Report on RCL-Cortez Workshop: Lithospheric Rupture in the Gulf of California

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 geological 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 directions for future research. Theoretical models and studies of other rifted margins 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 insights 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 weakening 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 deformation 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. Studies of marine for up to ~500 km of post-12.5 Ma rela- mal structure, and lithospheric strength. micropaleontology enable us to map and tive plate motion throughout the GEP. The point was made that, while the open- date ancient marine incursions in the Recent studies of volcanic rocks in ing of the Gulf can be viewed as a plate- Gulf-Trough region, which indicate coastal and interior Sonora reveal zones boundary “jump” related to the transfer where and when the Earth’s surface was of intense late Miocene dextral faulting of the Baja peninsula to the Pacific plate, lowered below sea level in response to and clockwise block rotation, strength- it can also be viewed as a localization of crustal thinning. It is now known that a ening this suggestion. strain following a long history of west- narrow marine seaway inundated the en- The location, magnitude and kinemat- ward migrating extension and tire Gulf of California and Salton Trough ics of faults that accommodated plate magmatism. This idea led to discussion by latest Miocene time (~6.0-6.5 Ma), motion during late Miocene time are of how the earlier period of extension with marine deposits as old as ~8 Ma poorly understood. One hypothesis calls may have conditioned the lithosphere and documented in the southern Gulf. Late for regional strain partitioning in which influenced the eventual evolution of rift- Miocene marine incursion appears to much of the dextral motion was taken up ing in the Gulf of California, generating record an abrupt increase in strain rate in by strike-slip offset on the Tosco- a number of questions: How did pre-12.5 the Gulf-Trough region at about 6.0-6.5 Abreojos fault, southwest of the Baja Ma extension and magmatism modify the Ma, but this is a matter of ongoing dis- peninsula, while orthogonal NE-SW ex- strength of the lithosphere? What was the cussion and debate. At least some of the tension occurred in the GEP. An alterna- role of Pacific/North American coupling plate-boundary strain became focused in tive hypothesis holds that relative plate in acceleration and localization of strain? the Gulf region starting ~12.5 Ma. Re- motion since ~12.5 Ma has been accom- Were strain rate and localization related worked microfossils provide indirect evi- modated by a single phase of strain that to external boundary conditions (e.g., dence for an earlier, middle Miocene extends beyond the traditional bounds of remnant slab, plate coupling), intrinsic marine incursion in the northern Gulf and the GEP. Resolution of these problems conditions (e.g., increasing temperature, Salton Trough, but the data for this pe- is critical for understanding the history decreasing strength), and/or other fac- riod are very sparse and incomplete. A of strain localization, plate boundary tors? What was the effect of the change complete record of middle Miocene ma- forces, and other fundamental controls on to more northerly-directed extension and rine deposition is central to our under- lithospheric rupture. the resultant onset of transtensional de- standing of the early stages of basin Page 12 MARGINS Newsletter No. 16, Spring 2006 RCL evoltuion. morphology, it appears that the Farallon, 4. Upper Mantle Processes Pescadero, and Carmen segments between 3. Domains of Deformation Alarcon and Guaymas rifted in a mode simi- Upper mantle processes clearly play an important role in the history and mechanics of New results from active-source seismology lar to Alarcon. This morphology and other similar characteristics suggest a north-cen- faulting and basin evolution in the Gulf- provide the first crustal-scale images of con- Trough region. For example, the prolonged tinent-to-continent conjugate rifted margins. tral domain that shares characteristics of both the northern and the south-central segments. period of Basin-and-Range extension, and its These images and related onland studies re- westward sweep toward the margin, was veal substantial along-axis variations in the The southernmost segment at the mouth of the Gulf represents a distinct domain charac- probably influenced by the depth and condi- geometry and style of extension that can be tion of the Farallon slab. It is likely that the broadly grouped into 3 to 4 structural domains terized by narrow, amagmatic rifting. The along-axis variability in crustal-scale slab continued to control rifting after it based on mapped upper crustal features and stopped actively subducting. Possible slab crustal thinning profiles. Onland, Late Ceno- deformation is unexpected, since the com- monly assumed primary forcing functions of windows may explain some of the observed zoic to modern low-angle normal faults are NW-SE variation in rift architecture. found only in the northern Gulf and Salton extensional style – strain rate, rheology, and mantle temperature – ought to be more or less Studies of upper mantle structure, fabric Trough, where the overall trend of the plate and state will provide an important next step margin is moderately oblique (30°-40°) to the constant throughout the Gulf of California. The underlying controls on this variability in understanding the dynamics of this rift relative plate motion. Within the northern system, and exciting new results are already Gulf, deformation occurs in a diffuse system thus remain unknown and require further study. Relevant factors likely include rift emerging. Recent shear-wave splitting stud- of oblique-normal faults with no evidence for ies using data from the NARS-Baja and other transform faults. This contrasts with the obliquity, strain rate, lithospheric strength and composition, disposition of the remnant slab, broadband seismic arrays reveal NE-SW to southern two-thirds of the Gulf where fault- ENE-WSW anisotropy beneath mainland ing occurs on discreet large transform faults mantle-wedge metasomatism, and history of arc magmatism. Voluminous sediment in the Mexico. These directions may reflect present- linked by short spreading centers (Fig. 1). day absolute motion of North America, or This first-order difference may be related to Salton Trough, northern Gulf, and Guaymas basin plays a major role in the production of could be inherited from now-inactive Mi- the change in obliquity from north to south ocene extension. E-W directions beneath the and/or to the large input of sediment from post-6 Ma crust, but the nature of its control on crustal rheology, styles of faulting, and Baja peninsula are similar to patterns seen in the Colorado River. It’s clear that a signifi- southern California and appear to record cant portion of crust in the northern Gulf and spatial variations in rift architecture remains poorly known. Numerical models predict that asthenospheric flow related to removal of the Salton Trough consists of young, voluminous, subducted Farallon slab. Preliminary tomo- Colorado River derived, sedimentary and heating and weakening should localize strain, so we might expect that thick sediments graphic analyses reveal the first images of metasedimentary rock. This young crust is lithospheric-asthenospheric structure beneath strongly affected by rapid burial, high heat would trap heat, reduce the strength of the lithosphere, and form discrete localized litho- the active rift. A low-velocity anomaly be- flow, and associated mafic to rhyolitic neath the Peninsular Ranges may be related magmatism. Consequently, crust in the Gulf sphere-scale faults. However, the observation of diffuse, distributed upper-crustal deforma- to delamination and sinking of dense lower of California is thickest in the north (up to lithosphere and associated upwelling of as- ~20 km) and decreases to as little as 7 km in tion in the thickly sedimented northern Gulf suggest the opposite. Thus the Gulf of Cali- thenosphere. Regional uplift related to dra- sediment-starved southern basins. matic thinning and/or heating of lithosphere The Guaymas basin and perhaps the fornia – Salton Trough focus site provides a unique opportunity to explore the role of large beneath the Peninsular Ranges may have be- northern Gulf seem to have ruptured in a nar- gun as recently as 1.0 to 1.5 Ma, but the tim- row-rift mode, with spreading and new igne- sediment input in controlling crustal composition, thickness, rheology, and architecture ing and cause of this recent uplift event are ous crust formation beginning ~6 Ma. There not well known. is controversy, however, concerning the ori- of rifted continental margins and nascent ocean basins. The NARS-Baja array has considerable gin of igneous crust at depth in the northern potential, but the existing array is too widely Gulf. Some workers infer that this crust con- The modern expression of faulting and related basin formation in the Gulf of Cali- spaced to accurately locate earthquakes sists of Cretaceous plutons that have experi- within the Gulf of California. In contrast, re- enced lower-crustal flow during fornia is in many respects asymmetric. Ac- tive strain is presently focused along the steep sults from the Salton Trough reveal the evo- large-magnitude stretching. Others suggest lution of an earthquake swarm on a single that the crust is entirely new (post-6-Ma) ig- southwestern margin of the Gulf, whereas the relatively inactive northeast side has a broad fault system that was precursory to a major neous crust overlain by sediment and slip event. Denser deployments of passive metasedimentary rock. This problem is cen- shelf and low topographic gradients. Spread- ing rates at the spreading centers indicate that seismic stations can address questions rang- tral to our understanding of lithospheric rup- ing from mantle dynamics and state to up- ture in this area and to our understanding of the margins have moved into the “passive” stage of development. Asymmetry exists in per-crustal stress relief and deformation. ”transitional” crust at other rifted margins as Small dense OBS arrays are currently de- well as the efficacy of lower-crustal flow in part because the center of extension shifted to the northwest in most rift segments at about ployed in the Gulf in Guaymas and Alarcon general. In the southern Gulf, seismic results basins, but a Gulf-scale passive OBS deploy- reveal that the Alarcon basin segment de- 3.5 Ma, resulting in an overall westward shift throughout the Gulf. The causes, timing, and ment would enable detailed imaging of litho- formed in a wider rift mode (Fig. 3), with sphere-asthenosphere geometry, disposition rupture and formation of new igneous crust synchronous versus diachronous nature of beginning at ~3.5 Ma. Based on seafloor this westward shift are not well known. See “Cortez” cont. on pg. 37 MARGINS Newsletter No. 16, Spring 2006 Page 37

“Cortez” cont. from pg. 12 achieved to date and address new questions • Reconstruction of large volcanic units of the relic slab and slab windows, and of broad global significance. This document across regions of uncertain fault offset many other targets that would shed con- does not attempt to prescribe specific direc- • Micropaleontology to track and date siderable light on the fundamental pro- tion for future research, but hopefully pro- marine incursions vides a useful overview of workshop out- cesses controlling lithospheric rupture in • Isotope and trace-element geochemistry comes that can be used in future planning and of volcanic rocks this focus site. steering of the MARGINS-RCL initiative. • Tectonic geomorphology and Some examples of research and data Concluding Remarks paleoseismology to document Quaternary needs: faults and slip rates The above summary is our attempt to syn- • Passive seismics in the Gulf of California thesize some of the main topics and ideas that • Acquisition/compilation of regional • Hi-resolution fault and basin imaging in potential field datasets were discussed at the workshop. Any sum- the southern Gulf of California mary of this nature cannot include everything, • Mapping, dating, and kinematic analysis Examples of modeling efforts needed: and we recognize that we may have missed of onland faults and basins • Community models for paleogeography topics of interest to some participants. It is through time clear that emerging data from this focus site • Geodetic studies of modern strain parti- are providing remarkable new insights into tioning and elevation change • Numerical models of lithospheric exten- processes of oblique rift formation, continen- • Paleo-elevation studies sion tal rupture, and plate-boundary deformation. • Geophysical studies onshore in the • Integration of strain recorded over three There was a strong consensus that future re- northern Gulf, Salton Trough, and Sonora time scales along multiple transects search is needed to continue the success • Ocean drilling