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Tectonic Mechanisms Associated with P–T Paths of Regional Metamorphism: Alternatives to Single-Cycle Thrusting and Heating

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Tectonic Mechanisms Associated with P–T Paths of Regional Metamorphism: Alternatives to Single-Cycle Thrusting and Heating Tectonophysics 392 (2004) 193–218 www.elsevier.com/locate/tecto Tectonic mechanisms associated with P–T paths of regional metamorphism: alternatives to single-cycle thrusting and heating John Wakabayashi* Wakabayashi Geologic Consulting, 1329 Sheridan Lane, Hayward, CA 94544, USA Available online 24 June 2004 Abstract Metamorphic pressure ( P)–temperature (T) paths are commonly used as tools to interpret the tectonic history of orogenic belts, those deformed belts of rocks that record past activity along active plate margins. Many studies and reviews relating P–T path development to tectonics have focused on thrusting–thermal relaxation cycles, with special emphasis on collisional processes. Other studies have assumed that P–T paths resulted from a single tectono-metamorphic event that accounted for the entire burial–exhumation history of the rocks. In many cases, such assumptions may prove invalid. This paper speculates on the relationship of tectonic processes other than thrusting–heating to P–T path development. The processes discussed herein include subduction initiation, triple-junction interactions, initiation and shut off of arc volcanism, subcontinental delamination, and hot spot migration. All of these processes may leave a signature in the metamorphic rock record. Examples are presented from a number of localities, most of which are from the Pacific Rim. Although thrusting– heating cycles have influenced metamorphic evolution in many orogenic belts, the potential impact of other types of tectonic mechanisms should not be overlooked. D 2004 Elsevier B.V. All rights reserved. Keywords: P–T paths; Tectonics; Orogenic belts; Subduction initiation; Triple-junction migration; Magmatic arcs 1. Introduction Studies linking tectonic environments to types of metamorphic rocks, with key examples from the The study of ancient, exhumed continental margins Pacific Rim and Alpine regions, were published as commonly focuses on deformed belts of rocks known plate tectonic theory became widely accepted (e.g., as orogenic belts, because these regions formed as a Miyashiro, 1967, 1973; Ernst, 1971). Oxburgh and consequence of active plate margin tectonics (e.g., Turcotte (1974) conducted pioneering thermal mod- Dewey and Bird, 1970; Moores, 1970). Within oro- eling of pressure ( P)–temperature (T)–time (t) paths genic belts, geoscientists use the spatial distribution of that represent the progressive metamorphic evolution different rock types, their ages, and their structural and of rocks. Oxburgh and Turcotte (1974) based the metamorphic history, to better understand the tecton- thermal models on a process that involved thrusting, ics of ancient plate boundaries. that buried rocks faster than they can equilibrate with the ambient geothermal gradient, followed by ther- * Fax: +1-510-887-2389. mal relaxation, which reestablished the ambient geo- E-mail address: [email protected] (J. Wakabayashi). thermal gradient in underthrust rocks. The thrusting– 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.04.012 194 J. Wakabayashi / Tectonophysics 392 (2004) 193–218 J. Wakabayashi / Tectonophysics 392 (2004) 193–218 195 thermal relaxation cycle was linked to subduction 2. Partial P–T paths and ‘apparent’ P–T paths followed by collision. Subsequent researchers refined thermal models based on thrusting–thermal relaxa- 2.1. Partial versus complete P–T loops tion cycles and evaluated clockwise ( P on the positive y axis) P–T paths of orogenic belts in this Mineral growth recording a P–T path will not context (e.g., England and Thompson, 1984; Thomp- reflect the complete burial and exhumation history son and England, 1984; Thompson and Ridley, 1987; of a rock. This is because metamorphic minerals do Spear and Peacock, 1989) (Fig. 1G). Since the late not grow during the lowest grade parts of the burial 1980s, an increasing number of studies have inter- and exhumation paths, and because successive recrys- preted P–T paths that differ significantly from tallization may erase earlier formed assemblages. clockwise P–T loops (Fig. 1, Table 1). These studies Collisional thrusting–exhumation, and subduction include those of some types of granulites (e.g., with syn-subduction exhumation can account for an Bohlen, 1987; Harley, 1989) (Fig. 1C), low-P/high- entire burial–exhumation cycle of a rock (Fig. 2A to T metamorphism (e.g., Clarke et al., 1987; Gram- J). Basinal sedimentation followed by basin inversion bling, 1988; Rubenach, 1992; Sisson and Pavlis, may also result in a complete burial–exhumation 1993; Johnson and Vernon, 1995; Brown, 1998a,b) cycle, but the maximum depths of burial are much (Fig. 1B), subduction–transform transition (Waka- less than in collisional and subduction settings (e.g., bayashi, 1996) (Fig. 1D, H), and of rocks associated Cooper and Williams, 1989). For other tectonic mech- subduction initiation (e.g., Wakabayashi, 1990; anisms, the rocks that record P–T paths associated Hacker, 1991; Parkinson, 1996; Hacker and Gnos, with the mechanism may be at significant depth when 1997) (Fig. 1A). the tectonothermal event begins and ends (Fig. 2K– This paper focuses on evaluation of P–T paths U). Multiple tectonic events must occur in order to formed by mechanisms other than thrusting–thermal complete a burial–exhumation cycle for such rocks. If relaxation. The mechanisms reviewed in this paper are there is no metamorphic record of exhumation and overlooked in many analyses of P–T paths. Fig. 1 initial burial, however, the P–T path recorded in such shows examples of some of the types of P–T paths rocks may be the product of a single metamorphic that I will discuss in the paper. These and other P–T event (Fig. 2K–U). paths are listed in tabular form in Table 1. The first sections of the paper discuss some general definitions 2.2. Time dependence and ‘apparent’ P–T paths and concepts related to tectonic interpretations of P–T paths. Following the general sections, I will speculate P–T paths recorded by overprinting mineral on specific tectonic settings of P–T path develop- assemblages, inclusions, and mineral zoning are ment. Most of the examples used in these discussions generally interpreted to result from a single evolving are from the Pacific Rim region, with particular tectono-metamorphic event (e.g., Oxburgh and Tur- emphasis on western North America. This paper is cotte, 1974; England and Thompson, 1984; Thomp- not intended as a comprehensive review of P–T paths son and England, 1984; Ernst, 1988). However, the and tectonics. My goal is merely to point out that P–T different stages of metamorphic mineral growth in paths we interpret from rocks are likely generated by a rocks can result from metamorphic events separated greater variety of tectonic processes than commonly by long periods of time and related to different proposed. tectonic events (e.g., Getty et al., 1993; Hand et al., Fig. 1. Examples of different P–T path types. Most examples are P–T paths determined from field-based studies. Modeled P–T paths are denoted by an asterisk after the acronym. Labels for P–T paths correspond to the following references: A02*: Aoya et al. (2002); B87: Bohlen (1987); B98: Baba (1998); H91*: Hacker (1991); HD92: Henry and Dokka (1992); HG97: Hacker and Gnos (1997); M85: Maruyama et al. (1985); ML88: Maruyama and Liou (1988); JV95: Johnson and Vernon (1995); OB90: Otsuki and Banno (1990); R92: Ring (1992); Rb92: Rubenach (1992); S89: Sisson et al. (1989); S99: Smith et al. (1999); SP89; Stu¨we and Powell (1989); SP99: Soto and Platt (1999); TR87*: Thompson and Ridley (1987); W90: Wakabayashi (1990); W96*: Wakabayashi (1996); Y82: Yardley (1982); P–T paths in ‘‘D’’ from Wakabayashi (1996). Facies abbreviations are Am: Amphibolite, Bs: Blueschist, EA: Epidote amphibolite, Ec: Eclogite, Gr: Granulite, Gs: Greenschist, PP/Z: Prehnite–pumpellyite and Zeolite. Facies boundaries from Takasu (1984). 196 J. Wakabayashi / Tectonophysics 392 (2004) 193–218 Table 1 Table 1 (continued) Examples of P– T paths generated by processes other than P–T path type Location Reference thrusting–thermal relaxationa Anticlockwise: Platta nappe, upper Phillip (1982) (y), P–T path type Location Reference inception of Engadine Valley, Deutsch (1983) (y) Anticlockwise: High-grade blocks, Wakabayashi (1990), subduction Switzerland Alps. inception of Franciscan Krogh et al. (1994) Anticlockwise Granulites, many Bohlen (1987), subduction Complex, (acw) or locations in listed Harley (1989) California, USA isobaric references (mostly Eclogites, Yukon– Perchuk et al. cooling (ibc), ibc, some acw) Tanana terrane, (1999) medium to Kohistan Arc, Yamamoto (1993) Canada high P Pakistan (acw) Structurally highest Brown et al. (1982) granulites Lewisian Complex, Baba (1998) part of Shuksan (y) Outer Hebrides, Suite, Washington, Scotland (acw) USA Early Henry and Dokka Raspberry Schist, Roeske (1986) (y) metamorphism, (1992) Alaska, USA Mojave extensional Tetagouche Group, Trzcienski et al. belt, California, New Brunswick, (1984), van Staal USA (acw) Canada et al. (1990) (y) Anticlockwise Chugach Complex, Sisson et al. Structurally highest Smith et al. (1999) (acw), isobaric Alaska, USA (ibh) (1989); Sisson and unit of Villa de cooling (ibc), Pavlis (1993) Cura belt, isobaric hairpin Ryoke Belt, Japan Brown (1998a,b) Venezuela (ibh), and other (ibh) High-grade blocks, Kato and Godoy complex paths: Cooma Complex, Johnson and Coastal Chile (1995) mostly high T, Australia (acw) Vernon (1995) Anglesey Gibbons and Gyopari low P Mount Isa Inlier,
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