T13E-01
Seismicity and Faulting in the Southern Gulf of California from the Sea of Cortez Ocean- Bottom Array (SCOOBA) Experiment
Accurate earthquake locations can help assess the mechanisms of faulting and deformation at rifted continental margins. The Pacific-North American plate boundary within the Gulf of California (GoC) provides a unique opportunity to explore such rifting, as continental extension in the north transitions to seafloor spreading in the south. From October 2005 to October 2006, an array of fourteen four-component ocean- bottom seismometers were deployed in the GoC to record earthquakes and other natural seismic signals as part of the Sea of Cortez Ocean-Bottom Array (SCOOBA) experiment. Over 30 well-located M>3.5 earthquakes and two M 6 strike-slip events within the southern Gulf were recorded by the array, along with numerous small local events that were not located by land-based networks. The two large events are each accompanied by several foreshocks and aftershocks. Most of the events are located on or near the major NW-SE strike-slip faults that delineate the plate boundary. However, some events appear to be located on faults well away from the major plate boundary faults. We systematically detect and relocate the earthquakes using data from the SCOOBA array and the onshore NARS-Baja experiment. For sufficiently well-recorded and paired events, we apply a double-difference algorithm to obtain high-precision relative locations. We anticipate that these locations will improve our understanding of the distribution of seismic deformation within the greater extensional zone in the southern GoC.
T13E-02
Normal Fault Basin Geometries From Gravity Analyses in the La Paz – Los Cabos Region, Baja California Sur, Mexico
The southern cape region of the Baja California peninsula is ruptured by an array of roughly north-striking, left-stepping active normal faults, which accommodate regional transtension. Dominant faults within this system include the Carrizal, San Juan de los Planes (SJP) (and offshore Espiritu Santo fault), La Gata, and San Jose del Cabo (SJC) faults. We conducted gravity surveys across the basins bounded by these faults to gain insight into fault slip rates and basin evolution to better understand the role of upper-crustal processes during development of an obliquely rifted plate margin. The geodetic location of each gravity observation station was measured to cm-scale accuracy with real-time kinematic GPS and the relative gravity was measured with a LaCoste and Romberg Model G gravity meter to an accuracy of 0.01 mgal. Gravity data were modeled as a 2D two-layer model with a bedrock density of 2.67 g/cm3 and a basin fill density of 2.1 or 2.2 g/cm3. The hanging wall of the east-dipping Carrizal fault hosts the La Paz basin. In the subsurface, this basin is a half-graben that is manifest as two smaller basins (few hundred meters deep) separated by a bedrock high, which likely reflects the two main east-dipping splays (Carrizal and Centenario faults). The SJP basin is a graben bound by the SJP fault on the west and the La Gata fault on the east and has a modeled maximum depth of approximately 1.5 km. This basin is marked by a series of relict normal faults dipping toward the basin center. The maximum depth to bedrock is just northwest of center, asymmetric toward the SJP fault, indicating that slip may be greater along the SJP than along the La Gata fault. It might also mark the possible location of basin inception, indicating that as the basin evolved, faulting moved outward to the presently active SJP and La Gata Faults. The SJC basin has a maximum depth of approximately 2.5 km. The favored gravity model depicts the SJC basin as resulting from slip along a series of normal faults predominantly dipping toward the basin center (east). Bedrock topography within the basin could be attributed to paleotopography; however, with up to 1 km of relief, the interpreted faults seem more likely. The SJC basin has the greatest depth to bedrock, signifying that it accommodates a greater slip rate than the other faults within this system or that it accommodates equal slip but was the first to initiate. The adjacent topography is also the highest, indicating that the modern deformation has been stationary spatially for some time in contrast to the SJP fault, which has low footwall topography. It is likely that the SJP basin is a graben that serves to transfer strain between the larger Carrizal and Espiritu Santo faults and the SJC fault. An offshore CHIRP survey completed in late August, 2008 will provide further insight into the extent and interactions of faults within this system.
T13E-03
New Constraints on the Timing, Magnitude and Style of Deformation in the Southern Gulf of California: Oblique Rifting Since ~14-12 Ma
A multichannel seismic (MCS) profile spanning 600 km across conjugate rifted margins in the southern Gulf of California provides insight into the spatial and temporal evolution of extension of the Baja peninsula away from mainland Mexico. Stratigraphic analysis of multiple rifted basins within the Alarcon spreading corridor indicates an initial stage of extension starting near or shortly after 14-12 Ma. This initial phase of extension was characterized by the formation of several large, widely distributed basins, that show little to no syn-rift sedimentation. A second phase of extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began at or near 6 Ma, and was characterized by the formation of smaller half-grabens along both conjugate margins, with syn- and post-rift sedimentary sequences. A key feature imaged in the MCS data is a highly reflective, ropey layer at the top of basement with a maximum thickness variation ranging between 250-500 m. Travel-time modeling of common mid-point (CMP) supergathers shows that this layer has a P-wave velocity of around 2.5 km/s, overlying a basement velocity of 4 km/s. The ropey reflectivity patterns along with its low P-wave velocity suggests a volcanic origin, which is interpreted to be either late-stage Comondu volcano-clastics ending near 11 Ma, and/or early rifting volcanics that range in age between 10-9 Ma. This layer is imaged continuously over the Tamayo bank and appears to be mantling the Tamayo trough, partially concealing the largest episode of rift-induced subsidence. Basic extrapolation of sediment thickness to time of deposition within the largest basins in this corridor (i.e., Tamayo trough) suggests that the onset of rifting began at ~14-11 Ma. These new spatial and temporal constraints, when combined with a crustal thickness tomographic profile obtained across the entire Alarcon corridor, support a model of NW-SE oblique extension within the Gulf of California starting at 14-12 Ma. This view of extensional history is in contrast to an earlier two-staged model of Gulf formation where orthogonal E-W extension dominated between 12-6 Ma, with 300 km of additional plate motion accommodated along the strike-slip Tosco-Abreojos fault, off the west coast of Baja California. The second stage of extension in this model abruptly occurs at 6 Ma, where the system quickly evolves into the dextral, oblique rift that defines the modern Gulf province. The additional evidence for early oblique rifting in the Gulf, suggests significant amounts (~200 km) of on-shore extension occurred prior to 6 Ma east of the Guaymas and Delfin basins of the central and northern Gulf province within the states of Sonora and Sinaloa. The observed distribution of extension within the Alarcon corridor is of similar magnitude across both conjugate margins implying symmetrical rifting within a pure shear environment, which is in contrast to simple shear extension (i.e., detachment faulting) observed within some segments of the northern Gulf of California.
T13E-04
2D Numerical Models of Ridge-Trench Collision: Implications for Slab Detachment Beneath Baja California
The approach of a buoyant spreading ridge to a subduction zone is a scenario that may lead to detachment of a subducted slab. Previous work has called upon the detachment process as a possible explanation for observed ridge abandonment and slab-window related magmatism in Baja CA/western Mexico, but such a scenario has not previously been tested using fully-dynamic numerical models. We use two-dimensional fully- dynamic models of ridge approach to a subduction zone to explore the dependence of detachment and resultant surface effects on subducted slab length, ridge-trench distance, spreading rate, and lithospheric yield strength. We find that our models, which include non-Newtonian rheology, demonstrate the following dynamics of ridge approach: (a) a decrease in subduction velocity as the ridge approaches the trench, (b) a shrinking surface plate that maintains a uniform subduction velocity, (c) rapid slab detachment at depths ranging from 55-95 km depth depending on the slab age (7-12 My) and (d) ridge abandonment distances of 125-225 km from the trench, and slab gap distances of 200-270 km from the trench. Slab gap distance is used as a proxy for the distance to a possible slab-window related magmatism. These results are consistent with observations in Baja CA, where detachment of the Cocos slab may explain abandonment of observed segments of the East Pacific Rise 50-200 km outboard of the trench and the presence of a non-arc magmatic pulse 100-250 km inboard of the trench, with geochemical signatures separate from that associated with the normal subduction history for the Farallon plate.
T13E-05
Baja transfer by partial coupling with the Pacific plate
The Baja California (BAJA) microplate was ruptured from the North American (NAM) plate ~ 12 Ma ago and since then translated with the Pacific (PAC) plate. The microplates' transport mechanism has been explained by partial coupling with the PAC plate. According to this theory, the young oceanic lithosphere from the Farallon-Pacific spreading center approaching North America was too buoyant to be subducted. Therefore a zone of increased lithospheric coupling developed between the partially subducted Farallon slabs and the overlying NAM margin. In consequence both, the subduction and the seafloor spreading slowed down and ceased. With the development of this coupling region west of BAJA the main PAC-NAM plate boundary jumped into the NAM continent, east of BAJA. After a phase of distributed extension in the the Protogulf region the new plate boundary localized along the Gulf of California. We use a numerical modeling technique to test the dynamic conditions of BAJA transport as seen from present-day and from geologic plate motion studies. Using the kinematic data we test the necessary coupling forces for BAJA transport, as well as, geometrical constraints along the PAC-BAJA coupling zone. Evaluating the transport conditions at different stages of the plate boundary evolution we want to learn about necessary pre-conditions for the BAJA transfer, in particular along the Gulf of California.
T13E-06 INVITED
Structure and Stratigraphy of the Rift Basins in the Northern Gulf of California: Results from Analysis of Seismic Reflection and Borehole Data.
The northern Gulf of California contains two parallel, north-south trending rift basin systems separated by a basement-high. The interpretation of several exploration wells, and ~4500 km of seismic reflection data from PEMEX (Mexican national oil company) indicate that the tectonically active basins to the west (Wagner- Consag and Upper Delfin basins) may have initiated synchronously with the now abandoned Tiburón- Tepoca-Altar basins to the east in the Sonora margin. In both basin systems the lower sequence (A) is marine mudstone-siltstone, has parallel reflectors and a largely uniform thickness that reaches up to1.5 km, and gradually pinches out toward the lateral margins. This suggests that the unit was deposited prior to their segmentation by transtensional faulting. Marine microfossils from borehole samples from sequence A in the Tiburón and Consag basins indicates middle Miocene (>11.2 Ma) proto-Gulf conditions. Sequence B conformably overlies sequence A, and is characterized by up to 2 km growth strata with a fanning geometry that show a clear genetic relationship to the major transtensional faults that control the segmentation of the two basin systems. Sequence C in the Tiburón and Tepoca basins is comparatively thin (<800 m) and includes several unconformities, but is much less affected by faulting. In contrast, sequence C in the active Wagner, Consag and Upper Delfin basin is a much thicker (up to 2 km) growth sequence with abundant volcanic intrusions. Marked variations in sequence C in the different basin systems clearly demonstrate a major westward shift of deformation and subsidence at this time. The modern depocenter in Wagner-Consag basins is controlled by the Consag and Wagner faults, which trend parallel to the north ~20 km apart, and show opposite normal offset. These two faults merge at an oblique angle (70°-50°, respectively) into the Cerro Prieto transform fault to the north and likely accommodate an important amount of dextral shear. To the south the Consag and Wagner faults connect with a diffuse zone of deformation defined by a series of NE trending faults with moderate normal displacement in the Upper Delfin basin. These NE-trending faults intersect the northern strand of the Ballenas transform fault along the Baja California margin, whereas the eastern end of the NE-trending faults is poorly defined along the western flank of the central antiform. In summary, sequence A was likely deposited across most of the northern gulf in the late Miocene, sequence B marks the onset of two discrete transtensional basin systems controlled by both low and high-angle faults in late Miocene-Pliocene time, and sequence C marks the regional migration of plate- margin shearing to its present location in the western gulf. Thermal effects associated with abundant volcanism and sedimentation along the western margin of the gulf likely controlled the asymmetric partitioning plate margin and shearing during the most recent phase of oblique rifting.
T13E-07
Crustal Structure in the Imperial Valley Region of California From Active-Source Seismic Investigations
New crust is being generated by rifting in the Salton Trough. As the rift opens, mafic intrusive rocks fill it from below as young sedimentary rocks fill it from above. Rifting and intrusion produce high heat flow and temperatures that metamorphose the sedimentary rocks to shallow depths, forming a metasedimentary basement in the central part of the Trough, or Imperial Valley, thus consolidating the new crust. The U.S. Geological Survey conducted an extensive seismic-refraction survey in the Imperial Valley region of California in 1979, and recorded additional data in 1992. Profile data were modeled using a combination of forward and inverse modeling techniques. First arrivals on profiles and arrays from all shots were combined in an inversion for a basement-depth model. Finally an an existing gravity profile across the Salton Trough was modeled. Results are as follows: (1) No first-order velocity discontinuity is observed between sedimentary and "basement" rocks in the Imperial Valley; whereas such a discontinuity is observed on West Mesa, west of the Imperial Valley. In the Imperial Valley, basement velocity is 5.65 km/s, and basement is as much as 6 km deep. On West Mesa, basement velocity is 5.9 km/s and is at most 2 km deep. In the Imperial Valley, basement shoals beneath known geothermal areas, and the deepest wells (approx. 4 km) have penetrated only the upper part of the known Cenozoic stratigraphic column in the Salton Trough. Based on these results, we interpret basement in the Imperial Valley to be sedimentary rocks metamorphosed to lower greenschist facies and basement on West Mesa to be crystalline rocks. (2) The Imperial fault offsets basement in a normal sense by as much as 1 km down to the northeast, and there is an irregular basement scarp as high as 3.5 km between West Mesa and the Imperial Valley, which we interpret as a rift suture between old crystalline and young metasedimentary basement. (3) "Subbasement" (Vp 6.9 km/s) is seen at depths as shallow as 12 km beneath the Imperial Valley. Modeling of gravity data requires that this layer deepen and/or pinch out beneath the bordering mesas and mountain ranges. This pinch-out is imaged in the 1992 data beneath the Chocolate Mountains. Based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the Imperial Valley, the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust. (4) Crustal thickness and upper-mantle velocity are 21-22 km and 7.6-7.7 km/s, respectively, beneath the Imperial Valley but increase to 27 km and 8.0 km/s, respectively, beneath the Chocolate Mountains. Our results from the Salton Trough may be contrasted with active-source seismic results from the northern Gulf of California (Guaymas basin; Lizarralde et al., 2007). These results show the crust to thin to 10-14 km within the Gulf. Below 3-4 km of sediment, the crust has a velocity of 6.8 km/s, interpreted to be new igneous (gabbroic) crust. Thus, the rifting process appears to have produced negligible metasedimentary basement and a crustal thickness as little as half that beneath the Salton Trough.
T13E-08 INVITED
How much do we understand the structure and evolution of the Salton Trough?
The Salton Trough, at the northern end of the Gulf of California, likely formed by different processes than the central and southern Gulf due to the weak rheology of its thick quartz-rich sedimentary fill and its proximity to the transpressional Big Bend and the Eastern California shear zone. Many of its key early structures are buried or poorly exposed, so it is unclear whether deep patches of mafic crust were produced by NE-trending mid-ocean-ridge segments, by E-W extension akin to that seen in the Wagner basin, or by some other process. Three distinct tectonic regimes produced the Salton Trough. (1) Late Miocene extension is poorly understood because much of the record is in the subsurface, and existing evidence from thermochronology in the Sierra el Major and Anza Borrego Park suggests extensional exhumation as old as 10-15 Ma, whereas the oldest stratigraphic evidence for extension is ca. 8-6 Ma. (2) Regional-scale, large-magnitude transtensional deformation began in late Miocene or early Pliocene time (ca. 8-6 Ma). The paleo-San Andreas fault took up most of the strain, with additional dextral shear and extension on detachment faults with breakaways in the W, central? and SE Salton Trough. (3) Pleistocene to modern wrench tectonics followed a massive reorganization at about 1.1-1.3 Ma. Detachment faults were cut, folded and largely abandoned, new dextral faults formed SW of the San Andreas fault, and the SE 2/3 of the paleo-San Andreas fault became inactive. The Imperial, Cerro Prieto, San Jacinto, Elsinore, and San Felipe faults and the Brawley seismic zone all date to this latest period of deformation. The Salton Trough has been interpreted to contain a smothered pair of oceanic spreading centers beneath the Salton Sea and Cerro Prieto geothermal field, two regions of high heat flow and latest Pleistocene volcanism. Patches of dense mafic crust at depth beneath 5-10? km of Pliocene to Holocene sediment and metasedimentary rocks produce two principal gravity highs beneath the central axis of the Trough. These data are consistent with the spreading-center hypothesis. However, the faults that currently deform the Salton Trough north of Cerro Prieto are not consistent with sea floor spreading models. Normal faults and normal focal mechanisms are less common than right- and left-lateral strike-slip faults and focal mechanisms along the San Jacinto fault zone. Quaternary dip-slip components of strain on the dextral faults are more commonly contractional than extensional along the margins of Coachella Valley and the San Jacinto, San Felipe and Elsinore fault zones. Focal mechanisms, topographic escarpments, and dip directions indicative of contraction and transpression over large areas. Pleistocene and older sediment all along the NE margin of the Salton Trough are being uplifted and faulted obliquely over the basin. If the southern San Andreas fault really dips NE in this area then the northern Salton Trough is currently behaving like a transpressional Laramide-style basin, not a complex pull-apart basin. Interpretation of the NNW-trending Brawley seismic zone as a buried spreading center is also problematic. Relocated microearthquakes show a ladder-like fault structure composed of left and right lateral faults where normal faults are predicted. Subsurface data in the Salton geothermal field show that the main conduit for hot fluids and perhaps the young magmas are vertical NE-striking left-lateral faults not the NE-striking normal faults expected at a spreading center. Much remains to be learned about the complex structure and evolution of the Salton Trough.