Results of Prior NSF Support G. J. Axen, PI EAR 9706432, $130,000, 7/1/97 to 6/30/01 (the project is in its second extension period) Effects of the Ballenas transform on the evolution of the Bahia de San Luis Gonzaga rift segment, Baja California (), The south end of a rift segment adjacent to the Ballenas transform originated as a west-vergent segment (from ~20-13 Ma) in the Gulf extensional province, as inferred by Axen [1995]. This segment was probably initially oblique to the rift margin and was subsequently overprinted by the passage of the Tiburón spreading center and by transform tectonics, as proposed by Stock [2000]. Field work in 2001 will further test this history. This grant supported an M.S. thesis [Parkin, 1998] and postdoctoral work by Ben-Fackler Adams. Fackler-Adams’ subsequent strongly teaching-oriented position (Skagit Community College) has slowed publication but two manuscripts are in preparation; one will be submitted in February, 2001. Also supported is Axen’s work on palinspastic reconstruction from the central Gulf to central California [e.g., Axen, 2000; Axen, 1999; and four other abstracts]. A. J. Harding and G. M. Kent, PIs OCE-9633774, $985,338, 1/1/97-12/31/99 ARAD 3-D Seismic Experiment The ARAD 3-D Seismic Experiment is an international collaborative project between investigators at IGPP/SIO and the University of Cambridge. The ARAD 3-D Seismic Experiment was conducted aboard the R/V Maurice Ewing during September-October of 1997, and was centered over the archetypal 9°03’N overlapping spreading center (OSC), East Pacific Rise; key elements of this survey included: (1) the first 3-D reflection survey of a mid-ocean spreading center, and (2) a coincident 3-D crustal tomography experiment. The images generated thus far have provided considerable insight into crustal structure and melt dynamics beneath this enigmatic feature. The observed distribution of crustal magma accumulations beneath the overlapper appear to be inconsistent with either a simple, broadly symmetrical structure for the OSC, or with models which depict the limbs of the OSC as attenuated ends of magmatic systems fed largely by horizontal flow of melt from distant sources. Peer-reviewed publications resulting from this work include Kent et al. [2000]. W. S. Holbrook, PI OCE-9302477, $123,452, 5/1/93–10/31/96 Seismic and thermal structure of gas hydrate deposits, Blake Ridge and Carolina Rise This award provided support for analysis of vertical-incidence and wide-angle ocean-bottom seismic data acquired in 1992 aboard the R/V Cape Hatteras, plus geothermal measurements acquired in 1991 and 1992, on the Blake Ridge and Carolina Rise. Traveltime inversion and amplitude-versus-offset of wide-angle data demonstrated that (1) P-velocities above the BSR are relatively low (1.9 km/s), suggesting low concentrations of hydrate; (2) strong lateral variations in BSR character are not associated with any changes in overlying velocity structure; and (3) free gas is present beneath the BSR. Waveform inversion of wide-angle data from the Blake Ridge and Carolina Rise showed that the BSR is caused by a combination of concentrated hydrate overlying free gas , a prediction that was later verified by drilling. These results provided important site survey information for the Leg 164 drilling. Peer-reviewed publications include:. Katzman et al. [1994], Korenaga et al. [1997].

D. Lizarralde, PI OCE-002417, $106,718 4/01 - 3/03 Oceanic upper mantle seismic structure from very large offset refraction measurements This project is designed to image upper mantle structure using airgun shooting techniques. The project is current, the cruise is scheduled for June 2001, and so no results yet exist for this project.

P. J. Umhoefer, PI EAR-9526506, $240,000 1/96 – 12/98 Active tectonics of a young oblique-rifted continental margin, Loreto area, Baja California Sur, Mexico We mapped a large area of the coastal belt that defined the 90-km-long Loreto rift segment. Segmentation formed as purely normal faulting in the late Miocene. The Loreto segment was modified in Pliocene time by strike-slip and continued normal faulting including the Loreto basin. Late Quaternary fault scarps and marine terraces indicate continued low level faulting along parts of the segment. A small basin on Carmen Island suggests a link between the Loreto fault and transform faults with Carmen Island rotating clockwise within the fault array. Publications include Dorsey and Umhoefer [2000] and Umhoefer et al. [In press].

C1 Collaborative Research: Seismic and Geologic study of Gulf of California Rifting and Magmatism

Co-Principal Investigators: D. Lizarralde Georgia Institute of Technology (GT) G.J. Axen University of California, Los Angeles (UCLA) G.M. Kent and A.J. Harding Scripps Institute of Oceanography (SIO) W.S. Holbrook University of Wyoming (UW) P.J. Umhoefer Northern Arizona University (NAU) International collaborators: J.M. Fletcher and Centro de Investigación Científica y de Educación A. González-Fernández Superior de Ensenada (CICESE)

INTRODUCTION Rifting of continental lithosphere is a fundamental process in the growth and evolution of continents, and it is one that has substantial societal relevance by virtue of the global petroleum reserves accumulated within basins formed through rifting. Rifting proceeds from the application of extensional stress to the accumulation and localization of strain until the lithosphere ruptures, whereupon seafloor spreading and production of oceanic lithosphere accommodate most extension. Continental breakup thus constitutes a dramatic expression of two fundamental geological processes: deformation and magmatism. Yet first-order questions exist about every process in the rift-to-drift sequence. We lack a full understanding of both the magnitude and cause of the stresses that drive rifting, the deformational mechanisms by which continental lithosphere responds to those stresses, and the key parameters that control this deformation. Similarly, the role of the rift-related magmas in localizing strain and advecting heat from asthenosphere to lithosphere is poorly understood, as are the controls on mantle melting during extension (e.g., mantle temperature, volatile content, small-scale convection). Understanding these processes is a fundamental goal of the Rupturing Continental Lithosphere (RCL) initiative of NSF's MARGINS program. In this proposal we suggest an integrated geological and seismological study that will address these issues by determining the spatial and temporal patterns of extension and magmatism in the Gulf of California, which has been identified as a MARGINS focus site. Together with other proposed and planned studies, our results will allow an assessment of the variation in extensional patterns as a function of such key parameters as lithospheric strength, lithospheric and asthenospheric temperature, magmatic input, and strain rate. Numerous observational and theoretical studies of continental rifting have identified general rift types and some likely deformational mechanisms associated with them. Continental rifts can be divided morphologically between narrow- and wide-rift end members, with the East Africa Rift and Basin and Range as examples, while the principle models of extension are pure and simple shear [e.g. Buck et al., 1988], though these do not necessarily correlate directly with morphological styles. Numerical models of lithospheric extension, which typically invoke a brittle upper crust, ductile lower crust, and brittle upper mantle [e.g. Ruppel, 1995], have helped to define conditions under which wide or narrow rifts can develop. Narrow rifts develop under a variety of conditions as a response to a necking instability [e.g. Zuber and Parmentier, 1986; Braun and Beaumont, 1987; Bassi, 1991]. Necking, or the localization of strain, can occur through a variety of mechanisms, from very simple plastic deformation to more complex patterns in brittle layers [e.g. Dunbar and Sawyer, 1989; Lavier et al., 2000], and necking can proceed to rupture with or without lower crustal flow, which may dramatically affect observables such as the crustal thinning profile and subsidence history [Hopper and Buck, 1998]. Necking is a manifestation of pure shear deformation (though this terminology is imperfect if significant lower-crustal flow occurs) and generally leads to symmetric patterns of thinning. Simple shear extension involves strain along crustal- or lithospheric-scale low-angle normal faults and leads to asymmetric rift structures [e.g. Wernike, 1985; Lister et al., 1986; Axen, 1992], but it may produce wide or narrow rifts. Simple shear may be important in the development of wide rifts, based on geological observations of large-offset upper-crustal low-angle normal faults, core complexes, and synthetically dipping normal faults over broad zones in wide rifts such as the Basin and Range [e.g. Wernicke, 1981; 1985]. Observational and theoretical studies of the role of simple shear in extension and the development of wide rifts remain inconclusive. Numerical wide rifts tend to require high geotherms and/or lower-crustal layers with weak rheological properties (e.g. quartz) [e.g. Kusznir and Park, 1984; Buck, 1991; Bassi, 1995; Hopper and Buck, 1996] that may not be characteristic of continental lower-crust composition, which is generally mafic [Rudnick, 1995; Christensen and Mooney, 1995]. Numerical studies have shown that lower crustal flow can produce asymmetries [e.g. Block and Royden, 1990], ductile shear zones [e.g. Braun and Beaumont, 1989; Hopper and Buck, 1996; McKenzie et al., 2000], and even core-complex faulting in simple parameterizations [Lavier et al., 2000], but again, these models

C2 require very weak rheologies. Crustal-scale observations of these processes are less conclusive still. While some studies show that lower-crustal flow is likely important during rifting [e.g. Block and Royden, 1990; Hopper and Buck, 1998; Karner and Driscoll, 2000], observations of crustal deformation across ancient rifted margins consistently suggest "upper plate" deformation, the so-called upper-plate paradox [e.g. Kusznir, 2000]. These observations led to the long held belief that all rifts were symmetric and deform in pure shear. Moreover, though the uppermost reaches of low-angle detachment faults have been imaged seismically [e.g. Taylor et al., 1999], nowhere has such a fault been imaged on a crustal scale so that the role of these features in accommodating strain can be assessed. It is clear that predictions about the style and mechanisms of rifting based on numerical modeling results and surface geologic observations exceed the crustal-scale observations needed to test them, and that we are currently observation-limited in our understanding of rifting processes. Our best observations of complete rifting at a lithospheric scale come from seismic studies across rifted continental margins. Any thorough study of continental rupture requires high-quality images of the velocity and impedance structure of the crust and uppermost mantle. These images, furnished by detailed reflection/refraction seismic profiles, provide essential information on patterns of deformation and crustal thinning, crustal composition, subsidence history, magmatic additions, and the onset of seafloor spreading. While decades of such observations have largely driven the theoretical and numerical modeling efforts, seismic studies of rifted margins have to date not conclusively addressed the degree of rift asymmetry or the importance of lower-crustal flow and simple-shear extension in the rupture process. This is largely because of the lack of unambiguous conjugate-margin transects enabling analysis of both sides of a single rift. Nowhere on Earth has a high-resolution, deep-penetration seismic reflection/refraction data set been acquired on paired rifted margins segments that are known to have once been exactly conjugate. Existing studies (e.g., the Labrador Sea, the Newfoundland-Iberia rift, the East Greenland-British margins) suffer from one or more limitations: they were either situated on ancient, inactive rifts; acquired with seismic systems of limited resolution; located over voluminous volcanic outpourings that mask the underlying deformational structures; and/or were located on margins where uncertainties in plate reconstructions preclude confidence that images on opposing margins can be restored to a common rift segment. Classification of rifted margins based on crustal seismic observations generally fall between "volcanic" and "non-volcanic" end members. Volcanic margins are characterized by voluminous syn-rift volcanism far in excess of that expected for passive decompression melting of normal asthenosphere. These margins are common [e.g. White and McKenzie, 1989; Holbrook and Kelemen, 1993; Eldholm et al., 2000], yet we lack a complete understanding of their development, particular in linking the thermal/dynamic models required to explain the effusive magmatism with the mechanical models that describe lithospheric deformation. Non- volcanic margins are thus more amenable to comparison with theoretical models of rifting. These margins often exhibit a relatively narrow region across which continental crust thins rapidly, bordered seaward by a wide (~100 km) "transition zone" containing crust of enigmatic origin (Fig. 3) [Dean et al., 2000]. More rarely, a non-volcanic margin may lack this transitional crust and show a simpler, broader zone of crustal thinning that gives way seaward to normal oceanic crust [Horsefield et al., 1993]. Transition-zone crust is relatively thin (2-5 km), highly laterally variable, and often contains strong seismic velocity gradients and high velocities (~7.2-7.5 km/s). The origin of this transitional crust is debated, with hypotheses including thin continental crust [e.g. Whitmarsh et al., 1990], thin, slow-spreading oceanic crust [e.g. Whitmarsh and Sawyer, 1996], or serpentinized upper mantle with little or no volcanic crust [e.g. Pickup et al., 1996; Discovery 215 Working Group, 1998]; the latter hypothesis seems favored by ODP drilling results [ODP Leg 173 Shipboard Scientific Party, 1998]. The cause of the relatively amagmatic transition zone is poorly understood but is thought to be related to a combination of low upper mantle temperatures and/or very slow spreading rate [e.g. Bown and White, 1996]. Non-volcanic margins also often show strong intrabasement seismic reflectors interpreted as low-angle faults accommodating extension, often in the late stages of rifting [e.g. Sibuet, 1992]. The exact role and age of these structures is unclear because of, again, the lack of unambiguous conjugate margin transects enabling reconstruction of the full rift-to-drift history.

The Gulf of California The Gulf of California provides an excellent location to study continental rifting processes, and particularly those aspects outlined in the MARGINS Science Plan, for a variety of reasons: 1) The Gulf is one of the very few actively rifting regions of the world where rift processes can be studied across an entire, complete rift from one rifted margin, across a spreading center, and then across the conjugate rifted margin. 2) The regional tectonic and geologic history, including the history of extension, is reasonably well understood. 3) Reconnaissance wide-angle and multi-channel and seismic profiles exist in the Gulf to guide design of comprehensive experiments [Dañobeitia, et al., 1997; Persaud, et al; 2000]. 4) Spreading segment boundaries are clearly identifiable in bathymetric and potential field data, ensuring that geophysical transects are securely

C3 situated within single rift segments spanning conjugate margins. 5) Extensional styles vary along the length of the Gulf, presumably because of differences in lithospheric strength and other key parameters of rift development, such as sediment input. 6) Continental rifting is active in the northern Gulf and active extension along low-angle normal faults is observed onshore. 7) Geological evidence suggests that this rift is likely non- volcanic (or modestly volcanic), so that we can expect to image rift-related structures unobscured by the thick lava flows that blanket volcanic margins. In addition to these scientific factors, the Gulf is located in a logistically convenient, highly accessible, politically stable, and well-studied part of the world, in a country with first-rate earth scientists, including scientists from CICESE, which is the premier earth science research institute of Mexico and is located on the Baja peninsula. A large contingent of scientists at CICESE are actively engaged in regional research and have established working relationships with U.S. scientists These characteristics ensure that the infrastructure exists to support long-term, coordinated interdisciplinary studies of the sort envisioned by the MARGINS program. For example, the Gulf's proximity to a major U.S. oceanographic institution makes staging a two- ship seismic experiment practical, which, as we will describe below, vastly enhances the return of such an experiment. In large part due to all of these factors, the Gulf of California was selected in January 2000 as one of two focus sites for the MARGINS RCL initiative. This selection makes it likely that the Gulf will host the number and diversity of allied studies that, taken together, promise a quantum step forward in our understanding of rift processes. In addition, most of our group are involved in a proposal to NSF's Continental Dynamics program to study the Baja Peninsula and the role of a possible underlying trapped slab as a driving force for extension. If funded, that project will add value to our proposed study of the adjacent rift. We believe that the crustal-scale transects we propose will constitute a linchpin of future work in this MARGINS focus site. Rifting in the Gulf of California initiated as a response to major plate-margin reorganization. The western margin of what is now the Baja Peninsula was a subduction boundary prior to ~29 Ma, with the Farallon plate descending eastward beneath North America [Atwater, 1970; Stock and Molnar, 1988]. The East Pacific Rise (EPR), separating the Pacific and Farallon plates, intersected the trench offshore of northern Baja California or southern California at about 28 Ma [Atwater, 1989; Atwater and Stock, 1998]. The Pacific- margin was initiated here as a short, but elongating, transform margin that grew by both triple-junction migration and southward ridge jumps that transferred Farallon-plate lithosphere to the . As subduction slowed and then ceased, parts of western North America became coupled to the Pacific plate along the former subduction zone [Atwater, 1989; Stock and Hodges, 1989; Nicholson et al., 1994; Bohannon and Parsons, 1995], driving rift initiation in the Gulf of California, the formation of a new plate boundary there, and the transfer of the Baja Peninsula lithosphere onto the Pacific plate. Once subduction ceased, extension in the Gulf of California began at ~12 Ma, in an east-northeast direction roughly orthogonal to the rift trend [Angelier, 1981; Atwater, 1989; Lonsdale, 1991; Stock and Hodges, 1989; Stock and Lee, 1994; Axen, 1995; Lee et al., 1996; Axen et al., 2000]. From ~12 to ~6 Ma, relative plate motion near Baja California is thought to have been partitioned between dextral faults of the continental borderland near the trench, such as the San Benito-Tosco-Abreojos fault system, and roughly margin-orthogonal extension in and east of the site of the modern gulf [Spencer and Normark, 1979; Stock and Hodges, 1989]. Recent reconstructions [Axen, 2000] suggest, however, that significant oblique rifting probably characterized the gulf since at least ~8 Ma, and possibly throughout its history. Since ~6.5 Ma (based on these reconstructions, on extrapolation of the data of DeMets [1995] northward along the gulf and on the cross-gulf ties of Oskin and Stock [1999]), most plate-margin slip has been concentrated in the gulf, where long dextral transforms link short spreading centers [e.g., Lonsdale, 1989]. This contrasts with previous commonly held ideas, based mainly on southern California geology, that most plate motion has been in the gulf only since ~4-6 Ma. At present nearly all of the modern Pacific-North American relative plate motion is accommodated within the Gulf and the Salton trough [e.g., Lonsdale, 1989; DeMets, 1995; Bennett et al., 1996]. The Baja California-North America spreading rate recorded by magnetic anomalies in the mouth of the gulf accelerated from ~43 mm/yr to ~51 mm/yr at ~0.78 Ma, attaining the nearly full Pacific-North American rate then [DeMets, 1995; DeMets and Dixon, 1999;Lonsdale, pers. comm.]. Significant north-to-south variations in rift-basin morphology exist within the Gulf that probably represent consequences of differences in key parameters, including geotherms, syn-rift sedimentation, and presumably initial crustal thickness. Rifting initiated throughout the Gulf at approximately the same time, and roughly the same amount of strain has accumulated across the various basins north to south [e.g. Axen, 2000]. In the south, for example, Alarcón Basin, which records only Baja California-North America relative motions (Figs. 1 and 2) appears to have necked and begun seafloor spreading at 3.58 Ma [DeMets, 1995]. Most of the extensional deformation here is present below sea-level, including an apparent failed rift within the segment, and the

C4 spreading center is lightly sedimented. In the north, extension in the region of the Delfín Basin has not achieved seafloor spreading [e.g. Nagy and Stock, 2000; González-Fernández, et al., 1999], much of the extensional deformation is subaerial, including an active low-angle detachment fault bounding the extensional province to the west (Fig. 2) [Axen and Fletcher, 1998; Axen et al., 1999, 2000b], and the current locus of extension is heavily sedimented and has scarps and bathymetry that trend N-S [e.g., Fenby and Gastil, 1991] rather than NE-SW. Presumably, the crustal structure within the Delfín basin is like that of the central Salton trough, where a granitic layer is absent and new crust is forming but is made of late Cenozoic sediments intruded, metamorphosed, and underlain by juvenile mafic igneous rocks [Fuis et al., 1982; Fuis and Kohler, 1984; Elders and Sass, 1988; Parsons and McCarthy, 1996]. The seismic structure of the Delfín basin shows strong vertical P- wave velocity gradients and smooth discontinuities, similar to the Salton trough structure (Fig. 5) [González- Fernández et al., 1999]. This segment also features a "failed" rift, the Tiburón Basin. occupies an intermediate position both geographically and in terms of rift morphology. Here, distinct NE-trending overlapping spreading-center grabens are contained within a bathymetrically deep rhombochasm with significant overlying sediment which is intruded by mafic spreading-center magmas [Lonsdale, 1989]. Strain in each of these basins has been partitioned between extension and transform motion, and the extensional strain has been distributed across the rift, with active extensional deformation still persisting all along the western Gulf margin [e.g. Gastil et al., 1985; Umhoefer and Stone, 1996; Umhoefer et al., 1997; Fletcher et al., 2000; Nagy, 2000]. It is likely that arc magmatism played a role in the varying patterns of extension and strain localization in the Gulf. Following intrusion of the Mesozoic arc-related plutons in the Peninsular Ranges, arc magmatism swept eastward into Mexico then returned west, with a largely Miocene volcanic arc being constructed along the present site of the Gulf of California [Hausback, 1984; Sawlan and Smith, 1984; Sawlan, 1991; Martín-Barajas et al., 1995]. Subduction magmatism was shut off from north (~16Ma) to south (~11 Ma) as the offshore triple junction migrated southward [e.g. Sawlan, 1991; Nagy and Stock, 2000]. Nagy and Stock [2000] thus speculate that strength differences due to different extensional and arc-magmatic histories have played a role in creating along-Gulf differences in extensional deformation. It is also likely that rifting in the gulf is "non-volcanic" in the sense described above. Previous studies of ancient non-volcanic rifted margins have produced valuable insights and hypotheses regarding strain partitioning during continental breakup. The identification of similar features (transition zone crust, serpentinite ridges, low-angle intrabasement faults) in the active Gulf of California rift would represent a major opportunity to both decipher the deformational roles and timing of these features, and to better understand the conditions (strain rate, temperature, rheology) necessary for their development, as these conditions vary along the length of the gulf.

SCIENTIFIC OBJECTIVES We propose a seismic experiment and structural geology program that will delineate the geometries and patterns of crustal extension and rift magmatism along three main conjugate-margin transects across the Alarcón, Guaymas, and Delfín Basins, and an east-west profile across the (Figs. 1 and 2). The work proposed in any one of these basins would represent the first crustal-scale transect across truly conjugate rifted continental margins and, as such, would provide a significant step toward understanding the partitioning of strain during continental rupture. Together, these three transects provide the framework for understanding an entire rift system and for characterizing along-strike changes in crustal architecture and rift processes. Because the along-Gulf variations in rift-basin morphology represent consequences of differences in key parameters (geotherms, syn-rift sedimentation, and presumably initial crustal thickness), characterizing patterns of extension and magmatism within these basins will enable us to develop an understanding of basic rifting phenomena and the influence of variations in key rifting parameters. Our scientific objectives follow from the broad goals of the MARGINS RCL Science Plan, which aim to address basic questions of how rifts behave as mechanical systems, how rift architecture evolves during extension, and what processes are important in the transition from rifting to initial seafloor spreading. Many of these basic questions can be framed in terms of competing hypotheses, such as: (a) Continental lithosphere deforms in simple shear, with extension along low-angle detachment faults and lower-crustal flow playing a fundamental role in developing rift asymmetry; or (b) Continental lithosphere deforms through depth- dependent pure shear, with lower-crustal ductile shear zones decoupling crustal and upper mantle deformation and resulting in symmetric extension with the possible exhumation of high-grade lower crust and upper mantle rocks during late stages of rifting. Similar hypotheses exist for the generation of rift magmatism, its role in rifting processes, and other key parameters involved in continental rupture. Each of our scientific objectives, summarized below, implies one or more key hypotheses within the Gulf of California that our work will test by seismically imaging patterns of crustal thinning, deformation features (faults and folds), stratigraphic patterns, crustal velocity structure (a proxy for composition), and ductile strain fabric in the lower crust; and

C5 by quantifying onshore extension through geologic measurements of fault orientation, slip, and the age of deformation. • Determine the spatial and temporal partitioning of strain across the full width and throughout the entire history of the rift. Our current view of lithospheric rupture relates extensional style (e.g. pure versus simple shear, or wide versus narrow rifts) to an interplay of lithospheric strength, crustal thickness, internal buoyancy forces, thermal conditions, magmatism, and strain rate [e.g. Buck et al., 1988; Buck, 1993; Bassi, 1991; Bassi, 1995; Hopper and Buck, 1996; Benes and Davy, 1996]. Spatial and temporal patterns of crustal extension are the primary proxy for the style of lithospheric extension. The work we propose will quantify these patterns of extension in the Gulf of California through (1) a series of seismic transects that will produce detailed pictures of crustal thickness, velocity structure, stratigraphy, and internal deformation throughout the submerged basins and across the margins; and (2) a structural geology program that will synthesize new and existing geologic data on the timing, sense, and magnitude of onshore deformation with the new seismic results. These linked efforts will produce a unified view of crustal extension across the province, including a reconstruction of the entire rift-to-drift evolution, enabling us to test models of continental rifting by assessing the symmetry of conjugate margin segments and the structures that accommodate strain. Hypothesis 1: The crustal thinning profile and structures that accommodate extension, including high- and low-angle normal faults, are symmetric on conjugate sides of a rift. Hypothesis 2: The lower crust behaves ductilely during continental extension and flows in response to lateral pressure gradients. Implied Targets: High- and low-angle faults; upper- and lower-crustal thinning profiles; and crustal velocity structure; ductile strain fabrics in lower crust; continent-ocean boundary; geologic and stratigraphic control on rift timing. • Constrain rift magmatism and its role in strain localization and subsidence. Constraints on syn-extensional magmatism within a rift are key to understanding the mechanics of continental rupture for several reasons. 1) Rift magmatism is an important indicator of asthenospheric mantle temperature [White and McKenzie, 1989] and dynamics [e.g. Boutiller, and Keen, 1999], both of which play a central role in the evolution of rifting. 2) It is likely that rift magmatism itself also directly impacts the mechanics and mode of extension through the rapid vertical advection of heat and the weakening effects of diking. 3) It is essential to measure the volume and distribution of new igneous material in order to properly assess continental crustal thinning [e.g. Lizarralde and Holbrook, 1997]. In addition, the relative roles of mantle temperature and convective processes in generating rift magmatism remains a matter of considerable debate, and the Gulf of California, through this study and focused passive seismic experiments proposed under the MARGINS program, may provide an excellent location to resolve a number of these issues. Geological evidence (e.g., the lack of flood basalts) suggests that the Gulf of California rift is relatively non-volcanic. Paradoxically, the Gulf of California is underlain by one of the largest negative upper-mantle shear-wave velocity anomalies on Earth [e.g. Grand, 1994; Ekström, et al., 1997.; Ritsema and van Heijst, 2000]. If this anomaly is due to higher temperatures, excessive magmatism throughout the Gulf of California would be expected. Possible explanations for this paradox include: 1) the margins of the Gulf contain more volcanic material than is suggested by the surface geology; 2) the upper-mantle Vs anomaly is caused by something other than temperature (e.g., composition, fluids); 3) the upper mantle anomaly is physically separated from the source region for Gulf of California melting (e.g., by a subducted slab); or 4) small-scale convection is essential to the development of volcanic margins, and robust convection requires special conditions not present in the Gulf, such as rifting adjacent to thick cratonic lithosphere. Our data will be a crucial step in resolving this paradox, as rift-related magmatic additions to the crust can generally be distinguished from extended continental crust by higher velocities of mafic mid- to lower-crustal intrusive rocks, and by the characteristic seismic expression of extrusive rocks or igneous rocks emplaced within sediments. Hypothesis 3: Despite low upper mantle shear-wave velocities, continental rifting in the Gulf of California is producing non-volcanic rifted margins. Implied Targets: Crustal velocity structure; transitional-crust velocities; "peridotite ridge" structures; reflection images of basement structure and extrusives; correlation of seismic images with gravity and magnetic potential-field data.

C6 • Relate along-axis differences in extensional style to possible controlling factors, such as the rheology of continental lithosphere, magmatic input, and sedimentation. The Gulf of California apparently has significant along-strike differences in extensional style and rift-to- drift structure. To the north, rifting in the Delfín and Wagner Basins has not proceeded to seafloor spreading but rather is accommodated by broad continental extension and generation of new crust [e.g., Nagy and Stock, 2000]. Farther north in the Salton Trough this new crust lacks a granitic layer and is composed of young sediments intruded, metamorphosed, and underlain by juvenile mafic magmas [e.g., Fuis et al, 1984; Elders and Sass, 1988; Parsons and McCarthy, 1996]. In the southern Gulf, the Alarcón Basin shows clear seafloor spreading anomalies and a well-defined mid-ocean spreading center. Because total extension is relatively constant along the entire length of the Gulf [e.g. Axen, 2000], these variations must be due to other factors, such as differences in initial crustal thickness, lithospheric composition or thermal state (and hence rheology), or the effects of sedimentation. If the apparent north-south variation in extensional style is borne out by crustal structure measurements, then our data will provide a unique opportunity to examine the effects of these parameters on deformation and magmatism during rifting. Hypothesis 4: Despite relatively constant amounts of total extension north to south in the Gulf, oceanic crust increases in width to the south, reflecting fundamental along-axis differences in strain partitioning. Hypothesis 5: Differences in extensional style along a rift reflect variations in composition and/or rheology of the continental lithosphere. Implied Targets: Continental crustal and upper mantle thickness and velocity; location of continent-ocean boundary; lower crust ductile strain fabric; width of oceanic crust; total crustal extension. • Assess the influence of sedimentation on deformation, subsidence, magmatic processes, and the formation of "transitional" crust. The strong variation in sediment input from north to south in the Gulf provides an unusual opportunity to study the impact of sedimentation on subsidence, crustal deformation, and magmatic style during rifting. Rapid sedimentation in the Delfín and Tiburón Basins may have inhibited the formation of "normal" ocean crust, thus preventing the development of a mid-ocean spreading center and contributing to a wide transition zone in which extension is accommodated by a combination of magmatic intrusion and extension of sedimentary crust (see above). Confined rift basins with substantial surrounding topography are not uncommon, and determining the influence of sediment input on margin architecture may help better understand the origin of "transitional" crust on ancient non-volcanic rifted margins. Hypothesis 6: Thick sediments suppress extrusive volcanism, profoundly altering the structure of new igneous crust and creating a type of transitional crust observed near the continent-ocean boundary of the southern Gulf basins. Implied Targets: Crustal velocity structure; sediment thickness; reflectivity structure of sediments and crust. The ultimate goal of our project, then, is to produce data needed to test a number of key hypotheses. To accomplish that, we will determine deformation structures, magmatic additions, crustal thinning profiles, and subsidence patterns in unambiguous conjugate margin segments spanning the entire Gulf of California. These data will enable us to reconstruct the tectonic, sedimentatary, and magmatic histories of the rift from initiation to seafloor spreading. Results from this work, however, will not answer all outstanding questions. Additional types of data are clearly needed, including passive seismic measurements of deeper mantle stucture and state, heat flow measurements across the Gulf, detailed swath bathymetry images, and further geologic work that extends beyond the scope of this proposal. The information resulting from our project will establish an observational framework of crustal architecture that will both address our scientific objectives and form a basis for anticipated future work in this MARGINS focus site.

PROPOSED RESEARCH The overriding goals of this research involve defining the crustal scale patterns and geometries of extensional deformation and magmatism throughout the Gulf of California rift. These goals are best achieved through a linked seismic and geologic program. Marine seismic methods are ideally suited for the detailed imaging of crustal features offshore at a variety of scales. Multi-channel seismic (MCS) images will delineate faults and basement structure in the Gulf and will image Moho in many places. Velocity images derived from wide-angle seismic data recorded on closely spaced (~10 km) ocean-bottom seismometers (OBSs) and onshore seismographs will delineate the large scale structure of the entire crust at a resolution comparable to the instrument spacing, providing estimates of composition and reliable means for distinguishing between extended continental crust and rift-related magmatic additions. Extensional deformation extends onshore as well, and in the Gulf of California this extension was initially normal to the rift as opposed to the present-day

C7 oblique extension. To properly assess the spatial and temporal patterns of extension onshore, we include an onshore/offshore seismic program and a linked structural geology program. In general, the onshore extensions of the instrumented transects will record seismic arrivals with bottoming points mid-way between the offshore sources and onshore receivers, and these bottoming points become progressively deeper as the source/receiver offset increases, resulting in decreasing upper-crustal resolution with distance inland. The history and patterns of upper crustal extensional deformation can be determined onshore from geologic mapping, and are in fact reasonably well known in many parts of the region and have been used to construct our present understanding of the evolution of this rift. A fundamental component of our research program is thus a structural geology component that will incorporate this important body of knowledge into our delineation of structure along the main transects of the experiment. This component includes both synthesis of existing data and new geologic data collection in key regions of interest. Crustal Seismic Experiment Experiment Design. The proposed seismic experiment consists of four main coincident wide-angle/MCS transects crossing the Alarcón, Guaymas, Delfín, and Wagner basins (Fig. 2). As described above, this range of basins has been chosen to capture the along-axis variability in rift processes that is a fundamental component of the proposed research. The Alarcón, Guaymas and Delfín transects follow the present-day flow lines of oblique rifting and cross the active rift centers of these basins. The Wagner basin transect runs perpendicular to the locus of transtensional deformation there and in the direction of extension in this northernmost region where, to the west, active extension is accommodated along the listric Sierra San Pedro Mártir fault. This transect thus provides our best opportunity for imaging an active low angle detachment fault and a style of deformation that may have been characteristic in the other basins at earlier stages of rifting. In addition to the wide-angle/MCS data along these main transects, ~800 km of additional MCS profiling will be acquired within each of the three basins. These profiles will focus on imaging the transition between oceanic and extended continental crust and assess three dimensional patterns of faulting and deformation. Our seismic program will build upon results from previous seismic cruises, including a 1978 DSDP Leg 64 site survey which collected MCS profiles within the Guaymas basin [Curry et al., 1982]; the 1996 Spanish CORTES-P96 experiment, the only recent wide-angle seismic experiment in the Gulf of California, which collected several coincident MCS/OBS profiles using a small number of widely spaced OBSs [Dañobeitia et al., 1997]; a 1998 joint SIO/CICESE 1998 that acquired single-channel seismic data across the Alarcón basin [Gonzalez- Fernandez, 2000]; and a high-resolution (short streamer, small group spacing) MCS experiment in the northern gulf that studied fault architecture on a basin-wide scale [Persaud et al., 2000]. The CORTES-P96 experiment is particularly relevant to the proposed work, as that experiment acquired a profile running parallel to our Delfín Basin transect and extending from just west of the Upper Delfín basin to just east of the Tiburón basin (Fig. 2,5). These data were acquired with a small (3000 in3) airgun array and employed only 6 OBSs and 7 onshore instruments. Although of limited resolution, results from these data will are usefull in the design of our experiment. Each of the main transects will be instrumented offshore with OBSs from the NSF OBSIP facility and along onshore extensions with 60 RefTek seismographs from the NSF PASSCAL instrument pool. As described below, the onshore extensions are required to maintain the resolution of the velocity model up to the coast, and provide important, though lower resolution, constraints on average crustal thickness and velocity inland. In particular, the onshore data provide good estimates of lower crustal velocity essential for estimating the composition (and thus rheologic properties) of the lower crust across the margin. The Guaymas basin transect also features conjugate onshore extensions oriented in the rift-normal direction designed to constrain crustal thinning associated with the earliest phase of extension. The Guaymas basin was chosen as the sole location for this onshore component for several reasons. First, mainland tectonic complexity due to proximity to the near Alarcón precludes a rift-normal onshore profile there. Second, the region of onshore extensional deformation near the Delfín transect is broader than near Guaymas and there is no corresponding coastal embayment, requiring a more extensive onshore deployment with diminishing rewards as source receiver offsets increase. Finally, because the resolution of crustal structure onshore will be significantly lower than offshore, it is likely that resolved differences in onshore crustal structure between Delfín and Guaymas rift-normal land profiles will not be significant. Marine Logistics. The experiment will be conducted using a two-ship strategy, with the R/V Maurice Ewing as the airgun source and MCS acquisition vessel and the R/V New Horizon tending solely to OBS operations. This two-ship strategy has two principle advantages. First, because the day-rate cost of the New Horizon is about half that of Ewing; the total cost of the experiment is reduced by using a less expensive ship to handle the non-specialized tasks of launching and recovering instruments. Second, substantial time is saved by a two-ship experiment, enabling the Ewing to acquire additional MCS profiles during OBS launch and

C8 recovery operations. The proponents have experience with two-ship experiments in the Aleutians and Newfoundland basin and are aware of the level of coordination between both ships and crews on shore. We have devised a flexible schedule that enables the order of MCS acquisition and OBS shooting to be switched if necessary and have allotted ample contingency time for both vessels. If operations run smoothly, the contingency time on the Ewing can be converted to additional MCS acquisition (3-days/600 km MCS). Seismic acquisition will start with the Alarcón basin, then move northward to the Guaymas basin, and finally, cross the Delfín/Wagner basins. We have planned for 42-day legs aboard the Ewing and New Horizon, with an additional 2 days of port time in Guaymas for the New Horizon. These operations will be staged out of La Paz, Mexico, with the New Horizon leaving two days in advance of the Ewing to deploy instruments ahead of the shooting ship. The Ewing will commence OBS shooting in Alarcón basin using repetition rates appropriate for refraction work in deeper waters (60-90s) and will then reshoot the transects at MCS firing rates. The New Horizon will recover and redeploy the instruments during the MCS reshoot. This order will be repeated in Guaymas basin, and then the New Horizon will make a two day port call in Guaymas to refuel and supply, causing the Ewing to enter the northern gulf first and begin acquiring MCS data in the Delfín basin area. Once the OBSs are deployed, the two northern instrumented profiles will be shot for both wide-angle and MCS data, as shallow water in this region reduces the deleterious effects of previous shot noise and so both data types can be acquired at MCS shot rates (20 s, or ~50 m). Upon completion the New Horizon will recover, while the Ewing collects the last of the additional 800-km of MCS data. Onshore Logistics. In order to meet our scientific objectives, we will need to determine the velocity structure of the continental crust beneath both Baja and mainland Mexico. To do this, we will deploy approximately 60 portable RefTek seismometers onshore to record the Ewing's airgun shots and thus extend the transects onto unrifted (or only slightly rifted) continent. In designing the onshore-offshore program, we have drawn on experience gained during similar experiments on the South Island of New Zealand, the U.S. East Coast, and Southeast Greenland, which one of our PIs (WSH) was heavily involved in. Data from those experiments has shown that onshore recordings of the Ewing's 20-gun array can produce high-quality phases (including PmP and Pn) out to distances exceeding 200 km. The deployments will be carried out by two crews, one working on Baja and one on the mainland. Each crew will be responsible for the deployment, recovery, data downloading, and re-deployment of 30 RefTeks. Crews will consist of 6 people, divided into two 2-person deployment teams and a 2-person headquarters team (at least one of the 12 people involved will be a PASSCAL engineer). Each deployment team will thus handle installation of 15 RefTeks, which is a reasonable number. Because of the great distance between the three transects, it is impractical to establish a single field center on each coast that would serve deployments on all three transects. Fortunately, the relatively small number of instruments (30) to be handled by each crew makes it feasible to establish separate headquarters for each transect, using motel rooms for data downloading, instrument programming, and battery charging operations. Prior to the experiment we will scout seismometer sites, prepare deployment notes, and identify hotels for field centers on each coast, in towns such as La Paz, Sta. Rosalia, and Mexicali on Baja, and Hermosillo, Culiacan, and Tepic on the mainland. During the experiment the onshore crews will migrate from south to north ahead of the Ewing, establishing new field centers for each transect. The onshore deployments include 15 instruments over 150 km on the east and west ends of the Alarcón transect, 15 instruments over 150 km on the west and 10 instruments over 100 km on the east of the main Guaymas transect, 10 instruments over 100 km in the east and 20 instruments over 200 km in the west on the rift-normal Guaymas transects, 10 and 6 instruments over 100 and 60 km on the eastern ends of the Delfín and Wagner transects, and 10 instruments over 50 km across Isla Tiburón. The shots across Isla Tiburón will be reversed and so the extra density here will enable consistent crustal velocity resolution along the entire Delfín basin transect. The onshore experiment design represents a balance between cost, logistical constraints, and scientific objectives. A more comprehensive characterization of the onshore continental crust (as was conducted, for example, in the New Zealand project) would involve explosive shots onshore and much denser station spacing. However, such a plan is unrealistic (and unnecessary) for several reasons: (1) The drilling, explosives, and large number of instruments needed for an onshore refraction survey would add enormous cost and logistical complexity. (2) At the NSF-sponsored Gulf of California meeting in Puerto Vallarta, our Mexican colleagues strongly urged us not to propose onshore explosive shooting, as that would necessitate involving the Mexican Army. (3) Much of the geological strain that our experiment is targeting occurs offshore or close to the coast (unlike, for example, the South Island of New Zealand, where the deformation is centered on the island). (4) Previous onshore-offshore experiments on rifted margins (the U.S. East Coast [Lizarralde and Holbrook, 1997] and Southeast Greenland [Korenaga et al, 2000]) have shown that a similar design can effectively map crustal thickness and average crustal velocity structure (especially lower-crustal velocity) without onshore shots. (5)

C9 Finally, our work will be coordinated with a project proposed to NSF's Continental Dynamics program that will include similar onshore-offshore imaging on the Pacific side of the Baja Peninsula; if that work is funded, we will have virtually complete images across the entire peninsula. It is important to note that a variety of permits from the Mexican government will be required to complete this experiment. Our colleagues from CICESE have ample experience in obtaining these types of permits (CORTES Experiment, Stock and others high-res MCS), and we will work closely with them. At least 6 months should be allotted for obtaining these permits. The Isla Tiburón requires special permitting, but scientific work has been allowed there and we foresee no difficulty deploying instruments across the island. Also, the preferred season for marine shooting is the summer when marine mammal activity is at a minimum. Data Analysis. The wide scope of the proposed research will produce a large volume of seismic data (1.25+ TB MCS; 200 GB OBS), requiring substantial, coordinated contributions from the participating institutions. Processing of the MCS data will take place at Georgia Tech, the Univ. of Wyoming, Scripps and CICESE. All of these institutions have the facilities to process large seismic datasets, including the software packages Focus, GeoDepth, SIOSeis and SU, SGI, Sun, and HP workstations, and other essential peripherals. Initial MCS processing will follow standard techniques, including deconvolution, dip moveout, velocity analysis, FK-based multiple suppression and alpha-trim median stacking to capitalize on the 6-km-long streamer, and depth migration. These post-stack, migrated images will serve as the foundation for more sophisticated pre-stack imaging schemes that are available through Paradigm Geophysical’s Geodepth. This enhanced processing will be most useful for those regions, such as the transition from oceanic to continental crust, where the overriding structure may be complex enough to prevent or seriously degrade coherent imaging at depth. All MCS/OBH data will be stored and made available on-line, through the IGPP digital library, which is a DLT-based tape-archive that is interfaced through a single Unix mount point facilitated by AMASS software. Analysis of the wide-angle seismic data will involve digital processing, phase interpretation, and an integrated tomographic/prestack depth migration velocity modeling approach. Wide-angle data processing involves a flow of band-pass filtering, deconvolution, range scaling and previous shot noise reduction. Phase interpretation will be aided by the tight instrument spacing, which provides multiple reciprocity relations between records, and by correlation with the MCS migrated images. Initial velocity models will be constructed with a ray-based traveltime inversion that enables detailed shallow structure imaged by the MCS data to be easily incorporated into the velocity model and which also yields an accurate, if coarse, parameterization of deeper structure [Zelt and Smith, 1992]. These initial velocity models will provide starting models for tomographic inversion using the code developed by van Avendonk et al. [1998]. This approach uses a graph-theory traveltime calculation for first-arrival tomography coupled with a regularized inversion for reflector depths, enabling the incorporation of wide-angle reflections into the tomographic inversion. Experience with wide-angle seismic modeling of data from the Aleutians [Lizarralde and Holbrook, 2000] and New Zealand [Holbrook et al., 1998] has shown that wide-angle depth migration of complex reflections from the base of the crust yields important feedback for velocity analysis if low velocity zones are present deep in the crust and can provide meaningful images of localized scattering bodies embedded in the lower crust or upper mantle. Wide-angle prestack depth migration will thus be integrated into our velocity analysis flow. Synergy between velocity models derived from prestack analysis of MCS data and wide-angle techniques will also be explored.

Structural Geologic Investigations The majority of what is presently known about the tectonic and magmatic history of the Gulf of California region, summarized briefly above, is based on many years of careful geologic mapping and analysis. While marine seismic methods have great advantages in imaging deformation on a crustal scale, geologic field mapping has corresponding advantages in the ability to directly study, measure, sample and date particular geologic features. Satellite images of the well exposed Baja Peninsula, coupled with the accumulated geologic knowledge of the area, provide a means to assess the complex three-dimensional patterns of deformation that have resulted from the long history of Gulf of California rifting. The aim of the geologic component of this project is to provide geologic constraints on the timing, sense and magnitude of extension on land. This will be done through integrating existing data with data collected in targeted locations near the main seismic transects. New mapping and structural analysis is critical for developing progressive deformation models of rifting in the Gulf extensional province. Modern plate motion across the Gulf of California is strongly transtensional and right-lateral wrenching exceeds margin-perpendicular rifting by a factor of ~3. Faulting is commonly thought of in terms of en echelon strike slip faults and normal faults associated with pull-apart basins. However,

C10 transtensional shearing produces highly three dimensional strain that can only be accommodated by a minimum of five independent slip systems [Fletcher and Munguía, 2000]. Therefore, kinematic considerations require at least three more fault sets. Detailed structural analysis have shown that faults and striae have a wide spectrum of orientations [e.g., Angelier et al., 1981; Lewis and Stock, 1998; Umhoefer and Stone, 1996; Zanchi, 1994] and that the number of independent slip systems greatly exceeds the minimum of five. Moreover, because shearing is strongly noncoaxial, finite strain axes, along with tectonic blocks and fault arrays, rotate systematically with progressive deformation. The progressive rotation of faults results is changes in the orientation and magnitude of resolved shear stress across that plane. Because faults typically form in orientations that maximize resolved shear stress, progressive rotation typically causes faults to become unfavorably oriented and abandoned. Given the complex nature of trantensional strain, progressive deformation models can only be established through detailed field studies. The characterization of progressive transtension begins with making a thorough inventory of the distribution, kinematics and relative ages of faults where they can be identified onshore. Enhanced thematic mapper (L7) scenes will be utilized to identify fault traces, define offset makers, and focus field studies to examine fault surfaces and characterize fault kinematics. Such information is critical for interpreting two dimensional seismic profiles that will likely traverse non–two- dimensional structures. The structural investigations will be concentrated at the NW and W ends of the main transects in Baja California. Investigations on the mainland side are not planned for the following reasons. In general, the Baja California side of the Gulf is more geologically active than the mainland side, where exposure and relief are low, inhibiting geologic studies. This is particularly true of the E and SE ends of the Wagner and Guaymas transects. The SE end of the Delfín-Tiburón transect is being studied actively by the Caltech group, with whom we have good communications and working relations. The SE end of the Alarcón transect lies in heavily vegetated country where the geologic information yield is lower than our targeted desert areas and is the subject of studies by Mexican research groups. All of the geological collaborators (Axen, Fletcher, and Umhoeffer) have long-standing and ongoing research programs in Baja California. Hence, the greatest progress will be made through work in Baja California. There is strong evidence that east-dipping low-angle normal faulting was important in the evolution of the Baja California peninsula where our northern transects come onshore. There, the rift is in a segment where the dominant transport direction of faults is top-to-the-east [Axen, 1995]. The steep rift flank is bounded on the west by the active Sierra San Pedro Mártir fault (Fig. 2) [Gastil et al., 1975]. This fault is listric and flattens to an unknown dip eastward [Dokka and Merriam, 1982; Stock and Hodges, 1990]. Immediately to the east is Valle San Felipe-Valle Chico, which is occupied by a dextral fault with poor surface expression [Stock et al., 1991]. East of this valley are a N-trending set of low ranges that are separated by ~ENE-striking faults. Some of these faults are known to be sinistral-normal and the southern ranges have experienced variable clockwise vertical axis-rotations (generally ~30°) since ~6 Ma that may have occurred above a low-angle fault at depth [Lewis and Stock, 1998a; 1998b; Nagy, 2000]. At least one of these ranges contains a gently E-dipping normal fault that was active in late Miocene-Pliocene time [Stock et al., 1991; 1996] and another is reported nearby [McEldowney, 1970]. Farther north, low-angle normal faults of Gulf age are common [Axen and Fletcher, 1998; Axen et al., 2000] and locally are probably still active [Axen et al., 1999]. Axen and a student will concentrate on reconnaissance and detailed geologic mapping around the NW end of our northern transect in order to evaluate the locations of low-angle normal faults, the direction and magnitude of slip on them, and their ages. Such data will be key to tying the fault and velocity structure imaged in the seismic surveys to exposed geology of the Peninsula and to evaluating the evolution of strain partitioning in this complex oblique rift. In addition several key samples of relevant volcanic rocks will be collected for 40Ar/39Ar dating of matrix and phenocrysts, providing age constraints on fault slip. Dates will be provided by the New Mexico Tech geochronology lab as an analytical service to UCLA (see letter of support), with which we have worked successfully in the past. The central transects cross onto the Baja California peninsula where Luis Delgado of CICESE is involved in major projects. Fletcher and Umhoefer will coordinate a summary and comparison of the geology onland with the results of the geophysical transect. Umhoefer will work on an initial structural project to fill in areas where L. Delgado has not worked. In the south, Fletcher has an extensive ongoing project on the tectonics of the plate margin on the Baja California peninsula and islands south of the Alarcón transect, and Umhoefer has led a major effort on the tectonics of the Loreto region ~50 – 150 km north of the Alarcón transect. Umhoefer is just starting an investigation of structures and pre- and syn-rift stratigraphy and basin analysis on San Jose Island near the transect with Tobias Schwennicke, a sedimentologist at the Universidad Autónoma de Baja California Sur in La Paz. The geology of the islands near the southern transect is especially key as they offer a window into the

C11 upper crust that can be studied in detail and compliment the geophysical data. The major objectives of the geology program in the south are to fill in the gap along the coast of Baja between La Paz and Loreto where the structural geology is relatively simple [e.g. Hausback, 1984; Schwennicke, pers. comm., Umhoefer reconnaissance], to continue the initial studies of Fletcher and Umhoefer on the islands, and to coordinate and link the databases of Umhoefer and Fletcher from north and south of the transect. Umhoefer and his graduate students will work on three related projects. The sedimentologic and stratigraphic aspects of these will be closely coordinated with Schwennicke (see letter of support, Supp. Doc.). They will study (a) the secondary structures along the relatively straight gulf escarpment near the coast ~50 km north and south of the southern transect, (b) a complex Pliocene to Quaternary(?) basin and the structures in it on San Jose Island (the island that the transect crosses), and (c) the structures in the pre-rift granites and Miocene Comondú Formation on the islands to understand the early rift history. Local stratigraphic studies of pre-rift units have been done near Timbabichi and San Juan de la Costa [Hausback, 1984; Fischer et al., 1995; Grimm, 2000; Plata-Hernández and Schwennicke, 2000; Schwennicke, pers. comm.] along the coast near the transect, but there have been virtually no structural studies. Despite the relatively simple geology along the coast near the transect, Umhoefer’s reconnaissance [and Hausback, 1984; Schwennicke, pers. comm.] has shown that there are numerous secondary faults that must be studied to determine the role of the gulf escarpment in the evolution of the margin. Near Loreto, the rift margin has distinct faults and a typical rift escarpment, but the boundary faults are not as large as typical rift boundary faults [e.g. Bosworth, 1985; Rosendahl, 1987], which emphasizes the need for understanding the structures on the islands and in the marine shelf from MCS where larger faults are likely. The initial week of work on San Jose Island immediately before this submission revealed a complex basin that has numerous microfossils for dating and paleobathymetry, is bounded in part by major oblique-slip faults, and shows syn-depositional faulting and major changes in relative sea level. The basin is larger than we thought and will require 1-2 additional field seasons beyond our work this winter. The project on the structures in the granites and Comondu Formation will be done with Fletcher, who will lead the thermochronology to determine the history of uplift. Granitic rocks are known from all four major islands near the transect but have not been studied and the Comondu is extensive on San Jose Island. Axen is currently creating map-view palinspatic restorations, at times 2, 4, 6.5, and 8-12 Ma, that cover the region from the central Gulf to the central San Andreas fault [e.g., Axen, 2000]. These incorporate all currently available data, including fault offsets, fault ages, neotectonic and geodetic slip rates, vertical-axis rotations, and Baja California-North America spreading rates from the mouth of the Gulf. In this project he will compare the map-view evolution with a compilation of existing and new crustal thickness data to evaluate strain partitioning in three dimensions and through time. Fletcher and Umhoefer will also create palinspastic restorations of the southern gulf that extend the effort begun by Axen on the northern gulf and southern California. These efforts will be joined to create unified palinspastic reconstructions for the entire Gulf.

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18˚N 118˚W 116˚W 114˚W 112˚W 110˚W 108˚W 106˚W 104˚W 102˚W

Figure 1. Bathymetric and topographic map of the Gulf of California and surrounding areas. Red lines indicate major proposed seismic transects within the Alarcón, Guaymas, Delfín and Wagner Basins. Detailed features of the experiment are shown in Fig. 2. The intersection of the East Pacific Rise (EPR) with a subduction zone west of Baja ended subduction there. The EPR then migrated and jumped southward to its current position, transferring the Baja microplate onto the Pacific plate and initiating rifting in the Gulf of California. The Gulf of California is presently an oblique rift, but earliest extension was probably normal to the rift. Total extension is approximately uniform from north to south, but in the southern basins rifting has gone to completion and nearly all of the extension there is taken up by seafloor spreading. In the north, continental rifting is still active and strain is partitioned between extension within the basins and motion along transform faults. Some of the north-to-south differences in extensional patterns may be due to along axis differences in lithospheric strength related to the southward propagating termination of arc magmatism within the Sierra Madre Occidental, the mountain chain on the mainland east of the Gulf.

13 114˚W 113˚W 112˚W 111˚W 110˚W 109˚W 108˚W 107˚W

32˚N 31˚N 30˚N 29˚N 28˚N

115˚W 106˚W

Wagner basin | |

| 116˚W32˚N | | | Delfín Basin 107˚W | Tiburón Isla Tiburón 26˚N | | | Basin | | || Roads | | | | | | | | | | | | | | ||

San Pedro Mártir Fault 31˚N Guaymas Basin 25˚N 117˚W

108˚W

30˚N

116˚W 115˚W 114˚W 113˚W 112˚W 111˚W 110˚W 109˚W 29˚N 28˚N 26˚N

111˚W 110˚W 108˚W 107˚W 105˚W 104˚W

26˚N 25˚N 24˚N 23˚N 22˚N

103˚W

112˚W

Alarcón Basin 26˚N

20˚N

25˚N 112˚W 111˚W 110˚W 109˚W 108˚W 107˚W 106˚W 105˚W 23˚N 21˚N

500 km

Figure 2. Layout of the proposed seismic experiment. White dots represent OBSs and green dots represent onshore seismographs. The experiment involves 70 OBSs and 60 RefTek seismographs. Additional red lines indicate approximate line lengths of MCS shooting that will be performed in addition to the main wide-angle/MCS transects. Final determination of the additional MCS shooting pattern will be made at sea based on real-time stacks of the MCS profiles. Road systems on the mainland are shown for the rift-normal Guaymas Basin transect to indicate feasibility of the deployment. Nominal instrument spacing on all of the transects is ~10 km. Approximate cross-Gulf tie points from Nagy and Stock [2000] are shown as stars on either end of the Delfín Basin transect to indicate the degree of conjugacy that can be expected across each basin. The thin blue line crossing Delfín and Tiburón Basins is the transect of the CORTES-P96 seismic profile (see Fig. 5). The San Pedro Mártir fault is an active low-angle normal fault with extension in the direction of the Wagner Basin transect, and so its expression at depth may be imaged along that transect and with additional MCS profiling. Note that the Alarcón Basin transect crosses an apparent failed rift towards the center of the profile. The Tiburón basin, which has no significant bathymetric expression on this figure, is a similar feature along the

14 Distance [km] 3) -20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 0

5 5.0 10 6.0 7.0 7.0 15 7.9

Depth [km] 20

25

V.E. 5 30 Figure 3. Velocity model across the southern Iberian Abyssal Plain non-volcanic margin, after Dean et al. [2000]. Notice the strong lateral variations in crustal velocity and the wide zone (from model km 80-220) containing velocities of about 7.3 km/s, interpreted as serpentinized upper mantle. Circles show locations of OBSs. This profile contained 13 instruments at an average spacing of about 23 km; our proposed station spacing is 10 km..

4a) 2 4b) 5 20 km 20 km 3 6 TWTT (s) 4 7

LCC 5 TWTT (s) 8

6 9

7 10 W E 8

9 Moho depth predicted by velocity model

10 Figure 4. Example of MCS data acquired by R/V Ewing during the summer of 2000 in the Newfoundland Basin (approx. conjugate to model in Fig. 3). Dashed lines show predicted two-way time of Moho, calculated from coincident velocity model (not shown). (a) The transition from seawardmost reflective lower continental crust to "transitional" crust. (b) Seawardmost part of line. The crust here shows numerous intracrustal reflections (mostly landward-dipping) that extend down to Moho and are likely faults. These are preliminary stacks with water-velocity migration, but they serve to demonstrate that the Ewing's seismic system, with its 6-km streamer and 20-gun array, will produce good deep-crustal images on a non-volcanic rifted margin.

Delfín Tiburón Distance [km] Figure 5. Crustal velocity model for the -70 -50 -30 40 10 30 50 70 90 110 130 150 170 190 210 Upper Delfín and Tiburón Basins based on 0 CORTES-P96 data (see Fig. 2 for line 4.0 location) [González-Fernández, et al., 1999]. 5.2 5 Blue circles are OBSs and green are land 5.8 stations. The velocity structure is apparently continental and shows thinnest 10 6.2 crust beneath the extending Delfín Basin. 6.5 The velocity model resulting from the proposed experiment will extend across the 15 entire rift and will be substantially higher 6.8 resolution because of the greater density of 20 instruments and the more powerful airgun source.

15