International Geology Review, Vol. 38, 1996, p. 979-994.
Copyright © 1996 by V. H. Winston & Son, Inc. All rights reserved.
Tectono-Metamorphic Impact of a Subduction-Transform
Transition and Implications for Interpretation of Orogenic Belts
JOHN WAKABAYASHI
1329 Sheridan Lane, Hayward, California 94544
Abstract
Subduction-transform tectonic transitions were common in the geologic past, yet their impact
on the evolution of orogenic belts is seldom considered. Evaluation of the tectonic transition in
the Coast Ranges of California is used as an example to predict some characteristics of exhumed
regions that experienced similar histories worldwide.
Elevated thermal gradients accompanied the transition from subduction to transform tec­
tonics in coastal California. Along the axis of the Coast Ranges, peak pressure-temperature
(P/T) conditions of 700 to 1000° C at a pressure of ~ 7 kbar, corresponding to granulite-facies
metamorphism, and cooling to 500° C, or amphibolite facies, within 15 million years, are
indicated by thermal gradients estimated from the depth to the base of crustal seismicity.
Greenschist-facies conditions may occur at depths of 10 km or less. These P / T estimates are
consistent with the petrology of crustal xenoliths and thermal models. Preservation of earlier
subduction-related metamorphism is possible at depth in the Coast Ranges. Such rocks may
record a greenschist or higher-grade overprint over blueschist assemblages, and late growth of
metamorphic minerals may reflect dextral shear along the plate margin, with development of
orogen-parallel stretching lineations.
Thermal overprints of early-formed high-P (HP), low-T (LT) assemblages, in association with
orogen-parallel stretching lineations, occur in many orogenic belts of the world, and have been
attributed to subduction followed by collision. Alternatively, a subduction-transform transition
may have caused the overprints and lineations in some of these orogenic belts. Possible examples
are the Sanbagawa belt of Japan and the Haast schists of New Zealand. P / T conditions of inferred
granulite-grade metamorphism in the Coast Ranges, and predicted cooling of these rocks
through lower thermal gradients, resemble the P / T evolution of many granulite belts, suggesting
that some granulite belts may have formed as a result of a subduction-transform transition. Arc­
like belts of plutons also can form as a consequence of subduction-transform transition.
Introduction
Coastal California has been the site of subduc­
TRIPLE-JUNCTION MIGRATION along trenches
involving either migrating transform faults or
spreading ridges is common in Earth history
(Sisson et al., 1994). Accordingly, it is reason­
able to surmise that past plate interactions,
similar to the subduction-transform transition
occurring in present-day northern California,
have left their imprint on many orogenic belts.
Nelson and Forsythe (1989) suggested that
ridge-trench collision is an important process
in crustal growth, and speculated that this pro­
cess played an even greater role in Archean
crustal growth. However, the impact of ridgetrench interactions or subduction-transform
transitions on the development of orogenic
belts still is largely unappreciated.
The Coast Ranges of California are a type
example of subduction-transform transition.
tion or transform tectonics for the last 160 m.y.
0020-6814/96/225/979-16 $10.00
(Engebretson et al. 1985). During the late
Mesozoic and Tertiary, the Franciscan subduc­
tion complex formed over a period of ≥140
m.y. of continuous subduction (Wakabayashi,
1992). Subduction terminated with the north­
ward passage of the Mendocino triple junction,
and a transform
plate boundary
developed
south of the triple junction (Atwater, 1970).
General geologic relations are shown in Fig­
ure 1.
In addition to the different style of deforma­
tion, higher thermal gradients followed the sub­
duction-transform transition (Dickinson and
Snyder, 1979; Lachenbruch and Sass, 1980;
Furlong, 1984). Dickinson and Snyder (1979)
proposed that a "slab window" trailing in the
979
980
JOHN
WAKABAYASHl
FIG. 1. Tectonic elements of the California Coast Ranges, showing major strike-slip faults of the San Andreas
system, and upper Cenozoic volcanic rocks (v). Also shown are exposed blueschist-facies rocks of the Franciscan
Complex (bsch.). Abbreviations: KJf/g = areas underlain by Franciscan Complex or Great Valley Group rocks
(includes areas where these rocks are overlain by Tertiary and Quaternary deposits); Ksb = granitic and high-grade
metamorphic basement of the Salinian Block and overlying sedimentary deposits. Map derived from Jennings
(1977).
wake of the triple junction allowed asthenospheric upwelling, causing a significant
increase in the thermal gradient. Thermal modeling based on the slab-window concept predicts
a thermal peak just after the passage of the
Mendocino triple junction, followed by cooling
as new lithosphere forms in the window region
(Furlong, 1984). This model is consistent with
heat-flow data of Lachenbruch and Sass (1980)
and the occurrence of late Cenozoic volcanism
in the Coast Ranges. Recently, the slab-window
concept has been challenged as a framework for
the late Cenozoic tectonics of coastal California
(Bohannon and Parsons, 1995; Beaudoin et al.,
1996), and no single model proposed thus far is
completely consistent with some of the recent
seismic data obtained (Hole, 1996). However,
the thermal effects associated with the passage
of the triple junction and the transform nature
of the plate boundary south of the Mendocino
triple junction are not disputed. These points of
consensus, rather than any specific tectonic
model, will serve as the foundation of this
paper.
This paper relates field, thermal, structural,
and geophysical data to inferred metamorphic
mineral assemblages and structures at depth
in the Coast Ranges of California, as a case
SUBDUCTION-TRANSFORM TRANSITION
981
FIG. 2. Longitudinal section of the California Coast Ranges along the line depicted in Figure 1, showing inferred
depth to the ~300° C isotherm based on the base of crustal seismicity from Hill et al. (1990). The screened dashed
line shows the line in the crust that experienced a temperature of 300° C during earlier passage of the thermal peak.
study; the attempt is to assess what the deep
levels of the transform plate margin in Califor­
nia may look like if exhumed and to speculate
on the imprint of similar, past plate inter­
actions on exhumed metamorphic belts of the
world. Among the topics addressed in this
paper are: (1) an alternative tectonic mecha­
nism to thrusting-thermal relaxation (subduction followed by collision) models for thermal
overprinting of some high-pressure, lowtemperature (HP/LT) metamorphic rocks; (2)
an alternative tectonic mechanism for develop­
ment of some granulite belts, or other hightemperature metamorphic belts; (3) problem­
atic arc-like belts of plutons; (4) possible
examples of exhumed equivalents of subduction-transform orogens; and (5) distribution of
blueschists and granulites in time.
An Example of a Subduction-Transform
Transition: The California
Coast Ranges at Depth
Elevated thermal gradients following
subduction-transform conversion
Modeling of heat-flow data by Lachenbruch
and Sass (1980) and Dumitru (1989) suggest
peak thermal gradients of 35° C/km, following
subduction-transform transition in coastal Cal­
ifornia. The modeling of Furlong (1984) pre­
dicts peak temperatures of about 700° C at
about 25 km depth, 1 m.y. after passage of the
triple junction, subsequently cooling to about
450° C at the same depth 20 m.y. after triplejunction passage. In addition to heat-flow data,
high thermal gradients in the California Coast
Ranges are indicated by the 10- to 18-km depth
of the base of crustal seismicity in the Coast
Ranges, which is inferred to coincide with the
brittle-ductile transition (e.g., see Hill et al.,
1990). The depth of the base of crustal seis­
micity varies from north to south along the
strike of the Coast Ranges in the wake of the
triple junction, first shallowing to an average of
about 10 km (locally as shallow as 7 to 8 km in
the Clear Lake area), then deepening pro­
gressively to the south and leveling out at an
average of about 15 km south of San Francisco
Bay (Fig. 2; cross-sectional view shown in Fig.
3). The brittle-ductile transition in quartz-rich
rocks, inferred to constitute most of the Coast
Ranges at depth on the basis of seismic veloc­
ities (Holbrook et al., 1996), is estimated to take
place at a temperature of ~300° C (Sibson,
1982). The base of crustal seismicity indicates
thermal gradients ranging from 30° C/km at
the thermal peak region (35-40° C/km in the
Clear Lake region), cooling to 20° C/km in the
southern Coast Ranges. The migration rate of
the Mendocino triple junction indicates that
the cooling to 20° C/km thermal gradients, in
the wake of the passage of the thermal peak,
took 10 to 15 m.y. All of the above studies
suggest high (≥30° C/km) peak thermal gra­
dients in the California Coast Ranges after the
subduction-transform transition, followed by
cooling to lower gradients. Superimposed on
the regional thermal effect noted above are local
effects, possibly related to space problems in
the area of the migrating triple junction (Dumi­
tru, 1991; Underwood, 1989; Underwood et al.,
1995). These local processes have resulted in
very high local uplift rates and elevated thermal
gradients of ≥50° C/km (Dumitru, 1991). In
addition, pull-apart basins along the major
982
JOHN
WAKABAYASHI
FIG. 3. Cross-section of the California Coast Ranges along the line depicted in Figure 1, showing major strike-slip
faults and thermal structure. Adapted from Fuis and Mooney (1990) with some modifications from Holbrook et al.
(1996) and Wakabayashi and Unruh (1995). " 3 0 0 " is the 3 0 0 ° C isotherm, and the screened " 3 0 0 " represents the
location of the corresponding isotherm at the time of the passage of the thermal peak. The 3 0 0 ° C isotherm is
estimated from the base of crustal seismicity (Hill et al., 1990). Lower-case single letters correspond to the locations
of hypothetical P / T paths on Figures 4, 7, and 8. Abbreviations: fr = mostly Franciscan Complex; gv - Great Valley
Group (sandstone and shale); fr/oph = Franciscan Complex (mostly sandstone and shale, subordinate volcanic
rocks and pelagic rocks), ophiolite, and underlying mantle rocks, and possibly Sierran basement (which may include
volcanic-arc and ophiolitic rocks); sal = Salinian Block (granitic basement and high-grade metamorphic rocks).
strike-slip faults may facilitate local upwelling
of hot material and/or intrusion of plutons
(Jové and Coleman, 1992) and may have an
impact on the local thermal structure.
Late Cenozoic volcanism in the Coast Ranges
locally has brought up xenoliths of deep crustal
material (Brice, 1953; Hearn et al., 1981;
Stimac et al., 1992; Jove and Coleman, 1992;
Nakata et al., 1993), including high-grade,
silicic metamorphic rocks. For example,
numerous xenoliths of high-grade, schistose
metamorphic rocks have been found in andesites from the 10-ka to 2-Ma Clear Lake volcanic
field (Brice, 1953; Hearn et al., 1981). The
mineral assemblages in these xenoliths include
orthopyroxene-clinopyroxene-plagioclasequartz ± biotite ± garnet and orthopyroxeneplagioclase-biotite ± sillimanite ± cordierite ±
spinel (Stimac et al., 1992). Thermobarometry
on these rocks has yielded estimates for their
crystallization conditions of 800 to 900° C at 5
to 8 kbar. These P / T estimates are consistent
with temperatures estimated for the deeper levels of the California Coast Ranges discussed
above (Fig. 4). On the basis of chemical and
isotopic data, Stimac et al. (1992) concluded
that these rocks most likely are metamorphosed
Franciscan greywackes, formed by regional
metamorphism in the wake of triple-junction
migration. The well-developed schistosity or
foliation of these rocks indicates regional
(rather than contact) metamorphism, and the
P / T conditions of metamorphism are grossly
incompatible with original subduction-related
metamorphism of Franciscan rocks, the country rocks of the Clear Lake area (McLaughlin
and Ohlin, 1984), or Sierra Nevada regional
metamorphic rocks, which have been suggested
to tectonically underlie the eastern Coast
Ranges (Jachens et al., 1995).
Jove and Coleman (1992) analyzed gabbroic
xenoliths from Pliocene volcanic rocks near
Coyote Lake and estimated crystallization conditions of 915 to 1000° C at 9 to 10.5 kbar,
based on thermobarometry. The P / T conditions
calculated for these gabbro xenoliths also are
consistent with the hypothetical thermal gradients discussed above (Fig. 4). Nakata et al.
(1993) reported xenoliths that include biotite
and sillimanite-corundum schists (in addition
to gabbroic xenoliths) from the andesite of
Dowdy Ranch in the Diablo Range, north of the
Quien Sabe volcanic field and east of the Coyote
Lake volcanics. Nakata et al. (1993) obtained
K/Ar ages of 8.5 to 12.3 Ma from igneous
xenoliths from the andesite of Dowdy Ranch,
although the ages may reflect resetting by the
heat from the andesite in which they were
entrained, which yielded a date of 8.2 Ma.
The mineral assemblages in schist xenoliths
described by Nakata et al. (1993) are indicative
of the same type of metamorphism under high
thermal gradients recorded in xenoliths studied
by Stimac et al. (1992) from the Clear Lake
SUBDUCTION-TRANSFORM TRANSITION
FIG. 4. Hypothetical P / T paths for metamorphic rocks at
depth in the California Coast Ranges following conversion
to transform tectonics. The dotted lines with arrows repre­
sent the prograde metamorphic path after the conversion.
Solid paths with arrows show cooling from the thermal peak
at locations in the crust (lower-case letters) shown on
Figure 3. The 20° C/km line corresponds to estimates of
the thermal gradient in the central and southern Coast
Ranges, 15 m.y. or more after the passage of the thermal
maximum, based on the depth to the base of crustal seismicity (Hill et al., 1990). The "late-subduction thermal
gradient" is the pre-transform thermal gradient north of the
Mendocino triple junction estimated from the base of
crustal seismicity (Hill et al., 1990). The peak thermal
gradient in the figure is 35 ° C/km, an average figure within
the range of estimates derived from several different
methods (see text). The 30° C/km line represents the hightemperature maximum of longer duration (see text) and
may be more representative of peak prograde assemblages.
Metamorphic conditions for Clear Lake xenoliths (Stimac
et al., 1992) and Coyote Lake xenoliths (Jove and Coleman,
1992) are shown.
volcanic field. The xenoliths from the Dowdy
Ranch volcanics probably formed in a setting
similar to that of the xenoliths found in the
Clear Lake volcanics, at a time when the triple
junction, and trailing thermal peak, was farther
south in the Coast Ranges.
The thermal pulse that followed the triplejunction migration also produced volcanism
(Johnson and O'Neil, 1984; Fox et al., 1985).
The volcanic rocks mostly are silicic to inter­
mediate rocks of calc-alkalic affinity (Hearn et
al., 1981, Johnson and O'Neil, 1984), and
apparently involved melting of the crustal rocks
above underplated or intruded basaltic magma
(Liu and Furlong, 1992). There is evidence of a
983
magma chamber at depth below the most recent
of these volcanic rocks, the Clear Lake volcan­
ics (Isherwood, 1981). This magma chamber is
estimated to have a diameter of 14 km and to
extend from 7 km to 21 km in depth (Isher­
wood, 1981), or alternatively to a depth of 30
km (Iyer et al., 1981), in the crust. Analogous
solidified magma chambers probably exist at
depth below older volcanic fields, such as the
Quien Sabe and Sonoma volcanics. Most of the
magma generated by partial melting of the crust
apparently does not reach the surface and
should form plutons at depth (Johnson and
O'Neil, 1984; Liu and Furlong, 1992). The
amount of melt generated may be linked to the
velocity of the triple-junction migration (Liu
and Furlong, 1992). Significant volumes of plutonic material are expected at depth in the
Coast Ranges, although they probably do not
form a contiguous belt similar to the Sierra
Nevada batholith.
Present-day plate boundary kinematics
The present-day California Coast Ranges are
part of the transform boundary between the
Pacific and North American plates. Dextral
shear in the Coast Ranges totals about 35 to 40
mm/yr, about 80% of the motion being parallel
to the plate boundary (DeMets et al., 1990) and
nearly all of the small contractional component
of ≤3 mm/yr being perpendicular to the plate
boundary (Argus and Gordon, 1991; DeMets et
al., 1990; Gordon and Argus, 1993). The domi­
nant structural features of the Coast Ranges are
the major strike-slip faults of the San Andreas
system. These strike-slip faults account for
essentially all of the margin-parallel plate
motion within the Coast Ranges (e.g., Kelson et
al., 1992).
The contractional component of plate
motion contributes to the development of foldand-thrust belts within the Coast Ranges and
the uplift of the range (Mount and Suppe, 1987;
Zoback et al., 1987); much of the shortening is
concentrated along the eastern border of the
Coast Ranges (Wakabayashi and Smith, 1994).
The present contractional component of plate
motion in the Coast Ranges has persisted since
a change in plate motions at 3.4- to 3.9-Ma
(Harbert, 1991). Local shortening and some
extension also occurs within the Coast Ranges
as a consequence of constrictional and releas­
ing bends in the major strike-slip faults (e.g.,
984
JOHN WAKABAYASHI
Aydin and Page, 1984). Prior to the 3.4- to 3.9Ma change in plate motions, the transform plate
boundary may have had a slight divergent component across it, resulting in the formation of
some of the Tertiary basins of the Coast Ranges
(Graham et al., 1984; Engebretson et al., 1985).
The pre-3.4- to 3.9-Ma history of the transform
margin constitutes, for most of the present
transform margin, the larger part of the total
elapsed time as a transform plate boundary.
Uplift rates in the Coast Ranges locally are high,
ranging up to several mm/yr (Merritts and Bull,
1989; Dumitru, 1991), but are generally
between 0.1 and 0.5 mm/yr (Lettis, 1982; Bürgmann et al., 1994; Lettis et al., 1994; K. Lajoie,
pers. commun., 1994; M. Angell, pers. commun., 1995). Apatite fission-track ages for most
of the northern and central Coast Ranges are
greater than 30 Ma, indicating that the average
Cenozoic uplift rate for rocks now exposed at
the surface has been relatively low (Dumitru,
1989). Because young (post-20 Ma) fissiontrack ages are rare, except in local areas of high
uplift rates, the average exhumation rate of
rocks exposed on the surface must have been
less than ~ 0 . 1 5 mm/yr since the thermal peak
that followed conversion to the transform plate
margin; otherwise, much more of the Coast
Ranges would yield younger apatite fissiontrack ages. These uplift rates place constraints
on the hypothetical retrograde P / T paths of
rocks at depth in the Coast Ranges.
Inferences regarding deep structure
The character of structures present at depth
in the Coast Ranges probably depends, in part,
on their depth relative to the brittle-ductile
transition. Above this transition, structures are
dominated by the strike-slip faults of the San
Andreas system. Second-order features are
folds and thrust faults that are a consequence
of both plate-normal contraction and local
restraining bends along major strike-slip faults.
A major reflector, interpreted as a possible
crustal detachment, has been imaged at a depth
of ~15 km at the latitude of San Francisco Bay;
this reflector is near the depth of the inferred
brittle-ductile transition in the area (Brocher et
al., 1994). A similar reflector is offset by major
strike-slip faults in the northern Coast Ranges
(Beaudoin et al., 1996), indicating that such a
reflector probably does not represent a detachment in the northern Coast Ranges. As an
alternative to a detachment, the reflector may
be a consequence of a major metamorphicfacies change, or a fluid-rich zone in the crust
(that also may be a consequence of metamorphic reactions). At present there are no direct
data to determine whether a detachment occurs
at the brittle-ductile transition, or if such a
structure is present locally, but not universally,
in the Coast Ranges. Numerous examples can
be found in exhumed orogenic belts of either
detachments at the brittle-ductile transition
(e.g., Coney, 1980) or discrete shear zones that
extend below the brittle-ductile transition (e.g.,
Hurlow, 1993). It follows that only indirect
inferences can be made regarding structures
below the brittle-ductile transition in the Coast
Ranges.
Although there are many permissible interpretations of deep structure, there is general
agreement that dextral strike-slip motion dominates the kinematics of the Coast Ranges and
should strongly influence structures at all levels of the crust. This dextral shear may be either
localized in discrete shear zones or accommodated in a broad zone of distributed ductile
deformation; present data do not favor or eliminate either end member. Shear-zone rocks
should reflect significant stretching parallel or
subparallel to the plate margin. As a result, new
mineral growth and stretching lineations developed below the brittle-ductile transition may be
subparallel or parallel to the plate margin (e.g.,
Ellis and Watkinson, 1987; Ave Lallemant and
Guth, 1990), although local complexities may
be expected in some shear zones because of the
small component of convergence (e.g., Robin
and Cruden, 1994; Tikoff and Teyssier, 1994).
Fuis and Mooney (1990) developed an interpretive cross-section of the Coast Ranges, based
on seismic refraction and reflection studies.
The cross-section in Figure 3 is adapted from
Fuis and Mooney's Figure 8.4, with some reinterpretation of subsurface structure per
Holbrook et al. (1996) and Wakabayashi and
Unruh (1995). The structure of the deeper
parts of the plate boundary are an unresolved
issue, and several competing models have been
proposed (Hole, 1996). The deep plate boundary may be expected to have some vertical
component of motion to accommodate the
contractional component of plate motion.
Although the magnitude of the shortening is
SUBDUCTION-TRANSFORM
TRANSITION
985
FIG. 5. Cross-section of the Coast Ranges along same transect as Figure 3, showing the distribution of the grade of
metamorphism from transform-related thermal effects and the pre-transform distribution of blueschist-facies
rocks. It should be noted that the facies boundaries are based on the temperatures attained at the thermal peak of 30
to 40 ° C/km that may be of limited duration and spatial extent (see text). The broader and longer-lived thermal high
defines a gradient of 30° C/km. If this longer-lived thermal maximum is used, then the facies boundaries will shift
downward by a few km. The distribution of blueschist-facies rocks at depth represents the minimum distribution of
rocks that at one point in their history were at least 20 km deep, according to the tectonic model of Wakabayashi and
Unruh (1995).
small relative to strike-slip displacement (Argus
and Gordon, 1991), the component of accumu­
lated vertical crustal motion may become signif­
icant if the present-day kinematics persist for a
long time (≥30 million years or so). Such longterm vertical movement would be important in
the future exhumation of the deeply buried
parts of the present Coast Ranges.
Inferred metamorphism at depth in the Coast
Ranges and relationship with deep structure
Higher thermal gradients associated with the
transform margin compared to those associated
with the previous subduction zone will produce
a quite different metamorphic suite at depth
than the earlier subduction (Franciscan) H P /
LT metamorphism for which the California
Coast Ranges are well known (e.g., Ernst,
1970). Instead of the facies series prehnitepumpellyite, blueschist, eclogite that charac­
terizes the subduction-zone metamorphism of
the Franciscan Complex, the predicted facies
series at depth in the present-day Coast Ranges
should be greenschist, amphibolite, granulite.
Cloos and Dumitru (1987), recognizing the
thermal significance of the ongoing subduction-transform transition, concluded that the
lack of greenschist overprints in exposed Fran­
ciscan rocks indicated that no subductiontransform transitions occurred during the span
of Franciscan accretionary history (approx­
imately 160 Ma to 20 Ma). The 300° C isotherm
in the present-day Coast Ranges corresponds to
greenschist-facies metamorphism (Figs. 2, 3,
and 4). The elevated temperatures at depth
should result in significant recrystallization
and growth of new metamorphic minerals. The
peak conditions of metamorphism at the base of
the Coast Ranges crust are predicted to be ~ 7 0 0
to 1000° C at a pressure of about 7 kbar, or
granulite grade (Fig. 4), with subsequent cool­
ing to lower thermal gradients. The distribution
of metamorphic facies boundaries in cross-sec­
tion view is shown in Figure 5. It should be
noted that if the Clear Lake region (300° C
isotherm at 7- to 8-km depth) is considered to be
a local effect, or too short-lived to cause signifi­
cant recrystallization, then the "sustained"
peak thermal gradient that is likely to result in
significant recrystallization is 30° C/km (300°
C isotherm at 10-km depth) (see Fig. 2). If the
30° C/km gradient is used as the thermal peak
that causes major recrystallization, then the
facies boundaries would shift a few km deeper
in the crust than shown in Figure 5, a result that
is in accord with recent estimates of seismic
velocities (John Hole, pers. commun., 1996).
On the basis of structural studies of Cenozoic
rocks along the eastern margin of the Coast
Ranges, the only significant period of regional
shortening and uplift affecting the Coast
Ranges since the passage of the triple junction
is the period of time since the plate-motion
change at 3.4 to 3.9 Ma (Namson and Davis,
986
JOHN WAKABAYASHI
FIG. 6. Approximate distribution of transform-related metamorphism, transform-related shallow plutons, and
blueschist-facies relics at 10-km depth in the Coast Ranges. This diagram is based on the point in the future when
this level of the crust is exhumed (tens of millions of years from now), so that the Mendocino triple junction has
migrated well north of the northern California border. Exposure of deeper levels of the crust will yield a
metamorphic belt with higher-grade metamorphism and a greater volume of plutons. Such a level of exposure may
appear more "arc-like."
1988). Calculation of P / T paths for rocks at
depth, as shown in Figure 4, is based on the
depth to the 300° C isotherm from the data of
Hill et al. (1990), with an assumed average
uplift rate of 0.3 mm/yr, applied only since the
plate-motion change at 3.4 to 3.9 Ma. Because
the peak metamorphism is inferred to correspond to a thermal transient, cooling will occur
under conditions of lower thermal gradient
with time. Extended uplift of these rocks
depends on their location relative to major fault
systems in the present and future Coast Ranges.
The deepening of the base of crustal seismicity
southward in the Coast Ranges (Hill et al.,
1990), shown in Figure 2, and the inferred rate
of triple-junction migration (from Engebretson
et al., 1985) indicate that rocks that experience
granulite-grade metamorphism cool to 500° C,
or amphibolite grade, within 15 million years
after the passage of the thermal maximum
(Fig. 4).
Preservation of the earlier Franciscan highP / T metamorphic assemblages at depth may
occur in the areas affected by recent greenschist (and possibly higher-grade) metamorphism, because the duration of heating by the
thermal peak is probably not sufficient to completely erase pre-existing high-P/T relics.
Because the notable recent metamorphic
recrystallization (greenschist grade and above)
SUBDUCTION-TRANSFORM TRANSITION
is associated with ductilely deforming rocks,
new metamorphic mineral growth may in part
define stretching lineations parallel or subparallel to the plate boundary, reflecting the
dominant sense of motion at this transform
plate boundary, with minerals such as amphibole elongated parallel to this lineation (Fig. 6).
Overprinted high-P/T rocks may occur as
two general types in the Coast Ranges and
represent types of rock with contrasting histo­
ries (Fig. 5). At significant (20- to 30-km)
depths in a relatively narrow belt along the
eastern margin of the Coast Ranges, metamor­
phic conditions were in the blueschist facies
prior to conversion to the transform margin,
and the rise in thermal gradient was relatively
small following subduction-transform transi­
tion (Fig. 5, points f and g). Following the
transition to a transform margin, these rocks
were heated to greenschist-facies conditions
(Fig. 5). In this case, rocks resided in high-P/T
conditions until the time of tectonic transition
and heating (path f, g in Fig. 7).
The other type of greenschist-overprinted
high-P/T rock at depth in the Coast Ranges
probably occupies a much greater volume of the
crust than those discussed above (Fig. 6). These
high-P/T rocks in the core of the Coast Ranges
were uplifted from their original depth of metamorphism (≥20 km) and should be overprinted
by greenschist or higher-grade assemblages fol­
lowing subduction-transform transition (Fig. 5;
path a', b' in Fig. 7). These rocks may preserve
their peak high-P/T assemblage, with negligible
mineral growth during synsubduction uplift,
typical of exposed Franciscan blueschists that
were exhumed under low thermal gradients
(e.g., Ernst, 1988). Thus the "apparent" P / T
path for overprinted rocks such as these in the
core of the Coast Ranges would be the super­
position of the peak overprint assemblage (and
subsequent retrograde assemblages) over the
peak high-P/T assemblage, yielding apparent
P / T paths similar to those shown in Figure 7
(paths a', b', f'). These paths should vary as a
function of uplift of the rocks prior to subduc­
tion-transform transition and the peak condi­
tions of high-P/T metamorphism experienced
by these rocks.
The likelihood of preservation of high-P/T
relics would decrease with depth, because the
grade of overprinting metamorphism would
987
FIG. 7. Hypothetical P / T path of rocks at depth in the
Coast Ranges, compared with P / T paths for the Sanbagawa
Belt (Otsuki and Banno, 1990) and the Haast schists of New
Zealand (Yardley, 1982). This diagram shows how "clock­
wise" P / T paths similar to those preserved in the San­
bagawa Belt and in New Zealand could form in the
California Coast Ranges as a result of the conversion from a
subducting to a transform plate boundary. Paths a, b, f, g,
and h correspond to P / T paths at the points on Figures 3
and 5. The arrows along the dashed "subduction thermal
gradient" line show the direction of P / T evolution for
Coast Range rocks experiencing uplift prior to conversion
to the strike-slip thermal regime. The "prograde" apparent
P / T path preserved in such rocks would be an overprint of
the peak subduction assemblage by the peak post-subduction assemblage; examples of such P / T paths are the
screened and dashed paths labeled a', b ' , and P. Depending
on the amount of uplift of blueschists prior to subductiontransform transition, different P / T trajectories are possi­
ble. The horizontal paths at a, b, f, g, and h show apparent
prograde paths of rocks that do not record an earlier uplift
history prior to transform-related metamorphism.
increase with resulting faster reaction kinetics.
In the case of older high-P/T rocks that have
been uplifted prior to overprinting, the highP / T metamorphism and overprinting may be
separated by 20 to 150 m.y., based on the
duration of Franciscan subduction and the tim­
ing of the subduction-transform transition. For
the deeper overprinted rocks, the temporal sep­
aration between high-P/T metamorphism and
overprinting may be much shorter.
988
JOHN WAKABAYASHI
The pattern of transform-related metamorphism prior to disruption by post-metamorphic
faulting may be approximately symmetrical,
with the highest-grade zone flanked by parallel
lower-grade zones, in contrast to the pronounced asymmetrical pattern of subductionrelated m e t a m o r p h i s m . P o s t - m e t a m o r p h i c
faulting, however, should greatly complicate the
distribution of transform-related metamorphism. For example, if the deep crustal plate
boundary is a relatively narrow zone of deformation, one side of the boundary should rise
relative to the other in response to the contractional component across the plate boundary,
although the dominant sense of motion would
be strike-parallel. An extended period of movement along such a boundary zone, during
which time the Coast Ranges cooled following
their thermal peak in the wake of triple-junction migration, would lead to higher-grade
rocks on the upthrown side and lower-grade
rocks on the downthrown side; with sufficient
time (tens of m.y.) the cumulative vertical displacement across the boundary would become
significant. A major deep crustal fault zone
striking obliquely across the metamorphic belt
also may juxtapose terranes formed under different thermal gradients, such as the axial Coast
Ranges (high thermal gradients) against the
eastern margin of the Coast Ranges (low thermal gradients). If the deep crustal plate boundary is a broader zone of distributed shear,
the exhumed plate boundary zone may have
an apparent inverted metamorphic gradient
across it.
Probability of future exposure
The probability of future exposure of
rocks from a given depth in the Coast Range
within a period of 200 m.y. or less should
decrease with increasing depth, because with
greater depth, the amount of exhumation and
time required to accomplish the exhumation
would increase. Accordingly, within that time
frame, exposure of the greenschist-facies level
of the Coast Ranges would be more likely than
exposure of the granulite level of metamorphism. Conversely, preservation of the shallower levels of the plate margin becomes less
likely with a large amount of elapsed time—say
500 million years or more—and exposure of the
deepest portions of the plate margins is more
likely.
Re-examination of Metamorphic Belts
Considering the Role of SubductionTransform Transitions
Past ridge-trench interactions have been
inferred for the Shimanto Belt of Japan (e.g.,
Hibbard et al., 1993), the Chile margin triplejunction area (both presently occurring and
exhumed rocks that experienced high thermal
gradients) (Forsythe and Nelson, 1985), and the
southern Alaska forearc (Sisson and Pavlis,
1993). The consequences of a ridge-trench
interaction may be similar to the transformtrench interaction described here, and the early
history of subduction-transform transition in
southern coastal California probably involved
ridge-trench interaction as well (e.g., Atwater,
1970, 1989; Bohannon and Parsons, 1995). The
following discussion focuses on the types of
features of metamorphic belts that can result
from a triple-junction migration of the California type, but may apply broadly to ridge-trenchtype interactions as well.
Thermal overprinting of some high-P/T rocks
associated with orogen-parallel stretching
lineations
As noted earlier, h i g h - P / T assemblages
should be overprinted by assemblages of higher
thermal gradient over large areas of the Coast
Ranges at depth. This type of overprint represents a "clockwise" P / T path that is typical of
most high-P/T terranes of the world (Ernst,
1988). Such P / T paths generally are suggested
to have been formed as a consequence of subduction, followed by collision of an island arc or
continental margin and subsequent cessation of
subduction. Clearly, the late Cenozoic thermal
event in the California Coast Ranges differs
because it involves no collision.
Major strike-slip faults typically are associated with belts of high-P/T rocks (Ernst, 1971),
and orogen-parallel stretching lineations also
are common features of many orogenic belts
(e.g.,. Faure, 1986; Ellis and Watkinson, 1987;
Brown and Talbot, 1989; Ratschbacher et al.,
1989; Ave Lallemant and Guth, 1990; Wallis,
1990). These stretching lineations typically
postdate high-P/T metamorphism and are texturally related to the thermal overprinting of
the earlier high-P/T metamorphic assemblages
(Ratschbacher et al., 1989; Hara et al., 1990;
Wallis, 1990). A subduction-transform transi-
SUBDUCTION-TRANSFORM TRANSITION
tion may be an alternative explanation to collisional orogenesis for development of orogenparallel stretching lineations and overprints of
high-P/T rocks in some of these orogenic belts.
The influence of a subduction-transform
transition on orogenesis versus the influence of
collision is difficult to evaluate for several reasons. The most important may be: (1) geochronologic studies are not sufficiently detailed
in many orogenic belts; and (2) collision apparently did occur at least at some time during the
development of many orogenic belts.
The Sanbagawa Belt of Japan and the Haast
schists of New Zealand are two possible examples of overprinted high-P/T belts with orogenparallel stretching lineations that may have
been affected by a subduction-transform transition. Figure 7 shows the similarity of the P / T
paths of metamorphism from the Sanbagawa
Belt (Otsuki and Banno, 1990) and the Haast
schists (Yardley, 1982) to the apparent P / T
paths that may form beneath the California
Coast Ranges as a consequence of the subduction-transform transition.
In the Sanbagawa, the clockwise P / T evolution has been attributed to subduction followed
by collision (e.g., Ernst, 1988), possibly of the
Kurosegawa tectonic zone in the Late Jurassic
(Maruyama et al., 1984). However, if the
Kurosegawa composite terrane had indeed
buoyantly clogged the subduction zone and
halted subduction, then it should form the
"lower plate" of the orogen and be more likely
to be overprinted with high-grade metamorphism, analogous to major collisional orogens
such as the Alps (e.g., Ernst, 1988). The
Kurosegawa tectonic zone apparently lacks Sanbagawa or younger metamorphism, and yields
metamorphic muscovite K/Ar ages significantly older than the Sanbagawa metamorphism (Maruyama et al., 1984), indicating that
heating of this terrane during and after the time
of Sanbagawa metamorphism did not exceed
K/Ar closure temperatures for muscovite.
Taira et al. (1983) suggested emplacement of
the older rocks of the Kurosegawa zone along a
major strike-slip fault as an alternative to collision. The timing of the Cretaceous metamorphism of the Sanbagawa rocks recorded by ArAr dates (Takasu and Dallmeyer, 1990) reflects
cooling from the thermal peak that, in turn,
may be the thermal overprint that followed the
high-P/T metamorphism (Hara et al., 1990).
989
The timing of this metamorphism is consistent
with the conversion of this margin from a
subduction zone to a transform margin as proposed by Osozawa (1994) in his plate recons t r u c t i o n s for t h i s area. A s u b d u c t i o n transform transition is a permissible alternate
to collision as the cause of the thermal overprinting in the Sanbagawa. The Median Tectonic line of Japan separates the Sanbagawa Belt
from the HT/LP Ryoke Belt. Brown and Nakajima (1994) concluded that the metamorphism
of the Ryoke Belt was a product of spreading
ridge-trench interaction. The Median Tectonic
line may be the exhumed analog of the plate
boundary at depth in coastal California, juxtaposing two terranes with different thermal
histories affected by subduction-transform
transition or ridge-trench interaction.
The Haast schists of New Zealand record a
similar thermal history of early blueschist-type
assemblages overprinted by later greenschist
assemblages (Yardley, 1982). Similar to the Sanbagawa Belt, the Haast schists are bordered by a
major strike-slip fault (the Alpine fault) that
separates them from high-grade rocks. The
schists also have stretching lineations that are
subparallel to the border of the belt (Mortimer,
1992). The origin of the structures and overprint of the Haast schists, like the Sanbagawa
Belt, generally is attributed to a collision (Mortimer, 1992). A subduction-transform transition may be a reasonable alternative model for
part of the tectonothermal evolution of these
schists.
Alternative explanation for some rock associations interpreted as ancient volcanic arcs
A belt of plutons and associated metamorphic rocks traditionally is interpreted to be the
exhumed root of an ancient volcanic arc. Such a
belt of plutons should be present, however,
beneath the California Coast Ranges (Liu and
Furlong, 1992). An analog of such a plutonic
belt may be exposed in the Gulf of Alaska,
although the origin of these plutons is still in
dispute (Barker et al., 1992).
A possibly similar association of granitic
rocks and strike-slip faults, in Hercynian shear
zones in Iberia and shear zones in the British
Caledonides, was noted by Hutton and Reavy
(1992). Although these authors suggest crustal
thickening during transpressional deformation
990
JOHN WAKABAYASHI
depending on the component of shortening (or
lack thereof) that accompanies the strike-slip
motion and drives uplift.
In addition to some granulite belts, other
belts of HT metamorphism may be a consequence of a subduction-transform transition,
representing somewhat shallower levels of
crust. An example of such a metamorphic belt
may be the Salmon River suture of Idaho, a
terrane that features regional metamorphism of
greenschist to upper amphibolite facies, with
structures suggestive of major strike-slip displacement along the suture (Lund and Snee,
1988). The main stage of regional HT metamorphism associated with this belt may be a consequence of a subduction-transform transition.
FIG. 8. Hypothetical retrograde P / T paths for rocks at
depth in the California Coast Ranges compared with P / T
paths of granulites compiled by Bohlen (1987, 1991). The
Coast Ranges paths show cooling from 3 5 ° C/km. The
thermal peak of longer duration (see text and figure captions for Figs. 4 and 5) is 3 0 ° C/km and may be more
representative of peak prograde assemblages, in which case
Coast Range retrograde P / T paths originate from the 3 0 °
C/km line, and the similarity of these P / T paths to the
granulite paths would be greater.
as the cause of crustal melting, an alternative
for the formation of some of the granitoids they
discussed may be a subduction-transform
transition.
Granulite belts and other HT metamorphic belts
In many granulite belts, granulite assemblages have undergone retrograde metamorphism under conditions of decreasing thermal
gradient, or a counterclockwise P / T path (e.g.,
Bohlen, 1991). Such metamorphism has been
attributed to metamorphism at the base of a
volcanic arc or subcontinental underplating
(Bohlen and Metzger, 1989). It should be noted
that the P / T conditions of metamorphism at
depth in the Coast Ranges and the predicted
cooling path (Fig. 8) are very similar to many of
the examples cited by Bohlen (1991). The
inferred cooling of deep rocks in the California
Coast Ranges may have been close to isobaric,
because of the minimal uplift since the passage
of the plate triple junction (see discussions in
earlier sections; Figs. 4 and 8). The cooling
paths of rocks in a general subduction-transform transition orogen may vary, however,
Probability of exposure and the age of granulite
and high-P/T belts
As indicated previously, the probability of
future exposure of different depths of the
present-day California Coast Ranges varies with
the amount of elapsed time in the future; that
is, exclusively shallow levels of exposure are
more likely with less elapsed time, and deep
levels are more likely with long elapsed time. As
one of several types of plate-boundary changes
that can influence the evolution of mountain
belts, the probability of preservation subduction-transform transition effects provides
insight into general problems of preservation of
various rock types on Earth. For example, it is
likely that a long-lived active plate margin will
experience a collision, subduction-transform
transition, or ridge-trench interaction at some
point in its history. The longer the elapsed time
since the formation of a blueschist belt, the less
likely blueschists along any plate margin will be
preserved, because: (1) there is increased probability of a plate-boundary transition that stops
subduction or causes an increase in thermal
gradients; and (2) the longer elapsed time
allows for greater exhumation of rocks, exposing deeper levels of the crust where older highP / T rocks are more likely to have been completely overprinted by the thermal effects of the
plate boundary interactions. It therefore is not
surprising that the vast majority of high-P/T
metamorphic belts are Phanerozoic in age
(Ernst, 1972; Liou et al., 1989). The scarcity of
older blueschist has been attributed to a
decrease in thermal gradients as the Earth
cooled (Burke et al., 1977). Although the
SUBDUCTION-TRANSFORM TRANSITION
decrease in thermal gradients with time may
have affected the age distribution of exposed
blueschists, exposure time to later thermal processes, as discussed here, also may play a major
role.
Conversely, any granulites that form in an
environment analogous to the present-day California Coast Ranges are not likely to be exposed
for a long time, because of the magnitude of
cumulative exhumation necessary to expose
them. The fact that the majority of granulite
belts are Precambrian (Bohlen and Metzger,
1989) is consistent with this observation.
REFERENCES
Argus, D. F., and Gordon, R. G., 1991, Current Sierra
Nevada-North America motion from very long baseline interferometry: Implications for the kinematics
of the western United States: Geology, v. 19, p.
1085-1088.
Atwater, T., 1970, Implications of plate tectonics for the
Cenozoic evolution of western North America: Geol.
Soc. Amer. Bull., v. 81, p. 3513-3536.
_____,
1989, Plate tectonic history of the northeast
Pacific and western North America, in Winterer,
E. L., Hussong, D. M., and Decker, R. W., eds., The
eastern Pacific Ocean and Hawaii: Geology of North
America, vol. N: Boulder, CO, Geol. Soc. Amer., p.
21-72.
Avé Lallemant, H. G., and Guth, L. R., 1990, Role of
extensional tectonics in exhumation of eclogites and
blueschists in an oblique subduction setting: Northeastern Venezuela: Geology, v. 18, p. 950-953.
Aydin, A., and Page, B. M., 1984, Diverse PlioceneQuaternary tectonics in a transform environment,
San Francisco Bay region, California: Geol. Soc.
Amer. Bull., v. 95, p. 1303-1317.
Barker, F., Farmer, G. L., Ayuso, R. A., Plafker, G., and
Lull, J. S., 1992, The 50 Ma granodiorite of the
eastern Gulf of Alaska: Melting in the accretionary
prism of the forearc: Jour. Geophys. Res., v. 97 , p.
6757-6778.
Beaudoin, B. C , Godfrey, N. J., Klemperer, S. L., Lendl,
C , Trehu, A. M., Henstock, T. J., Levander, A., Holl,
J. E., Meltzer, A. S., Luetgert, J. H., and Mooney,
W. D., 1996, Transition from slab to slabless: Results
from the 1993 Mendocino triple junction seismic
experiment: Geology, v. 24, p. 195-199.
Bohannon, R. G., and Parsons, T., 1995, Tectonic
implications of post-30 Ma Pacific-North American
relative plate motions: Geol. Soc. Amer. Bull., v. 107,
p. 937-959.
991
Bohlen, S. R., 1987, Pressure-temperature-time paths
and a tectonic model for the evolution of granulites:
Jour. Geol., v. 95, p. 617-632.
_______, 1991, On the formation of granulites: Jour.
Metamor. Geol., v. 9, p. 223-229.
Bohlen, S. R.,and Metzger, K., 1989, Origin of granulite
terranes and the formation of the lowermost continental crust: Science, v. 244, p. 326-329.
Brice, J. C , 1953, Geology of the Lower Lake quadrangle, California: San Francisco, Calif. Div. Mines
and Geol., Bull. 166, 72 p.
Brocher, T. M., McCarthy, J., Hart, P. E., Holbrook,
W. S., Furlong, K. P., McEvilly, T. V., Hole, J. A„ and
Klemperer, S. L., 1994, Seismic evidence for a lower
crustal detachment beneath San Francisco Bay, California: Science, v. 265, p. 1436-1439.
Brown, M., and Nakajima, T., 1994, High-T, Low-P
metamorphism in the Ryoke Belt of Japan: Consequence of ridge subduction [abs.]: Geol. Soc. Amer.
Abs. Prog., v. 26, p. 214.
Brown, E. H., and Talbot, J. L., 1989, Orogen-parallel
extension in the north Cascades crystalline core,
Washington: Tectonics, v. 8, p. 1105-1114.
Biirgmann, R., Arrowsmith, R., Dumitru, T., and
McLaughlin, R., 1994, Rise and fall of the southern
Santa Cruz Mountains, California, from fission
tracks, geomorphology and geodesy: Jour. Geophys.
Res., v. 99, p. 20181-20202.
Burke, K., Dewey, J. F., and Kidd, W. S. F., 1977, World
distribution of sutures—The sites of former oceans:
Tectonophysics, v. 40, p. 69-99.
Cloos, M., and Dumitru, T. A., 1987, Blueschist terranes in the Franciscan Complex of California:
Their future character and implications for past
plate interactions [abs.]: Geol. Soc. Amer. Abs. Prog.
v. 23, p. 14.
Coney, P. J., 1980, Cordilleran metamorphic core complexes—an overview, in Crittenden, M. D., Jr.,
Coney, P. J., and Davis, G. H., editors, Cordilleran
metamorphic core complexes: Boulder, CO, Geol.
Soc. Amer. Memoir 153, p. 7-31.
DeMets, C , Gordon, R. G., Argus, D. F., and Stein, S.,
1990, Current plate motions: Geophys. Jour. Int., v.
101, p. 425-478.
Dickinson, W. R., and Snyder,W. S., 1979, Geometry of
triple junctions related to San Andreas transform:
Jour. Geophys. Res., v. 84, p. 561-572.
Dumitru, T. A., 1989, Constraints on uplift in the
Franciscan subduction complex from apatite fission
track analysis: Tectonics, v. 8, p. 197-220.
_______,
1991, Major Quaternary uplift along the
northernmost San Andreas fault, King Range, northwestern California: Geology, v. 19, p. 526-529.
Ellis, M., and Watkinson, A. J., 1987, Orogen-parallel
extension and oblique tectonics, the relation
992
JOHN WAKABAYASHl
between stretching lineations and relative plate
motions: Geology, v. 15, p. 1022-1026.
Engebretson, D. C., Cox, A., and Gordon, R. G., 1985,
Relative motion between oceanic and continental
plates in the Pacific basin: Boulder, CO, Geol. Soc.
Amer. Spec. Pap. 206, 59 p.
Ernst, W. G., 1970, Tectonic contact between the Franciscan melange and the Great Valley Sequence,
crustal expression of a Late Mesozoic Benioff Zone:
Jour. Geophys. Res., v. 75, p. 886-902.
_______,
1971, Metamorphic zonations on presumably subducted lithospheric plates from Japan, California, and the Alps: Contrib. Mineral. Petrol., v. 34,
p. 43-59.
________, 1972, Occurrence and mineralogic evolution of blueschist belts with time: Amer. Jour. Sci., v.
272, p. 657-668.
, 1988, Tectonic history of subduction zones
inferred from retrograde blueschist P / T paths: Geology, v. 16, p. 1081-1084.
Faure, M., 1986, Microtectonic evidence for eastward
ductile shear in the Jurassic orogen of S.W. Japan:
Jour. Struct. Geol., v. 7, p. 175-186.
Forsythe, R. D., and Nelson, E. P., 1985, Geologic
manifestations of ridge collision: Evidence from the
Golfo de Penas-Taitao Basin, southern Chile: Tectonics, v. 4, p. 477-495.
Fox, K. F., Fleck, R. J., Curtis, G. H., and Meyer, C. E.,
1985, Implications of the northwestwardly younger
age of volcanic rocks in west-central California:
Geol. Soc. Amer. Bull., v. 96, p. 647-652.
Fuis, G. S., and Mooney, W., 1990, Lithospheric structure and tectonics from seismic refraction and other
data, in Wallace, R. E., ed., The San Andreas Fault
system, California: Washington, DC, U.S. Geol.
Surv. Prof. Paper 1515, p. 207-238.
Furlong, K. P., 1984, Lithospheric behavior with triple
junction migration: An example based on the Mendocino triple junction: Phys. Earth Planet. Inter., v.
36, p. 213-223.
Gordon, R. G., and Argus, D. F., 1993, The San Andreas
fault system in central California as the boundary
between the Pacific and the Sierra Nevada-Great
Valley microplate: Kinematics from VLBI geodesy:
EOS (Trans. Amer. Geophys. Union), v. 74, no. 43,
p. 64.
Graham, S. A., McCloy, C , Hitzman, M., Ward, R., and
Turner, R., 1984, Basin evolution during change
from convergent to transform continental margin in
central California: Amer. Assoc. Petrol. Geol. Bull.,
v. 68, p. 233-249.
Hara, I., Shiota, T., Hide, K., Okamoto, K., Takeda, K.,
Hayasaka, Y., and Sakurai, Y., 1990, Nappe structure
of the Sanbagawa belt: Jour. Metamor. Geol., v. 8, p.
441-456.
Harbert, W., 1991, Late Neogene motion of the Pacific
and North American plates: Tectonics, v. 10, p. 1-16.
Hearn, B. C , Donelly-Nolan, J. M., and Goff, F. E., 1981,
The Clear Lake volcanics: Tectonic setting and
magma sources, in McLaughlin, R. J., and DonellyNolan, J. M., eds. Research in the Geysers-Clear
Lake geothermal area: Washington, DC, U.S. Geol.
Surv., Prof. Paper 1141, p. 25-45.
Hibbard, J. P., Laughland, M. M., Kang, S. M., and
Karig, D., 1993, The thermal imprint of spreading
ridge subduction on the upper structural levels of an
accretionary prism, southwest Japan, in Underwood,
M. B., ed., Thermal evolution of the Tertiary Shimanto Belt, southwest Japan: An example of ridgetrench interaction: Boulder, CO, Geol. Soc. Amer.
Spec. Pap. 273, p. 83-101.
Hill, D. P., Eaton, J. P., and Jones, L. M., 1990, Seismicity, 1980-1986, in Wallace, R. E., ed., The San
Andreas Fault system, California: Washington, DC,
U.S. Geol. Surv. Prof. Paper 1515, p. 115-151.
Holbrook, W. S., Brocher, T. M., ten Brink, U. S., and
Hole, J. A., 1996, Crustal structure of a transform
plate boundary: San Francisco Bay and the central
California continental margin: Jour. Geophys. Res.,
v. 101, p. 22311-22334.
Hole, J. A., 1996, Wide-angle seismic constraints on the
evolution of the deep plate boundary beneath the
California Coast Ranges: EOS (Trans. Amer.
Geophys. Union), v. 77, no. 46, p. F743.
Hurlow, H. A., 1993, Mid-Cretaceous strike-slip and
contractional fault zones in the western Intermontane terrane, Washington, and their relation to the
North Cascades-Southeastern Coast Belt orogen:
Tectonics, v. 12, p. 1240-1257.
Hutton, D. H. W., and Reavy, R. J., 1992, Strike-slip
tectonics and granite petrogenesis: Tectonics, v. 11,
p. 960-967.
Isherwood, W. F., 1981, Geophysical overview of the
Geysers, in McLaughlin, R. J., and Donelly-Nolan,
J. M., eds. Research in the Geysers-Clear Lake geothermal area: Washington, DC, U.S. Geol. Surv.,
Prof. Paper 1141, p. 83-95.
Iyer, H. M., Oppenheimer, D. H., Hitchcock, T., Roloff,
J. N., and Coakley, J. M., 1981, in McLaughlin, R. J„
and Donelly-Nolan, J. M., eds., Research in the
Geysers-Clear Lake geothermal area: U.S. Geol.
Surv., Prof. Paper 1141, p. 97-115.
Jachens, R. C., Griscom, A., and Roberts, C. W., 1995,
Regional extent of Great Valley basement west of the
Great Valley, California: Implications for extensive
tectonic wedging in the California Coast Ranges:
Jour. Geophys. Res., v. 100, p. 12769-12790.
Jennings, C. W., compiler, 1977, Geologic map of California: Sacramento, California Division of Mines
and Geology.
SUBDUCTION-TRANSFORM TRANSITION
Johnson, C. M., and O'Neil, J. R., 1984, Triple junction
magmatism: A geochemical study of Neogene volcanic rocks in western California: Earth Planet. Sci.
Lett., v. 71, p. 241-262.
Jové, C. F., and Coleman, R. G., 1992, Evidence for
mantle upwelling along the San Andreas fault system
using thermobarometry of xenoliths in Pliocene
basalts invading the Calaveras fault: EOS (Trans.
Amer. Geophys. Union), v. 73, p. 656.
Kelson, K. I., Lettis, W. R., and Lisowski, M., 1992,
Distribution of geologic slip and creep along faults in
the San Francisco Bay region, in Borchardt, G., et
al., eds., Proc. Second Conf. on Earthquake Hazards
in the Eastern San Francisco Bay Area: Sacramento,
California Division of Mines and Geology Special
Publication 113, p. 31-38.
Lachenbruch, A. H., and Sass, J. H., 1980, Heat flow and
energetics of the San Andreas fault zone: Jour.
Geophys. Res., v. 85, p. 6185-6222.
Lettis, W. R., 1982, Late Cenozoic stratigraphy and
structure of the western margin of the central San
Joaquin Valley, California: Washington, DC, U.S.
Geol. Surv. Open File Report 82-526.
Lettis, W. R., Kelson, K. L, Wesling, J. R., Hanson,
K. L., and Hall, N. T., 1994, Quaternary deformation
of the San Luis Range, San Luis Obispo County,
California, in Alterman, I. B., McMullen, R. B.,
Cluff, L. S., and Slemmons, D. B., eds., Seismotectonics of the central California Coast Ranges: Boulder, CO, Geol. Soc. of Amer. Spec. Paper 292, p.
133-150.
Liou, J. G., Graham, S. A., Maruyama, S., Wang, X.,
Xiao, X., Carroll, A. R., Chu, J., Feng, Y., Hendrix,
M. S., Liang, Y. H., McKnight, C. L., Tang, Y., Wang,
Z. X., Zhao, M., and Zhu, B., 1989, Proterozoic
blueschist belt in western China: Best documented
Precambrian blueschists in the world: Geology, v.
17, p. 1127-1131.
Liu, M., and Furlong, K. P., 1992, Cenozoic volcanism in
the California Coast Ranges: Numerical solutions:
Jour. Geophys. Res., v. 97, p. 4941-4951.
Lund, K., and Snee, L. W., 1988, Metamorphism, structural development, and age of the continent-island
arc juncture in west-central Idaho, in Ernst, W. G.,
ed., Metamorphism and crustal evolution of the
western United States, Rubey Volume VII:
Englewood Cliffs, NJ, Prentice-Hall, p. 1035-1060.
Maruyama, S., Banno, S., Matsuda, T., and Nakajima, T.,
1984, Kurosegawa zone and its bearing on the development of the Japanese islands: Tectonophysics, v.
110, p. 47-60.
McLaughlin, R. J., and Ohlin, H. N., 1984, Tectonostratigraphic framework of the Geysers-Clear Lake
region, California, in Blake, M. C., Jr., ed., Franciscan geology of northern California: Los Angeles,
993
Pacific Section, Soc. Econ. Paleontol. Mineral., v.
43, p. 221-254.
Merritts, D., and Bull, W. B., 1989, Interpreting Quaternary uplift rates at the Mendocino triple junction,
northern California, from uplifted marine terraces:
Geology, v. 17, p. 1020-1024.
Mortimer, N., and Graham, I. J., 1992, Terrane characterization and timing of metamorphism of the Otago
schist, New Zealand, using Rb-Sr and K-Ar geochronology: New Zealand Jour. Geol. Geophys., v.
35, p. 391-401.
Mount, V. S., and Suppe, J., 1987, State of stress near
the San Andreas fault: Implications for wrench
tectonics: Geology, v. 15, p. 1143-1146.
Nakata, J. K., Sorg, D. H., Russell, P. C , Meyer, C. E.,
Wooden, J., Lanphere, M. A., McLaughlin, R. J.,
Sarna-Wojcicki, A. M., Saburomaru, J. Y., Pringle,
M. S., and Drinkwater, J., 1993, New radiometric
ages and tephra correlations from the San Jose and
the northeastern part of the Monterey 1:100,000
map quadrangles, California: Isochron/West, v. 60,
p. 19-32.
Namson, J. S., and Davis, T. L., 1988, Seismically active
fold and thrust in the San Joaquin Valley, central
California: Geol. Soc. Amer. Bull., v. 100, p.
257-273.
Nelson, E. P., and Forsythe, R. D., Ridge collision at
convergent margins: Implications for Archean and
post-Archean crustal growth: Tectonophysics, v.
161, p. 307-315.
Osozawa, S., 1994, Plate reconstruction based upon age
data of Japanese accretionary complexes: Geology, v.
22, p. 1135-1138.
Otsuki, M., and Banno, S., 1990, Prograde and retrograde metamorphism of hematite-bearing basic
schists in the Sanbagawa belt in central Shikoku:
Jour. Metamor. Geol., v. 8, p. 425-439.
Ratschbacher, L. , Frisch, W., Neubacher, F., Schmid,
S. M., and Neugebauer, J., 1989, Extension in compressional orogenic belts: The eastern Alps: Geology,
v. 17, p. 404-407.
Robin, P.-Y. F., and Cruden, A. R., 1994, Strain and
vorticity patterns in ideally ductile transpression
zones: Jour. Struct. Geol., v. 16, p. 447-466.
Sibson, R. H., 1982, Fault zone models, heat flow, and
depth distribution of earthquakes in the continental
crust of the United States: Seismol. Soc. Amer.
Bull., v. 72, p. 151-163.
Sisson, V. B., and Pavlis, T. L., 1993, Geologic consequences of plate reorganization: An example from
the Eocene southern Alaska forearc: Geology, v. 21,
p. 913-916.
Sisson, V. B., Pavlis, T. L., and Prior, D. J., 1994, Effects
of triple junction interactions at convergent plate
margins: GSA Today, v. 4, p. 248-249.
994
JOHN WAKABAYASHI
Stimac, J. A., Goff, F. E., and Glassley, W. E., 1992,
Crustal xenoliths in the Clear Lake volcanics, California: Transformation of Franciscan assemblage
rocks to continental crust?: EOS (Trans. Amer.
Geophys. Union), v. 73, p. 505.
Taira, A., Saito, Y., and Hashimoto, M., 1983, The role
of oblique subduction and strike-slip tectonics in the
evolution of Japan, in Hilde, T. W. C., and Uyeda, S.,
eds., Geodynamics of the western Pacific-Indonesia
region: Washington, DC, Amer. Geophys. Union
Geodynamics Series, v. 11, p. 303-316.
Takasu, A., and Dallmeyer, R. D., 1990, 40 Ar/ 39 Ar mineral age constraints for the tectonothermal evolution
of the Sanbagawa metamorphic belt, central
Shikoku, Japan: A Cretaceous accretionary prism:
Tectonophysics, v. 185, p. 111-139.
Tikoff, B., and Teyssier, C., 1994, Strain modeling of
displacement-field partitioning in transpressional
orogens: Jour. Struct. Geol., v. 16, p. 1575-1588.
Underwood, M. B., 1989, Temporal changes in geothermal gradients, Franciscan subduction complex,
northern California: Jour. Geophys. Res., v. 94, p.
3111-3125.
Underwood, M. B., Laughland, M. M., Shelton, K. L.,
and Sedlock, R. L., 1995, Thermal-maturity trends
within Franciscan rocks near Big Sur, California:
Implications for offset along the San Gregorio-San
Simeon-Hosgri fault zone: Geology, v. 23, p.
839-842.
Wakabayashi, J., 1992, Nappes, tectonics of oblique
plate convergence and metamorphic evolution
related to 140 million years of continuous subduction, Franciscan Complex, California: Jour. Geol., v.
100, p. 19-40.
Wakabayashi, J., and Smith, D. L., 1994, Evaluation of
recurrence intervals, characteristic earthquakes,
and slip rates associated with thrusting along the
Coast Range-Central Valley geomorphic boundary,
California: Bull. Seismol. Soc. Amer., v. 84, p.
1960-1970.
Wakabayashi, J., and Unruh, J. R., 1995, Tectonic wedging, blueschist metamorphism, and exposure of
blueschists: Are they compatible?: Geology, v. 23, p.
85-88.
Wallis, S. R., 1990, The timing of folding and stretching
in the Sanbagawa belt: The Asemigawa region,
central Shikoku: Jour. Geol. Soc. Japan, v. 96, p.
345-352.
Yardley, B. W. D., 1982, The early metamorphic history
of the Haast Schists and related rocks of New Zealand: Contrib. Mineral. Petrol., v. 81, p. 317-327.
Zoback, M. D., Zoback, M. L., Mount, V., Suppe, J.,
Eaton, J. P., Healy, J. H., Oppenheimer, D., Reasenberg, P., Jones, L., Raleigh, C. B., Wong, I. G., Scotti,
0., and Wentworth, C. M., 1987, New evidence on
the state of the San Andreas fault: Science, v. 238, p.
1105-1111.