass Lakes pluton – host rock system represents the
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earliest documented regional dextral transpression in the Cretaceous central Sierra
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Nevada batholith.
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1. Introduction
The study of modern arcs suggests that a variety of parameters affect the style and
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kinematics of deformation within the overriding plate (Dewey, 1980; Uyeda, 1982;
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Isacks, 1988; Dewey and Lamb, 1992; Tobisch et al., 1995). These parameters include
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convergence angle and velocity, lithospheric thickness and temperature variations within
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the down-going slab. While emphasis has been placed on the partitioning of deformation
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in the fore-arc regions and along intra-arc fault systems (e.g., Jarrard, 1986), long-lived
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volcanic arcs and their magmatic underpinnings represent thermal anomalies where
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deformation may also be localized (e.g., Saint Blanquat et al., 1998).
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The nature of strain regimes within an arc during pluton emplacement has long
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been controversial. Arc-perpendicular shortening, arc perpendicular extension and
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regional transpression are all well represented in nature and in the literature (e.g.,
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Apperson, 1991; Glazner, 1991; Hamilton, 1995; Paterson and Miller, 1998; Horsman et
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al., 2008). Knowledge of the movement history and timing of shear zones is vital for
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understanding the tectonic/magmatic evolution of an arc and the construction of sub-
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volcanic magma chambers. Faults and ductile shear zones provide anisotropies in the
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shallow to deep crust and have been interpreted as “pathways” for magma ascent and
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pluton emplacement (e.g., Tobisch and Cruden, 1995).
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Fabric patterns in plutons may provide information regarding regional
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deformation when emplacement-related (e.g., “ballooning”) and other processes (e.g.,
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internal magmatic convection) can be ruled out (Paterson et al., 1998). Magmatic
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foliation patterns within plutons can vary significantly depending on the depth of
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intrusion, emplacement dynamics and tectonic stresses present during the assembly of a
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given mapped intrusion. Several authors suggest that magmatic fabrics may record strain
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caused by regional tectonic processes (Tobisch et al., 1995; Fowler and Paterson, 1996;
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Paterson et al., 1998; Benn et al., 2001). Therefore, magmatic foliations may be
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analogous to solid-state „regional‟ shear zones in that they record increments of
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regionally imposed deformation as the pluton is cooling. These fabrics may be used in
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conjunction with the orientation of metamorphic fabrics and radiometric age dating to
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gain a better understanding of the evolution of the regional strain field during batholith
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construction.
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Within the middle Cretaceous Sierra Nevada batholith, central California, the
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presence of several regionally extensive shear zones developed in the metamorphic
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framework rocks has been documented (Fig. 1; Table 1). These shear zones show both
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dip-slip and strike-slip displacement. For example, Tong (1994), Tobisch et al. (1993,
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1995) and McNulty (1995) showed that ductile shear zones that formed prior to 90 Ma
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exhibit dip-slip motion. In contrast, Glazner (1991), Tikoff and Teyssier (1992), and
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Tikoff and Saint Blanquat (1997) proposed large-scale transcurrent shear zones and faults
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induced by oblique subduction are responsible for the emplacement of the post-90 Ma
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portion of the batholith. The work reported here indicates that ca. 98 Ma magmatism was
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affected by dextral oblique shearing (transpression), thus extending the record of oblique
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subduction in the arc back 10 million years. This is consistent with Engebretson et al.
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(1985) who concluded that around 100 Ma, orthogonal and sinistral convergence between
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the North American and Farallon plates switched to oblique, right-lateral convergence.
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2. Geologic Setting
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2.1 Jackass Lakes Pluton
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The Jackass Lakes pluton - host rock system is located in the central Sierra
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Nevada batholith (Figs. 1, 2) and provides an excellent opportunity to study the
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interactions between magma emplacement and regional strain field evolution. The near
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100% exposure and high relief provide the opportunity to determine contact relationships
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and structural features in three-dimensions. Extensive field mapping over the past 50
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years by the United States Geological Survey (USGS) and academia has resulted in a
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large database of maps and structural information for the region (Fig. 1; summarized in
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Bateman, 1992; Peck, 1980; Peck and Van Kooten, 1983; Saleeby, 1990; Tobisch et al.,
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1993; 1995; 2000; McNulty et al., 1996; Pignotta et al., 2010). We present new detailed
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mapping of the southwestern quarter of the Merced Peak 1:62,500 geologic quadrangle
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sheet (e.g., Peck, 1980), mapped at 1:10,000 scale in Figure 3.
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The Jackass Lakes pluton (98.5 ± 0.3 to 97.1 ± 0.7 Ma, McNulty et al., 1996) is
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exposed as an approximately 13 x 17 km rectangular body and has been interpreted as an
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incrementally emplaced, resurgent pluton that intruded its own volcanic ejecta (Fiske and
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Tobisch, 1978; Peck, 1980; McNulty et al., 1996). These metavolcanic rocks form the
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144-132 Ma and 101-98 Ma Minarets Caldera sequence adjacent to the eastern margin of
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the pluton and include tuffs, lapilli tuffs and tuff breccias as well as minor hypabyssal
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intrusive rocks (Fiske and Tobisch, 1994). Numerous roof pendants, screens and large
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xenoliths within the pluton (Figs. 2, 3, 4) have been interpreted to be a part of the
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Minarets Caldera sequence by Fiske and Tobisch (1994). Host rocks to the pluton also
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include sparse Jurassic (?) - Paleozoic (?) metasedimentary rocks, most of which crop out
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near the western contact, but also crop out in other locales (Fig. 4; Peck, 1980). Locally
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these metasedimentary rocks are apparently in stratigraphic contact with the metavolcanic
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rocks of the Minarets Caldera sequence, although contacts are commonly deformed. The
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western contact of the Jackass Lakes pluton cuts the slightly older 99.1 ± 0.1 Ma
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granodiorite of Illilouette Creek (Fig. 3; Tobisch et al., 1995) and contains xenoliths of
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the Illilouette Creek granodiorite. These slightly older host rocks are characterized by a
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weak/moderate north-northwest-striking magmatic foliation and steep lineation (>60º)
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defined by elongated phenocrysts of hornblende and biotite aggregates. Euhedral
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hornblende phenocrysts up to 2 cm long are diagnostic within the granodiorite of
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Illilouette Creek as compared to the Jackass Lakes granodiorite. Sparse dioritic
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magmatic enclaves are found within the Illilouette Creek and are elongated parallel with
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the local magmatic fabric. Unobscurred contacts between the Illilouette Creek and other
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intrusive units or host rocks are not observed. However, dikes of granodiorite ranging in
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thickness from < 1m to over 10 m identical to that of the Jackass Lakes pluton directly
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east were emplaced along the contacts. It is likely that the Illilouette Creek represents an
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earlier phase of the Jackass Lakes plutonic system and is separated by a contact zone of
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co-magmatic granodioritic dikes.
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The Jackass Lakes pluton is cut on its northern boundary by the Red Devil Lake
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pluton (95.1 ± 2 Ma, Tobisch et al., 1995 and McNulty, 1995) and the Half Dome phase
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of the Toulumne Intrusive Suite (88-90 Ma, Coleman et al., 2004). The southern margin
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was intruded by the Mount Givens pluton (ca. 90 Ma, Tobisch et al., 1995; McNulty et
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al., 2000). In addition, the southern margin of the Jackass Lakes pluton is locally
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bounded by an undifferentiated Cretaceous (?) quartzofeldspathic gneiss that crops out
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between the granodiorite of Jackass Lakes and the Mt. Givens granodiorite (Fig. 2).
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The Jackass Lakes pluton is predominately comprised of medium-grained
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granodiorite that generally crops out as bulbous to elongate intrusive bodies that are in
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gradational and/or intrusive contact and define several distinct compositional units.
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These compositional units include elongate and equidimensional dioritic intrusives,
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porphyritic leucogranite, swarms of mafic enclaves and other texturally heterogeneous
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hybrid compositions. Coyne et al. (2004) identified at least eight different lithologies
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ranging from diorite to leucogranite within the Jackass Lakes pluton and Pignotta et al.
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(2010) provided a detailed account of the assembly of the pluton by multiple pulses and
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host rock deformation mechanisms. Aluminum-in-hornblende barometry calculations by
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Ague and Brimhall (1988) yielded an emplacement depth of 13-15 km for the
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granodiorite of Jackass Lakes. However, the presence of miarolitic cavities and regional
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intrusive relations with the Minarets volcanic ejecta (see below) point to crystallization at
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significantly shallower depths (Fiske and Tobisch, 1978, 1994; Peck, 1980; McNulty et
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al., 1996; Wolak, 2004) than the aluminum-in-hornblende barometry indicate. Pignotta
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et al. (2010) calculated a minimum crystallization pressure of 280 MPa, using the
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aluminum-in-hornblende geobarometer adjusted for temperature (Anderson and Smith,
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1995). This value will be used throughout this paper.
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Only the two most aerially-extensive and distinct plutonic rock types of the
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Jackass Lakes pluton are shown in Figure 3. However, local variations of lithology from
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diorite to granite are common. Three east-west geologic cross sections across the pendant
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and igneous rocks are illustrated in Figure 5. The most voluminous plutonic phase is
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referred to as the granodioritic unit of the Jackass Lakes pluton (e.g., Peck, 1980; Fig. 3).
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It is a coarse-to medium-crystalline hornblende + biotite granodiorite with accessory
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apatite, zircon, titanite, and muscovite (Fig. 6). Nearly all of the xenoliths present in the
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field area are enclosed by the granodioritic unit.
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The second phase is a leucogranitic unit (Fig. 6) that crops out in the southern half
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of the field area, beneath the inferred roof contact with the Sing Peak pendant (Figs., 3,
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4). It is fine- to medium-crystalline with hypidiomorphic texture and contains
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approximately 5% mafic minerals, principally biotite and sparse hornblende. Contacts
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between the leucogranitic and granodioritic unit are variable from sharp to gradational
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over short map distances (e.g., Fig. 6). Inclusions of the leucogranitic rocks are observed
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within the granodioritic unit and vice versa. Dikes of the leucogranitic rocks are also
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observed cutting the granodioritic unit and again vice versa. Schlieren banding is also
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observed between the two rock types. The various contact relations suggest that these two
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lithologies are probably similar in age but vary in their rheology over relatively short time
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periods and map distances depending on degree of crystallization. A well-developed
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north-northwest-striking and steeply west-dipping magmatic foliation occurs throughout
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the granodiorite of Jackass Lakes. A moderate to shallow plunging lineation is present
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and defined by elongated biotite aggregates, prismatic hornblende crystals, and aligned,
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prolate mafic magmatic enclaves. Poles to magmatic foliations and lineations are plotted
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in Figure 7. The frequent observation of parallel magmatic foliations that cross the
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contact between these two units (e.g., Fig. 3) indicates that the compositional segregation
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of the leucocratic and granodioritic phases occurred prior to formation of the magmatic
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foliations.
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In addition to these principal phases of the Jackass Lakes pluton, granodiorite
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intrusions crop out along the western margin and have been previously described as
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distinct plutons (e.g., Peck, 1980). The granodiorite of Breeze Lake is the youngest unit
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within the field area based on cross cutting relationships (Fig. 3). Peck (1980) described
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the intrusion as an elongate north-northwest trending stock located in the western part of
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the field area. It is an undated, fine-grained biotite ± hornblende granodiorite to granite
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with hypidiomorphic texture. Relatively small (< 20 cm) mafic magmatic enclaves are
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scarcely observed. The fine-grained phaneritic texture makes fabric recognition in the
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field difficult. However, a north-northwest-striking foliation is present with some local
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variation (Fig. 3). Contacts between the granodiorite of Breeze Lake and the Jackass
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Lakes pluton are sharp and distinct with inclusions of the Jackass Lakes granodiorite
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contained in the granodiorite of Breeze Lake, implying that the Jackass Lakes is older.
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Elsewhere, mafic magmatic enclave swarms within the Jackass Lakes granodiorite are
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concentrated near stepped, angular contacts with the granodiorite of Breeze. We interpret
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these relationships to indicate that the mafic enclaves in the Jackass Lakes magma
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collected adjacent to the rigid Breeze Lake contact. Because contact relations are
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sometimes ambiguous and lithologies are similar, we interpret these as separate, but
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probably related (comagmatic) intrusions with the Jackass Lakes pluton.
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2.2 Metamorphic host rocks
Host rocks of the Jackass Lakes pluton consist of metasedimentary rocks of
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possible Paleozoic to Mesozoic age and middle Cretaceous metavolcanic rocks (Peck,
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1980). Highly deformed Jurassic (?) metasedimentary rocks crop out sporadically
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throughout the field area (Figs. 3, 4; ages from Peck, 1980). The rocks are primarily
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thinly-bedded, micaceous quartzites, quartzite, and semi-pelitic schists and contain the
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metamorphic mineral assemblage of muscovite + biotite (after cordierite?) + quartz +
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plagioclase. A cordierite precursor to the micas suggests lower amphibolite grade
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metamorphic conditions (~6000C; Spear, 1993). Outcrop- and map-scale folds deform
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the metasedimentary rocks and are truncated by the contact of the Jackass Lakes pluton.
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These structures are discordant to the main foliation in the metavolcanic rocks and the
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Jackass Lakes pluton and are interpreted to reflect regional Jurassic-Cretaceous (?)
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deformation that affected the framework rocks of the Sierra Nevada batholith prior to ~98
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Ma. (e.g., Tobisch et al., 1989)
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Metavolcanic rocks of the mid-Cretaceous Minarets Caldera sequence comprise
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the majority of screens, wall rocks, roof pendants and xenoliths within the intrusive units
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of the field area (Fig. 3). Fiske and Tobisch (1978; 1994) interpreted these rocks to be
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remnants of a caldera-fill complex that they called the Minarets Caldera. Thick
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sequences of the caldera fill are well-documented within the Ritter Range pendant, 15 km
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to the east of the field area (Fiske and Tobisch, 1978; 1994). The western-most exposures
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of the metavolcanic rocks within the Jackass Lakes pluton-host rock system occur in the
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Sing Peak pendant and are some of the most extensive exposures within the pluton (Fig.
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2).
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2.3 Sing Peak pendant
The Sing Peak pendant is comprised of two distinct metavolcanic units that vary
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significantly in terms of their stratigraphic characteristics, structure, contact relations and
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deformation intensity. The southern portion of the pendant is primarily plagioclase-
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phyric dacitic to rhyolitic metatuff (Fig. 8), containing plagioclase + biotite/chlorite +
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hornblende/actinolite + quartz ± garnet ± muscovite ± potassium feldspar. Metamorphic
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foliations, primarily defined by elongated hornblende and biotite aggregates, strike north-
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northwest and dip steeply to the west. Poles to metamorphic foliations and the trend and
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plunge of metamorphic lineations are plotted in Figure 7. Lithic fragments are rare but
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when observed are elongated within the metamorphic foliation. The sharp intrusive
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contact with the leucogranitic unit of the Jackass Lakes pluton is shallowly dipping in the
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southern portion of the pendant near Madera Peak (Fig. 4). Locally, 5 m-wide dikes of
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the granodiorite of Jackass Lakes intrude the pendant. Deformed porphyritic granodiorite
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sills and dikes spatially related or connected to the underlying intrusion are common (Fig.
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8B, C) and show various states of crystal-plastic and solid-state deformation.
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The northern section of the Sing Peak pendant is characterized by the same meta-
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plagioclase-phyric dacite to rhyoltic metatuff rock types found in the southern portion of
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the pendant and a more mafic, meta-lapilli tuff and tuff breccia consisting of plagioclase
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+ biotite/chlorite + hornblende/actinolite + epidote + garnet + quartz ± muscovite ±
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potassium feldspar. A weak to strong north-northwest-striking foliation is defined in
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both rock types by porphryoclasts of plagioclase surrounded by strongly recrystallized
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and elongated biotite, quartz, ± epidote and ± amphibole. Throughout the northern
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portions of the pendant, the contact with the granodiorite of Jackass Lakes is steeply
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dipping either westward or eastward indicating a keel-like shape of the northern pendant
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in three dimensions (Figs. 3, 5). Discontinuous lenses of porphyritic granodiorite are
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found parallel to the north-trending metamorphic foliation within the metavolcanic rocks.
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These intrusions show various stages of solid-state deformation (e.g., Fig. 8B). Mafic
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magmatic enclaves mingling with deformed felsic lapilli meta-tuff xenoliths are locally
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preserved within these sills of porphyritic granodiorite (Fig. 8C).
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3. Structural analysis
The structural fabric of the Jackass Lakes pluton-host rock system is dominated
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by magmatic and subsolidus foliations and lineations and a newly recognized high-strain
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zone located in the southwestern portions of the pluton. Detailed mapping (1:10,000; Fig.
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3) within the Illilouette Creek, Breeze Lake, and Jackass Lakes granodiorites documented
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variations in the intensity and orientation of magmatic fabrics within each intrusive unit.
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The magmatic fabrics are defined by aligned biotite ± hornblende phenocrysts and
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lensoidal mafic magmatic enclaves. Microstructural observations indicate minimal
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crystal-plastic overprint of the fabric; therefore, following the criteria of Paterson et al.
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(1989), we interpret mineral alignment to have occurred in the hypersolidus state. Planar
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magmatic fabrics such as foliations, schlieren banding, and mafic magmatic enclave
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swarms show a strong preferred orientation with a north-northwesterly strike (Fig. 7).
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Magmatic foliation display little to no deflection in orientation when crossing from one
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intrusive phase to another (Fig. 3), even where sharp contact relations indicate a definite
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younger/older relationship (e.g., the contact of the northern Breeze Lake phase with the
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Jackass Lakes granodiorite, Fig. 3).
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Magmatic lineations are moderately to well-developed in the foliation plane in the
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field area. These fabrics are particularly well developed in the northern region of the field
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area and around the northern tip of the Sing Peak pendant (Fig. 3) and are moderate to
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shallowly plunging with a north trend (Fig. 7). This shallow north plunge and trend is in
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contrast to more steeply plunging magmatic lineations in the interior and eastern portions
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of the pluton, as reported by Pignotta et al. (2010).
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3.1 Sing Peak Shear Zone
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A previously undocumented shear zone is located throughout the northern and
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central portions of the pendant (Fig. 3) and affects both the metavolcanic rocks and the
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surrounding granodiorite of the Jackass Lakes pluton. The deformation is characterized
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by i) a zone of mylonitic metavolcanic rocks with a sub-vertical north-northwest-striking
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foliation defined by stretched and flattened volcanic breccias and lapilli (Figs. 8, 9), ii)
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hypersolidus foliations and subsolidus mylonitic foliations and S-C structures within the
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granodiorite (Fig. 9B), iii) asymmetric, dextral prophyroclasts of plagioclase, epidote,
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biotite and lithic fragments (Fig. 10), and iv) a moderate (30-45°) north-plunging
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lineation in both the metavolcanic rocks and granodiorite (Fig. 7).
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Microstructural analysis of the shear zone samples reveals kinematic indicators
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and conditions of deformation. Magmatic fabrics within the host Jackass Lakes
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granodiorite show a continuous transition from near-solidus to subsolidus deformation
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mechanisms within the shear zone along the margins of the pendant. Aligned euhedral to
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subhedral feldspars and amphibole phenocrysts in the granodiorite become progressively
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deformed approaching the shear zone, displaying evidence for grain boundary migration
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recrystallization indicative of high-temperature recrystallization. Sigma-type feldspar
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porphyroclasts (e.g., Passchier and Trouw, 2005) of plagioclase and potassium feldspar
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within mylonitic rocks of the Sing Peak shear zone contain recrystallized tails of
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feldspars, biotite, muscovite, and quartz (Fig. 10). These asymmetric porphyroclasts
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indicate a dextral sense of shear in thin section and on the outcrop. Magmatic foliations
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and lineations in the surrounding granodiorite are parallel to planar and linear fabrics in
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the metavolcanic rocks (e.g., Fig. 7) and display the same dextral-oblique shear sense
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indicators. Both of the mafic and felsic lithologies in the pendant contain porphyroclasts
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of plagioclase and potassium feldspar as well as metamorphic epidote, and relict
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hornblende phenocrysts in a fine-grained recrystallized quartz matrix. Epidote and
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hornblende are more prevalent in the mafic metatuff and are primarily responsible for the
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variation in pendant rock types. Anhedral/euhedral garnet porphyroblasts are found
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within the metatuffs in thin bands that contain the shallow to moderately plunging
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lineation. Dynamically recrystallized quartz comprises the matrix within the pendant
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rocks independent of deformation. Based on the presence of garnet and the degree of
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recrystallization of plagioclase and potassium feldspar, deformation was probably in the
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range of ~500-600 ºC and pressures of 300-400 MPa (Spear, 1993; Passchier and Trouw,
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2005).
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In addition to these structures, granodioritic and granitic dikes that emanate from
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the host Jackass Lakes pluton are folded within metavolcanic xenoliths and record
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approximately east-west shortening during dike emplacement and crystallization. Dikes
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ranging in width from mm to several meters define open to isoclinal folds with axial
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planes that are parallel with regional magmatic foliations in the pluton and gneissic to
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mylonitic foliations within the host rocks (Fig. 11). In outcrop, these dikes display a
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variety of igneous fabrics (e.g., phase layering, magmatic foliations, xenolith alignment)
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and commonly have an axial planar foliation defined by the alignment of amphibole,
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biotite, plagioclase and/or lithic fragments that is parallel with the regional foliations
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(Fig. 11). Lithic fragments and deformed volcanic lapilli within the host rocks are aligned
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parallel to the regional magmatic fabric orientation in the Jackass Lakes pluton.
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Microstructural analysis of the folded dikes indicates that axial planar foliations are
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defined by the alignment of plagioclase feldspar laths, biotite (010) planes, and
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amphibole crystals (Fig. 12). Quartz displays some grain boundary bulging and subgrain
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development as well as local, discreet bands of crystal-size reduction parallel to the axial
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plane. Thin granitic dikes tend to display greater amounts of crystal-plastic strain in the
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form of dislocation creep in quartz (Fig. 12C, D). K-feldspars contain patchy extinction,
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kinking, and some micro-fracturing (Fig. 12D). However, the minimal to non-existent
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evidence for crystal plastic deformation mechanisms such as dislocation creep in
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feldspars is consistent with axial planar foliation development in the presence of melt
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(e.g., Vernon, 2000).
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4. Discussion
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Deformation within the Sing Peak shear zone affects both metavolcanic rocks and
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igneous rocks of the Jackass Lakes pluton and contains moderate/shallow north-plunging
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lineations and evidence of dextral, transcurrent shear at ca. 98 Ma. The following
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discussion will place the above observations into a regional geologic context. We will
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then explore the implications of the results to shear zone development, magma
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emplacement, and the regional plate kinematics between the Farallon and North
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American plates during the middle to Late Cretaceous construction and evolution of the
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Sierra Nevada batholith.
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4.1 ca. 98 Ma Deformation – Regional or „emplacement‟-related?
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Establishing a link between parallel, well-developed magmatic foliations and
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lineations within the granodiorite of Jackass Lakes and metamorphic foliations and
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lineations in the Sing Peak pendant and xenoliths is essential in constraining the timing
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and nature of deformation within this part of the central Sierra Nevada batholith. The
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moderately to shallowly plunging lineation and dextral kinematic indicators throughout
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the metavolcanic rocks of the pendant and the adjacent granodiorite of Jackass Lakes are
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consistent with transpressional kinematics. Two hypotheses may explain these structural
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relationships. First, the Sing Peak shear zone may be a region of localized ductile
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deformation that is related to emplacement of batches of magma. For example, during
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magma intrusion, localization of deformation may occur along the margins of the igneous
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body during inflation of a growing magma chamber. If this occurred during assembly of
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the Jackass Lakes pluton, then inflation of the intrusion involved shearing, as opposed to
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bulk flattening (e.g., “ballooning”, Sylvester et al., 1978). In such a scenario, fabric
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formation is produced by buoyancy forces attending magma emplacement during cooling
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and crystallization and is “local” in extent. The second hypothesis involves regional
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tectonic deformation that is localized within the evolving arc (e.g., Tikoff and Teyssier,
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1992; Sharp et al., 2000; Horseman et al., 2008).
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Magmatic fabrics observed in the granodiorite of Jackass Lakes are parallel
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throughout the pluton, including across distinct compositional zones and with mafic
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magmatic swarms (this study; McNulty et al., 1996; Pignotta et al., 2010). This
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observation, and the lack of any significant deflection of magmatic foliations around
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xenoliths, suggests that fabric formation occurred after xenolith incorporation and likely
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late in the history of chamber construction (Wolak, 2004; Yoshinobu et al., 2009;
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Pignotta et al., 2010; cf. Fowler and Paterson, 1996). Thus, the magmatic fabric is
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interpreted to preserve the youngest increment of strain and probably has overprinted any
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evidence for earlier flow or strain of the magma during emplacement into the reservoir. It
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is reasonable then to infer that the planar and linear fabrics within the map area record the
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ambient regional deformation field at the time of fabric formation.
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We argue that the structural and timing relations described above are most
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consistent with shear zone localization due to regional deformation partitioning based on
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the following. First, metamorphic fabrics within the roof pendant and xenoliths, including
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syn-magmatic fold axes and axial planes, have a consistent northwest trend (Fig. 7).
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These fabrics are parallel to magmatic foliations and lineations observed within the
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granodiorite of Jackass Lakes (Figs. 7, 11). Second, the magmatic foliations cross cut
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various phases of the pluton (e.g., the leucocratic and granodioritic phases; cf. Pignotta et
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al., 2010) and therefore post-date, or are diachronous with, the formation of the various
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phases at the site of final solidification. The parallelism of these structures at the km-
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scale is consistent with syn-magmatic folding and fabric development contemporaneous
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with magma emplacement and pluton assembly over a crystallization interval from ~98 to
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97 Ma. Third, shallowly- to moderately-plunging hypersolidus and subsolidus lineations
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as mapped in this study are in contrast to steeply plunging hypersolidus lineations
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mapped in the central and eastern portions of the pluton (Pignotta et al., 2010). We
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suggest that the spatial transition from steeply to shallowly plunging lineations reflects a
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hypersolidus dextral-oblique, “transpressional” shear zone developed within the Jackass
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Lakes pluton, similar to the Gem Lake-Cascade Lake shear zone in the eastern Sierra
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(Tikoff and Greene, 1997; Tikoff et al., 2005). Therefore, the “boundaries” of the Sing
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Peak shear zone can thus include the western granodiorite of Jackass Lakes and extend
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further northward out of the mapping area, possibly even including much more of the
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pluton (Fig. 13).
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The lack of significant solid-state deformation within the granodiorite of Jackass
Lakes suggests that the transcurrent + shortening strain regime was active prior to (?) and
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during emplacement of the granodiorite at 97-98 Ma. Reconnaissance work in the ~99
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Ma Illilouette Creek pluton to the west has revealed magmatic fabrics of similar
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orientation and intensity (Fig. 3) suggesting that the strain regime existed prior to
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emplacement of the Jackass Lakes pluton. The ~95 Ma Red Devil Lake pluton to the
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north displays similar patterns in magmatic fabric orientation but different kinematics of
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deformation in the form of post ~95 Ma discreet extensional shears along the Bench
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Canyon shear zone within the Red Devil Lake pluton is preserved (McNulty, 1995).
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4.2 Implications for ca. 98 Ma deformation in the central Sierra Nevada batholith
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Cretaceous shear zones in the central Sierra Nevada are typically broken up into
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two groups; 1) those that show steeply plunging stretching lineations and 2) those that
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show mostly oblique and/or sub-horizontal stretching lineations (Table 1; Tobisch et al.,
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1995). Mid-Cretaceous shear zones tend to be defined by steeply plunging lineations,
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whereas Late Cretaceous shear zones (ca. 90 Ma or younger) tend to have a component
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of oblique or strike-slip displacement. Two shear zones, the Bench Canyon and Quartz
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Mountain shear zones are located in close proximity to the Sing Peak shear zone and
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have been extensively studied (Fig. 1; McNulty, 1995; Tobisch et al., 1995). The Bench
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Canyon shear zone, located near the eastern contact between the granodiorite of Jackass
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Lakes and the 132-144 Ma rocks of the Minarets Caldera sequence, has a long and
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complex deformational history. McNulty (1995) concluded that the Bench Canyon
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underwent three distinct periods of deformation; 1) 101-95 Ma extension, 2) 95-90 Ma
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contraction and 3) 90-78 Ma extension. The early (101-95 Ma) deformational history is
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cryptic and was based on the observation of extensional structures (normal faults,
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extensional low-angle ductile shears and asymmetric extensional fractures) outside of the
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shear zone proper (McNulty, 1995). None of these structures was observed within the
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present field area nor in the Jackass Lakes granodiorite to the east (Pignotta et al., 2010).
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The Quartz Mountain shear zone is located approximately 5 km to the west of the
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field area (Fig. 1). Deformation is preserved within solid-state fabrics of the granodiorite
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of Illilouette Creek and the granodiorite of Ostrander Lake (Tong, 1994). Steeply north
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plunging stretching lineations and a lack of extensional structures in the region suggested
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that deformation occurred during a period of contractional strain within the arc (Tong,
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1994).
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Shear zones along strike with the Quartz Mountain shear zone, including the
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Kaiser Peak and Courtwright-Wishon, display similar deformational histories (Tobisch et
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al., 1995) (Fig. 1). Pre-90 Ma, contractional deformation was predominant along the
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Quartz Mountain and Kaiser Peak shear zones (Tobisch et al., 1995). Extension
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associated with the construction of the Mount Givens granodiorite at 90 Ma. is preserved
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within the Kaiser Peak and Courtwright-Wishon shear zones (Tobisch et al., 1995). The
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similar deformational history and strike orientation of these shear zones suggests they
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may have been part of a larger shear zone system prior to their separation during the
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construction of the Mount Givens granodiorite (Tobisch et al., 1995, McNulty et al.,
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2000).
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The above kinematic history and that observed within shear zones located farther
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to the east and southeast of the field area suggest a change in kinematics around 90 Ma
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during construction of the Sierran arc. Greene and Schweickert (1995) mapped the Gem
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Lake shear zone and observed moderately/steeply plunging stretching lineations that were
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accompanied by several types of dextral kinematic indicators (S-C fabrics, asymmetric
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porphyroclasts and crenulations). Field constraints suggest that the dextral transpression
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was initiated as early as 91 Ma continued until 80 Ma within the shear zone. Moderate
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and/or shallow plunging lineations are observed to the southeast within the Rosy Finch
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shear zone and are also interpreted to be the result of regional transpression (Tikoff and
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Teyssier, 1992, 1994). Several authors propose that the oblique component of
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convergence became dominant around 90 Ma and was responsible for the strike-slip
442
motion observed on these two eastern shear zones (Engebretson et al., 1985; Glazner,
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1991; Tikoff and Teyssier, 1992, 1994; Tobisch et al., 1995; McNulty, 1995). The
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similarities in kinematics and the age of deformation along these two shear zones lead
445
some authors to propose that they are part of the Sierra Crest shear zone that may extend
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more than 150 km along strike (Tikoff and Saint Blanquat, 1997; Tikoff and Greene,
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1997).
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Moderate/shallow plunging stretching lineations within the Sing Peak shear zone
449
suggests a transpressional strain regime during emplacement of the granodiorite of
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Jackass Lakes at ~97-98 Ma. Engebretson et al. (1985) noted that around 100 Ma there
451
was a significant component of obliquity between the subducting Farallon plate and the
452
North American plate (Fig. 13B). Tobisch et al. (1995) suggested that around 100 Ma the
453
convergence vector made an angle ~ 20º to the arc normal. It was around this time that
454
the convergence vector passed through a “critical point/angle” initiating the dextral
455
deformation (Tobisch et al., 1995). We suggest that dextral-oblique shearing, or
456
“transpression” in the common vernacular, likely began around ~100 Ma and was active
457
during the syntectonic assembly of the granodiorite of Jackass Lakes. Figure 14 depicts
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an oblique, schematic view of our interpretation of shear zone development in the context
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of mid-Cretaceous assembly of the Jackass Lakes pluton. We envision a system of
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braided shear zones including the Sing Peak, Bench Lake and other shear zones, in which
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contractional and transcurrent displacement was partitioned during deformation of the
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overriding plate and batholith growth. In this hypothesis, the site of pluton assembly is
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not directly linked to shear zone behavior. Rather, the growing pluton – assembled by a
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variety of mechanisms – provides a focus for deformation localization because of the
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inherent temperature-dependent viscosity contrast between the magmas and the host
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rocks.
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The Sing Peak shear zone differs substantially from the previously mentioned
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central Sierra Nevada shear zones in several characteristics. First, the Sing Peak shear
469
zone is the only shear zone in which deformation is preserved in both the solid and
470
magmatic state. Each of the shear zones listed in Table 1 is defined by solid-state fabrics
471
either in host rocks and/or plutonic rocks (cf. Tikoff et al., 2005 for evidence for a solid-
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state overprint of hypersolidus fabrics). This interpretation poses questions regarding 1)
473
the dimensions of shear zones as mapped in plutonic bodies, and 2) the importance and
474
significance of identifying magmatic fabric trends and their causes. We hypothesize that
475
fabrics within the entire granodiorite of Jackass Lakes may record regional, dextral-
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transcurrent strains imparted by plate motions rather than local, “emplacement-related”
477
deformation. Given the area of the pluton (viz. ~13 km wide and at least 17 km long), this
478
shear zone is much wider than any of the central Sierra Nevada shear zones (e.g., Fig. 1).
479
Since shear zones in the central Sierra Nevada have traditionally been defined by “solid-
480
state” deformation, the idea that syntectonic plutons can represent zones of pervasive
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shearing warrants further testing. Detailed mapping of solid-state shear zones and their
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nearby plutons may improve our understanding of strain variations during arc
483
construction.
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5. Conclusions
486
Detailed mapping, coupled with structural analysis, allows for a concise structural
487
evolution to be extracted for the Jackass Lakes pluton – host rock system that is related to
488
middle Cretaceous plate kinematics. The Sing Peak shear zone is the earliest documented
489
regional transpressional shear zone in the Cretaceous central Sierra Nevada batholith,
490
suggesting that significant oblique-slip motion was active within the arc prior to 90 Ma.
491
The Jackass Lakes pluton-host rock system preserves evidence for syn-emplacement
492
shortening and transcurrent motion. Assembly of the Jackass Lakes granodiorite pluton
493
and shear zone development occurred simultaneously. Parallel magmatic and
494
metamorphic fabrics including moderate to shallowly plunging lineations, dextral
495
kinematic indicators, and synmagmatic folding of granodioritic dikes of the Jackass
496
Lakes pluton indicate that a regional dextral transpressional strain field was present ca. 98
497
Ma. The regional transpressional strain field was active from approximately 98 Ma to no
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later than 95 Ma. This study further indicates that the orientation of magmatic fabrics
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may preserve important information on the geometry of paleostrain fields within ancient
500
magmatic arcs and provides a new tool that can be used to unravel plate kinematics in
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exhumed magmatic arcs.
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Acknowledgements
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Research was funded in part by NSF grants EAR-0106557 and EAR-0439750 to
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Yoshinobu. We thank Brendan McNulty, Geoff Pignotta, Scott Paterson, and Jeannette
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Wolak for their insights into the Jackass Lakes pluton, and Dallas Peck for his
507
impeccable mapping of the Merced Peak quadrangle that set the stage for our work. We
508
gratefully acknowledge the field assistance of Jeannette Wolak, David Martin, Mike
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Blevins, Nate Zimermann, Don, Natalie, Maaike, Jacob and Peiter Weilenga, and Celeste,
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Weston, Miles, and Galen Yoshinobu. Stereonets were made with Rick Allmendinger‟s
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software.
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Figure Captions
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Figure 1. Tectonic map of the central Sierra Nevada batholith depicting the major
636
Cretaceous shear zones, intrusive suites, and plutons noted in the text. After Bateman
637
(1992).
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Figure 2. Simplified geologic map of the Jackass Lakes pluton and host rocks and
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location of detailed mapping of Sing Peak pendant. Modified from Peck (1980).
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Figure 3. Geologic map of the Sing Peak pendant, southwest quarter of the Jackass Lakes
643
pluton (see Fig. 2 for location). Topographic base from 1:24,000 Sing Peak and Timber
644
Knob U.S. Geological Survey quadrangles. Geodetic Datum NAD27, UTM Grid Zone
645
11.
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Figure 4. Panorama images of the Sing Peak pendant and enclosing plutonic rocks. A.
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Eastward view of Madera Peak with metasedimentary (Jm) and metavolcanic
649
(Kfm/Kmm) xenoliths in the foreground enclosed in granodiorite (Kja). B. Westward
650
view of Madera Peak illustrating the sub-horizontal nature of the leucogranite (Klja)
651
beneath the southern Sing Peak pendant. Approximate location of the field of view and
652
legend in Figure 3.
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Figure 5. Geologic cross sections of the Sing Peak Pendant. Legend same as Figure 3.
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See Figure 3 for location of cross section lines.
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Figure 6. Field photographs of contact relations between granodiorite (Kja) and
658
leucogranite (Klja) of Jackass Lakes pluton. A. Cross-cutting contact indicating younger
659
Klja with respect to Kja. B-C. Complex contact relationships where schlieren bands and
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modal igneous layers separate the two units.
661
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Figure 7. Lower-hemisphere, equal area stereonet displaying poles to magmatic and
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metamorphic foliations and magmatic and metamorphic lineations within the region
664
shown in Figure 3.
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Figure 8. Field photographs of metavolcanic rocks in the Sing Peak pendant. A. Picture
667
of a backpack resting on interlayered mafic (Kmm) and felsic (Kfm) andesitic to rhyo-
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dacitic volcanic rocks. B. Sill of the Jackass lakes granodiorite deformed in the plane of
669
foliation within meta-andesites. Compass for scale. C. Meta-andesite (e.g., Kmm)
670
intruded by granodiorite of Jackass Lakes (Kja). Note the presence of elongate mafic
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magmatic enclave (black arrow) and deformed, felsic lapilli tuff within the granodiorite.
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Figure 9. Field photographs of A. deformed volcanic breccia and lapilli tuff in the
674
northern Sing Peak shear zone. B. „S-C‟ structures within metavolcanic rocks on a sub-
675
horizontal surface. North is to the right; lineation plunges shallowly to the right,
676
indicating dextral, oblique-slip displacement.
677
678
Figure 10. Microstructural kinematic indicators within the Sing Peak shear zone. A.
679
Plane-light photomicrograph of a plagioclase porphyroclast with recrystallized
680
qtz+fld+bte defining dextral „sigma‟ tails from the metavolcanic rock shown in Fig. 9A.
681
B. Plagioclase „delta‟ porphyroclast with qtz+bte tails indicating dextral rotation. Both
682
images represent „kinematic‟ sections and are cut parallel to the lineation and
683
perpendicular to the foliation.
684
685
Figure 11. Field photographs of folded granodiorite dikes contained within metavolcanic
686
xenoliths and screens in the Jackass Lakes pluton. A. Medium-crystalline, 1 m-wide
687
granodiorite dike intruded into meta-andesite and folded about north-striking axial planes
688
parallel to metamorphic foliations in xenolith (from Yoshinobu et al., 2009). Axial planar
689
metamorphic foliation in the xenolith is parallel with magmatic foliations within the dike
690
(see Yoshinobu et al., 2009 for details). B. Line drawing of A highlighting the folded
691
igneous layering within the sea-monster-like dike. C. Late granitic dike cutting Kja and
28
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
Krueger and Yoshinobu, Plutons and Plate Motions… submitted to Tectonophysics, 2012
692
meta-andesite and folded about axial planar metamorphic foliations within the xenolith.
693
D. Dike of the granodiorite of Jackass Lakes folded about axial planes that are parallel
694
with magmatic foliations in the pluton and metamorphic foliations within the xenolith. E.
695
Folded granitic dike; axial plane of fold is parallel to metamorphic foliation in meta-
696
andesite. F. Close-up view of dike in E displaying fine igneous textures within the hinge
697
region of the folded dike. G. Asymmetrically folded granitic dike in meta-andesite. H.
698
Northward view of synformal granodioritic dike with axial planes parallel to north-
699
striking metamorphic foliations within the enclosing meta-andesite. I. Outcrop view of
700
folded granodioritic dike in meta-andesite. J, K. Close up of dike in Fig. 10I. Note
701
magmatic foliation (parallel to hammer handle and pencil) that is at high angles to dike
702
wall and is within 10° of the metamorphic foliation within the xenolith (black arrows).
703
704
Figure 12. Magmatic and crystal-plastic structures. White dashed line is orientation of
705
foliation in all samples. A. Thin-section view in cross-polarized light of granodiorite of
706
Jackass Lakes displaying magmatic foliations defined by aligned plagioclase feldspar (P)
707
and amphibole (A). B. Thin-section view in cross-polarized light of magmatic foliation
708
with overprinting crystal-plastic microstructure. Quartz (Q) displays evidence for discreet
709
bands of recrystallization (white arrows) and crystal-size reduction. C. Polished slab of
710
deformed metavolcanic xenolith (kfm) and folded granitic dike displaying axial-planar
711
foliation. D. Thin-section photomosaic in cross-polarized light of folded dike in C
712
displaying axial-planar foliation defined by aligned k-feldspar (K) porphyroclasts, and
713
coarse- and fine-crystalline, recrystallized quartz (black and white arrows, respectively).
714
Kfm = Cretaceous felsic meta-volcanic rocks.
715
716
Figure 13. Trend lines of magmatic and metamorphic foliations (in metavolcanic rocks)
717
that define the magmatic-crystal plastic Sing Peak shear zone.
718
719
Figure 14. A. Oblique, disected perspective sketch of ca. 98-97 Ma intra-arc deformation
720
and dextral-oblique plate convergence. Hypothesized relationship between the Jackass
721
Lakes pluton, Sing Peak shear zone, and temporally and spatially-related shear zones in
29
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
Krueger and Yoshinobu, Plutons and Plate Motions… submitted to Tectonophysics, 2012
722
the central Sierra Nevada batholith is shown in the subsurface. Fabrics within the Jackass
723
Lakes pluton (JLP) are the result of transpressional deformation attending arc evolution
724
and assembly of the pluton. BL: Bench Lake shear zone; GL-CL: Gem Lake-Cascade
725
Lake shear zone. See Figure 1 for comparison of regional geology. B. Farallon/Kula plate
726
kinematics relative to fixed North America during the middle Cretaceous (modified from
727
Engebretson et al., 1985).
728
729
730
731
732
733
734
30
Table
Click here to download Table: Krueger-Yoshinobu-Table-1.pdf
Krueger and Yoshinobu, Plutons and plate motions...
submitted to Tectonophysics
Table 1. Summary of mid-Cretaceous shear zones in the central Sierra Nevada batholith.
Shear Zone Name
Age of activity
Lineation, other structures
Interpreted
(Ma)
Strain Field
Bench Canyon
early
101-95
local, dicsreent shear bands, faults,
(weak early extension)
main
95-90
steep to moderate plunging lineations
contractional
late
90-78
steep to moderate foliations
Quartz Mountain
~98
steep to north-plunging, steep
contractional
foliation, retrograde T-t path >600°400°; reverse kinematics
Sing Peak
98-97
moderate to shallowly north plunging
dextral
hypersolidus-subsolidus foliations,
transpressional
S-C mylonites
Kaiser Peak
102-91 with
steep lineation/foliation, S-C fabrics,
contractional
~95 Ma peak
reverse kinematics
Courtwright-Wishon
early
90
steep foliation/lineation;
weakly extensional
normal kinematics
late
post-90
steep folaition/lineation;
contractional
co-axial shortening
Sierra Crest Shear Zone System
~90
steep foliations, moderate to steep
Cascade Lake
dextral
lineations, S-C fabrics
transpression
91-80
steep foliations, steep to moderately
dextral
Gem Lake
north-plunging lineations,
transpressional
S-C fabrics, etc.
Rosy Finch
88-84
steep and shallow, north-plunging
dextral
steep hypersolidus-subsolidus foliations,
transpressional
S-C fabrics, etc.
Reference
McNulty, 1995
Tobisch et al., 1995
Tong, 1994
this study
Tobisch et al., 1995
Tobisch et al., 1993; 1995
Tikoff et al., 2005
Greene and Schweickert, 1995;
Tikoff and Greene, 1997
Tikoff and Teyssier, 1992;
Saint Blanquat and Tikoff, 1997;
Tikoff and Saint Blanquat, 1997
Figure1
38° N
120° W
119° W
?
Bench Lake
Shear Zone
Gem Lake/
Cascade Lake
shear Zone
Sing Peak
Shear Zone
Western
Metamorphic
belt
Ritter Range
roof pendant
Rosy Finch
Shear Zone
?
Fig. 2
Quartz Mtn.
Shear Zone
CA
37° N
Map
Area
Sierra Nevada
Batholith
118° W
~90 Ma.
Mt. Givens Pluton
Kaiser Peak
Shear Zone
Cenozoic rocks
undifferentiated
95-85 Ma Tuolumne Intrusive
Suite
92-88 Ma John Muir Intrusive Suite
98-97 Ma Jackass Lakes Pluton
107-95 Ma Washburn Lake, Buena
Vista Crest and Merced Peak
Intrusvive Suites
135-97 Ma Yosemite Valley, Fine Gold,
and Shaver Lake Intrusive Suites
undifferentated
Paleozoic and Mesozoic
Metamorphic Host Rocks
N
CourtwrightWishon Shear Zone
Krueger and Yoshinobu, Figure 1.
Tectonic map of the central Sierra Nevada batholith depicting the major Cretaceous shear
zones, intrusive suites, and plutons noted in the text. After Bateman (1992).
0
20
Km
Figure2
Sierra
Nevada
batholith
K
ca. 95-85 Ma plutons
85
Minarets
Caldera
Sequence
Field
area
Khd
60
80
107-99 Ma
plutons
70
K
K
50
O
Fig. 3
K
Y
K
85
85
80
Y
K
O
85
O Y
85
75
85
80
K
75
K
K?
70
37°30’
K?
50
0
90 Ma Mount Givens pluton
98 Ma Jackass Lakes pluton
Leucocratic phase of the Jackass
Lakes pluton
Metasedimentary and metavolcanic
xenolith fields
K
Jr
N
60
Jr
Jurassic (Jr) metasedimentary and
Cretaceous (K) metavolcanic rocks
undifferentiated
1
2
km
119°30’
Magmatic foliation inclined; trace
Metamorphic foliation inclined; vertical
O
Y Older/younger intrusive
contact relationship
Krueger and Yoshinobu, Figure 2.
Figure 2. Simplified geologic map of the Jackass Lakes pluton and host rocks and
location of detailed mapping of Sing Peak pendant. Modified from Peck (1980).
Figure3
Ki
metasedimentary orgin
Kb Breeze Lake granodiorite
82
Kmq
Illilouette granodiorite
77
83
85
73
83
80
Qa
82
50
48
86
Kja
76
85
71
Kfm
34
71
85
55
85
Klja
119° 22' 30''
85
43
84
78
60
24
30 83
68
83
73
78
48
73
73
85
79
70
78
73
73
61
Kja
87
76
79
68
7
67
Klja
65
65
79
Kja
72
78
65
75
82
83
75
85
77
86
63
75
87
82
70
Kfm
75
Kja
Kfm
69
80
Jm
Kfm
33
78
85
Kmm
70
Kja
61
Kmm
84
69
Kb
80
80
82
75
86
84
87
31
Ki
58
64
81
85
82
73
60
80
50
82
87
Ki
80
74
79
73
57
O
Qa
72
79
60
72
82
m
86
Klja
84
45
Km
49
75
82
68
70
Klja
Kja
42
86
Kja 76
75
Jm
50
74
65
87
Jm
Qa
59
46
s
Jm
Jm
67
82
Qa
74
Kmq
s
87
78
39
86
87
75
s
Kja
47
80
Y
79
86 79
Qa
79
71
Qa
64
Kmm
45
49
Jm
39
85
68
Kmq
45
Jm
Kja
86
85
81
85 Ki
79
85
70
85
81
70
86
64
86
Jm
Kja
71
75
63
75
Jm
A
Kmm
65
Kfm
80
70
65
72
82
Kja
B
Jm Jurassic (?) phyllites, schists, gneisses of
86
78
Klja Jackass Lakes lueco granite
O
Y
Klja
7
60 4
Kfm Felsic metavolcanic rocks (rhyolite & dacite)
Xenolith-rich locality
Ki
field of
view Fig.
4A
69
Kmm Mafic metavolcanic rocks (andesite)
Jm
Kja Jackass Lakes granodiorite
Contour interval 40 feet; topog
Timber Knob USGS Quadrangles
37° 32' 30''
Kmq Cretaceous (?) Quartzofeldspathic gneiss (metatuff?)
Quaternary undifferentiated
s
Qa
75
82
88
83
71
80
85
Kja
Kja
64
78
79
80
Qa
67
77
65
Qg
77
Kja
72
B'
73
87
61
field of view
Fig. 4B
60
79
73
A'
71
85
80
80
74
79
80
37° 32' 30''
Krueger and Yoshinobu, Figure 3
Figure 3. Geologic map of the Sing Peak pendant, southwest quarter of the Jackass Lakes pluton
(see Fig. 2 for location). Topographic base from 1:24,000 Sing Peak and Timber Knob U.S. Geological
Survey quadrangles. Geodetic Datum NAD27, UTM Grid Zone 11. This figure is spread over two pages
Qa
tour interval 40 feet; topographic base from 1:24,000 Sing Peak and
Knob USGS Quadrangles. Geodetic Datum NAD27 UTM Grid Zone 11.
Kja
81
76
77
70
86
Kb
82
82
87
81
Jm
Y
84 O
85
85
Qa
70
84
84
82
24
Kja
Kfm
83
83
40
47
83
47
82
Kfm
45
56
76
54
76
72
47
80
69
72
81
71
84
82
85
70
77
Kja
70
Kja
72
72
86
78
86
82
74
75
61
Klja
37° 35' 00''
Structural
Symbols
84
71
61
74
86
73
76
Kja
77
80
Qa
85
74
84
38
45
119°22'30''
Kmm
85
80
54
38
Qa
73
79
82
79
70
77
85
85
46
70
89
45
85
66
82
89
77
48
75
72
86
86
73
19
86
Kfm
73
52
Kja
82
78
Qa
Kja
45
73
85
Kmm
79
78
85
81
86
78
82
76
86
0 83
68
88
69
Qa
73
60
77
88
76
78
m
Qa
O
82
Kja
65
78
Y
83
68
77
81
Kb
75
87
82
76
Kb
85
80
78
75
79
Jm
Kja
89
70
75
Kja
O
Y 87
79 Kja
83
66
85
70
82
85 Ki
Jm
meters
75
87
80
61
Ki
70
500
N
84
75
71
C
72
37° 35' 00''
0
84
Magmatic foliation,
lineation, inclined/vertical
81
83
Metasedimentary
bedding inclined/vertical
70
74
C'
73
80
Metamorphic foliation,
lineation, inclined/vertical
Kja
s
B'
Qa
Plunging,
asymmetric fold
Syncline
Lithological contacts
Plunging anticline
O/Y
Contact relationships
Older vs. Younger
Krueger and Yoshinobu, Figure 3
Figure 3. Geologic map of the Sing Peak pendant, southwest quarter of the Jackass Lakes pluton
(see Fig. 2 for location). Topographic base from 1:24,000 Sing Peak and Timber Knob U.S. Geological
Survey quadrangles. Geodetic Datum NAD27, UTM Grid Zone 11. This figure is spread over two pages
Figure4
A
view to east
Madera Peak
Kfm
Kfm
Klja
Kfm/Kmm
Jm
Kja
Jm
Jm
Kja
Jm
Kja
Jm
Klja
B
view to west
Madera Peak
Kfm
Klja
Kja
Kja
Lady Lake
Krueger and Yoshinobu, Figure 4
Figure 4. Panorama images of the Sing Peak pendant and enclosing plutonic rocks.
A. Eastward view of Madera Peak with metasedimentary (Jm) and metavolcanic (Kfm/Kmm)
xenoliths in the foreground enclosed in granodiorite (Kja). B. Westward view of Madera Peak
illustrating the sub-horizontal nature of the leucogranite (Klja) beneath the southern Sing Peak
pendant. Approximate location of the field of view and legend in Figure 3.
Figure5
A
11,000
Kja
Kja
Jm
Jm
Kmq
10,000
9,000
Kfm Madera Peak
Jm
10,000
Klja
9,000
?
Kja
8,000
8,000
B
B'
10,000
Ki
Jm
9,000
Kfm
Ki
Kmm
8,000
?
7,000
Kfm
?
10,000
Klja
Kja
9,000
8,000
?
?
7,000
C
10,000
9,000
C'
Kmm
Kb
Ki
Kja
10,000
Kfm
Kja
8,000
7,000
elevation in feet
elevation in feet
A'
11,000
Kja
?
9,000
8,000
7,000
V= H
Krueger and Yoshinobu, Figure 5 .
Figure 5. Geologic cross sections of the Sing
Peak Pendant. Legend same as Figure 3. See
Figure 3 for location of cross section lines.
Figure6
A
Klja
Kja
B
Klja
Kja
C
Krueger and Yoshinobu, Figure 6
Figure is in color for web version,
black-and-white for printed version.
Figure 6. Field photographs of contact
relations between granodiorite (Kja) and
leucogranite (Klja) of Jackass Lakes
pluton. A. Cross-cutting contact indicating
younger Klja with respect to Kja. B-C.
Complex contact relationships where
schlieren bands and modal igneous layers
separate the two units.
Klja
Kja
Figure7
Equal Area
Magmatic foliations, n=197
Metamorphic foliations in screens/xenoliths, n=72
Magmatic lineations, n=13
Metamorphic lineations in screens/xenoliths, n=20
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Scatter
N =
Plot:
11 ;
Symbol =
Plot:
43 ;
Symbol =
Plot:
27 ;
Symbol =
Plot:
116 ;
Symbol =
Plot:
6 ;
Symbol =
Plot:
13 ;
Symbol =
Plot:
34 ;
Symbol =
Plot:
19 ;
Symbol =
Plot:
13 ;
Symbol =
Plot:
20 ;
Symbol =
Krueger and Yoshinobu, Figure 7.
Figure 7. Lower-hemisphere, equal area stereonet displaying
poles to magmatic and metamorphic foliations and magmatic
and metamorphic lineations within the region shown in Figure 3.
Figure8
A
B
deformed Kja sill
Kmm
Kfm
C
Kmm
Kmm
Kja
Krueger and Yoshinobu, Figure 8 .
Figure is in color for web version;
black-and-white for print version.
Figure 8. Field photographs of metavolcanic rocks in the Sing Peak pendant.
A. Picture of a backpack resting on interlayered mafic (Kmm) and felsic (Kfm) andesitic
to rhyo-dacitic volcanic rocks. B. Sill of the Jackass lakes granodiorite deformed in the
plane of foliation within meta-andesites. Compass for scale. C. Meta-andesite (e.g., Kmm)
intruded by granodiorite of Jackass Lakes (Kja). Note the presence of elongate mafic
magmatic enclave (black arrow) and deformed, felsic lapilli tuff within the granodiorite.
Figure9
A
B
C
S
Krueger and Yoshinobu, Figure 9 .
Figure is in color for web version,
black-and-white for printed version.
Figure 9. Field photographs of A. deformed volcanic breccia and lapilli tuff in the
northern Sing Peak shear zone. B. ‘S-C’ structures within metavolcanic rocks on
a sub-horizontal surface. North is to the right; lineation plunges shallowly to the
right, indicating dextral, oblique-slip displacement.
Figure10
A
B
1 mm
1 mm
Krueger and Yoshinobu, Figure 1 0
Figure 10. Kinematic indicators within the Sing Peak
shear zone. A. Plane-light photomicrograph of a
plagioclase porphyroclast with recrystallized
qtz+fld+bte defining dextral ‘sigma’ tails from
the metavolcanic rock shown in Fig. 9A.
B. Plagioclase ‘delta’ porphyroclast with qtz+bte
tails indicating dextral rotation. Both images
represent ‘kinematic’ sections and are cut parallel
to the lineation and perpendicular to the foliation.
Figure11-1
A
C
Kmm
D
Kja
Kja
B
Kja
Kmm
C
Kmm
E
Kmm
G
Kmm
Kmm
Krueger and Yoshinobu, Figure 11.
Figure is in color for
web version;
black-and-white for
print version.
F
H
Klja
Kmm
Kja
Kmm
Figure 11. Field photographs of folded granodiorite dikes contained within metavolcanic xenoliths and screens in the Jackass
Lakes pluton. A. Medium-crystalline, 1 m-wide granodiorite dike intruded into meta-andesite and folded about north-striking axial
planes parallel to metamorphic foliations in xenolith (from Yoshinobu et al., 2009). Axial planar metamorphic foliation in the
xenolith is parallel with magmatic foliations within the dike (see Yoshinobu et al., 2009 for details). B. Line drawing of A highlighting the folded igneous layering within the sea-monster-like dike. C. Late granitic dike cutting Kja and meta-andesite and folded
about axial planar metamorphic foliations within the xenolith. D. Dike of the granodiorite of Jackass Lakes folded about axial
planes that are parallel with magmatic foliations in the pluton and metamorphic foliations within the xenolith. E. Folded granitic
dike; axial plane of fold is parallel to metamorphic foliation in meta-andesite. F. Close-up view of dike in E displaying fine igneous textures within the hinge region of the folded dike. G. Asymmetrically folded granitic dike in meta-andesite. H. Northward
view of synformal granodioritic dike with axial planes parallel to north-striking metamorphic foliations within the enclosing
meta-andesite. I. Outcrop view of folded granodioritic dike in meta-andesite. J, K. Close up of dike in Fig. 11I. Note magmatic
foliation (parallel to hammer handle and pencil) that is at high angles to dike wall and is within 10° of the metamorphic foliation
within the xenolith (black arrows).
Figure11-2
I
Kmm
Fig. J
Kja
Fig. K
Kmm
J
K
Kja
Kmm
Kja
Kmm
Kmm
Krueger and Yoshinobu, Figure 11, continued.
Figure is in color for web version;
black-and-white for print version.
Figure 11, continued. Field photographs of folded granodiorite dikes contained within metavolcanic xenoliths and screens in the
Jackass Lakes pluton. I. Outcrop view of folded granodioritic dike in meta-andesite. J, K. Close up of dike in Fig. 11I. Note magmatic foliation (parallel to hammer handle and pencil) that is at high angles to dike wall and is within 10° of the metamorphic
foliation within the xenolith (black arrows).
Figure12
A
5 mm
B
5 mm
P
P
Q
Q
P
A
A
P
P
P
C
P
A
kfm
D
kfm
K
kfm
10 mm
Krueger and Yoshinobu, Fig. 12
Color in web version;
black-and-white in print version
Figure 12. Magmatic and crystal-plastic structures. White dashed line is orientation of foliation in all samples.
A. Thin-section view in cross-polarized light of granodiorite of Jackass Lakes displaying magmatic foliations
defined by aligned plagioclase feldspar (P) and amphibole (A). B. Thin-section view in cross-polarized light of
magmatic foliation with overprinting crystal-plastic microstructure. Quartz (Q) displays evidence for discreet
bands of recrystallization (white arrows) and crystal-size reduction. C. Polished slab of deformed meta-rhyodacite (kfm) and folded granitic dike displaying axial-planar foliation. D. Thin-section photomosaic in crosspolarized light of folded dike in C displaying axial-planar foliation defined by aligned k-feldspar (K) porphyroclasts, and coarse- and fine-crystalline, recrystallized quartz (black and white arrows, respectively). Kfm =
Cretaceous felsic meta-volcanic rocks.
119° 22' 30''
80
79
78
87
Kja
77
79
87
87
60
82
65
46
82
83
79
68
45
79
59
75
82
77
85
73
75
85
82
86
67
64
82
Klja
50
88
75
Kja
74
47
85
65
85
84
79
39
45
70
64
63
49
75
73
87
76
50
72
65
Qa
65
86
Kja69
Kfm
79
72
K
mm
68
75
86
8
72 6
79
Qa
80
68
78
Klja
63
71
67
61
78
75
83
84
80
31
81
64
33
60
80
43
70
Qa
79
48
71
85
73
61
78
61
76
75
70
60
73
74
73
70
87
Ki
Kb
85
77
73
73
83
80
78
Kja
86
75
81
85
Kja
24
79
78
30 83
68
82
87
Kfm
82
80
Jm
Kja 73
72
84
Kja
70
Kmm
82
Kfm 80
Kmm
85
Kfm
71
Ki
84
72
72
85
86
75
61
70
79
86
83
76
Klja
82
83
45
Qa
Kja
Qa
85
70
81
74
82
86
82
89
70
82
76
69
83
81
78
85
78
84
72
81
47
82
85
82
38
82
72
83
Kja
81
Kmm Qa 52
Kja
88
84
O
Y 87
75
Kb Kb
Kja
85
82
77
19
89
84
85
86
79
75
Kja
77
75
79
Qa
Kja
Ki
Qa
Kja
68
70
Kmm80
79
73
84
87
70
Jm
Y
O
82
Kfm
83
45
85
70
84
81
70
Kfm
86
65
80
82
86
76
71
69
84
38
77
76
86
74
40
85
85
77
47
86
85
77
72
78
YO
66
66
89
80
45
48
84
Kja
71
54
45
78
72
56
77
76
46
80
70
88
54
37° 35' 00''
Kja74
75
Qa
Kfm 76
82
Kja
37° 35' 00''
0
80
80
69
Kja
75
85
45
82
86
82
N
85
34
74
85
YO
Ki
55
71
75
65
79
39
85
60
58
42
Kfm
O
Y
119°22'30''
37° 32' 30''
85
73
87
80
Jm82
Kja76
Jm
Kmm
Qa 86 79
85
Klja86
70
57
s
80
87
78
49
85
81
86
Qa Kmq 67 48
Jm
Kja Jm
Kmq 73
85
70
Jm
65
80
70
86
47
Kja
80
Jm
74
Qa
71
83
77
Kja
Kmm
64
Ki
69
Klja
Qa
Klja
Jm
86
71
65
Kmq
75
82
s
s
s
83
50
Jm
79
82
Jm
78
7
60 4
Kja
Figure13
KILOMETERES
1.0
Krueger and Yoshinobu, Figure 13.
Figure 13. Trend lines of magmatic and metamorphic foliations (in metavolcanic rocks) that
define the magmatic-crystal plastic Sing Peak shear zone.
Figure14
Figure 14. A. Oblique, disected perspective sketch of ca. 98-97 Ma intra-arc deformation and dextraloblique plate convergence. Hypothesized relationship between the Jackass Lakes pluton, Sing Peak
shear zone, and temporally and spatially-related shear zones in the central Sierra Nevada batholith is
shown in the subsurface. Fabrics within the Jackass Lakes pluton (JLP) are the result of transpressional
deformation attending arc evolution and assembly of the pluton. BL: Bench Lake shear zone; GL-CL: Gem
Lake-Cascade Lake shear zone. See Figure 1 for comparison of regional geology. B. Farallon/Kula plate
kinematics relative to fixed North America during the middle Cretaceous (modified from Engebretson et
al., 1985).