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 tion or transform tectonics for the last 160 m.y. involving either migrating transform faults or (Engebretson et al. 1985). During the late spreading ridges is common in Earth history Mesozoic and Tertiary, the Franciscan subduc (Sisson et al., 1994). Accordingly, it is reason tion complex formed over a period of ≥140 able to surmise that past plate interactions, m.y. of continuous subduction (Wakabayashi, similar to the subduction-transform transition 1992). Subduction terminated with the north occurring in present-day northern California, ward passage of the Mendocino triple junction, have left their imprint on many orogenic belts. and a transform plate boundary developed Nelson and Forsythe (1989) suggested that south of the triple junction (Atwater, 1970). ridge-trench collision is an important process General geologic relations are shown in Fig in crustal growth, and speculated that this pro ure 1. cess played an even greater role in Archean crustal growth. However, the impact of ridge- In addition to the different style of deforma trench interactions or subduction-transform tion, higher thermal gradients followed the sub transitions on the development of orogenic duction-transform transition (Dickinson and belts still is largely unappreciated. Snyder, 1979; Lachenbruch and Sass, 1980; The Coast Ranges of California are a type Furlong, 1984). Dickinson and Snyder (1979) example of subduction-transform transition. proposed that a "slab window" trailing in the

0020-6814/96/225/979-16 $10.00 979 980 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 astheno- 1996), and no single model proposed thus far is spheric upwelling, causing a significant completely consistent with some of the recent increase in the thermal gradient. Thermal mod• seismic data obtained (Hole, 1996). However, eling based on the slab-window concept predicts the thermal effects associated with the passage a thermal peak just after the passage of the of the triple junction and the transform nature Mendocino triple junction, followed by cooling of the plate boundary south of the Mendocino as new lithosphere forms in the window region triple junction are not disputed. These points of (Furlong, 1984). This model is consistent with consensus, rather than any specific tectonic heat-flow data of Lachenbruch and Sass (1980) model, will serve as the foundation of this and the occurrence of late Cenozoic volcanism paper. in the Coast Ranges. Recently, the slab-window This paper relates field, thermal, structural, concept has been challenged as a framework for and geophysical data to inferred metamorphic the late Cenozoic tectonics of coastal California mineral assemblages and structures at depth (Bohannon and Parsons, 1995; Beaudoin et al., 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 of the base of crustal seismicity in the Coast levels of the transform plate margin in Califor Ranges, which is inferred to coincide with the nia may look like if exhumed and to speculate brittle-ductile transition (e.g., see Hill et al., on the imprint of similar, past plate inter 1990). The depth of the base of crustal seis actions on exhumed metamorphic belts of the micity varies from north to south along the world. Among the topics addressed in this strike of the Coast Ranges in the wake of the paper are: (1) an alternative tectonic mecha triple junction, first shallowing to an average of nism to thrusting-thermal relaxation (subduc- about 10 km (locally as shallow as 7 to 8 km in tion followed by collision) models for thermal the Clear Lake area), then deepening pro overprinting of some high-pressure, low- gressively to the south and leveling out at an temperature (HP/LT) metamorphic rocks; (2) average of about 15 km south of San Francisco an alternative tectonic mechanism for develop Bay (Fig. 2; cross-sectional view shown in Fig. ment of some granulite belts, or other high- 3). The brittle-ductile transition in quartz-rich temperature metamorphic belts; (3) problem rocks, inferred to constitute most of the Coast atic arc-like belts of plutons; (4) possible Ranges at depth on the basis of seismic veloc examples of exhumed equivalents of subduc- ities (Holbrook et al., 1996), is estimated to take tion-transform orogens; and (5) distribution of place at a temperature of ~300° C (Sibson, blueschists and granulites in time. 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 An Example of a Subduction-Transform Clear Lake region), cooling to 20° C/km in the Transition: The California southern Coast Ranges. The migration rate of Coast Ranges at Depth the Mendocino triple junction indicates that the cooling to 20° C/km thermal gradients, in Elevated thermal gradients following the wake of the passage of the thermal peak, subduction-transform conversion took 10 to 15 m.y. All of the above studies Modeling of heat-flow data by Lachenbruch suggest high (≥30° C/km) peak thermal gra and Sass (1980) and Dumitru (1989) suggest dients in the California Coast Ranges after the peak thermal gradients of 35° C/km, following subduction-transform transition, followed by subduction-transform transition in coastal Cal cooling to lower gradients. Superimposed on ifornia. The modeling of Furlong (1984) pre the regional thermal effect noted above are local dicts peak temperatures of about 700° C at effects, possibly related to space problems in about 25 km depth, 1 m.y. after passage of the the area of the migrating triple junction (Dumi triple junction, subsequently cooling to about tru, 1991; Underwood, 1989; Underwood et al., 450° C at the same depth 20 m.y. after triple- 1995). These local processes have resulted in junction passage. In addition to heat-flow data, very high local uplift rates and elevated thermal high thermal gradients in the California Coast gradients of ≥50° C/km (Dumitru, 1991). In Ranges are indicated by the 10- to 18-km depth 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). "300" is the 300° C isotherm, and the screened "300" represents the location of the corresponding isotherm at the time of the passage of the thermal peak. The 300° 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 P/T conditions of metamorphism are grossly of hot material and/or intrusion of plutons incompatible with original subduction-related (Jové and Coleman, 1992) and may have an metamorphism of Franciscan rocks, the coun• impact on the local thermal structure. try rocks of the Clear Lake area (McLaughlin Late Cenozoic volcanism in the Coast Ranges and Ohlin, 1984), or Sierra Nevada regional locally has brought up xenoliths of deep crustal metamorphic rocks, which have been suggested material (Brice, 1953; Hearn et al., 1981; to tectonically underlie the eastern Coast Stimac et al., 1992; Jove and Coleman, 1992; Ranges (Jachens et al., 1995). Nakata et al., 1993), including high-grade, Jove and Coleman (1992) analyzed gabbroic silicic metamorphic rocks. For example, xenoliths from Pliocene volcanic rocks near numerous xenoliths of high-grade, schistose Coyote Lake and estimated crystallization con• metamorphic rocks have been found in ande- ditions of 915 to 1000° C at 9 to 10.5 kbar, sites from the 10-ka to 2-Ma Clear Lake volcanic based on thermobarometry. The P/T conditions field (Brice, 1953; Hearn et al., 1981). The calculated for these gabbro xenoliths also are mineral assemblages in these xenoliths include consistent with the hypothetical thermal gra• orthopyroxene-clinopyroxene-plagioclase- dients discussed above (Fig. 4). Nakata et al. quartz ± biotite ± garnet and orthopyroxene- (1993) reported xenoliths that include biotite plagioclase-biotite ± sillimanite ± cordierite ± and sillimanite-corundum schists (in addition spinel (Stimac et al., 1992). Thermobarometry to gabbroic xenoliths) from the andesite of on these rocks has yielded estimates for their Dowdy Ranch in the Diablo Range, north of the crystallization conditions of 800 to 900° C at 5 Quien Sabe volcanic field and east of the Coyote to 8 kbar. These P/T estimates are consistent Lake volcanics. Nakata et al. (1993) obtained with temperatures estimated for the deeper lev• K/Ar ages of 8.5 to 12.3 Ma from igneous els of the California Coast Ranges discussed xenoliths from the andesite of Dowdy Ranch, above (Fig. 4). On the basis of chemical and although the ages may reflect resetting by the isotopic data, Stimac et al. (1992) concluded heat from the andesite in which they were that these rocks most likely are metamorphosed entrained, which yielded a date of 8.2 Ma. Franciscan greywackes, formed by regional The mineral assemblages in schist xenoliths metamorphism in the wake of triple-junction described by Nakata et al. (1993) are indicative migration. The well-developed schistosity or of the same type of metamorphism under high foliation of these rocks indicates regional thermal gradients recorded in xenoliths studied (rather than contact) metamorphism, and the by Stimac et al. (1992) from the Clear Lake SUBDUCTION-TRANSFORM TRANSITION 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 plu- FIG. 4. Hypothetical P/T paths for metamorphic rocks at tonic material are expected at depth in the depth in the California Coast Ranges following conversion Coast Ranges, although they probably do not to transform tectonics. The dotted lines with arrows repre form a contiguous belt similar to the Sierra sent the prograde metamorphic path after the conversion. Nevada batholith. Solid paths with arrows show cooling from the thermal peak at locations in the crust (lower-case letters) shown on Present-day plate boundary kinematics Figure 3. The 20° C/km line corresponds to estimates of the thermal gradient in the central and southern Coast The present-day California Coast Ranges are Ranges, 15 m.y. or more after the passage of the thermal part of the transform boundary between the maximum, based on the depth to the base of crustal seis- Pacific and North American plates. Dextral micity (Hill et al., 1990). The "late-subduction thermal shear in the Coast Ranges totals about 35 to 40 gradient" is the pre-transform thermal gradient north of the mm/yr, about 80% of the motion being parallel Mendocino triple junction estimated from the base of to the plate boundary (DeMets et al., 1990) and crustal seismicity (Hill et al., 1990). The peak thermal nearly all of the small contractional component gradient in the figure is 35 ° C/km, an average figure within of ≤3 mm/yr being perpendicular to the plate the range of estimates derived from several different boundary (Argus and Gordon, 1991; DeMets et methods (see text). The 30° C/km line represents the high- temperature maximum of longer duration (see text) and al., 1990; Gordon and Argus, 1993). The domi may be more representative of peak prograde assemblages. nant structural features of the Coast Ranges are Metamorphic conditions for Clear Lake xenoliths (Stimac the major strike-slip faults of the San Andreas et al., 1992) and Coyote Lake xenoliths (Jove and Coleman, system. These strike-slip faults account for 1992) are shown. essentially all of the margin-parallel plate motion within the Coast Ranges (e.g., Kelson et volcanic field. The xenoliths from the Dowdy al., 1992). Ranch volcanics probably formed in a setting The contractional component of plate similar to that of the xenoliths found in the motion contributes to the development of fold- Clear Lake volcanics, at a time when the triple and-thrust belts within the Coast Ranges and junction, and trailing thermal peak, was farther the uplift of the range (Mount and Suppe, 1987; south in the Coast Ranges. Zoback et al., 1987); much of the shortening is The thermal pulse that followed the triple- concentrated along the eastern border of the junction migration also produced volcanism Coast Ranges (Wakabayashi and Smith, 1994). (Johnson and O'Neil, 1984; Fox et al., 1985). The present contractional component of plate The volcanic rocks mostly are silicic to inter motion in the Coast Ranges has persisted since mediate rocks of calc-alkalic affinity (Hearn et a change in plate motions at 3.4- to 3.9-Ma al., 1981, Johnson and O'Neil, 1984), and (Harbert, 1991). Local shortening and some apparently involved melting of the crustal rocks extension also occurs within the Coast Ranges above underplated or intruded basaltic magma as a consequence of constrictional and releas (Liu and Furlong, 1992). There is evidence of a ing bends in the major strike-slip faults (e.g., 984 JOHN WAKABAYASHI

Aydin and Page, 1984). Prior to the 3.4- to 3.9- alternative to a detachment, the reflector may Ma change in plate motions, the transform plate be a consequence of a major metamorphic- boundary may have had a slight divergent com• facies change, or a fluid-rich zone in the crust ponent across it, resulting in the formation of (that also may be a consequence of metamor- some of the Tertiary basins of the Coast Ranges phic reactions). At present there are no direct (Graham et al., 1984; Engebretson et al., 1985). data to determine whether a detachment occurs The pre-3.4- to 3.9-Ma history of the transform at the brittle-ductile transition, or if such a margin constitutes, for most of the present structure is present locally, but not universally, transform margin, the larger part of the total in the Coast Ranges. Numerous examples can elapsed time as a transform plate boundary. be found in exhumed orogenic belts of either Uplift rates in the Coast Ranges locally are high, detachments at the brittle-ductile transition ranging up to several mm/yr (Merritts and Bull, (e.g., Coney, 1980) or discrete shear zones that 1989; Dumitru, 1991), but are generally extend below the brittle-ductile transition (e.g., between 0.1 and 0.5 mm/yr (Lettis, 1982; Bürg- Hurlow, 1993). It follows that only indirect mann et al., 1994; Lettis et al., 1994; K. Lajoie, inferences can be made regarding structures pers. commun., 1994; M. Angell, pers. com- below the brittle-ductile transition in the Coast mun., 1995). Apatite fission-track ages for most Ranges. of the northern and central Coast Ranges are Although there are many permissible inter• greater than 30 Ma, indicating that the average pretations of deep structure, there is general Cenozoic uplift rate for rocks now exposed at agreement that dextral strike-slip motion domi• the surface has been relatively low (Dumitru, nates the kinematics of the Coast Ranges and 1989). Because young (post-20 Ma) fission- should strongly influence structures at all lev• track ages are rare, except in local areas of high els of the crust. This dextral shear may be either uplift rates, the average exhumation rate of localized in discrete shear zones or accommo• rocks exposed on the surface must have been dated in a broad zone of distributed ductile less than ~0.15 mm/yr since the thermal peak deformation; present data do not favor or elimi• that followed conversion to the transform plate nate either end member. Shear-zone rocks margin; otherwise, much more of the Coast should reflect significant stretching parallel or Ranges would yield younger apatite fission- subparallel to the plate margin. As a result, new track ages. These uplift rates place constraints mineral growth and stretching lineations devel• on the hypothetical retrograde P/T paths of oped below the brittle-ductile transition may be rocks at depth in the Coast Ranges. subparallel or parallel to the plate margin (e.g., Inferences regarding deep structure Ellis and Watkinson, 1987; Ave Lallemant and Guth, 1990), although local complexities may The character of structures present at depth be expected in some shear zones because of the in the Coast Ranges probably depends, in part, small component of convergence (e.g., Robin on their depth relative to the brittle-ductile and Cruden, 1994; Tikoff and Teyssier, 1994). transition. Above this transition, structures are dominated by the strike-slip faults of the San Fuis and Mooney (1990) developed an inter• Andreas system. Second-order features are pretive cross-section of the Coast Ranges, based folds and thrust faults that are a consequence on seismic refraction and reflection studies. of both plate-normal contraction and local The cross-section in Figure 3 is adapted from restraining bends along major strike-slip faults. Fuis and Mooney's Figure 8.4, with some rein- A major reflector, interpreted as a possible terpretation of subsurface structure per crustal detachment, has been imaged at a depth Holbrook et al. (1996) and Wakabayashi and of ~15 km at the latitude of San Francisco Bay; Unruh (1995). The structure of the deeper this reflector is near the depth of the inferred parts of the plate boundary are an unresolved brittle-ductile transition in the area (Brocher et issue, and several competing models have been al., 1994). A similar reflector is offset by major proposed (Hole, 1996). The deep plate bound• strike-slip faults in the northern Coast Ranges ary may be expected to have some vertical (Beaudoin et al., 1996), indicating that such a component of motion to accommodate the reflector probably does not represent a detach• contractional component of plate motion. ment in the northern Coast Ranges. As an 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 in the present-day Coast Ranges corresponds to and Gordon, 1991), the component of accumu greenschist-facies metamorphism (Figs. 2, 3, lated vertical crustal motion may become signif and 4). The elevated temperatures at depth icant if the present-day kinematics persist for a should result in significant recrystallization long time (≥30 million years or so). Such long- and growth of new metamorphic minerals. The term vertical movement would be important in peak conditions of metamorphism at the base of the future exhumation of the deeply buried the Coast Ranges crust are predicted to be ~700 parts of the present Coast Ranges. to 1000° C at a pressure of about 7 kbar, or granulite grade (Fig. 4), with subsequent cool Inferred metamorphism at depth in the Coast ing to lower thermal gradients. The distribution Ranges and relationship with deep structure of metamorphic facies boundaries in cross-sec Higher thermal gradients associated with the tion view is shown in Figure 5. It should be transform margin compared to those associated noted that if the Clear Lake region (300° C with the previous subduction zone will produce isotherm at 7- to 8-km depth) is considered to be a quite different metamorphic suite at depth a local effect, or too short-lived to cause signifi than the earlier subduction (Franciscan) HP/ cant recrystallization, then the "sustained" LT metamorphism for which the California peak thermal gradient that is likely to result in Coast Ranges are well known (e.g., Ernst, significant recrystallization is 30° C/km (300° 1970). Instead of the facies series prehnite- C isotherm at 10-km depth) (see Fig. 2). If the pumpellyite, blueschist, eclogite that charac 30° C/km gradient is used as the thermal peak terizes the subduction-zone metamorphism of that causes major recrystallization, then the the Franciscan Complex, the predicted facies facies boundaries would shift a few km deeper series at depth in the present-day Coast Ranges in the crust than shown in Figure 5, a result that should be greenschist, amphibolite, granulite. is in accord with recent estimates of seismic Cloos and Dumitru (1987), recognizing the velocities (John Hole, pers. commun., 1996). thermal significance of the ongoing subduc- On the basis of structural studies of Cenozoic tion-transform transition, concluded that the rocks along the eastern margin of the Coast lack of greenschist overprints in exposed Fran Ranges, the only significant period of regional ciscan rocks indicated that no subduction- shortening and uplift affecting the Coast transform transitions occurred during the span Ranges since the passage of the triple junction of Franciscan accretionary history (approx is the period of time since the plate-motion imately 160 Ma to 20 Ma). The 300° C isotherm 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 of triple-junction migration (from Engebretson depth, as shown in Figure 4, is based on the et al., 1985) indicate that rocks that experience depth to the 300° C isotherm from the data of granulite-grade metamorphism cool to 500° C, Hill et al. (1990), with an assumed average or amphibolite grade, within 15 million years uplift rate of 0.3 mm/yr, applied only since the after the passage of the thermal maximum plate-motion change at 3.4 to 3.9 Ma. Because (Fig. 4). the peak metamorphism is inferred to corre• Preservation of the earlier Franciscan high- spond to a thermal transient, cooling will occur P/T metamorphic assemblages at depth may under conditions of lower thermal gradient occur in the areas affected by recent green- with time. Extended uplift of these rocks schist (and possibly higher-grade) metamor• depends on their location relative to major fault phism, because the duration of heating by the systems in the present and future Coast Ranges. thermal peak is probably not sufficient to com• The deepening of the base of crustal seismicity pletely erase pre-existing high-P/T relics. southward in the Coast Ranges (Hill et al., Because the notable recent metamorphic 1990), shown in Figure 2, and the inferred rate recrystallization (greenschist grade and above) SUBDUCTION-TRANSFORM TRANSITION 987 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 amphi- bole 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 FIG. 7. Hypothetical P/T path of rocks at depth in the Coast Ranges, compared with P/T paths for the Sanbagawa and heating (path f, g in Fig. 7). Belt (Otsuki and Banno, 1990) and the Haast schists of New The other type of greenschist-overprinted Zealand (Yardley, 1982). This diagram shows how "clock high-P/T rock at depth in the Coast Ranges wise" P/T paths similar to those preserved in the San probably occupies a much greater volume of the bagawa Belt and in New Zealand could form in the crust than those discussed above (Fig. 6). These California Coast Ranges as a result of the conversion from a high-P/T rocks in the core of the Coast Ranges subducting to a transform plate boundary. Paths a, b, f, g, were uplifted from their original depth of meta- and h correspond to P/T paths at the points on Figures 3 morphism (≥20 km) and should be overprinted and 5. The arrows along the dashed "subduction thermal by greenschist or higher-grade assemblages fol gradient" line show the direction of P/T evolution for Coast Range rocks experiencing uplift prior to conversion lowing subduction-transform transition (Fig. 5; to the strike-slip thermal regime. The "prograde" apparent path a', b' in Fig. 7). These rocks may preserve P/T path preserved in such rocks would be an overprint of their peak high-P/T assemblage, with negligible the peak subduction assemblage by the peak post-subduc- mineral growth during synsubduction uplift, tion assemblage; examples of such P/T paths are the typical of exposed Franciscan blueschists that screened and dashed paths labeled a', b', and P. Depending were exhumed under low thermal gradients on the amount of uplift of blueschists prior to subduction- (e.g., Ernst, 1988). Thus the "apparent" P/T transform transition, different P/T trajectories are possi path for overprinted rocks such as these in the ble. The horizontal paths at a, b, f, g, and h show apparent core of the Coast Ranges would be the super prograde paths of rocks that do not record an earlier uplift position of the peak overprint assemblage (and history prior to transform-related metamorphism. subsequent retrograde assemblages) over the peak high-P/T assemblage, yielding apparent increase with resulting faster reaction kinetics. P/T paths similar to those shown in Figure 7 In the case of older high-P/T rocks that have (paths a', b', f'). These paths should vary as a been uplifted prior to overprinting, the high- function of uplift of the rocks prior to subduc P/T metamorphism and overprinting may be tion-transform transition and the peak condi separated by 20 to 150 m.y., based on the tions of high-P/T metamorphism experienced duration of Franciscan subduction and the tim by these rocks. ing of the subduction-transform transition. For The likelihood of preservation of high-P/T the deeper overprinted rocks, the temporal sep relics would decrease with depth, because the aration between high-P/T metamorphism and grade of overprinting metamorphism would overprinting may be much shorter. 988 JOHN WAKABAYASHI

The pattern of transform-related metamor- Re-examination of Metamorphic Belts phism prior to disruption by post-metamorphic Considering the Role of Subduction- faulting may be approximately symmetrical, Transform Transitions with the highest-grade zone flanked by parallel lower-grade zones, in contrast to the pro• Past ridge-trench interactions have been nounced asymmetrical pattern of subduction- inferred for the Shimanto Belt of Japan (e.g., related metamorphism. Post-metamorphic Hibbard et al., 1993), the Chile margin triple- faulting, however, should greatly complicate the junction area (both presently occurring and distribution of transform-related metamor• exhumed rocks that experienced high thermal phism. For example, if the deep crustal plate gradients) (Forsythe and Nelson, 1985), and the boundary is a relatively narrow zone of deforma• southern Alaska forearc (Sisson and Pavlis, tion, one side of the boundary should rise 1993). The consequences of a ridge-trench relative to the other in response to the contrac- interaction may be similar to the transform- tional component across the plate boundary, trench interaction described here, and the early although the dominant sense of motion would history of subduction-transform transition in be strike-parallel. An extended period of move• southern coastal California probably involved ment along such a boundary zone, during ridge-trench interaction as well (e.g., Atwater, which time the Coast Ranges cooled following 1970, 1989; Bohannon and Parsons, 1995). The their thermal peak in the wake of triple-junc• following discussion focuses on the types of tion migration, would lead to higher-grade features of metamorphic belts that can result rocks on the upthrown side and lower-grade from a triple-junction migration of the Califor• rocks on the downthrown side; with sufficient nia type, but may apply broadly to ridge-trench- time (tens of m.y.) the cumulative vertical dis• type interactions as well. placement across the boundary would become Thermal overprinting of some high-P/T rocks significant. A major deep crustal fault zone associated with orogen-parallel stretching striking obliquely across the metamorphic belt lineations also may juxtapose terranes formed under dif• ferent thermal gradients, such as the axial Coast As noted earlier, high-P/T assemblages Ranges (high thermal gradients) against the should be overprinted by assemblages of higher eastern margin of the Coast Ranges (low ther• thermal gradient over large areas of the Coast mal gradients). If the deep crustal plate bound• Ranges at depth. This type of overprint repre• ary is a broader zone of distributed shear, sents a "clockwise" P/T path that is typical of the exhumed plate boundary zone may have most high-P/T terranes of the world (Ernst, an apparent inverted metamorphic gradient 1988). Such P/T paths generally are suggested across it. to have been formed as a consequence of subduction, followed by collision of an island arc or Probability of future exposure continental margin and subsequent cessation of The probability of future exposure of subduction. Clearly, the late Cenozoic thermal rocks from a given depth in the Coast Range event in the California Coast Ranges differs within a period of 200 m.y. or less should because it involves no collision. decrease with increasing depth, because with Major strike-slip faults typically are associ• greater depth, the amount of exhumation and ated with belts of high-P/T rocks (Ernst, 1971), time required to accomplish the exhumation and orogen-parallel stretching lineations also would increase. Accordingly, within that time are common features of many orogenic belts frame, exposure of the greenschist-facies level (e.g.,. Faure, 1986; Ellis and Watkinson, 1987; of the Coast Ranges would be more likely than Brown and Talbot, 1989; Ratschbacher et al., exposure of the granulite level of metamor• 1989; Ave Lallemant and Guth, 1990; Wallis, phism. Conversely, preservation of the shal• 1990). These stretching lineations typically lower levels of the plate margin becomes less postdate high-P/T metamorphism and are tex- likely with a large amount of elapsed time—say turally related to the thermal overprinting of 500 million years or more—and exposure of the the earlier high-P/T metamorphic assemblages deepest portions of the plate margins is more (Ratschbacher et al., 1989; Hara et al., 1990; likely. Wallis, 1990). A subduction-transform transi- SUBDUCTION-TRANSFORM TRANSITION 989

tion may be an alternative explanation to colli- The timing of this metamorphism is consistent sional orogenesis for development of orogen- with the conversion of this margin from a parallel stretching lineations and overprints of subduction zone to a transform margin as pro• high-P/T rocks in some of these orogenic belts. posed by Osozawa (1994) in his plate recon• The influence of a subduction-transform structions for this area. A subduction- transition on orogenesis versus the influence of transform transition is a permissible alternate collision is difficult to evaluate for several rea• to collision as the cause of the thermal over• sons. The most important may be: (1) geo- printing in the Sanbagawa. The Median Tec• chronologic studies are not sufficiently detailed tonic line of Japan separates the Sanbagawa Belt in many orogenic belts; and (2) collision appar• from the HT/LP Ryoke Belt. Brown and Naka- ently did occur at least at some time during the jima (1994) concluded that the metamorphism development of many orogenic belts. of the Ryoke Belt was a product of spreading The Sanbagawa Belt of Japan and the Haast ridge-trench interaction. The Median Tectonic schists of New Zealand are two possible exam• line may be the exhumed analog of the plate ples of overprinted high-P/T belts with orogen- boundary at depth in coastal California, jux• parallel stretching lineations that may have taposing two terranes with different thermal been affected by a subduction-transform transi• histories affected by subduction-transform tion. Figure 7 shows the similarity of the P/T transition or ridge-trench interaction. paths of metamorphism from the Sanbagawa Belt (Otsuki and Banno, 1990) and the Haast The Haast schists of New Zealand record a schists (Yardley, 1982) to the apparent P/T similar thermal history of early blueschist-type paths that may form beneath the California assemblages overprinted by later greenschist Coast Ranges as a consequence of the subduc• assemblages (Yardley, 1982). Similar to the San• tion-transform transition. bagawa Belt, the Haast schists are bordered by a major strike-slip fault (the Alpine fault) that In the Sanbagawa, the clockwise P/T evolu• separates them from high-grade rocks. The tion has been attributed to subduction followed schists also have stretching lineations that are by collision (e.g., Ernst, 1988), possibly of the subparallel to the border of the belt (Mortimer, Kurosegawa tectonic zone in the Late Jurassic 1992). The origin of the structures and over• (Maruyama et al., 1984). However, if the print of the Haast schists, like the Sanbagawa Kurosegawa composite terrane had indeed Belt, generally is attributed to a collision (Mor• buoyantly clogged the subduction zone and timer, 1992). A subduction-transform transi• halted subduction, then it should form the tion may be a reasonable alternative model for "lower plate" of the orogen and be more likely to be overprinted with high-grade metamor• part of the tectonothermal evolution of these phism, analogous to major collisional orogens schists. such as the Alps (e.g., Ernst, 1988). The Alternative explanation for some rock associa• Kurosegawa tectonic zone apparently lacks San• tions interpreted as ancient volcanic arcs bagawa or younger metamorphism, and yields metamorphic muscovite K/Ar ages signifi• A belt of plutons and associated metamor• cantly older than the Sanbagawa metamor• phic rocks traditionally is interpreted to be the phism (Maruyama et al., 1984), indicating that exhumed root of an ancient volcanic arc. Such a heating of this terrane during and after the time belt of plutons should be present, however, of Sanbagawa metamorphism did not exceed beneath the California Coast Ranges (Liu and K/Ar closure temperatures for muscovite. Furlong, 1992). An analog of such a plutonic Taira et al. (1983) suggested emplacement of belt may be exposed in the Gulf of Alaska, the older rocks of the Kurosegawa zone along a although the origin of these plutons is still in major strike-slip fault as an alternative to colli• dispute (Barker et al., 1992). sion. The timing of the Cretaceous metamor• A possibly similar association of granitic phism of the Sanbagawa rocks recorded by Ar- rocks and strike-slip faults, in Hercynian shear Ar dates (Takasu and Dallmeyer, 1990) reflects zones in Iberia and shear zones in the British cooling from the thermal peak that, in turn, Caledonides, was noted by Hutton and Reavy may be the thermal overprint that followed the (1992). Although these authors suggest crustal high-P/T metamorphism (Hara et al., 1990). 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 conse• quence 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 dis• placement along the suture (Lund and Snee, 1988). The main stage of regional HT metamor• phism associated with this belt may be a conse• quence of a subduction-transform transition. Probability of exposure and the age of granulite FIG. 8. Hypothetical retrograde P/T paths for rocks at and high-P/T belts depth in the California Coast Ranges compared with P/T paths of granulites compiled by Bohlen (1987, 1991). The As indicated previously, the probability of Coast Ranges paths show cooling from 35° C/km. The future exposure of different depths of the thermal peak of longer duration (see text and figure cap• present-day California Coast Ranges varies with tions for Figs. 4 and 5) is 30° C/km and may be more the amount of elapsed time in the future; that representative of peak prograde assemblages, in which case is, exclusively shallow levels of exposure are Coast Range retrograde P/T paths originate from the 30° more likely with less elapsed time, and deep C/km line, and the similarity of these P/T paths to the levels are more likely with long elapsed time. As granulite paths would be greater. one of several types of plate-boundary changes that can influence the evolution of mountain as the cause of crustal melting, an alternative belts, the probability of preservation subduc• for the formation of some of the granitoids they tion-transform transition effects provides discussed may be a subduction-transform insight into general problems of preservation of transition. various rock types on Earth. For example, it is Granulite belts and other HT metamorphic belts likely that a long-lived active plate margin will experience a collision, subduction-transform In many granulite belts, granulite assem• transition, or ridge-trench interaction at some blages have undergone retrograde metamor- point in its history. The longer the elapsed time phism under conditions of decreasing thermal since the formation of a blueschist belt, the less gradient, or a counterclockwise P/T path (e.g., likely blueschists along any plate margin will be Bohlen, 1991). Such metamorphism has been preserved, because: (1) there is increased prob• attributed to metamorphism at the base of a ability of a plate-boundary transition that stops volcanic arc or subcontinental underplating subduction or causes an increase in thermal (Bohlen and Metzger, 1989). It should be noted gradients; and (2) the longer elapsed time that the P/T conditions of metamorphism at allows for greater exhumation of rocks, expos• depth in the Coast Ranges and the predicted ing deeper levels of the crust where older high- cooling path (Fig. 8) are very similar to many of P/T rocks are more likely to have been com• the examples cited by Bohlen (1991). The pletely overprinted by the thermal effects of the inferred cooling of deep rocks in the California plate boundary interactions. It therefore is not Coast Ranges may have been close to isobaric, surprising that the vast majority of high-P/T because of the minimal uplift since the passage metamorphic belts are Phanerozoic in age of the plate triple junction (see discussions in (Ernst, 1972; Liou et al., 1989). The scarcity of earlier sections; Figs. 4 and 8). The cooling older blueschist has been attributed to a paths of rocks in a general subduction-trans• decrease in thermal gradients as the Earth form transition orogen may vary, however, cooled (Burke et al., 1977). Although the SUBDUCTION-TRANSFORM TRANSITION 991 decrease in thermal gradients with time may Bohlen, S. R., 1987, Pressure-temperature-time paths have affected the age distribution of exposed and a tectonic model for the evolution of granulites: blueschists, exposure time to later thermal pro• Jour. Geol., v. 95, p. 617-632. cesses, as discussed here, also may play a major ______, 1991, On the formation of granulites: Jour. role. Metamor. 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