Tectonics
Supporting Information for
Linking Sierra Nevada, California, uplift to subsidence of the Tulare basin using a
seismically derived density model
Will Levandowski1,2*+, Craig H. Jones1,2 ; 1University of Colorado, 2CIRES
*Now at USGS Geologic Hazards Science Center, Golden, CO
+
Corresponding Author: wlevandowski@usgs.gov
Contents of this file
Text S1
Figures S1.1 to S1.6
Table S1.1
Additional Supporting Information (Files uploaded separately)
Datasets of seismic velocity and density model
MATLAB codes for modeling density and flexure
Introduction
This supplemental file contains a fuller discussion of the beamed receiver functions
(essentially an excerpt of Levandowski [2007]) and their implications for structure and
lithology in the Sierra and foothills (S1). Additionally, we provide a .zip file (Flex2D.zip)
that contains the full set of velocity models, MATLAB codes used to model flexure, and
the final density model.
1
Supplemental File S1: Beam formed receiver functions and implications for lower
crustal deformation
Although not essential to the primary interpretation of the main text, thickened
crust beneath the Tulare basin and the southwestern Sierran foothills provides
observational evidence in support of the conceptual model of Zandt et al. [2004] and
numerical models of the interaction between foundering lithosphere and the overlying
crust [Hoogenboom and Houseman, 2006; Molnar and Houseman, 2013]. A
geometrically complex cusp of entrained crustal material, a gradational crust-mantle
boundary, reverberations from basin sediments, and reflections or conversions from
deformed Franciscan rocks could all account for the absence of a clear Moho P-s
conversion recorded on seismometers located at the eastern edge of the Tulare basin.
Nevertheless, such signal-generated noise should not arrive simultaneously at adjacent
stations and therefore should interfere destructively if several stations’ recordings for a
single event are stacked. As long as crustal thickness is comparable beneath adjacent
stations, the low-amplitude Moho arrivals should interfere constructively when stations’
traces are stacked.
Therefore, in an effort to image the Moho beneath the western foothills and
southeastern San Joaquin Valley, we examined “beam-formed” receiver functions [Jones
and Phinney, 1998] on three-station subarrays in the southern Sierra Nevada
[Levandowski, 2007; Levandowski et al., 2007]. In order to reduce stochastic and signal
generated noise, the stations’ radial and vertical traces for a given event are stacked, and
the receiver function is the deconvolution of the stacked vertical from the stacked radial;
effectively, three stations together function as a single seismometer. This approach
2
diminishes both random and signal-generated noise that is not common to all three
stations, producing a pre-deconvolution increase in signal-to-noise ratio of √3.
Consequently, common arrivals (e.g., the Moho P-s conversion if the crustal thickness
varies more slowly than near-surface structure) are magnified in the receiver function.
The data used in this processing were from ~30 broadband seismometers (Figure
S1.1) deployed for ~6 months in the southern Sierra as part of the Sierra Paradox
Experiment. Radial and vertical traces from the three stations encompassing 10 seconds
before the direct-P arrival to 50 seconds after were slant-stacked and filtered from 0.1-4
Hz. A cosine taper was applied to the first and last 3 seconds of each stacked trace. These
filtered, tapered traces were used to calculate the receiver function by iterative, timedomain deconvolution [Ligorría and Ammon, 1999]. At least one beamed receiver
function passed quality control--variance reduction of 80%-- for 90 teleseismic events. 61
events had an interpretable Moho arrival (assumed to be the greatest positive arrival
between 3 and 7.5 seconds after the direct-P). Notably, beams that straddle rapid
transitions (i.e., of shorter wavelength than station spacing) in crustal thickness do not
recover the Moho P-s conversion, since these conversions arrive at different times and
interfere destructively. As such, subarrays spanning the eastern Sierran front into the
Basin and Range returned no interpretable Moho signal.
In regions under which crustal thickness is relatively uniform, however, this
receiver function analysis (Table S1.1) reveals similar patterns to previous estimates of
crustal thickness in the Sierra Nevada region (e.g., Frassetto et al., 2011). Specifically,
the lower elevation western foothills overlie thicker crust than the topographically highest
southern Sierra (Figure S1.2). Additionally, this analysis presents the first receiver
3
functions that sample Moho depths beneath the Tulare basin (Figure 4 in the main text);
the trend of thickening crust beneath diminishing topography extends into the basin. We
do not convert Moho P-s time to depth within the Sierra because we lack the necessary
controls on vp/vs and because previous work has clearly imaged the Moho there. In the
foothills, however, arrival times from local earthquakes do allow such a conversion.
4
Beam
name
Mean
Elevation
of vertices
NW
arrival
time
# NW
Events
SW
arrival
time
# SW
Events
SE
arrival
time
# SE
Events
WESTERN FOOTHILLS: SOUTH TO NORTH
SW1
WC1
WC4
NW2
NW1
990 m
612 m
1076 m
1272 m
802 m
5.4
6.4
6.2
5.8
?????
4
2
2
3
4
NC1
1768 m
5.8
2
5.1?
6.1
5.9
5.8
5.9 or
7.7
?????
3
5
5
4
4
5.1
5.5
?????
5.8
?????
7
3
3
8
8
4
5.8
7
CENTRAL SIERRA: SOUTH TO NORTH
SC1
SW2
SW3
HS4
HS3
NC3
2097 m
2358 m
1780 m
1581 m
2038 m
1926 m
4.8
-------------5.7
6.2
6.1
5
0
0
3
3
9
4.8
4.5
5.0
??????
5.4
5.6
6
2
2
8
5
7
4.8
?????
?????
6.2
?????
5.8
6
1
1
5
10
7
EASTERN SIERRA: SOUTH TO NORTH
ES1
2029 m
4.6
2
4.4
14
4.8
11
ES1
2268 m
4.7
6
4.7
11
4.6
12
HS1
2521 m
4.9
4
4.7
4
4.3
7
NE2
2407 m
4.8
4
?????
3
4.3
8
NE1
2108 m
?????
8
?????
10
4.3?
10
NC2
2138 m
4.8
3
5.3
6
?????
11
Table S1.1: Arrival times for Moho Ps conversion on N-S transects through the Sierra. To
highlight backazimuthal variations, events have been separated by quadrant. On each
transect, but particularly in the Central Sierra, the crust generally thickens from south to
north. This pattern is superimposed on a steeper gradient of westward thickening.
5
Figure S1.1: Sierran Paradox Experiment seismometers overlain on basemap from Zandt
et al. (2004).
a: Beams discussed
here are labeled and
comprise the seismic
stations located at the
vertices.
b: Moho P-s lag time. Note the general
WNW-ESE gradient, nearly perpendicular
to topographic strike (see Figure S1.2).
6
Figure S1.2: Elevation and crustal thickness are negatively correlated. The
topographically highest eastern Sierra has the thinnest crust (<40 km) while the
southwestern foothills overlie >50 km thick crust.
Beam-formed receiver functions from the western foothills
Only in the southwestern Sierran foothills do we convert Moho P-s lag time to
depth. Arrival times from local earthquakes in the western foothills of the central Sierra
(~100 km north of the region in question) suggest a crustal vp/vs of ~1.72 [Hurd et al.,
2006]. Crustal P-velocity is ~6.75 km/s [Thurber et al., 2009]. We bin and stack
receiver functions in the southwestern foothills by backazimuth quadrant, select the time
to Moho from this stack and calculate depth using the values above. In order to estimate
the conversion point for these receiver functions, we use the median backazimuth and ray
parameter for each beam-quadrant bin and backproject from the center subarray. As
shown in Figure 4 of the main text, these receiver functions image the Moho as far west
as the center of the Tulare basin. Similar conversion point estimation was not performed
for beams in the central or eastern Sierra.
There are two notable observations of crustal thickness in the southeastern Great
Valley (Figures S1.1b & S1.3a-f, and Figure 4 in the main text): the west dip of the Moho
7
continues under the Tulare basin, and there is an additional northward dip in the Tulare
basin, with crustal thickness reaching nearly 60 km at its north end. Systematically, the
crust under the basin is some 10-15 km thicker than topographically higher areas
immediately ENE. Additionally, there is a step from arrivals at ~6s on beams NE1,
NW1, NW2, WC1, and WC4 to arrivals at <5.2s on SW1 (Figure S1.1b, S1.3a-f) and
SW3 (Figure S1.4d).
The Moho P-s conversion is visible on most beams from the northwest and
southwest, but is of low amplitude (compare to eastern Sierra beams, Figure S1.5).
Southeastern backazimuth events show no, or lower amplitude still, Moho Ps, suggesting
profound variations in crustal thickness beneath the transition from the range to the
foothills at wavelengths comparable to station spacing. The backazimuthal dependence of
Moho P-to-s lag provides additional evidence for a dipping Moho under the Tulare basin.
For example, beams SW1, WC1, and WC4 (Figure S1.3d-f) are centered east or southeast
of the deepest part of the basin, and Moho conversions from the northwest that sample
beneath the basin arrive ~0.3 s after those from the southwest or southeast.
8
Figure S1.3: Beamed RFs from north to south along the western foothills of the Sierra.
Receiver functions from NW, SE, and SW quadrants are stacked separately and their
means are plotted. The seismometers that constitute the beam and their vertices are listed.
Figure S1.3a: Beam NC1
FLL: 37.28°N, 118.97°W, 2237 METERS
SRF: 36.97°N, 118.63°W, 1807 METERS
9
BRR: 36.91°N, 119.04°W, 1259 METERS
Figure S1.3b: Beam NW1
BGR: 36.63°N, 119.02°W, 954 METER
BRR: 36.91°N, 119.04°W, 1259 METERS
HVY: 36.7°N, 119.32°W, 193 METERS
10
Figure S1.3c: Beam NW2
BGR: 36.63°N, 119.02°W, 954 METER
BRR: 36.91°N, 119.04°W, 1259 METERS
CPR: 36.8°N, 118.58°W, 1603 METERS
11
Figure S1.3d: Beam WC4
CCC: 35.52°N, 117.36°W, 670 METERS
CPR: 36.8°N, 118.58°W, 1603 METERS
BGR: 36.63°N, 119.02°W, 954 METERS
12
Figure S1.3e: Beam WC1
CCC: 35.52°N, 117.36°W, 670 METERS
BGR: 36.63°N, 119.02°W, 954 METERS
LMC: 36.36°N, 119.03°W, 211 METERS
13
Figure S1.3f: Beam SW1
LMC: 36.36°N, 119.03°W, 211 METERS
WMD: 36.2°N, 118.58°W, 2592 METERS
PDC: 36.03°N, 118.98°W, 167 METERS
14
Beam-formed receiver functions from the central Sierra
Two notable patterns emerge in the central Sierra. First, the step in Moho Ps
arrivals is once again observed near 36.5°N. SC1, SW2, and SW3 have arrivals at <5s
(Figure S1.4d-f), compared to ~5.5s arrivals on HS3 and NC3 (Figure S1.4a-b). Second,
large, negative mid-crustal signals at ~2.5s are observed on SW3, SW2, and SC1 (Figure
S1.4d-f). The amplitude of this negative arrival decreases to the north (Figure S1.4a-c).
15
Figure S1.4: Same as Figure S1.3, but through the central Sierra.
Figure S1.4a: Beam NC3
CPR: 36.8°N, 118.58°W, 1603 METERS
SRF: 36.97°N, 118.63°W, 1807 METERS
BPC: 37.13°N, 118.43°W, 2370 METERS
16
Figure S1.4b: Beam HS3
CPR: 36.8°N, 118.58°W, 1603 METERS
SRF: 36.97°N, 118.63°W, 1807 METERS
OVY: 36.78°N, 118.33°W, 2704 METERS
17
Figure S1.4c: Beam HS4
CPR: 36.8°N, 118.58°W, 1603 METERS
CCC: 35.52°N, 117.36°W, 670 METERS
JUN: 36.58°N, 118.41°W, 2471 METERS
18
Figure S1.4d: Beam SW3
LMC: 36.36°N, 119.03°W, 211 METERS
WMD: 36.2°N, 118.58°W, 2592 METERS
MKW3: 36.45°N, 118.61°W, 2358 METERS
19
Figure S1.4e: Beam SW2
WMD: 36.2°N, 118.58°W, 2592 METERS
TWR2: 36.35°N, 118.41°W, 1946 METERS
MKW3: 36.45°N, 118.61°W, 2358 METERS
20
Figure S1.4f: Beam SC1
SFT: 36.23°N, 118.06°W, 1753 METERS
ARC2 WMD: 36.2°N, 118.58°W, 2592 METERS
TWR2: 36.35°N, 118.41°W, 1946 METERS
21
Beam-formed receiver functions from the eastern Sierra
A transect along the eastern front of the Sierra displays two important patterns.
First, the apparent Moho step near 36.5°N is absent; P-s conversions arrive consistently at
4-5s along the entire eastern front. Second, the negative arrival noted in the southcentral
Sierra (SW3, SW2, and SC1) is pervasive along strike and deeper (~3.5s compared to
~2.5 s). If anything, the signal degrades southward, opposite to the central Sierra.
22
Figure S1.5: Same as B3-4, but beams along the topographically highest Eastern Sierra
from north to south.
Figure S1.5a: Beam NC2
FLL: 37.28°N, 118.97°W, 2237 METERS
SRF: 36.97°N, 118.63°W, 1807 METERS
BPC: 37.13°N, 118.43°W, 2370 METERS
23
Figure S1.5b: Beam NE1
SRF: 36.94°N, 118.11°W, 2148 METERS
SRF: 36.97°N, 118.63°W, 1807 METERS
BPC: 37.13°N, 118.43°W, 2370 METERS
24
Figure S1.5c: BeamNE2
SRF: 36.94°N, 118.11°W, 2148 METERS
OVY: 36.78°N, 118.33°W, 2704 METERS
BPC: 37.13°N, 118.43°W, 2370 METERS
25
Figure S1.5d: Beam HS1
WHP: 36.59°N, 118.22°W, 2388 METERS
JUN: 36.58°N, 118.41°W, 2471 METERS
OVY: 36.78°N, 118.33°W, 2704 METERS
26
Figure S1.5e: Beam ES1
WHP: 36.59°N, 118.22°W, 2388 METERS
JUN: 36.58°N, 118.41°W, 2471 METERS
TWR2: 36.35°N, 118.41°W, 1946 METERS
27
Figure S1.5f: Beam ES1
WHP: 36.59°N, 118.22°W, 2388 METERS
TWR2: 36.35°N, 118.41°W, 1946 METERS
SFT: 36.23°N, 118.06°W, 1753 METERS
28
Mid-crustal negative
The trend of beams that record a negative mid-crustal arrival forms a ~60° angle
with topographic strike, SSW-NNE vs. NNW-SSE (Figure S1.6). The negative convertor
is deeper to the north (compare Figures S1.5d and S1.4d) and/or east (Figures S1.4d,
S1.4e, S1.5d, and S1.5e form an east-west transect, for example). The amplitude,
impulsivity, and consistency of the signal decay rapidly away from the main SSW-NNE
trend, within roughly 30 km. A mid-crustal negative has previously been observed in the
eastern Sierra [Jones and Phinney, 1998; Zandt et al., 2004; Frassetto et al., 2011] and
was interpreted as ductile shear zone, where Basin and Range extension is encroaching
on the Sierra Nevada. Indeed, the NNE strike of this negative arrival closely parallels the
Kern Canyon Fault through the southernmost Sierra (Figure S1.6). The confinement of
negative conversions to the lower crust beneath the range (and absence in the foothills) is
consistent with a shear zone that has served to destabilize the lowermost crust and the
mantle lithosphere from beneath the Sierra but has not removed material from beneath
the western foothills.
29
Figure S1.6: Characterization of the midcrustal negative (MCN) arrival overlain on
basemap from Zandt et al. [2004]. Note narrow expanse, coincidence with the Kern
Canyon Fault [Nadin and Saleeby, 2010], sub-orthogonal strike to the Sierra, and absence
beneath the western foothills.
30
A crude upper bound on lower crustal viscosity
Comparing the putative viscous thickening of the lower crust to the density
anomaly and dimensions of the Isabella Anomaly places an upper bound on the viscosity
of the lower crust. Given the density anomaly of 14.1 kg/m3 mentioned in the main text
and a thickness (H) of 200 km, the Isabella Anomaly represents a load per unit area of:
𝜎𝑧𝑧 = ∆𝜌𝑔𝐻 = 68.6 𝑀𝑃𝑎
(S1.1)
If we infer that there is no other mechanism of support for this load, 68.6 MPa is
the maximum normal stress that the Isabella Anomaly could exert on the Moho. Further,
we can approximate that the crust (elsewhere ~40 km, but ~50 km beneath the Tulare
basin) has undergone 25% strain. If this deformation has occurred since 10 Ma (the age
̇ is ~8.0x10-16/s. Conversely, if this deformation
of pyroxenitic xenoliths), strain rate (𝜀)
has occurred since 3 Ma (the age of peridotitic xenoliths), strain rate is ~2.7x10-15/s. The
vertical normal stress (if we assume lithostatic pressure to be approximately laterally
equal at a given depth in the crust) generates strain following:
𝜎𝑧𝑧 = 2𝜂𝜀̇ 𝜂 = 𝜎𝑧𝑧 / 2𝜀̇
(S1.2)
For the two timeframes listed above, the calculated lower crustal viscosity is of the order
1022 Pa s (1.3-4.3 x 1022 Pa s).
Given the low heat flow [Saltus and Lachenbruch, 1991], high Pn velocities
[Buehler and Shearer, 2010], and deep seismicity [Hurd et al., 2006] in the western
foothills of the Sierra, Moho temperatures are likely lower than in most non-cratonic
regions. Using seismic models and topography, Levandowski et al. [2013] estimated a
Moho temperature near 350°C in the western foothills, or an average geotherm of a mere
~7°C/km. Therefore, if viscosity in the lower crust were governed mostly by temperature
31
variations, one might suspect that the western foothills of the Sierra are a decent proxy
for the maximum viscosity of lower crust.
Furthermore, it is unlikely that the upper portion of the crystalline crust has
accommodated thickening, since seismic velocity [Gilbert et al., 2012; Jones et al.,
2014] and density [Levandowski et al., 2013] are relatively high in the middle crust in
the southwestern foothills. If the entire crust column had telescoped (e.g., by pure shear
lateral squashing), then lower-density upper crust would have advected to previously
middle crustal depths and would now manifest in low velocity and density. If this logic
holds, then the percent strain accrued in the lower crust is greater still, for example by a
factor of two if only the lower half of the crust has deformed, and the maximum plausible
viscosity of the cold, mafic lower crust in the foothills decreases proportionally.
32
References
Buehler, J. S., and P. M. Shearer (2010), Pn tomography of the western United States
using USArray, JGR, 115(B9), B09315, doi:10.1029/2009JB006874.
Frassetto, A. M., G. Zandt, H. Gilbert, T. J. Owens, and C. H. Jones (2011), Structure of
the Sierra Nevada from receiver functions and implications for lithospheric,
Geosphere, 7(4), doi:10.1130/GES00570.1.
Gilbert, H., Y. Yang, T. J. Owens, C. H. Jones, G. Zandt, and J. C. Stachnik (2012),
Imaging lithospheric foundering in the structure of the Sierra Nevada,
Geosphere, 8(6), doi:10.1130/GES00790.1.
Hammond, W. C., and E. D. Humphreys (2000), Upper mantle seismic wave
attenuation: Effects of realistic partial melt distribution, JGR.
Hoogenboom, T., and G. A. Houseman (2006), Rayleigh–Taylor instability as a
mechanism for corona formation on Venus, Icarus, 180(2), 292–307,
doi:10.1016/j.icarus.2005.11.001.
Hurd, O., A. Frassetto, G. Zandt, H. Gilbert, and T. J. Owens (2006), Deep crustal
earthquakes and repeating earthquakes in the west-central Sierra Nevada,
western USA, AGU Fall Meeting, S43A-1360.
Jackson, I., and U. H. Faul (2010), Grainsize-sensitive viscoelastic relaxation in
olivine: Towards a robust laboratory-based model for seismological application,
Physics of the Earth and Planetary Interiors, 183(1-2), 151–163,
doi:10.1016/j.pepi.2010.09.005.
Jones, C. H., and R. A. Phinney (1998), Seismic structure of the lithosphere from
teleseismic converted arrivals observed at small arrays in the southern Sierra
Nevada and vicinity, California, JGR, 103(B5).
Jones, C. H., H. Reeg, G. Zandt, H. Gilbert, T. J. Owens, and J. Stachnik (2014), P-wave
tomography of potential convective downwellings and their source regions,
Sierra Nevada, California, Geosphere, 10(3), doi:10.1130/GES00961.1.
Lee, C.-T. A. (2003), Compositional variation of density and seismic velocities in
natural peridotites at STP conditions: Implications for seismic imaging of
compositional heterogeneities in the upper mantle, JGR, 108(B9), 2441,
doi:10.1029/2003JB002413.
Levandowski, W. B. (2007), Beam-formed receiver function analysis of the southern
Sierra Nevada, California [A.B. Undergraduate]: Princeton University.
Levandowski, W. B., C. H. Jones, G. Nolet, and R. A. Phinney (2007), Mysterious Moho
Beneath the Southern Sierra Nevada, Analyzed with Beam-Formed Receiver
33
Functions, AGU Fall Meeting, T33A-1148.
Levandowski, W., C. H. Jones, H. Reeg, A. Frassetto, H. Gilbert, G. Zandt, and T. J.
Owens (2013), Seismological estimates of means of isostatic support of the
Sierra Nevada, Geosphere, 9(6), 1552–1561, doi:10.1130/GES00905.1.
Ligorría, J. P., and C. J. Ammon (1999), Iterative deconvolution and receiver-function
estimation, BSSA, 89(5), 1395–1400.
Molnar, P., and G. A. Houseman (2013), Rayleigh-Taylor instability, lithospheric
dynamics, surface topography at convergent mountain belts, and gravity
anomalies, J. Geophys. Res. Solid Earth, 118(5), 2544–2557,
doi:10.1002/jgrb.50203.
Nadin, E. S., and J. B. Saleeby (2010), Quaternary reactivation of the Kern Canyon
fault system, southern Sierra Nevada, California, GSAB, 122(9-10), 1671–1685,
doi:10.1130/B30009.1.
Saltus, R. W., and A. H. Lachenbruch (1991), Thermal evolution of the Sierra Nevada:
Tectonic implications of new heat flow data, Tectonics, 10(2).
Thurber, C., H. Zhang, T. Brocher, and V. Langenheim (2009), Regional threedimensional seismic velocity model of the crust and uppermost mantle of
northern California, JGR, 114(B1), B01304, doi:10.1029/2008JB005766.
Zandt, G., H. Gilbert, T. J. Owens, J. Saleeby, and C. H. Jones (2004), Active foundering
of a continental arc root beneath the southern Sierra Nevada in California,
Nature, 431, 1–6.
34