International Review

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Whither the megathrust? Localization of large- scale 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.

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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 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 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 ; 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 (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 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 a sedimentary mélange. We also review studies that have been locally displaced by out-of-sequence low-angle suggest that similar relationships may be common in other faults (Wakabayashi 2015). accretionary complexes. The Franciscan Complex records at least one long episode of non-accretion of 50–60 million year duration spanning from ca. 170 to 120 Ma in the northern part of El Cerrito Quarry in a regional Franciscan context Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 the Franciscan (Dumitru et al. 2010), and from ca. 170 The Franciscan Complex of the San Francisco Bay to110MaintheSanFranciscoBayareaandmuchof region comprises a stack of thrust nappes that accreted the Diablo Range (Wakabayashi 2015). Such a gap in from about ca. 114 to 80 Ma following a >50 million year accretion corresponds to ~3800–4400 km of subduction, period of non-accretion or subduction erosion (Snow based on the margin-normal component of Pacific- et al. 2010; Wakabayashi 2012, 2015)(Figure 1). One Farallon plate motion for this time (Engebretson et al. of the most prominent contacts between nappes separates 1985). The Franciscan may have also been non-accret- the structurally blueschist facies Angel Island nappe ing from ca. 75 to 55 Ma (Dumitru et al. 2015; above from the prehnite-pumpellyite facies Alcatraz Wakabayashi 2015) and this would correspond to nappe below (Figure 2; Wakabayashi 1992). A shale about 2200 km of subduction based on the matrix mélange occupies the contact zone at the best Engebretson et al. (1985) plate motion model. Shorter exposure at El Cerrito Quarry (Wakabayashi 1992, periods of non-accretion are difficult to resolve with 2011)(Figures 1–3). existing geochronology. A period of non-accretion The upper Angel Island nappe comprises mainly between the accretion of many nappes is suggested by jadeite-glaucophane-lawsonite-bearing metagreywacke, significant differences in adjacent nappes of sandstone whereas the lower Alcatraz nappe comprises mostly Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 856 .WkbysiadCD Rowe C.D. and Wakabayashi J.

Figure 1. Location map showing general Franciscan Complex exposures of the San Francisco Bay area and inset (lower right) of general relationships within greater Franciscan. Map adapted from Wakabayashi (2015). International Geology Review 857

Figure 2. Geologic map of El Cerrito Quarry and Vicinity. Stereoplot of structural orientations shown (plotted using Stereonet for OSX v. 2.2, Cardozo and Allmendinger). and bedding poles include data from all three units, whereas stretching data are

Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 derived primarily from the upper Angel Island nappe with three lineations from the fault zone between the mélange and the base of the Angel Island nappe. Locations of various outcrop photos and samples (those not associated with outcrop photos) are shown.

Figure 3. Field photographs showing larger-scale relationships at El Cerrito Quarry. (A) Photograph taken from near the eastern end of Schmidt Avenue looking NW showing locations of some of the outcrop photographs shown in Figure 5. (B) Photograph from about 100 m W of the viewpoint in (A) looking NNE. 858 J. Wakabayashi and C.D. Rowe

greywacke of prehnite-pumpellyite grade. These two Geology of El Cerrito Quarry and environs units have yielded indistinguishable maximum deposi- Summary of local field relationships tional ages of ca. 100 Ma, based on the youngest popula- In the vicinity of El Cerrito Quarry, the Franciscan units, tion of detrital zircon dated by U/Pb methods (Snow from structurally high to low comprise: the poorly exposed et al. 2010). Of the 45 zircons dated from the structurally Tiburon mélange, the blueschist facies Angel Island higher Angel Island nappe, seven (15%) range from 1.2 nappe, thrust in turn over the Hillside mélange, below to 2.9 Ga, and the rest range from ~100 to 175 Ma. In which lies the Alcatraz nappe of prehnite-pumpellyite contrast, the 45 zircons dated from the structurally lower facies greywacke (Figures. 1 and 2). Each of these units Alcatraz nappe produced no zircons older than 157 Ma. is regionally extensive across the San Francisco Bay area All but four of the Alcatraz nappe ages are ca. 137– but variable in structural thickness and orientation. 157 Ma, whereas 15 zircons from the Angel Island Locally, the contacts between the Franciscan units strike nappe gave ages of <130 Ma. NW and dip NE (Figure 2), whereas folding results in The samples of Alcatraz and Angel Island nappes used different dip directions elsewhere in the region (Figure 1). for detrital zircon chronology also show differences in rare Rhyolitic volcanic rocks unconformably overlie the earth element (REE) and trace element geochemistry Franciscan rocks. These have been mismapped in the past (Ghatak et al. 2013). Normalized to chondrite, REE con- as Jurassic rocks of the Coast Range ophiolite (e.g. centrations of the Angel Island nappe show a negative Eu Wakabayashi 1992, 2005), but detailed field studies and anomaly, whereas those of the Alcatraz Island nappe do geochemical and geochronologic analyses have shown not. The Alcatraz nappe is also less enriched in REEs than similar rocks in this area to be Miocene, postdating the Angel Island nappe. N-MORB-normalized trace ele- Franciscan subduction (Murphy 2001;Murphyet al. ment concentrations show low Nb and Ta compared with 2002). The late Cenozoic (and currently active) dextral La for the Angel Island nappe, but not for the Alcatraz Hayward fault transects the Tiburon mélange, and the dis- nappe. Although the primary lithologies are similar and placement of Franciscan and Jurassic Coast Range ophiolite the maximum depositional ages are indistinguishable, the rocks across this trace is less than 5 km, with much of the zircon age distributions and geochemical differences show >35 km of cumulative late Cenozoic dextral slip on the these two units to have distinctly different sedimentary greater Hayward fault accommodated on a now-dormant sources, rather than being differentially exhumed correla- strand to the east (E in Figure 2) (Wakabayashi 1999). tive units. Below, we present field and microstructural observations The contact between the Angel Island and Alcatraz of the hanging wall and footwall rocks (Angel Island and nappes extends for >15 km along strike and >20 km Alcatraz nappes, respectively), and then the fault zone across strike and exhibits a folded low-angle geometry between them. We will then discuss the characteristics of (Figure 1).Thecontactmayextendfor>70kmalong the fault zone, and comparisons to other exhumed mega- strike (Figure 1), but the possible northern extent of this thrust zones, in the context of subduction interface processes. contact (dotted red line on Figure 1) has not been examined in detail by either field or geochronologic studies. Out-of-sequence low-angle faults have been interpreted in the Franciscan at scales of kilometres Angel Island nappe (Hanging wall) and greater (Figures 4–6, 10, 11, 15 of Wakabayashi 2015). Although such faults locally form the contact The Angel Island nappe consists of massive metagrey- between two accreted nappes, they truncate nappe con- wacke that has a penetrative foliation/ but pre- Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 tacts elsewhere. serves relics of its original clastic texture (Figures 4A–C WesuggestthatthecontactbetweentheAngel and 5A, B). In the El Cerrito Quarry area this unit has a Island and Alcatraz nappes is a palaeomegathrust hor- structural thickness of several hundred metres. It does not izon because it is of regional extent and separates two appear to exceed 500 m in structural thickness in local distinct nappes along its entire length. Metamorphic exposures but may exceed 1 km in the northwestern San burial depth contrast of the sort recorded between the Francisco Bay area (Figure 1). Most of this greywacke is nappes at El Cerrito Quarry is atypical of Franciscan massive with rare interbeds of shale, conglomerate, or palaeomegathrust horizons. Many (most) horizons sedimentary breccia. Locally, the greywacke includes between nappes accreted at different times show no blocks of serpentinite up to 0.5 m in size (Wakabayashi metamorphic contrast, because there has been no differ- 2013). Some chert bodies up to tens of metres long crop ential exhumation of the upper versus the lower nappe out within this unit, but the contact exposures are insuffi- (Wakabayashi 2015). cient to determine whether these chert bodies are structural We now examine the detailed field relationships of the imbricates within the greywacke unit or detrital blocks contact between the two nappes at El Cerrito Quarry to surrounded by a greywacke matrix. Abundant tightly assess the localization of the slip there. folded quartz veins suggest intense internal deformation International Geology Review 859

Figure 4. Photomicrographs of rocks from El Cerrito Quarry. Outcrop location of samples for A, I, and J shown in Figure 2. Mineral abbreviations: gln: glaucophane, jd: jadeitic clinopyroxene, qtz: quartz. (A) Angel Island nappe blueschist facies metagreywacke from the top of the quarry wall approximately 100 m (structurally) above the base of the unit. The glaucophane shows several different textural relationships including syntectonic and postectonic (grows across fabric) neoblastic growth. The rounded jadeite aggregate (lower centre) may be a detrital grain or early neoblastic growth that has been pulled apart with later neoblastic overgrowth forming locally sharp terminations. The large grain to the lower right is a metavolcanic lithic clast replaced primarily by glaucophane along with very fine lawsonite (too small to label). The internal glaucophane growth is truncated along the sub-rounded detrital grain boundary, but there are local neoblastic growths of acicular

Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 glaucophane on this grain, such as those above the ‘gln’ label in the upper left corner of the grain. The brownish grain in the lower left, partly between two jadeite grains, is a talc schist lithic clast. A shale clast is seen in the upper left, partly overlapped by the figure label ‘A’. (B) Angel Island nappe 15 m (structurally) above the base of the unit (outcrop of Figure 5A, location in Figures 2 and 3A). Rounded clots of jadeite are present, some of which have been pulled apart and feature late acicular overgrowths. Glaucophane occurs as medium-sized syntectonic grains or aggregates such as that labelled grain and as fine syn- to post-tectonic needles (many of the fine needle-like grains seen in this view.) (C) Angel Island nappe 5 m (structurally) above the base of the unit (outcrop of Figure 5B, location on Figure 2). Mineralogic and textural relationships are similar to those seen in (A) and (B), with syn- to post-tectonic glaucophane needles and jadeite that may reflect both detrital and neoblastic origins. Many small, tabular, colourless medium-relief grains at upper right centre are lawsonite (some labelled). (D) Greywacke from top of the fault zone zone (outcrop of Figure 5C, location on Figures 2 and 3A). Note the large jadeite grains. The large one on the right appears to have a syntectonic growth of a ‘tail’ on its lower left. (E) Another part of the same thin section in (D) showing texturally early glaucophane (blue core beneath label), with later syntectonic and post-tectonic growth of more needle-like crystals. (F) Greywacke from base of the fault zone in contact with the top of the Hillside mélange (outcrop of Figure 5D, location on Figures 2 and 3A). This rock lacks glaucophane and jadeite, but lawsonite of different textural generations is common. The labelled grains (law) are post-tectonic grains growing across the foliation. The brownish grains are altered plagioclase. Altered plagioclase is present (alt plg, brownish grains, lower left) and the alteration minerals consist of albite and fine pumpellyite. Twinning can be seen in cross-polarized light but the contrast is poor for photomicroscopy, so the cross-polarized view is not shown. (G) Prehnite-pumpellyite facies greywacke block from the top of the Hillside mélange (outcrop of Figure 5D, location on Figures 2 and 3A). Only rarely is fine neoblastic pumpellyite found in this rock, in addition to chlorite and carbonate. Plagioclase is altered to albite with fine white mica (sericite), carbonate, and pumpellyite. This greywacke closely resembles the greywacke of the Alcatraz nappe viewed in (I) Both display some but lack a pronounced cleavage or foliation and have similar quartz-feldspar-lithic grain proportions. This sample from the top of the mélange also contains detrital k-feldspar as shown in (H), similar to that seen in the Alcatraz nappe greywacke shown in (J). 860 J. Wakabayashi and C.D. Rowe

Figure 4. (Continued)

in the sandstones (Figure 5A); these are not found in the apart jadeite clots, and pressure shadows around jadeite underlying units. clots and some lithic grains. Foliation strikes NW and dips Blueschist facies minerals occur both as detrital grains NE and the lineation plunges nearly parallel to dip and neoblastic crystals and aggregates growing across grain associated with a consistent up-dip or tops to the SW boundaries. Jadeitic clinopyroxene has completely replaced sense-of-shear (Figure 2), most commonly expressed by the plagioclase/albite, quartz has been recrystallized, and shear bands and a weakly developed C-S fabric, centi- lithic clasts include metavolcanic, metachert, and siltstone/ metre-scale asymmetric folds, and asymmetric recrystal- shale (Figure 4A–C). Some of these lithic clasts show evi- lized tails of quartz on clots or segregations of quartz dence of pre-depositional blueschist facies metamorphism, (Figures 4 and 5). We observed no systematic gradation in such as the glaucophane that is truncated along the rounded the intensity of penetrative strain or shear fabric, including detrital grain boundary of the volcanic clast in Figure 4A. along the margins of this unit, as illustrated by comparison Neoblastic jadeitic clinopyroxene occurs as fan-like aggre- of metagreywacke samples from the structural base of the gates with sharp crystal terminations growing across detrital unit compared to intermediate exposed levels (Figure 4C, grain boundaries. Locally, neoblastic jadeite forms over- B, A). Variation in foliation and lineation orientation growths on what may have been detrital jadeite grains. (Figure 3)mayreflect later folding. The trace of the basal Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 Lawsonite also appears to grow as late, unstrained neoblastic metagreywacke contact over topography and the map dis- tablets as well as the rounded polycrystalline clots and bent tribution of domains of similar foliation and lineation orien- and rotated grains that may either be early neoblastic growths tation suggest folding on scales of up to several hundred or detrital grains. Neoblastic fine-grained sodic amphibole metres (Figure 2) similar to folding in similar rocks else- grows across detrital grain boundaries, and commonly where in the Franciscan (Wakabayashi 2015). Metre-scale parallel to foliation planes, and as well as forming over- folds are locally observed in outcrop. growths on detrital green (calcic or sodic-calcic) and sodic amphibole. The occurrence of recycled blueschist meta- morphic detritus accords with recent field and petrographic Coherent greywacke of the Alcatraz nappe (lower part of studies of Franciscan clastic rocks (e.g. Wakabayashi 2011, footwall) 2012, 2015), indicating that a blueschist facies source The coherent rocks of the Alcatraz nappe consist primarily contributed to the sedimentary provenance of the Angel of massive unfoliated greywacke that ranges from lithic Island nappe greywackes. poor to moderately lithic rich; local shale and pebbly These rocks display a weak lineation, defined by a mild sandstone beds are present. The structural thickness of preferred elongation of lithic grains, quartz ribbons, pulled- this unit is difficult to constrain in the vicinity of El International Geology Review 861

Figure 5. Outcrop photographs spanning a range of exposures from structurally high to low within the Hillside mélange. Locations of outcrops shown on Figures 2 and 3. (A) Angel Island nappe blueschist facies metagreywacke 15 m above the base of the nappe. This exposure Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 is slightly oblique to the stretching lineation and shows top-to-the-west sense-of-shear defined by local centimetre-scale asymmetric folds (some visible below the arrow), and a C-S fabric, best observed by zooming in on the sandstone fabric (not the quartz veins or quartz segregations) in the right-hand part of the photo. Many deformed quartz veins are visible. This outcrop is representative of most of the Angel Island nappe exposures. (B) Angel Island nappe 5 m above base of the nappe. Top to the south sense-of-shear is recorded by C-S fabric and asymmetric tails off of quartz segregations. The finer-grained (darker) zones appear to include fault rocks similar to those in the fault zone structurally below (compare to (C) and upper right part of (D)). (C) Fault zone cross-cut by sharp fault surfaces about 3 m below the base of Angel Island nappe. This rock consists of thin sandy interbeds, metashale, quartz veins, and black fault rock. Top to the south sense-of-shear is recorded by C-S fabric, imbrications, and asymmetric folds of quartz segregations, and asymmetric tails off quartz segregations. (D) Base of the fault zone with sharp contact atop sedimentary Hillside mélange. The fault zone is composed primarily of sandy interbeds or lenses, metashale, and black fault rock. Volcanic (v) and chert clasts or blocks are scarce in the fault zone compared with the Hillside mélange below, and most of these are restricted to the lower part of the the fault zone where they may be derived from the structurally underlying Hillside mélange. Tops to the west sense-of-shear are seen in the C-S fabric in the mélange and fault zone. Note that only a few of the volcanic clasts (greenish) are labelled for the Hillside mélange. (E) and (F) Exposure of the Hillisde mélange that preserves primary sedimentary block-in-matrix relation- ships. This rock is silty to sandy matrix sedimentary breccia with some rounded clasts (some exotic lithologies, including pyroxenite (px)) and no foliation. The exotic clasts are identical in lithology to other exotic blocks found elsewhere in the mélange unit. A pyroxenite clasts (px) is labelled. Although most clasts are angular, rounded clasts are present as well. One is located at the lower left of the photograph in line with the pen cap. (G) Temporary excavation along Schmidt Avenue (now covered). Base of the mélange (grey, right), resting on greywacke of the Alcatraz nappe to the west (left, tan-coloured massive rocks) along a locally steeply dipping (about 70° to the northeast) contact. The texture of the mélange here has been disturbed by excavator teeth but undisturbed (unsmeared) domains can be observed in direct contact with the greywacke along the upper part of this exposure (foliation ‘blades’ are smeared along the lower part), and these preserve sedimentary textures. Accordingly the lower contact of this mélange on the Alcatraz nappe greywacke is depositional, rather than tectonic. Some volcanic clasts (green) are labelled. The long dimension of view is approximately 3 m. 862 J. Wakabayashi and C.D. Rowe

Figure 5. (Continued)

Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 Cerrito Quarry, but it appears to range between several Alcatraz nappe. Similar to the upper Angel Island nappe, hundred metres and >1 km in more complete exposures this unit strikes NW and dips NE (Figure 2). along strike (mainly to the south), and as much as 3 km in the western San Francisco Bay area (Wahrhaftig 1984; Blake et al. 1984; Wakabayashi 1992)(Figure 1). Hillside mélange of the Alcatraz nappe (upper part of In addition to abundant detrital quartz and plagioclase, footwall) this sandstone has volcanic, chert, and clastic sedimentary This unit consists of blocks of chert, greywacke, metaba- (siltstone, mudstone) clasts and a trace of detrital salt, and rare lithologies such as pyroxenite, in a shale or K-feldspar (Figure 4I, J). Neoblastic fine needles of pum- sandstone matrix. In the El Cerrito Quarry area the struc- pellyite occur locally (Snow et al. 2010) and some expo- tural thickness of this unit ranges from about 100 to 200 m sures show some pressure solution features with (Figures 2 and 3). This appears consistent with other interlocking grain boundaries, without development of a observations along strike, as well as on Angel Island, cleavage. The weak tectonic fabric, preservation of alkali where the maximum structural thickness appears about feldspar, and paucity of metamorphic minerals suggest 100 m (Wahrhaftig 1984). Greywacke comprises the only slight metamorphism and deformation affected the most common block type as well as the largest blocks, International Geology Review 863

ranging to 60 m in size. The mélange blocks and matrix exhibit evidence of prehnite-pumpellyite facies meta- morphism similar to the Alcatraz nappe coherent grey- wackes, with growth of needles of neoblastic pumpellyite and some chlorite. Blocks lack penetrative fabrics and most of these greywacke blocks resemble coherent grey- wacke of the Alcatraz nappe (compare Figure 4G, J mélange blocks to Figure 4I, J Alcatraz nappe). Outcrop photos, showing the range in textures in this mélange from structurally high to low, are shown in Figure 5D–G. Most of the unit is matrix and clast-supported sedimen- tary breccia and conglomerate with rounded and angular clasts and a sandy to shaley matrix (Figure 5E, F). The matrix is variably foliated, forming an aligned scaly fabric with chaotic folds and changes in orientation (Figure 5D, G). The block or clast population in the least deformed exposures of sedimentary breccia and conglomerate is identical to the more highly deformed parts of the mélange and the expo- sures show gradations between strongly and weakly foliated mélange matrix, similar to those observed in other Franciscan mélanges (Wakabayashi 2011, 2012, 2015; Platt 2015). However, there is no systematic gradation in foliation intensity on the scale of the quarry face exposure. The lower contact of mélange appears to be a depositional (little deformed) contact on sandstone of the Alcatraz nappe, although the matrix is foliated several centimetres above this contact (Figure 5G). Matrix foliation strikes NW and dips NE parallel to bounding contacts of the mélange. A shear fabric is defined mainly by shear bands, a C-S fabric, and asymmetric (apparently cataclastic) tails on some clasts, most noticeable with greenish volcanic clasts owing to the colour contrast between these and the grey matrix. (Figure 5D). Transport direction can be inferred as normal to intersection of C and S surfaces by the tails (in three dimensions Moore 1978), on clasts, and preferred elongation of clasts. The sense-of- shear is up-dip and tops to the southwest (Figures 2 and 5).

Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 Fault zone (between Hillside mélange and Angel Island nappe) Figure 6. (A) Thin fault surfaces of black rock (black lines) The metamorphic contrast between the prehnite-pumpellyite separate differently oriented sections of the melange fabric (white Hillside mélange and the blueschist facies Angel Island lines). (B) Thin fault surfaces of aphanitic black rock (black nappe is accommodated in a fault zone 20–30 m thick. The lines) bound a coherent tabular piece of sandstone and cut fabrics above, below, and within the sandstone (white lines). Lens cap is metamorphic contrast indicates that this represents the actual 54 mm. (C) Backscatter electron image of a 1.5 mm-thick fault boundary between the footwall and hanging wall, so this surface of aphanitic black rock (vertical in centre of image). zone must represent a major fault surface. The fault zone Black rock contains quartz clasts with rough and embayed consists primarily of variably disrupted layers of metagrey- edges (inset) and sharply cross-cuts fabric in the wallrock fi wacke and shale, with blocks of basalt and chert confined to (right, where quartz and phyllosilicate layers de ne wavy meta- morphic fabric in metagreywacke). The matrix of the black rock the lower part of the zone (Figure 5C, D). The fault zone is is homogenous compared with syngenetic gouge layers (left of strongly foliated compared with the block-in-matrix fabric dashed line) and clasts are generally more equant and aligned. No seen in the Hillside mélange (Figure 5C, upper part of 5D; individual grains could be observed in the black rock matrix Figure 6A, B compared to lower part of Figure 5D). above 2 µm in size. 864 J. Wakabayashi and C.D. Rowe

Greywacke layers and blocks in the fault zone are foliated in (Dumitru 1989; Ernst 1993; Ernst and McLaughlin 2012). contrast to the greywacke blocks of the Hillside mélange, but The transport direction, indicated by stretching lineations, metavolcanic and metachert blocks found in the lower part of is approximately down-dip (Figure 2). Assuming pure dip- the fault zone lack foliation. slip displacement, a minimum estimate of displacement The fault zone spans an inverted metamorphic gradi- between the two nappes would be 15 km/(sin (average ent. The structurally lowest metagreywacke layers or subduction dip between 15 and 30 km)) depth). The pre- blocks, directly above the Hillside mélange, have fine sent-day dip of subduction-related fabrics in the El Cerrito neoblastic, and texturally late, lawsonite, but they have Quarry is about 30°, but its regional map pattern suggests albite and lack other blueschist facies minerals a much shallower regional dip (Figure 1). Folding of (Figure 4F). Structurally higher in the mélange, albite is original nappe contacts is ubiquitous in the Franciscan replaced by jadeitic clinopyroxene and quartz, and neo- (Wakabayashi 2015), so local dips do not generally repre- blastic glaucophane is present (Figure 4D, E), and the sent the original subduction interface geometry. As mod- abundance of jadeitic clinopyroxene and glaucophane as ern subduction zones dip 10–20° at the exhumed depth well as their grain size increases structurally upward range of the fault (Clowes et al. 1987; Kopp et al. 2001; within the fault zone. The blueschist facies minerals of Tréhu et al. 2008; Singh et al. 2011), we will use this as the upper part display textural relationships similar to our dip estimate. Using this subduction zone dip range and those in the Angel Island nappe. Jadeitic clinopyroxene, the ~15 km burial depth difference yields an approximate glaucophane, and lawsonite are present as apparent detrital minimum slip estimate of about 45–85 km. Owing to the grains, as well as neoblastic grains that show syntectonic many sources of uncertainty and assumptions made in this growth relative to the foliation and texturally late neoblas- estimate, its main utility is to illustrate that the slip on this tic growth that cross-cuts the foliation (compare fault was tens of kilometres or more. The preserved Figure 4D, E of fault zone to Figure 4A–C of the Angel across-strike (down-dip) dimension of this fault of Island nappe). >20 km (Figure 1) also suggests a minimum displacement The top of the fault zone is tectonically interleaved of tens of kilometres. with the base of the Angel Island nappe, with zones of finer-grained rocks up to about 1 m in thickness separating slabs or lenses of Angel Island nappe that range up to 5 m Localization of subduction megathrust displacement: in thickness. The uppermost few metres of the fault zone synthesis and other examples have abundant ultracataclasites and frictional melts (‘black Taken together, field relationships indicate that significant fault rocks’ or BFR of Rowe et al. 2005; Menghini et al. palaeomegathrust displacement between the Alcatraz and 2010)(Figure 6). Angel Island nappes recorded was probably accommo- dated in the 20–30 m-thick fault zone. The shear fabrics in the Hillside mélange and Angel Island nappe show that some of this subduction displacement may have been Discussion accommodated through distributed strain, but the preva- Minimum estimate of subduction zone slip between the lence of sharp discrete shears in the fault zone suggest Alcatraz and Angel Island nappes localization of slip there (e.g. Figures 6 and 7). A robust estimate for the amount of subduction slip A generic megathrust horizon in an accretionary com- accommodated between the Angel Island and Alcatraz plex has three stages of development (Figure 7). A new nappes is not possible because of the lack of well-con- megathrust horizon forms as a nappe is accreted and the Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 strained accretion ages. The difference in accretionary age older megathrust is abandoned. During accretion, the nappe ranges permitted by the detrital zircon data ranges from is internally shortened and slip will progressively accrue on zero to >10 Ma. This reflects variability in the difference the new megathrust as sections of the old megathrust are between the youngest detrital zircon ages and the actual abandoned (Figure 7A3, B3, details on A3 inset). In this depositional ages for sandstones in the Franciscan first stage, displacement is accommodated by both move- Complex and related units (Dumitru et al. 2015; ment on the developing megathrust and faults and penetra- Wakabayashi 2015). A minimum estimate of fault slip tive deformation that shorten upper nappe. can be made for a fault that accommodated syn-subduction For the general megathrust case that includes a gap in exhumation based on restoration of differential burial accretion between two nappes, the second stage of mega- depth with a dip consistent with that of modern accreting thrust slip is confined to the new megathrust during the subduction zones (Figure 7). period of non-accretion (the stage between Figure 7A3 and The metamorphic contrast suggests a difference in 7A4, and between Figure 7B3and7B4). As noted in the peak burial of about 15 km (about 15 versus 30 km burial Introduction, gaps in accretion between accretion of nappes depth) between the two nappes, based on PT estimates of are likely owing to the contrast between the rocks of each rocks with similar mineral assemblages in the Franciscan nappe. For El Cerrito Quarry, this contrast is demonstrated International Geology Review 865 Downloaded by [University of California, Berkeley] at 09:19 04 April 2016

Figure 7. Cross-sectional cartoons illustrating subduction, accretion, and accumulation of slip on the subduction megathrust for two different examples: (A) for the general case of subduction and accretion without differential exhumation of accreted nappes along the megathrust and (B) for cases of differential exhumation of accreted nappes along the megathrust, similar to the palaeomegathrust at El 866 J. Wakabayashi and C.D. Rowe

Figure 7. (Continued) Cerrito Quarry. Amounts of slip on the megathrust for each stage are given on each frame. The small numbers represent reference points to track the subduction-accretion, and slip process. The letters ‘h’ and ‘f’ refer to hanging wall and footwall cut-offs, respectively. 1 and 2 in Frame A2 mark the leading and trailing edge of material on the downgoing plate that will become the first (nappe 1) and structurally highest nappe of two accreted nappes in these examples. As of Frame A2, nappe 1 is about to accrete. In frames A3 and B3, nappe 1 begins to accrete at blueschist facies depth, and the accretion involves internal shortening as well as transfer of primary megathrust slip to the lower contact of nappe 1. The details of the transfer of megathrust slip and the internal shortening of an accreting nappe are schematically shown in the inset for A3. The new megathrust slip begins to accumulate and this slip is tracked by the hanging wall cut-offs 1 h and 2 h, and footwall cut-offs 1 f and 2 f. In this example the material that will form the structurally lower nappe begins to subduct just as nappe 1 is accreting, so there is a gap in accretion. Reference points 3 and 4 mark the leading and trailing edge of material on the downgoing plate that will become nappe 2. Frames A4 and B4 show the material for nappe 2 in place beneath nappe 1 just before accretion of nappe and transfer of the

Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 megathrust slip to the lower boundary of nappe 2. This is the end of activity on the megathrust horizon that separates nappes 1 and 2. For (B) partial exhumation of nappe 1 takes place prior to accretion in Frame B4. This exhumation is shown by the light grey hanging wall cut- offs 1 h and 2 h which mark the position of the upper nappe prior to partial exhumation to the position against nappe 2 marked by the black 1 h and 2 h. Insets X and Y (on Diagram (B)) are zoomed in views that illustrate the hypothetical impact of subduction erosion on previously accreted material, including part of a megathrust fault zone. This results in the migration of the position of the megathrust zone into the hanging wall (nappe 1 in this case). Inset Y also shows how many nappes are composed of interlayered and interfingered mélanges (sedimentary mélanges from submarine mass transport deposits) with turbidites (non-block-in-matrix sandstones, shales, conglomerate) considered the ‘coherent’ part of the nappe. This interfingering and interbedding can take place at such fine scales as to make coherent and mélange gradational and difficult to distinguish (Wakabayashi 2015). Many megathrusts may form along a mélange-coherent contact, as at El Cerrito Quarry. The lower right inset shows a schematic diagram of field relationships at El Cerrito Quarry. Although it is linked to the subduction erosion diagram (Y), this is not intended to imply that there is definitive evidence that subduction erosion took place during the development of the megathrust preserved at El Cerrito Quarry. International Geology Review 867

by the difference in detrital zircon age populations and plate stratigraphy without true exotic blocks (e.g. Kimura geochemistry of the sandstones in each nappe. et al. 2011). This type of mélange is similar to the Rodeo Movement on a megathrust ceases as the next nappe is Cove of the Marin Headlands (Meneghini and accreted beneath and the megathrust is progressively aban- Moore 2007), in contrast to mélanges with exotic blocks doned during the accretion of this the lower nappe. These such as the Hillside mélange and those in Sunol Regional stages of megathrust development suggest accommodation Wilderness (Wakabayashi 2015). of megathrust slip along the main fault separating two In the central , the boundary between the nappes, as well as by internal deformation of the nappes. Austroalpine (upper plate) and subjacent nappes Internal deformation of nappes may be accommodated by (from subducted oceanic plate), thought to preserve a moderate distributed strain, as at El Cerrito Quarry, or by palaeomegathrust, exhibits pseudotachylites at and near duplexing along discrete faults (e.g. Kimura et al. 1992 the base of the upper (Austroalpine) plate but not within (Figure 7). the mélange (uppermost Penninic nappes) (Bachman et al. If subduction erosion occurred during the life of a 2009). The mélange units certainly have been strongly megathrust, the megathrust horizon may move upward deformed, but the localization of the pseudotachylites into the hanging wall (Figure 7B, insets X and Y). Thus, above the mélange may reflect the accommodation of the fault zone between the Angel Island and Alcatraz significant megathrust slip on the upper contact of the nappes may accommodate only part of the original sub- mélange. duction slip on the fault zone between the two nappes, A recent compilation of the thickness of subduction because subduction erosion may have removed some of faults from a depth range of 0–15 km shows that a typical the original fault zone. Whereas geochronologic studies palaeo-decollement has one or more high-strain strands have shown evidence for significant periods of non-accre- about 5–35 m thick, which accommodate both distributed tion during Franciscan history, removal of previously and localized (including seismic) plate motion (Rowe accreted material is more difficult to assess (Wakabayashi et al. 2013). The El Cerrito Quarry fault, with its 20– 2015). Relatively brief alternating periods of accretion and 30 m-thick fault zone including pseudotachylytes, is simi- subduction erosion have been proposed to have taken lar to single strands in the compilation, whereas the total place during greater time spans of net accretion in the thickness of the megathrust zones from 2–15 km depths is Franciscan, based on the comparative timing of wide- 100 –350 m (Rowe et al. 2013). spread olistostromal deposition, forearc subsidence, and exhumation (Wakabayashi 2015). El Cerrito Quarry is the best exposure of a likely Conclusions palaeomegathrust contact found in the Franciscan Mélanges have been interpreted to preserve subduction Complex to date. Whereas comparable fault rocks have megathrust horizons within subduction complexes (e.g. not been studied in detail elsewhere in the Franciscan Fisher and Byrne 1987; Cloos and Shreve 1988a, 1988b; Complex, the apparently narrow (several hundred metres Kitamura et al. 2005; Kimura et al. 2011), but field and and less) zone of permissible megathrust positions petrographic relationships within the Franciscan Complex between two nappes is common (Wakabayashi 2015). In suggest sedimentary origins for many mélanges tradition- the regions with the most detailed mapping and geochro- ally considered to have formed by tectonism (e.g. Aalto nology, such as the Yolla Bolly region of the northernmost 1989; MacPherson et al. 1990; Wakabayashi 2011, 2012, Coast Ranges (Dumitru et al. 2010) and the Pacheco 2015; Platt 2015). Our study of an exhumed subduction (Ernst et al. 2009) and Panoche Pass regions megathrust in the Franciscan Complex suggests that sig- Downloaded by [University of California, Berkeley] at 09:19 04 April 2016 (Wakabayashi 2015) of the Diablo Range, the zone nificant subduction slip is localized along the upper con- between nappes is commonly <50 m thick and lacks tact of a mélange rather than distributed through its full mélange. thickness (Figure 7). The likely zone of slip accommoda- Observations from other mélanges in other orogenic tion is marked by ultracataclasites and pseudotachylites belts may reflect a localization of displacement on the (black fault rocks) and ranges in thickness from about upper contacts of mélanges, similar to that observed at 20–30 m (Figure 7). El Cerrito Quarry. In the Shimanto Belt of Japan, pseudo- The El Cerrito Quarry fault zone shows a uniform tachylites have been noted from the upper contact (roof seaward-vergent sense-of-shear, as observed thrust) of mélanges (Ikesawa et al. 2003; Kitamura et al. in subduction fault zones from 2–15 km depths (Rowe 2005). The localization of the pseudotachylites reflects et al. 2013). Accordingly the conclusion of Rowe et al. localization of seismic slip along the upper contact of (2013), that subduction faults show no evidence of sub- those mélanges. In the Shimanto Belt examples, the duction channel return flow predicted in numerical models block-in-matrix fabric may have resulted from tectonic (e.g. Cloos and Shreve 1988a, 1988b), may be extended to strain, in contrast to most Franciscan mélanges, because depths of 30 km. The thickness of the palaeomegathrust the block-in-matrix fabric may be nearly restored to ocean zone and the lack of evidence for accommodation of this 868 J. Wakabayashi and C.D. Rowe

slip in mélange at El Cerrito Quarry and other Franciscan Society of America Bulletin, v. 89, p. 1415–1423. examples is also incompatible with numerical models of doi:10.1130/0016-7606(1978)89<1415:OOBCRI>2.0.CO;2 subduction channel flow along the subduction interface. Cowan, D.S., 1985, Structural styles in Mesozoic and Cenozoic mélanges in the western Cordillera of North America: Geological Society of America Bulletin, v. 96, no. 4, p. 451– 462. doi:10.1130/0016-7606(1985)96<451:SSIMAC>2.0.CO;2 Acknowledgement Dumitru, T.A., 1989, Constraints on uplift in the Franciscan fi We thank the reviewers for their constructive comments. Subduction Complex from apatite ssion track analysis: Tectonics, v. 8, p. 197–220. doi:10.1029/TC008i002p00197 Dumitru, T.A., Ernst, W.G., Hourigan, J.K., and McLaughlin, R. J., 2015, Detrital zircon U–Pb reconnaissance of the Disclosure statement Franciscan subduction complex in northwestern California: No potential conflict of interest was reported by the authors. International Geology Review, v. 57, p. 767–800. doi:10.1080/00206814.2015.1008060. Dumitru, T.A., Ernst, W.G., Wright, J.E., Wooden, J.L., Wells, R. E., Farmer, L.P., Kent, A.J.R., and Graham, S.A., 2013, Funding Eocene extension in Idaho generated massive sediment Parts of this research were supported by National Science floods into the Franciscan trench and into the Tyee, Great Foundation [grant number EAR-0635767]. Valley and Green River basins: Geology, v. 41, p. 187–190. doi:10.1130/G33746.1 Dumitru, T.A., Wakabayashi, J., Wright, J.E., and Wooden, J.L., 2010, Early Cretaceous transition from nonaccretionary References behavior to strongly accretionary behavior within the Aalto, K.R., 1981, Multistage melange formation in the Franciscan Franciscan subduction complex: Tectonics, v. 29, p. 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