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C1 Results of Prior NSF Support 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.
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