Collaborative Research: Is the Isabella anomaly a fossil slab or the foundered lithospheric root of the Sierra Nevada batholith? Intellectual Merit: This project will simultaneously test two hypotheses regarding the tectonic origin of the Isabella seismic anomaly in the upper mantle of California’s southern Great Valley. Both hypotheses are viable based on existing seismic imaging that uses data from stations spaced about 70 km apart, but they have dramatically different implications for subduction termination and continental arc evolution. One hypothesis attributes the Isabella anomaly to the foundered mafic root of the Sierra Nevada batholith. The other attributes Isabella to a fossil slab that is a continuation of the Monterey microplate coherently translating beneath the Great Valley because it is mechanically coupled to the Pacific plate. Importantly, the latter hypothesis places the fossil slab beneath the along-strike extent of the section of the San Andreas fault that dominantly deforms by aseismic creep and hosts deep crustal tectonic tremor, which might be caused by fluids from the slab. Passive source seismic imaging with a dense array extending from the coast to the Sierra Nevada foothills can robustly test the two hypotheses with detailed mapping of lithospheric interfaces and identification of whether or not they are continuous across a plate bounding fault with >300 km of cumulative right lateral displacement. The proposed seismic study will advance understanding of the structural legacy of subduction termination, post-subduction evolution of the Sierra Nevada arc, and present day basal boundary conditions on the creeping section of the San Andreas fault by addressing the following questions:

1. Is the Isabella seismic anomaly structurally continuous with overlying lithosphere east or west of the San Andreas fault?

2. Is the strength (effective viscosity) of young oceanic lithosphere sufficient for a fossil slab to remain intact through hundreds of km of along-strike translation?

3. Is an along-strike gap in coastal block post-subduction volcanism a result of a fossil slab?

4. If Isabella is connected to Great Valley lithosphere, is it delaminating from North America along a localized detachment surface?

5. Is the creeping section of the San Andreas fault underlain by a dehydrating fossil slab?

Broader Impact: Viability of the fossil slab hypothesis is generally important for understanding the 3D structural legacy of plate boundary re-organization events and how inherited structures can influence subsequent geodynamic activity. Because the southern Sierra Nevada serves as the world’s premier case study of continental arc evolution and small-scale lithospheric convection, determining whether the Isabella anomaly has a fossil slab or Sierra Nevada batholith root origin has basic importance for multidisciplinary scientists seeking to understand convergent margin systems.

The proposed project will aid in the development of a new geophysics group at UNM by supporting a beginning-career PI, a graduate student, and an undergraduate researcher. Expansion of geophysics research and teaching at the state of New Mexico’s largest institution has outstanding potential to attract under-represented minorities to opportunities in the geosciences. A graduate student and undergraduate researcher will be supported at Caltech. The project will develop a new collaboration between UNM and Caltech.

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TABLE OF CONTENTSFor font size and page formatting specifications, see GPG section II.B.2.

Total No. of Page No.*Pages (Optional)*

Cover Sheet for Proposal to the National Science Foundation

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Project Description (Including Results from Prior

NSF Support) (not to exceed 15 pages) (Exceed only if allowed by aspecific program announcement/solicitation or if approved inadvance by the appropriate NSF Assistant Director or designee)

References Cited

Biographical Sketches (Not to exceed 2 pages each)

Budget (Plus up to 3 pages of budget justification)

Current and Pending Support

Facilities, Equipment and Other Resources

Special Information/Supplementary Documents(Data Management Plan, Mentoring Plan and Other Supplementary Documents)

Appendix (List below. )

(Include only if allowed by a specific program announcement/solicitation or if approved in advance by the appropriate NSFAssistant Director or designee)

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TABLE OF CONTENTSFor font size and page formatting specifications, see GPG section II.B.2.

Total No. of Page No.*Pages (Optional)*

Cover Sheet for Proposal to the National Science Foundation

Project Summary (not to exceed 1 page)

Table of Contents

Project Description (Including Results from Prior

NSF Support) (not to exceed 15 pages) (Exceed only if allowed by aspecific program announcement/solicitation or if approved inadvance by the appropriate NSF Assistant Director or designee)

References Cited

Biographical Sketches (Not to exceed 2 pages each)

Budget (Plus up to 3 pages of budget justification)

Current and Pending Support

Facilities, Equipment and Other Resources

Special Information/Supplementary Documents(Data Management Plan, Mentoring Plan and Other Supplementary Documents)

Appendix (List below. )

(Include only if allowed by a specific program announcement/solicitation or if approved in advance by the appropriate NSFAssistant Director or designee)

Appendix Items:

*Proposers may select any numbering mechanism for the proposal. The entire proposal however, must be paginated.Complete both columns only if the proposal is numbered consecutively.

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1. Introduction When subduction terminates where do slabs detach from the unsubducted portion of their tectonic plates? Recent tomographic images show high-velocity structures dipping into the asthenosphere landward of three former subduction zones in western North America (Zhang et al., 2008; Forsyth and Rau, 2009; Schmandt and Humphreys, 2011; Wang et al., submitted to PNAS). The locations and geometries of these high-velocity anomalies and the geological records of post-subduction magmatism in the corresponding regions motivate the hypothesis that slab detachment can occur at depths ≥130 km and perhaps, as great as ~500 km. Detachment at depth is proposed to leave behind a fossil slab segment that remains mechanically coupled to its unsubducted lithosphere over time scales >10 Myr, while the detached portion of the slab sinks independently. Each of the recently hypothesized fossil slabs in western North America is important for its potential to place post-subduction regional geologic activity in a tectonic framework and to constrain fundamental aspects of subduction dynamics. The scientific advances that could stem from detailed constraints on fossil slab morphology and physical properties merit further scrutiny of the structures. A deeper investigation is also merited by the considerable uncertainty that remains in uniquely identifying the high-velocity anomalies as subducted oceanic lithosphere. Smooth tomography models produced with existing datasets are not specifically diagnostic of this tectonic origin and alternative viable interpretations such as drip-like instabilities of continental arc lithosphere lead to significantly different models of regional tectonic evolution and subduction termination dynamics. A robust test of fossil slab origin is to determine the presence or absence of a continuous slab interface with a ~7 km layer of basaltic crust that dips landward directly connecting unsubducted oceanic lithosphere to the high-velocity anomaly in the mantle. Such a test can be conducted with modern passive source seismic imaging methods, though it requires temporary deployment of a dense and optimally located array of broadband seismometers (Rondenay et al., 2001; Audet et al., 2009; Abers et al., 2009; Kim et al., 2010, 2012; Pearce et al., 2012). This proposal focuses on testing the origin of one hypothesized fossil slab that is putatively attached to the Monterey microplate (Forsyth and Rau, 2009; Pikser et al., 2012; Wang et al., submitted to PNAS). This microplate is a remnant of the former Farallon plate that was captured by the Pacific plate about 19 Ma and presently resides offshore of central California (Fig. 1; Lonsdale, 1991; Wilson et al, 2005). The fossil slab is proposed to be a continuation of Monterey microplate oceanic lithosphere that dips shallowly eastward under coastal California and the San Andreas fault (SAF) and dips more steeply beneath the Great Valley accounting for the Isabella high velocity anomaly (Fig. 2; Forsyth and Rau, 2009; Pikser et al., 2012; Wang et al., submitted PNAS). Validation of the hypothesis would demonstrate that the strength (effective viscosity) of young oceanic lithosphere is sufficient to resist substantial deformation during >300 km of along strike translation after subduction ended (Pikser et al., 2012). Given the young age of Monterey oceanic lithosphere this would identify the chemical boundary layer (CBL) created as a result of ocean crust formation (e.g., Hirth and Kohlstedt, 1996; Evans et al. 2005) as essential to accurate characterization of young slab dynamics and

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convergent margin evolution – an effect that has long been recognized but is conventionally ignored to simplify modeling efforts. In addition to consequences for long-term geodynamic processes, the proposed Monterey fossil slab could also play a significant role in present day dynamics of the Pacific-North America transform boundary. The hypothesis places subducted oceanic lithosphere just beneath the section of the SAF that dominantly deforms by aseismic creep (e.g., Titus, 2006) and appears to support anomalously low shear stress (Hickman and Zoback, 2004; Provost and Houston, 2001, 2003). Resulting slab dehydration and cooler temperatures in the deep crust could be key facilitators of tectonic tremor that has been located in the deep crust of the central SAF (Nadeau and Dolenc, 2005; Peng et al., 2009; Shelly et al., 2009). An important and controversial implication of the Monterey fossil slab hypothesis is its suggestion of an alternative origin for the long-studied Isabella anomaly (Aki, 1982; Biasi and Humphreys, 1992; Zandt and Carrigan, 1993; Boyd et al., 2004; Yang and Forsyth, 2006; Schmandt and Humphreys, 2010), which has been integrated into a highly influential model for Sierra Nevada arc lithospheric evolution (Fig. 2; Saleeby et al., 2003; Zandt et al., 2004; Saleeby et al., 2012). This regional model serves as the archetype for geodynamic models of lithospheric instability and is a key motivation for a recent global model of cyclic evolution of continental arc systems (DeCelles et al., 2010). Validation of the fossil slab hypothesis would challenge widely held views regarding post-subduction Sierra Nevada arc evolution and understanding of the process (but not necessarily occurrence) of convective removal of mafic cumulates from beneath granitic batholiths. Existing seismic data do not constrain the Isabella anomaly’s connection to overlying lithosphere as a result of ~70 km mean station spacing above the anomaly. This prevents clear identification of whether it is a Rayleigh-Taylor drip-like instability of lithosphere east of the SAF or delamination along a localized failure surface east of the SAF or a fossil slab that is contiguous with the Monterey microplate outboard of the SAF. Consequently, basic aspects of the process underlying the hypothesized removal of Sierra Nevada arc lithosphere are currently inferred from dynamic modeling studies that require strong a priori assumptions (e.g., introduction of localized weakness, a tuned radial viscosity structure, and no plate motion) in order to achieve consistency with the large volume, close proximity to the overlying lithosphere, and western location of the Isabella anomaly (Le Pourhiet et al., 2006; Saleeby et al., in press). Such a complex lithospheric instability model is viable, but so too is the fossil slab hypothesis. We seek to scrutinize the Monterey fossil slab hypothesis and the origin of the Isabella anomaly because of their unique and far-reaching implications for fundamental geodynamic processes (subduction of young oceanic lithosphere, convective removal of mafic cumulates from continental arc lithosphere), the tectonic evolution of southwestern North America through the subduction to transform transition, and ongoing fluid input at the base of the central SAF. A clear opportunity for scientific advance is identified not only by the importance of these potential implications, but also by the existence of two well-defined hypotheses for lithospheric-scale structure (Fig. 2) that can be robustly tested with the passive source seismic imaging methods. 2. The Monterey microplate and subduction termination

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The plate boundary system that divides western North America from the Pacific is in the midst of a transition from subduction to right lateral transform deformation. It began near the latitude of southernmost California in the Oligocene when the former spreading ridge that separated the Pacific and Farallon plates began to subduct (Atwater, 1989). Bilateral along-strike growth of the transform margin continues as subduction terminates farther north and south, with the northward propagating Mendocino Triple Junction and the southward propagating Rivera Triple Junction bounding the transform margin. The Monterey microplate is a remnant of the former Farallon plate that resides along a section of the transform margin where the former spreading ridge did not subduct and Monterey was captured by the Pacific plate about 19 Ma (Lonsdale, 1991; Atwater and Stock, 1998). This is likely an indication that the older and colder portion of the subducted Farallon slab detached before the Monterey segment of the spreading center reached the trench, thereby removing the “slab pull” force required to subduct young and buoyant lithosphere near the spreading ridge (Burkett and Billen, 2009). The down-dip location of slab detachment and the extent of subducted slab that remains attached to Monterey lithosphere today are not well constrained by geophysical imaging. Oceanic crust of the Monterey microplate has been tracked from beneath the Santa Maria and Santa Lucia offshore basins to beneath the California coast by active source refraction and reflection imaging (Brocher et al., 1999; Trehu et al., 1991; Miller et al., 1992; Meltzer and Levander, 1991). However, the continuous landward extent of the oceanic crust is unknown. Brocher et al. [1999] previously inferred from several distinct active source profiles that a fossil slab underlies much of the California coastal block and is translating coherently with the Pacific plate. The fossil slab hypothesis discussed herein has evolved in response to recent larger-scale passive source imaging and we focus on the possibility of a Monterey fossil slab that is unique along strike in California because of its putative down-dip extension across the SAF. Post-subduction volcanism in coastal California is often interpreted as providing a record of when the slab fell away from the base of the forearc lithosphere allowing asthenosphere to rise to shallow depths and melt (e.g., Wilson et al., 2005, Severinghaus and Atwater, 1990; Dickinson 1997). So-called “slab window” volcanism has occurred along nearly all of California’s transform margin, but it is curiously absent on a portion of the coastal block the lies between the Monterey microplate and the SAF (Fig 1; Wang et al., submitted to PNAS; Walker et al., 2004; Wilson et al., 2005). Following the slab window model the volcanic gap is a potential indication that the slab has remained in place beneath the forearc while this section of the coastal block has translated northward. Translation of a Monterey fossil slab has also been inferred to apply basal tractions that contributed to clockwise rotation of the western Transverse Ranges in the Miocene (Nicholson et al., 1994). A numerical modeling attempt to test whether or not a stalled/fossil Monterey slab remains beneath the coastal block concluded that heat flow data and thermal modeling are insufficient to robustly discriminate between the two scenarios due to the young slab age (van Wijk et al., 2001). Resolution of the landward extent of oceanic lithosphere that is contiguous with the Monterey microplate can place valuable bounds on the process of subduction termination,

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and the physical properties of young oceanic lithosphere and the regional asthenosphere. If a Monterey fossil slab exists, the post-subduction offset of the SAF implies that it remained intact while being dragged through the asthenosphere during >300 km of right lateral displacement. In addition to requiring buoyancy compared to the asthenosphere, this tectonic circumstance could place a strong lower bound on the effective viscosity contrast between the young oceanic lithosphere and the asthenosphere. A simple finite-element model that takes into account compositional effects on the strength of young melt depleted ocean lithosphere finds it is plausible, though the minimum viscosity contrast varies substantially (~2-3 orders of magnitude) depending on whether the fossil slab is truncated near the coast or it extends across the SAF and accounts for the Isabella anomaly (Pikser et al., 2012). Young slab age rules out a purely thermal origin for a large viscosity contrast, instead this setting emphasizes the role of the CBL created as a result of oceanic crust formation at the spreading center (Cloos, 1993; Hirth and Kohlstedt, 1996; Afonso et al., 2007). It is widely recognized that melt extraction leads to dehydration and increased buoyancy in a ~50 km thick mantle section, and the resulting viscosity increase may be vital to focused melting at the ridge (Hirth and Kohlstedt, 1996; Braun et al., 2000; Evans et al., 2005). However, the CBL is often omitted in numerical simulations of ridge-trench collisions (e.g., Burkett and Billen, 2009, 2010). Verification of the fossil slab hypothesis would demonstrate that the CBL is an essential ingredient for realistic modeling of young slab dynamics. In addition to a large viscosity contrast between the Monterey slab and asthenosphere, preservation of a fossil slab that extends beneath the SAF would also require a translating “subduction” interface with low frictional strength. This interface would function as a dipping strike-slip fault that separates Great Valley lithosphere from the northward migrating Monterey slab. Forearc surface heat flow and seismic imaging commonly suggest the presence of serpentinized mantle that is isolated from corner flow (Furukawa, 1993; Brocher et al.; Peacock and Wang, 1999; Bostock et al., 2002; Currie et al., 2004; Abers et al., 2006; Wada et al., 2008). Stagnant serpentinized mantle in the wedge tip requires a decoupling zone that extends down-dip to the base of the upper plate lithosphere (Abers et al., 2006; Wada et al., 2008). Weakness of the interface may vary widely (e.g., Tan et al., 2012), but seismic imaging and geodynamic modeling suggest interfaces with low frictional strength in subduction zones with a variety of different slab dips and ages (Abers et al., 2006; Wada et al., 2008; Perez-Campos et al., 2008; Song et al., 2009; Van Avendonk et al., 2010). Hence, the Monterey fossil slab hypothesis is dynamically viable and resolving its veracity – and if true, its detailed morphology – would provide a sensitive gauge of lithospheric-scale plate boundary dynamics.

Driving questions regarding subduction termination and the hypothesized Monterey fossil slab: a. What is the present day landward extent of Monterey oceanic lithosphere? - Does it underlie the gap in “window” volcanism? - Does it cross beneath the SAF and connect to Isabella? b. Are the strength of young Monterey oceanic lithospheric and weakness of its

coupling to the mantle wedge east of the SAF sufficient to resist substantial deformation during >300 km of along-strike translation?

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3. The rootless southern Sierra Nevada and continental arc evolution The southern Sierra Nevada mountains stand out for their anomalously high topography compared to adjacent sections of the continental arc, but a there is not a sufficiently thick crustal root to compensate for the topography (Wernicke et al., 1996). This straightforward observation combined with evidence for increased exhumation rates since the Miocene (Clark et al., 2005; McPhilips and Brandon, 2010) suggests a recent increase in mantle buoyancy. Removal of lithospheric mantle between ~10-4 Ma and replacement with asthenosphere at depths as shallow as about 40 km is strongly supported by petrological constraints from multiple episodes of southern Sierra Nevada volcanism (Ducea and Saleeby, 1998; Saleeby et al., 2003; Feldstein and Lange, 1999; Farmer et al., 2002; Elkins-Tanton and Grove, 2003), and consistent with present-day geophysical imaging of low-velocity (Jones et al., 1994; Fliedner et al., 1996; Boyd et al., 2004; Yang and Forsyth, 2006) and high-conductivity (Park et al., 2004) uppermost mantle beneath the high topography. The fate of the removed lithosphere and specifically whether it was displaced westward to create the Isabella anomaly beneath the Great Valley are not well constrained. If the Isabella anomaly represents dense lithosphere removed from beneath the high southern Sierra Nevada then it is necessary to explain why the instability has been displaced westward and remains near the base of North America lithosphere. Zandt [2003] noted that if complete lithospheric removal occurred by 3-4 Ma then the dense removed mass would be expected to have already sunk through the asthenosphere, and so the Isabella anomaly was suggested to be a cool tail left in the wake of the removed lithosphere, though its westward displacement was difficult to explain in this context. Horizontal flow predicted by absolute motion of the Sierra Nevada/Great Valley block to the west (Zandt, 2003; Argus and Gordon, 1991; Wernicke and Snow, 1998), SKS splitting (Silver and Holt, 2002), and global flow models that take into account sinking of tomographically imaged slabs beneath the continental interior (e.g., Becker and O’Connell, 2001) predict that the dense anomaly should be offset east of the high southern Sierra Nevada rather than west. Later, the Isabella anomaly was inferred to include the original mass removed from beneath the southern Sierra Nevada (not just a cold tail) and additionally it was suggested that the instability has not completely detached from North America lithosphere (Fig. 2; Saleeby et al., 2003; Zandt 2004). Receiver function imaging of a dim and/or steeply dipping Moho, dubbed the “Moho hole”, beneath the Sierra Nevada foothills was proposed to mark active delamination that had progressed from east to west beneath the batholith (Zandt et al., 2004). The westward extent of the Moho hole and its correlation with the lateral boundaries of the Isabella anomaly are poorly known because temporary seismic deployments have focused on the Sierra Nevada. There is no broadband survey across the Isabella anomaly itself. Efforts to investigate the dynamics of lithospheric removal beneath the batholith have lead to a 2-D model of instability evolution since 20 Ma that is broadly consistent with the present position of the Isabella anomaly (Le Pourhiet et al., 2006; Saleeby et al., in press), but the model requires strong a priori assumptions about rheology and omits influences from plate motion and background mantle flow. Necessary assumptions to

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achieve consistency with Isabella include the presence of a weak interface to facilitate east-to-west progression of delamination and tuned viscosity increase with depth to keep the delaminated mass from sinking too rapidly away from the lithosphere (Le Pourhiet et al., 2006; Saleeby et al., in press). Both assumptions are plausible, and perhaps neglecting the background mantle flow field also is reasonable considering the potential uncertainties in flow velocity estimates derived from numerical models driven by density structure inferred from global tomography. The circumstance that the area of poorest structural constraint is precisely the area where active detachment is predicted to be occurring and the existence of a dynamically viable alternative hypothesis (the Monterey fossil slab) for the origin of the Isabella anomaly make the lack of observational constraints on structure between the Sierran foothills and coast a strong impediment to progress in understanding regional lithospheric dynamics.

4. Aseismic creep and deep crustal tremor on the central SAF If a Monterey fossil slab exists, does this legacy of plate boundary re-organization affect present-day plate boundary dynamics? North of Cholame and south of San Juan Batista the SAF dominantly deforms by aseismic creep and shows indications of anomalously low shear stress (Titus et al., 2006; Zoback 1987; Hickman and Zoback, 2004; Provost and Houston, 2001, 2003). This creeping section of the SAF coincides with the along-strike extent of the Monterey microplate to the west and the Isabella anomaly to the east. If the Monterey slab hypothesis is correct then dehydration and thermal insulation provided by the slab may be important basal boundary conditions on the central SAF (Pikser et al., 2012). Ongoing slab dehydration could be a fundamental contributor to mantle-derived fluids found in the SAF system and in San Joaquin oil fields (Kennedy et al. 1997; Wiersberg and Erzinger, 2008) and the presence of talc-bearing serpentinized ultra-mafic rocks along the creeping section of the SAF (Moore and Rymer, 2007). Fluid

Driving questions regarding the rootless southern Sierra Nevada and its relation to the Isabella anomaly: a. Is the Isabella anomaly structurally continuous with overlying

lithosphere east or west of the San Andreas fault? b. Does the Isabella anomaly have a sharp upper interface indicative of

delamination along a localized interface? Or is it more consistent with a coalescing (internally deforming) Rayleigh-Taylor drip?

c. What is the western extent of the “Moho hole”? Is it the result of a dim

contrast, dipping interface, or both?

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input to the lower crust could create the near lithostatic pore pressure conditions necessary for tremor triggered by small stress perturbations such as tidal loading and teleseismic earthquake waves (Peng et al., 2009; Thomas et al., 2009). Fulton and Saffer [2008] suggested underplated serpentinized mantle as the source of fluids in the central SAF system, and while this hypothesis is plausible similar post-subduction conditions should exist along the entire length of the SAF, and hence this line of reasoning cannot explain the limited along strike extent of the creeping segment.

4. Hypotheses Hypothesis 1. A fossil slab is attached to the Monterey microplate, it extends landward beneath the SAF, and it is the primary origin of the Isabella high-velocity anomaly beneath the Great Valley (Fig. 2). This fossil slab has remained essentially intact through >300 km of right lateral displacement, demonstrating a high effective viscosity contrast between young ocean lithosphere and the surrounding asthenosphere and a weak interface separating it from the base of North America. New opportunities to place quantitative bounds on these rheological parameters will be afforded by detailed constraints on fossil slab morphology. Proximity of the fossil slab to deep crustal tremor on the central SAF will be constrained. Lithosphere removed from beneath the southern Sierra Nevada between ~10-4 Ma has already sunk away from the base of North America and the detached volume may have been smaller than the Isabella anomaly. Hypothesis 2a. A fossil slab attached to the Monterey microplate is truncated beneath or just west of the SAF and translates coherently with the coastal block/Pacific plate (Fig. 2). This small fragment of underplated fossil slab stifled “window” volcanism between the Monterey microplate and the SAF. Convective loss of the southern Sierra Nevada batholith’s mafic root is the origin of the Isabella anomaly, and this ongoing removal process extends westward to at least the axis of the Great Valley. New seismic imaging will constrain the western edge and internal structure of the Isabella anomaly, the extent of the disrupted/dim Moho, and determine whether or not Isabella is delaminating from east to west along a localized failure surface.

Driving question regarding along-strike heterogeneity in SAF dynamics: Does a fossil slab provide a localized source of fluid input at the base of the central SAF?

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Hypothesis 2b: A fossil slab is truncated nearer to the coast than the SAF and thus cannot explain the gap in “window” volcanism. This would demonstrate that volcanism was not ubiquitous upon slab removal from the base of the fore-arc, and suggest stronger upper plate control on post-subduction coastal volcanism. Otherwise this hypothesis is identical to 2a. Note: Several refinements of 1 and 2 are conceivable, but the essential distinction this experiment seeks to make is whether or not a continuous east-dipping slab interface extends from the coast to the Isabella anomaly across the transform plate boundary. 5. The need for a dense broadband array study The existence of a large seismic anomaly beneath the Great Valley has been recognized for about three decades, but its tectonic origin remains unclear because there is insufficient data for scrutiny of lithospheric-scale structure in the corridor between the coast adjacent to the Monterey microplate and the Sierra Nevada foothills. In southwestern North America the combination of EarthScope, NARS, and regional observatory data (SCSN, NCSN) has recently provided a large-scale smoothed view of upper mantle structure along the transform margin (Wang et al., submitted to PNAS). This view motivated the hypothesis that captured fragments of the former Farallon have fossil slabs that dip into the asthenosphere and translate coherently with the Pacific plate after subduction termination (Pikser et al., 2012; Wang et al., submitted to PNAS). In the corridor from Monterey to the Sierra Nevada foothills, the current mean station spacing is similar the 70 km coverage provided by the EarthScope’s Transportable Array. A different scale of data collection is necessary to robustly identify the tectonic origin of the Isabella anomaly and test the fossil slab hypothesis. A broadband profile with less than ~10 km station spacing can determine whether Isabella is a relict subduction zone or an instability of North America lithosphere. Passive source imaging with dense broadband array methods has successfully identified and characterized the properties of subducted oceanic (Fig. 3; Rondenay et al., 2001; Audet et al., 2009; Kim et al., 2010, 2012) and continental (Pearce et al. 2012) crust. Such a profile extending from the coast across the San Andreas fault, directly over the middle of the Isabella anomaly, and ending in the Sierra Nevada foothills is capable of discriminating between the hypotheses given above (section 4; Fig. 2). In areas with only TA data it is not possible to obtain robust images of the Juan de Fuca-Gorda slab interface. Determining the presence or absence of a slab interface through strongly heterogeneous crustal structure including basins like the Great Valley is feasible with a dense profile. Broadband deployments with similar density to the array proposed below (section 6) have successfully imaged crust and upper mantle interfaces in regions with complex shallow structure including deep basins and young volcanic fields (Figure 3). For example, in Mexico the transition from the flat to steeply dipping Cocos slab beneath the region of Mexico City was resolved by Kim et al., [2010, 2012], and the normal and shallow dip segments of the Nazca slab interface were successful imaged through basins and young volcanic fields of the Peruvian Andes by Philips et al., [2012] and Philips and Clayton, [submitted to GJI].

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6. Proposed seismic array We propose to deploy 35 broadband seismometers for 18 months. The two basic elements of the array are a dense line and several perimeter stations (Fig. 4). The linear array will be composed of 29 seismometers spanning 210 km, with a nearly uniform spacing of ~7 km. The line will extend from the California coast near the middle of the along-strike extent of the Monterey microplate landward across the SAF and the center of the Isabella anomaly with the eastern terminus of the line in the Sierra Nevada foothills. In this corridor of California it will be possible to achieve excellent array geometry for scattered wave migration imaging because only minor deviations from linearity will be required to facilitate nearby road access. The second element of the array is composed of 6 stations that will be deployed in a perimeter 40-50 km away from the line. These sites will be strategically placed to fill gaps in regional observatory coverage (Fig. 4), and provide good sampling for noise correlations, and teleseismic body-wave and surface wave studies. The eastern end of the array in the Sierra Nevada foothills will provide a seamless connection to prior temporary arrays in the Sierra Nevada (Boyd et al., 2004; Zandt et al., 2004; Frassetto et al., 2011; Gilbert et al., in press). The instruments at each site will consistent of a Reftek 130 datalogger, Guralp 3T broadband seismometer, and GPS clock. Caltech owns this equipment and it is specifically available for the proposed deployment. As with prior experiments using these Caltech instruments, the data collected will be available to the community following the standard NSF/IRIS 2-year embargo period. 7. Passive source imaging methods The principal structural seismology objective is to identify the Isabella anomaly’s connection to the overlying lithosphere and specifically whether it is contiguous with the lithosphere east or west of the SAF. To accomplish this we will work iteratively between scattered wave migration imaging and tomographic inversions to create a detailed fully consistent structural model beneath the dense line, and its projection across prior temporary arrays in the Sierra Nevada (Zandt et al., 2004; Frassetto et al., 2011). A complementary larger-scale context will be obtained by incorporating the new array data into regional-scale tomography and receiver function imaging and compilations of anisotropy measurements that employ the cumulative regional dataset. Teleseismic P-wave scattering We will use both traditional single-station receiver function estimation methods (Langston, 1979; Ligorria and Ammon, 1999), and a multi-channel methodology that estimates the three-component teleseismic Green’s function (Mercier et al., 2006; Hansen and Dueker, 2009; Schmandt et al., 2012). Initially these will be used to create common conversion point images, which will identify most major structures with dip angles less than about 40 degrees (e.g., Kim et al., 2010). The teleseismic Green’s functions will be further used for more rigorous migration imaging based generalized radon transform (GRT) (e.g., Rondenay et al., 2001; Kim et al., 2010, 2012) and reverse-time migration (RTM) imaging methods (Shang et al., 2012), which can accurately image interfaces with

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steeper dips and truncations. Use of tomography constraints on smoothed velocity structure will help to focus scattered wave imaging and maximize use of teleseismic P-wave bandwidth (e.g., Shang et al., 2012). Salient features revealed by scattered wave imaging will be scrutinized with elastic finite-difference forward modeling. Surface-wave tomography The accuracy of scattered wave imaging of interfaces is in part controlled by the validity of the background velocity model used to migrate the scattered wave-field back down into the imaging volume. Large lateral variations in crustal velocity are expected because the array crosses the SAF and the Great Valley. Construction of a high-resolution velocity model will be achieved with broadband surface wave dispersion measurements using surface waves isolated by ambient noise interferometry (e.g., Bensen et al., 2007; Lin et al., 2008) and those from teleseismic earthquakes (Forsyth and Li, 2005; Yang and Forsyth, 2006). Noise interferometry with the proposed array will allow use of Rayleigh waves at periods as short as 3-4 sec (e.g., Calkins et al., 2011), thus providing good control on strongly heterogeneous crustal velocities. Receiver functions from the new stations and active source constraints will be used to create an accurate parameterization of sedimentary basin structure (Fliedner et al., 1996; Wentworth et al., 1984; Holbrook and Mooney, 1987), and Rayleigh wave ellipticity measurements (H/V ratio) will be used to better constrain Vp/Vs in the shallow crust (Lin et al., 2012). Teleseismic body-wave tomography P and S tomography models of the southwestern U.S. will be updated (Schmandt and Humphreys, 2010) with new data from the proposed array. Parameterization will be locally adapted to make optimal use of the dense station spacing. These models will damp deviations from an a priori crustal model derived from surface wave tomography. This is straightforward for Vs, but Vp/Vs will require re-scaling of the Vs models from surface waves. Where feasible we will use Vp/Vs constraints from Rayleigh wave ellipticity (in sediment layer) and free-surface multiples of teleseismic P waves for an estimate of integrated Vp/Vs through the crust. Where these types of constraint are not available or highly uncertain reference values will be used and we will test model sensitivity to the choice of the reference value (experience suggests that realistic changes usually have little effect on the morphology of structures in the tomography models). Sp receiver functions Compared to teleseismic P waves, S waves are longer period (low SNR at periods < 5 sec), and a more restricted event distance range is suitable for scattered wave analysis (Wilson et al., 2006). However, analysis of Sp scattering has the advantage that forward-scattered P-waves arrive as precursors to the much larger amplitude S-wave are not complicated by interference from free-surface multiples. We will calculate Sp receiver functions for the array using the multi-channel spectral deconvolution method of Hansen and Dueker [2009] and construct CCP images, which will be useful for detecting deeper and more gradual velocity gradients such as the base of the putative young slab’s CBL. In areas of dense data coverage Sp receiver functions can image sharp contrasts across tectonic boundaries and provide a valuable complement to Ps analysis (e.g., Hansen and Dueker, 2009; Lekic et al., 2012). Through ongoing collaboration with K. Dueker’s

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group at University of Wyoming, Schmandt has access to Sp receiver functions for all existing broadband stations in the region. SKS splitting Analysis of teleseismic SK(K)S splitting will be used to evaluate the width and depth of the transform boundary deformation zone (e.g., Platt and Becker, 2010; Titus et al., 2007) and to test the viability of multi-layer lithospheric models beneath the coastal block (e.g., Bonnin et al., 2010). We will use the code package of Wustefld et al. [2007]. Splitting parameters derived from data filtered in lower and higher frequency bands will be compared to better constrain the origin of the anisotropic signals (e.g., Wirth and Long, 2010). Forward modeling Salient features identified by construction of scattered wave images will be further tested with elastic finite-difference forward modeling (e.g., Yan and Clayton, 2007; Kim et al., 2010). Probable targets include truncated edges of discontinuities and steeply dipping discontinuities. This approach will be used to establish the resolution limits of the migration imaging for features important to hypothesis testing, and to evaluate the consistency of Ps and Sp receiver functions constraints. Particular emphasis will be on constraining uncertainties regarding interface dip, sharpness, and continuity by generating forward models that sample a small parameter space. 8. Work plan Site locations will be confirmed July-September 2013. Array installation will be in October 2013. After installation the sites will be visited every 3-4 months. Instrument removal is scheduled for March 2015. Data will be archived at UNM, Caltech, and also delivered to the IRIS DMC (for open use beginning 2 years after the array is removed). Schmandt will take the lead on surface-wave tomography, body-wave tomography, Sp receiver functions, and shear-wave splitting. Clayton will lead GRT and RTM imaging with teleseismic P-waves. Schmandt and Clayton will work together on forward modeling efforts, which will be tailored to test significant features revealed by batch processing methods. A UNM graduate student will focus on surface-wave (noise interferometry + earthquake) and teleseismic body-wave tomography. Schmandt will conduct Ps and Sp receiver function analysis and he intends that much of the shear wave splitting work will be completed through a UNM undergraduate thesis project. A Caltech student will focus on GRT and RTM imaging with teleseismic P-waves, which will employ tomography constraints on smooth velocity structure. During years 2 and 3 the Caltech and UNM groups will meet in person at least twice per year and will be in frequent remote communication. One group meeting will occur at each Caltech and UNM, and other in-person meetings will be coordinated with conferences (AGU, SCEC, or IRIS). Visits to each institution will give students valuable opportunities to give presentations and engage in detailed discussions of their research outside of their home institutions.

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9. Broader Impact The proposed project will have fundamental implications for understanding of plate boundary evolution, continental lithospheric instabilities, and the potential importance of mantle derived fluid inputs for controlling crustal fault dynamics. The project will advance the development of a new geophysics group at UNM by supporting a beginning-career PI, a graduate student, and an undergraduate researcher. Expansion of geophysics research and teaching at the state of New Mexico’s largest institution has outstanding potential to attract under-represented minorities to opportunities in the geosciences. Caltech funding will support a graduate student researcher and provide opportunities for undergraduate and minority involvement through the SURF and MURF programs. The experiment will also develop a new collaboration between UNM and Caltech. Schmandt plans to initiate outreach activities with public schools in Albuquerque with the expectation that he and the UNM graduate student working on this project will participate twice per year. This participation will include presentations about the geological formation of well-known features in New Mexico, and what it’s like to be a scientist. These efforts will build upon experience with the well-developed outreach activities coordinated by Caltech. 10. Prior support B. Schmandt is a beginning career investigator. R. Clayton, EAR-1045683, Collaborative Research: The Peru Subduction Zone Experiment (PeruSE), a Seismic Investigation of the Role of Water in the Lithospheric Dynamics of Subduction Zones, $172K, 03/01/2011-03/01/2013. The flat-to-normal slab dip regime in southern Peru is imaged with dense linear arrays of broadband stations. The key discoveries are a pervasive mid-crustal interface beneath the high Andes that is interpreted as underthrusting of the Brazilian Shield, and a continuous transition between steep and shallow subduction (rather than a tear) despite the abrupt change in slab dip. This study has so far resulted in three publications: 1) Phillips et al, 2012, 2) Phillips and Clayton, 2012, submitted to GJI, and 3) Ma et al, 2012, in revision JGR. The research from this project is the basis of a PhD thesis, and a major part of another. The PI led a class trip (with private funding) to the field area (southern Peru) that included 6 undergraduates and 12 graduate students. The data from this experiment will be sent to the IRIS-DMC two years after the experiment is ended. Parts of the data are being shared with two other groups conducting seismic surveys in this region and with the Institute of Geophysics in Peru.

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124˚ 122˚ 120˚ 118˚ 116˚ 114˚ 112˚ 110˚

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Figure 1. Alignment of the Monterey microplate, gap in coastal block volcanism, and the east-dipping Isabella high-velocity anomaly (IA). The VS tomography model was created by iterative inversion of Rayleigh wave and S-wave data (Wang et al., submitted PNAS). Volcanic ages are from the NAVDAT database (Walker et al., 2004). The location of the cross-section (lower right) is shown on the VS maps at 70 and 100 km depth.

Joint Rayleigh & S-wave VS tomography

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Figure 2. Schematics structures predicted by the two leading hypotheses for the origin of the Isabella anomaly. The extent of the proposed broadband array is shown on both, note that actual station spacing would be about twice as dense as depicted(~7 km). The key testable difference between the two hypotheses is whether a continuous subducted slab interface extends from the coast beneath the San Andreas fault and connects into the Isabella anomaly under the Great Valley. Regardless of specific outcome the landward extent of Monterey ocean crust and the Isabella anomaly’s connection to overlying lithosphere will be constrained.

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Examples of what can be achieved with a dense broadband profile

MASE Central Cascadia

Northern Cascadia (CAFE)

Figure 3. Prior results demonstrating detectability of subduction zone interface structure. Top left – The Middle America Subduction Experiment (MASE) imaged the flat Cocos slab landward of Acapulco and its transition to a steep dip beneath the Trans Mexican Volcanic Belt (TMVB). The transition was imaged through the deep basins and young volcanic fields surrounding Mexico City (black star). From teleseismic GRT imaging of Kim et al. [2012]. Top right – Central Cascadia 1993 deployment pioneered use of ~5 km broadband profiles(Nabelek et al., 1993). The moderate dip slab interface is imaged to 110 km depth, from teleseismic GRT image of Rondenay et al., [2001] and Bostock et al., [2002]. Bottom – Northern Cascadia profile from Cascadia Arrays for EarthScope (CAFE). Combination of dense profile and perimeter stations allowed high-resolution surface wave imaging and teleseismic GRT imaging, from Calkins et al., [2011] and Abers et al., [2009]. The array proposed here and nearby permanent broadband stations will provide comparable opportunities.

B B’ VS

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Proposed array

TA + Sierra Nevada PASSCAL arrays Active regional stations

Figure 4. Proposed broadband array and regional broadband stations. Top – The proposed array is composed of a dense line and 6 perimeter stations. The line nearly follows the cross-section line shown in Figure 1 and crosses the Isabella anomaly, gap in coastal block volcanism, and creeping section of the SAF. The perimeter stations fill holes in coverage by currently active regional network stations (lower right). Lower left – Broadband stations of the TA, regional observatories, and the 1997 and 2005-07 Sierra Nevada PASSCAL deployments. Position of the Isabella anomaly at 100 km depth from the joint Rayleigh and S-wave inversion (oval), the gap in coastal block volcanism (rectangle), and the creeping section of the SAF (dashed red line) are shown for reference. Lower right – Broadband stations that are currently active and will be complementary to the proposed temporary array data.

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