Published bi-annually by the MARGINS Office A Seismic and Geologic Study of Gulf of Lamont-Doherty Earth Observatory of Columbia University California Rifting and Magmatism 61 Route 9W, P.O. Box 1000 D. Lizarralde1, G.J. Axen2, J.M. Fletcher3, A. González-Fernández3, G.M. Kent4, Palisades, NY 10964-8000 A.J. Harding4, W.S. Holbrook5, and P.J. Umhoefer6 USA 1. School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 221 Bobby Dodd Way, Atlanta, GA 30332-0340, USA 2. Earth & Space Sciences, University of California, Los Angeles, 595 Young Drive East, Los Angeles, CA 90095-1567, USA 3. Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Km. 107 Carretera Tijuana - Ensenada, Código Postal 22860, Apdo. Postal 2732, Ensenada, B.C. México 4. Scripps Institute of Oceanography, San Diego, CA 92124, USA 5. Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006, USA 6. Northern Arizona University, Building 12, Knoles Drive, Flagstaff AZ 86011-4099, USA The Rupturing Continental Lithosphere volcanic, so that rift-related structures can (RCL) initiative of the MARGINS be seismically imaged unobscured by program aims to understand the processes thick lava. In addition to these scientific and controlling parameters of continental factors, the Gulf is located in a logis- rifting. This initiative focuses on basic tically convenient, highly accessible, questions about the stresses that drive politically stable, and well-studied part rifting; the deformational mechanisms of the world, in a country with first-rate responding to those stresses, the role of earth scientists, including scientists from magmatism in localizing strain; para- CICESE, which is the premier earth Newsletter No. 7, FallNewsletter 2001 meters such as crustal thickness, com- science research institute of Mexico and position and temperature that control is located on the Baja California penin- deformational style; and parameters such sula. These characteristics ensure that the as mantle temperature, volatile content, infrastructure exists to support long-term, and small-scale convection that control coordinated interdisciplinary studies of magmatic input. The Gulf of California the sort envisioned by the MARGINS was chosen as one of two focus sites for program and make it likely that the Gulf this initiative (the central/northern Red of California will host the number and Sea is the other) at the RCL TEI in diversity of allied studies that, taken Snowbird, Utah, January 2000 (see together, promise a quantum step forward MARGINS Newsletter no. 5). in our understanding of rift processes. In This Issue: The Gulf of California is an excellent Here we describe an integrated Science Article location to study continental rifting for a geological and seismological study of variety of reasons. It is one of the very Gulf of California rifting that will begin Gulf of California rifting ..........................1-5 few regions of the world where active rift in the Fall of 2002. This program will Editorial processes can be studied across an entire, determine the first-order spatial and Red Sea RCL status, Saudi visit .....................6 complete rift. The regional tectonic and temporal patterns of extension and Workshop reports geologic histories are reasonably well magmatism in the Gulf of California and Central America SEIZE & SubFac ...........7-10 understood, and reconnaissance seismic thus will provide a regional foundation Source-to-Sink Research Program ......11-13 profiles exist to guide future experiments. for subsequent studies in this focus area. EarthScope workshop...........................14-15 Extensional style varies along the length An excellent review of the tectonic News & Announcements of the province, presumably because of history of the region was presented by MARGINS Data Policy .................................16 differences in key parameters affecting Martín-Barajas et al. in MARGINS MARGINS Post-doc ...............................16-17 rift development. Continental rifting is Newsletter no. 6, and so here we will Funded Programs, addendum ....................10 InterMARGINS Office move .......................17 active in the northern Gulf and active focus on describing the design and CSM workshop Announcement.................17 extension along low-angle normal faults principal objectives of our program with EUROMARGINS first CFP............................17 is observed onshore. Spreading segment only a minimum of geologic background. Office Update..........................................18 boundaries are clear in bathymetric and (The full text of the proposal for this Contact Information ...........................19 potential field data, enabling geophysical program is available from the MARGINS transects to be located within single rift web site.) segments. The rift is likely only modestly Page 2 MARGINS Newsletter No. 7, Fall 2001 Science Article The Gulf of California is present below sea-level, and the deformation throughout the basins and Rifting in the Gulf of California initiated spreading center is lightly sedimented. across the margins, and through a as a response to major plate-boundary Further north in the Guaymas Basin, structural geology program that will reorganization. The western margin of distinct NE-trending overlapping spread- synthesize new and existing geologic what is now the Baja California Peninsula ing-center grabens are contained within data on the timing, sense, and magnitude was a subduction boundary prior to ~29 bathymetric deeps with significant over- of onshore deformation with the new Ma, with the Farallon plate descending lying sediment, which is intruded by seismic results. These linked efforts will eastward beneath North America. The spreading-center magmas. In the north- produce a unified view of crustal ex- East Pacific Rise intersected the trench ern Gulf, where extension in the region tension across the province, enabling us offshore of northern Baja California at of the Delfín Basin has not achieved to test models of rifting by assessing the ~28 Ma, initiating the Pacific-North seafloor spreading, much of the defor- symmetry of conjugate margins in terms American plate boundary as a transform mation is subaerial, including active of crustal thinning and structures accom- margin that grew southward by ridge high- and low-angle normal faults within modating strain. jumps, transferring Farallon-plate litho- the extensional province to the west (Fig. sphere to the Pacific plate and shutting 1). The current locus of extension is • Constrain rift magmatism and its role off subduction magmatism from north to heavily sedimented and has scarps and in strain localization and subsidence. south as the offshore triple junction bathymetry that trend N-S rather than migrated southward. As subduction NE-SW. Nagy and Stock [2000] specu- Constraints on syn-extensional magmat- slowed and then ceased, parts of western late that strength differences due to ism within a rift are key to understanding North America became coupled to the different arc-magmatic and extensional the mechanics of continental rupture. In Pacific plate along the former subduction histories have played a role in creating addition to the need to measure the zone, driving rift initiation in the Gulf of along-Gulf differences in extensional volume and distribution of new igneous California and the transfer of the Baja deformation. material in order to assess extensional Peninsula lithosphere onto the Pacific crustal thinning, rift magmatism is an plate. Extension in the Gulf of California Scientific objectives indicator of asthenospheric mantle began at ~12 Ma in an east-northeast Our seismic and structural geologic temperature and dynamics, which play a direction roughly orthogonal to the rift program will delineate the geometries central role in the evolution of rifting. margins, with the remaining component and patterns of crustal extension and rift Rift magmatism also directly impacts the of Pacific-North American relative plate magmatism along three main conjugate- mechanics of extension through advec- motion taken up along dextral faults of margin transects across the Alarcón, tion of heat and the weakening effects of the continental borderland near the Guaymas, and Delfín Basins, and an east- diking. Also, the relative roles of mantle trench. Since ~6.5 Ma, most plate- west profile across the Wagner basin (Fig. temperature and convective processes in boundary slip has been concentrated in 1). The primary scientific objectives of generating rift magmatism remains a the Gulf, where long dextral transforms this program include the following. matter of considerable debate, and studies link short spreading centers, and at in the Gulf of California provide an present nearly all of the modern Pacific- • Determine the partitioning of strain opportunity to study these issues. The North American relative plate motion is throughout the extensional province. Gulf of California is underlain by a large accommodated within the Gulf and the negative upper-mantle shear-wave Salton Trough. Our current view of lithospheric rupture velocity anomaly, which, if due to higher Significant north-to-south variations relates extensional style (e.g. pure versus temperatures, should imply excessive in rift-basin morphology exist within the simple shear, or wide versus narrow rifts) regional rift magmatism. Geological Gulf that probably represent consequ- to an interplay of lithospheric strength, evidence suggests