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 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).