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Report on RCL-Cortez Workshop: Lithospheric Rupture in the Gulf of California – Salton Trough Region Rebecca J

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Report on RCL-Cortez Workshop: Lithospheric Rupture in the Gulf of California – Salton Trough Region Rebecca J MARGINS Newsletter No. 16, Spring 2006 Page 9 Report on RCL-Cortez Workshop: Lithospheric Rupture in the Gulf of California – Salton Trough Region Rebecca J. Dorsey1, Raul Castro2, John Fletcher2, Daniel Lizarralde3, and Paul J. Umhoefer4 1Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, USA; Email: [email protected], 2División de Ciencias de la Tierra, CICESE, Ensenada, Baja California, México, 3Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA, 4Dept. of Geology, Box 4099, Northern Arizona University, Flagstaff AZ 86011, USA -115˚ -110˚ -105˚ Introduction 35˚ 35˚ From January 9 to 13, 2006, a group of about 70 professional and student re- Colorado R. searchers gathered at the Hotel Coral in Ensenada, Baja California, Mexico, to discuss the status of geophysical and geo- logical research in the MARGINS Gulf of California – Salton Trough focus site 30˚ 30˚ (Fig. 1). The main goals of the workshop were to summarize emerging new data and results, identify existing gaps in knowledge, and suggest possible direc- tions for future research. Theoretical models and studies of other rifted mar- gins allowed participants to compare and 25˚ 25˚ contrast results from this region. A 2-day field trip, led by John Fletcher and Gary Axen, illustrated a well-studied example of low-angle normal faulting in the La- NARS-Baja Stations guna Salada area, and provided new in- sights into Late Cenozoic extension and Seismic Transects transtensional tectonics in the Salton Faults 20˚ 20˚ Trough (Fig. 2). Spreading Centers Financial support for U.S., Mexican, and other international participants came -115˚ -110˚ -105˚ Figure 1. Shaded relief map of Gulf of California and Salton Trough. Pink shading denotes from the MARGINS and International the area of Tertiary Basin-and-Range extension. programs of NSF. The workshop was presentations, is available at http:// site has been very successful; (2) emerg- sponsored by NSF and CICESE (Centro www.rcl-cortez.wustl.edu/index.html. ing results have generated new questions de Investigación Científica y de Oral presentations were organized that a diverse community of scientists Educación Superior de Ensenada). Work- into thematic sessions, interspersed with believes are important to address; and (3) shop conveners were Rebecca Dorsey, poster sessions and group discussions. this region provides an excellent natural Raul Castro, John Fletcher, and Daniel Each thematic session had a 4- to 5-mem- laboratory for tackling such questions. Lizarralde. Julie Morris, Paul Wyer, and ber working group, which encouraged all Participants recognized many dynamic Meredith Berwick from the MARGINS participants to contribute directly to the links among the topics that were dis- Office provided invaluable support at all talks and discussions. On the final day, cussed, and it was widely agreed that in- levels, from the mundane to the profound. 5-minute “pop-up” presentations distilled terdisciplinary research is needed to Federico Graef, Director of CICESE, and critical “knowns” and “unknowns.” improve our understanding of the com- Enrique Gómez, Director of the Earth Group discussions then explored broader plex systems involved in rupturing litho- Science Division at CICESE, welcomed themes that unify the different topics and sphere and creating a new ocean basin. the group with introductory remarks that point to needs for future work. From this highlighted the current atmosphere of we conclude that: (1) recent work has Summary of Workshop international cooperation and collabora- produced a huge amount of new data and Outcomes tive research. Additional information insights into processes of lithospheric about the workshop, including partici- Below we synthesize the workshop pro- rupture, and thus the first generation of pants list, technical program, working ceedings in order to provide guidance to MARGINS-funded research in this focus groups, and downloadable oral and poster NSF and the community of researchers Page 10 MARGINS Newsletter No. 16, Spring 2006 RCL Photo by D. Lizarralde Photo by M. Berwick Photo by G. Karner Photo by B. Dorsey Figure 2. RCL-Cortez Salton Trough field trip photos (Laguna Salada area). working in this and related areas, and to end of subduction along the western mar- nia indicates that Baja California has been encourage new researchers to become gin of the Baja California peninsula mostly coupled to the Pacific plate since active in the Gulf of California – Salton around 12-14 Ma. Through an incom- ~6.1-6.3 Ma, and ~300 km of relative Trough focus site. Toward this end, we pletely understood sequence of events, plate motion has been accommodated by group the major scientific advances and this resulted in the transfer of the Baja opening in the northern Gulf since that unresolved problems that were discussed California peninsula to the Pacific plate. time. New seismic data from the south- at the workshop into four broad themes: Today the Gulf of California represents ern Gulf reveal ~350 km of opening by (1) Plate-Boundary Kinematics; (2) His- virtually all Pacific-North America (P- extension and seafloor spreading within tory of Strain Localization; (3) Domains NA) relative motion (48 mm/yr). The the Gulf. Additional crustal thinning of Deformation; and (4) Upper-Mantle Baja peninsula presently acts as a rigid across the rifted margins in the south may Processes. We hope these unifying con- microplate, moving slowly (~2-6 mm/yr) increase the total amount of extension to cepts will help stimulate thinking about relative to the Pacific plate. The modern 450-500 km, though the timing of this underlying processes of lithospheric rup- plate boundary in the Gulf of California extension is uncertain. In the Guaymas ture and motivate future research in this and Salton Trough is an oblique rift sys- basin, ~300 km of extension appears to region. Given the volume of material to tem with a large component of dextral have been accommodated by nascent sea- be covered, we cannot individually rec- shear, in which the angle between the floor spreading since ~6 Ma. ognize personal contributions here, but overall trend of the rift and the direction Substantial uncertainty exists regard- we thank all who participated and refer of relative plate motion varies from about ing how and where strain was partitioned readers to the online workshop presenta- 15-20° in the south to 30-40° in the north. in the Gulf extensional province (GEP) tions for more detailed information. The obliquity of the active rift is accom- between ~12.5 and 6.0 Ma. The above modated by short spreading centers observations suggest that the Gulf region 1. Plate-Boundary Kinematics linked by long transform faults (Fig. 1). evolved in two stages, with a pronounced The plate boundary in the Gulf of Cali- The global plate circuit requires up to increase in strain rate in the modern Gulf fornia/Salton Trough region formed in 640 km of P-NA relative motion since at about 6 Ma. It seems that the 300 km response to the southward migration of ~12.5 Ma. Correlation of volcanic cen- of post-6.1 Ma P-NA motion inferred for the Rivera triple junction and subsequent ters across the northern Gulf of Califor- the northern Gulf can be accounted for MARGINS Newsletter No. 16, Spring 2006 Page 11 50 150 250 350 450 550 650 750 850 km formation at ~12 Ma? The view of Gulf of California rifting 10 as related to Basin-and-Range extension motivates an assessment of analogies Sutherland et al. 20 between GEP processes and processes in the Mexican and U.S. Basin-and-Range 30 provinces. New data from Sonora show Alarcon Segment that volcanic episodes tended to imme- diately precede deformation episodes, suggesting that strain becomes localized in areas of magmatic heating and weak- ening of the lithosphere. Comparison of DiLuccio, Clayton and Persaud the late Miocene Gulf of California to the Figure 3. Integrated upper-mantle and crustal cross section from recent work presented at modern Walker Lane belt in western Ne- the RCL-Cortez workshop. See http://rcl-cortez.wustl.edu/Presentations.html vada shows that transtensional deforma- tion navigates around large batholiths in the southern Gulf, though the timing 2. History of Rift Localization (e.g., Peninsular Ranges and Sierra Ne- of extension there is not well known. In Recent structural and geochronologic vada). Both regions reveal a narrow belt the Alarcon segment (the most sub- studies show that there was a long pe- of deformation focused at the eastern merged segment of the Gulf), at least 150 riod (~15-20 m.y.) of Basin-and-Range margin of a large Cretaceous plutonic km of the remaining 350 km of 12.5-6 style extension and magmatism in main- province. These similarities suggest that Ma P-NA motion is accounted for by ex- land Mexico during late Oligocene to a common set of factors and processes tension across the continental margins, middle Miocene time, prior to relocation may be involved in oblique rifting and as imaged by marine seismics (Fig. 3). of the P-NA plate boundary inboard of ultimate rupture of continental lithos- Isostatic and gravity modeling suggest the Baja Peninsula. Discussion about the phere. that similar amounts of extension may factors that may have caused protracted The record of marine deposition pro- have occurred beneath the continental “unsuccessful” extension included the vides useful information about the his- margins of the Gulf, perhaps accounting role of strain rate, slab geometry, ther- tory of rift localization.
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