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Wakabayashi Intgeolr International Geology Review ISSN: 0020-6814 (Print) 1938-2839 (Online) Journal homepage: http://www.tandfonline.com/loi/tigr20 Whither the megathrust? Localization of large- scale subduction slip along the contact of a mélange John Wakabayashi & Christie D. Rowe To cite this article: John Wakabayashi & Christie D. Rowe (2015) Whither the megathrust? Localization of large-scale subduction slip along the contact of a mélange, International Geology Review, 57:5-8, 854-870, DOI: 10.1080/00206814.2015.1020453 To link to this article: http://dx.doi.org/10.1080/00206814.2015.1020453 Published online: 09 Mar 2015. Submit your article to this journal Article views: 189 View related articles View Crossmark data Citing articles: 1 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tigr20 Download by: [University of California, Berkeley] Date: 04 April 2016, At: 09:19 International Geology Review, 2015 Vol. 57, Nos. 5–8, 854–870, http://dx.doi.org/10.1080/00206814.2015.1020453 Whither the megathrust? Localization of large-scale subduction slip along the contact of a mélange John Wakabayashia* and Christie D. Roweb aDepartment of Earth and Environmental Sciences, California State University, Fresno, CA, USA; bDepartment of Earth and Planetary Sciences, McGill University, Montreal, Canada (Received 13 February 2015; accepted 14 February 2015) Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of mélanges in subduction complexes has spawned proposals that these mélanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan mélanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100–200 m-thick mélange zone separates the nappes, this mélange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglom- erate deposited on the underlying coherent sandstone, so the mélange is part of the lower nappe. A 20–30 m-thick fault zone between the top of the mélange, and the base of the Angel Island nappe displays an inverted metamorphic gradient with jadeite-glaucophane-lawsonite above lawsonite-albite assemblages. This zone has a strong seaward (SW)-vergent shear fabric and hosts ultracataclasite and pseudotachylite. These relationships suggest that significant subduction megathrust displacement at depths of 15–30 km was accommodated within the 20–30 m-thick fault zone. Field studies elsewhere in the Franciscan Complex suggest similar localization of megathrust slip, with some examples lacking mélanges. The narrow megathrust zone at El Cerrito Quarry, its uniform sense-of-shear, and the localization of slip along the contact of, rather than within a mélange, contrast sharply with the predictions of numerical models for subduction channels. Keywords: subduction tectonics; subduction megathrust; fault rocks; mélanges Introduction footwall cut-offs into the mantle during the time when Thousands of kilometres of oceanic plate subduct along material does not accrete in a subduction complex. Most reaches of modern convergent plate margins, with sub- of the length of modern convergent margins is non-accret- ducted-along-palaeosubduction zones now preserved as ing or undergoing subduction erosion (von Huene and horizons within accretionary complexes. Seismicity Scholl 1991; Clift and Vannucchi 2004; Scholl and von marks the position of modern subduction megathrusts, Huene et al. 2007), so continuous accretion is not but the location of the palaeomegathrusts in the rock expected during the history of a subduction zone. record is less certain. Subduction complexes preserve for- Determination of accretion age of materials in a sub- fi mer megathrust surfaces as tectonic contacts between duction complex is dif cult (e.g. Dumitru et al. 2015). thrust nappes that young structurally downward in Uncertainty in accretion ages and the structural complexity Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 fi accretionary age (e.g. Karig and Sharman 1975). of subduction complexes hinder identi cation and charac- Subduction-accretion is a process in which material is terization of palaeomegathrust zones in the rock record transferred from the top of the subducting plate to the (Wakabayashi 2015). subduction complex that lay above the plate boundary. Most mélanges (block-in-matrix units) within pre- This process results in the abandonment of the segment served accretionary wedges have been interpreted as either of the megathrust that once lay above the accreting mate- formed entirely by tectonic shearing of previously coher- rial, and the formation of a new megathrust segment ent units (Cloos 1982; Moore and Byrne 1987; Fisher and beneath the accreting material as it decouples from the Byrne 1987; Kimura and Mukai 1991), or as sedimentary subducting plate (e.g. Hashimoto and Kimura 1999; breccias and conglomerates variably overprinted with tec- Wakabayashi 2015). Displacement on subduction mega- tonic fabric (e.g. Aalto 1981; Moore et al. 1985; Larue and thrust horizons cannot be balanced or restored by geo- Hudleston 1987; Osozawa et al. 2011; Platt 2015; metric approaches, owing to the subduction of the Wakabayashi 2015). Proposals for tectonic origins of *Corresponding author. Email: [email protected] © 2015 Taylor & Francis International Geology Review 855 mélanges in subduction complexes commonly suggest that composition (Jayko and Blake 1984;Ghataket al. 2013) the formation of the block-in-matrix fabric took place and detrital zircon age populations (Ernst et al. 2009; along the subduction megathrust. Snow et al. 2010;Dumitruet al. 2010, 2015), and Changes in strain distribution with depth down the conglomerate composition (Seiders 1991). subduction thrust may result in thick zones of mild strain Recent work in the Franciscan has shown that most at shallow depths cut by highly localized zones of intense shale, serpentinite, and sandstone-matrix mélanges pre- strain at depth (e.g. Rowe et al. 2011). A recent compila- serve sedimentary block-in-matrix fabric, with interbed- tion of fault thickness observations shows that most of the ding and interfingering with coherent sandstones and plate boundary strain is taken up on fault strands ~5–35 m shales (Wakabayashi 2011, 2012, 2015). Felsic-intermedi- thick at any given stage of deformation, although the total ate igneous blocks of arc affinity do not resemble the accumulated thickness of deformed rock may be much coherent slices of mid-ocean-ridge basalt (MORB) and greater (Rowe et al. 2013). The mélanges in the ocean island basalt (OIB) of the Franciscan Franciscan and other accretionary complexes are highly (Wakabayashi 2015). Thus, their presence in nearly all variable in lithologic components and tectonic fabrics, and Franciscan mélanges indicates derivation from the upper may have a variety of origins (Cowan 1985; Wakabayashi crust of a magmatic arc, which can only be explained by 2011, 2015). submarine sliding of debris into the trench from the upper The Franciscan subduction complex of coastal plate of the subduction system (MacPherson et al. 1990; California consists of a stack of thrust nappes composed Wakabayashi 2015). These characteristics indicate that the of varying proportions of coherent and mélange compo- mélanges have a sedimentary origin (Cowan 1978; Aalto nents (Wakabayashi 2015), and the accretionary (incor- 1989; Wakabayashi 2012, 2015). poration into subduction complex) ages of these nappes If most of the volume of mélanges represent deformed young structurally downward (Ernst et al. 2009; Snow sedimentary deposits (olistostromes) rather than shear et al. 2010; Dumitru et al. 2010, 2015; Wakabayashi zones, the plate boundary (megathrust) displacement may 2015)(Figure 1). Following initiation of subduction at be accommodated in comparatively narrow zones (Rowe ca. 170 Ma, nappes of the Franciscan accreted episodically et al. 2013). Wakabayashi (2011) suggested accommoda- during >150 million years of continuous E-dipping sub- tion of large displacements on the upper contact of duction, during which over 13,000 linear kilometres of mélanges based on the position of the metamorphic con- oceanic plate subducted (Engebretson et al. 1985; trast between jadeite-bearing metagreywacke and a struc- Wakabayashi 1992; Ernst et al. 2009; Snow et al. 2010; turally lower prehnite-pumpellyite facies shale matrix Dumitru et al. 2010, 2013, 2015). Thus, multiple palaeo- mélange at El Cerrito Quarry in the eastern San megathrust horizons between accreted nappes collectively Francisco Bay area (Figures 1 and 2). In this paper we accommodate over 13,000 km of subduction slip within present new field observations from the El Cerrito Quarry this subduction complex. During and after accretion, the locality that strongly suggest the localization of plate nappes were internally deformed by both discrete faults boundary slip into a ~ 20–30 m-thick fault zone at the and penetrative deformation, and primary nappe contacts top of
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