T11A-1838
Does the West Salton Detachment extend through San Gorgonio Pass, southern California?
Rift-related extension and low-angle crustal detachment are key structural elements of the late Cenozoic southern San Andreas Fault system, as manifested by the West Salton Detachment (WSD). The most northwestern exposure of the WSD is in the Santa Rosa Mts (SRM), where the Zosel Fault bottoms a hangingwall sequence of upper Cenozoic marine and terrestrial sedimentary deposits that include stratigraphic units well known throughout the Salton Trough region. We have used geologic and geophysical data to investigate the distribution of the WSD system in the northern Salton Trough, including its possible extension into and beyond San Gorgonio Pass. Although the WSD is not exposed north of the SRM, late Miocene marine and terrigenous sedimentary rocks at Garnet Hill probably are hangingwall deposits squeezed up within the San Andreas Fault zone. West of Garnet Hill lie San Gorgonio Pass (SGP) and the 3 km-high northern escarpment of the San Jacinto Mountains (SJM). In SGP, upper Cenozoic sedimentary rocks south of the Banning strand of the San Gabriel Fault include the marine Imperial Formation and associated terrestrial deposits, a sequence similar to that in the WSD hangingwall throughout the greater Salton Trough region. We propose that the WSD originally extended from the NW head of Coachella Valley west into SGP, where the detachment may form the base of the Cenozoic marine and terrestrial sedimentary sequence. The WSD probably continues west beyond SGP, with extensional translation decreasing until the detachment intersects the Banning Fault near Calimesa. There, we propose that the WSD underlies a subsurface sedimentary package north of the San Timoteo badlands and south of the Banning Fault that a gravity low suggests is 2 km thick, and that reportedly contains marine sediment penetrated in boreholes. When ~44 km of right-slip is restored on the Banning Fault (Matti and Morton, 1993), the Calimesa low restores opposite a similar low in the northwestern Coachella Valley. The juxtaposed gravity lows mark a late Cenozoic depocenter that formed at the NW head of the Salton Trough during evolution of the San Gabriel and San Andreas Faults (10 Ma to 1.2 Ma). This reconstruction has several implications: (1) the WSD was active while the late Cenozoic sedimentary sequence in SGP accumulated in its hangingwall at 7 Ma (marine Imperial Fm) and probably as early as 10 Ma (Hathaway Fm); (2) At that time the San Jacinto Mts (SJM) began to rise in the WSD footwall, shedding sediment and landslide breccia into the SGP basin. Simultaneously, Transverse Ranges sources shed sediment southwest, south, and southeast into the SGP basin and the adjoining San Timoteo basin; (3) Prior to disruption by right-slip on the Banning Fault, the WSD probably extended around the NW head of the Salton Trough, where the detachment would have separated footwall crystalline rocks of SGP from hangingwall deposits of the Salton Trough (Coachella Fanglomerate, Imperial and Painted Hill fms). The enigmatic Whitewater Fault in the SE San Bernardino Mts may be part of the WSD. (4) Because extensional translation on the WSD diminished westward through SGP, it is doubtful that >3 km of topographic relief on the WSD footwall in the SJM resulted from footwall uplift alone during the period 10 Ma to 1.2 Ma. Post-WSD Quaternary uplift must account for an unknown component of this relief.
T11A-1839
Interaction Between Early San Andreas Strike-Slip Faulting and Extensional Tectonism in the Chocolate Mountains: A Prologue to Growth of the Salton Trough Along the Plate Boundary in Southern California
The Chocolate Mountains (CM) along the NE margin of the southern Salton Trough (ST) lie NE of the post-5- Ma San Andreas fault (SAF) and SW of the early and middle Miocene Clemens Well-Fenner-San Francisquito strand (CW-F-SF) of the SAF system. The CM are a highly extended terrain that evolved during the late Oligocene-middle Miocene and is bounded by the CW fault. Constrained by reconstruction of a compelling array of paleogeologic patterns, the approximately 300 km displacement on the SAF NW of the Garlock fault is distributed to the SE on the SAF (ca 160 km, 0 to 5 Ma), San Gabriel fault (ca 40 km, 0-5 to 12 Ma), and CW-F-SF fault (ca 100 km, 13 to 17-22 Ma). The youngest rocks yet shown to be offset 300 km in southern CA are basalts in the Diligencia and Plush Ranch formations, as young as 22 Ma. Lack of evidence for a large-displacement dextral fault in AZ on-trend with the CW fault requires the existence of a tectonic mechanism for absorbing its dextral displacement to the SE. Structure in the CM manifests late Oligocene-middle Miocene extensional tectonism that culminated in exhumation of Orocopia Schist by tectonic denudation. In its early stages, tectonism was accompanied by sedimentation and by voluminous magma-generation producing a batholithic-to-volcanic edifice. The principal structural feature is a complexly faulted, NW-trending array of en echelon antiforms that runs the length of the range and continues SE into AZ and NW into the Orocopia Mts. In the anticlinorium core, Orocopia Schist is intruded by a late Oligocene composite batholith of mafic to felsic plutons. A succession of tectonic plates separated by detachment faults overlies the schist and plutons. The structurally lowest fault is ductile and juxtaposes mylonite against the schist. Three higher faults, all brittle, vertically stack plates of (1) Mesozoic orthogneiss, (2) little deformed Triassic and Jurassic plutonic rocks, Proterozoic gneiss and anorthosite, and dacitic to rhyolitic late Oligocene hypabyssal intrusive rocks, and (3) moderately to steeply tilted supracrustal sedimentary and volcanic rocks. Field relations and age data allow us to bracket sequential stages in the late Oligocene to middle Miocene (ca 28 and 13 Ma) magmatic-tectonic evolution of the CM. Our 40Ar-39Ar and K-Ar dates and published U- Pb indicate plutonism at 24 Ma, dacitic to rhyodacitic volcanism at 24 to 22 Ma. 20 Ma, and 17 Ma. At least the highest detachment is post-17 Ma and pre-13 Ma, the oldest flow age (ca 13 to 9 Ma) from untilted sections of basalt and conglomerate. This interval is coeval with displacement on the CW-F-SF fault. The basalt and conglomerate sections are fully offset on the modern SAF, pre-date growth of the ST, and span much of the interval between 13-Ma cessation of the SF-F-CW fault and 5-Ma start of the SAF. Spatial and temporal linkage between dextral displacement on the CW fault and extension in CM is compatible with a transfer mechanism whereby right-slip on the CW fault is accommodated to the SE by hyper-extension in the Orocopia-Chocolate Mts block. This strain pattern prefigures later development of the West Salton detachment that was associated with growth of the ST and that began perhaps as early as ca 10 Ma (Matti and Langenheim, this session). Unlike this later strain pattern, the extensional accommodation proposed here was not linked to opening of the Gulf of California, but rather occurred in an extensional zone between the CW fault and a reconstructed zone of sinistral shear along the southern boundary of the Transverse Ranges and SW margin of the CM.
T11A-1840
New constraints on the tectonic evolution of the Salton Trough
The Salton Trough is a critical structure where two very different styles of deformation meet; spreading-center dominated deformation to the south in the Gulf of California and dextral strike-slip deformation along the San Andreas fault system(SAF) to the north. Seismic CHIRP data acquired in the Salton Sea provide new constraints on the interaction between the San Andreas, San Jacinto and Imperial fault systems and reveal distinct changes in deformational style from north to south. Based on the stratal geometry observed in CHIRP profiles, we propose three distinct phases of tectonic deformation: (1) Late- Pleistocene transpression north of the Extra Fault Zone (EFZ) replaced by (2) late-Holocene differential subsidence south of the EFZ and (3) recent formation of the Brawley Seismic Zone (BSZ), a through-going crustal shear zone. An angular unconformity is observed to separate the folded and faulted (late?) Pleistocene strata of the Brawley Formation from the overlying Holocene Cahuilla Formation (CF). North of the EFZ reflectors in the CF suggest little to no active deformation. Conversely, south of the EFZ reflectors exhibit marked divergence with their dip systematically increasing with depth. Such a pattern of divergence indicates that the rate of sedimentation has kept pace with the rate of tectonically-induced accommodation. As such, it appears that the EFZ is a tectonic hinge zone delineating the northern limit of active subsidence, high heat flow, and volcanism. Furthermore, given the observed subsidence pattern, we predict the existence of a NE trending basin-bounding normal fault, or series of normal faults, near the southern shoreline of the Salton Sea. In our conceptual model, the early distributed faulting and transrotation between the San Andreas and San Jacinto faults accounts for the compressional folding observed in the Brawley Formation, but later gave way to extension-dominated deformation as significant slip became focused along the Imperial Fault. Reactivation of Miocene extensional structures by the Imperial – San Andreas releasing bend may account for the differential subsidence in the southern sea. In addition, we propose that the BSZ is a young feature similar to through-going shear zones formed in pull-apart basin analog models. We suggest that seismicity patterns in the BSZ do not reflect the dominant tectonic processes or kinematics occurring beneath the Salton Sea, but rather short-term, low magnitude stress release. Finally, we have established a chronostratigraphic framework for the Salton Sea by correlating seismic horizons in the CHIRP data with Lake Cahuilla stratigraphy observed in onshore paleoseismic excavations. The EFZ shows evidence for multiple events during the last ~1,500 years, and the most recent event occurred ~1700 AD, suggesting the potential for coincident ruptures along the SAF and EFZ. Initial estimates suggest that the vertical subsidence rate across the EFZ has been ~ 6 mm/yr for the last 800 years.
T11A-1841
Recent Rift Volcanism in the Northern Gulf of California and the Salton Through: Why a Preponderance of Evolved Magmas?
Quaternary volcanoes and shallow intrusions throughout the northern Gulf Extensional Province provide a unique opportunity to characterize active crustal accretion associated with extreme continental rifting. In the Lower Delfin basin and Isla San Luis volcanic rocks have compositional continuity from basaltic andesite (>54 % SiO2) to sub-alkaline rhyolite, whereas Roca Consag in the Wagner basin, and Cerro Prieto are homogeneous, low-K, lithoidal, microlithic dacites. Salton Buttes surface lavas and a seamount in the Upper Delfin basin are dominantly rhyolitic. Basaltic xenoliths, intrusive basaltic sills and altered subsurface rhyolites are known from the Salton Trough and Cerro Prieto. All Quaternary volcanic rocks in the region have depleted (relative to CHUR) Nd isotopic compositions with εNd of +8.5 and +6.3 in the Salton Buttes and marginally lower values (+6.5 to +4.1) for Roca Consag, Lower Delfin basin and Isla San Luis. Rhyolite from the Upper Delfin basin yielded εNd of +2.2. These values are consistent with overall depleted 87Sr/86/Sr ratios (0.70353-0.70382). Only rhyolites from Lower and Upper Delfin basin have higher 87Sr/86Sr (0.70492 -0.70661) compared to coexisting andesites, which implies hydrothermal alteration and/or minor contamination by continental crust and/or sediments. Volcanic rocks within individual basins thus represent variably differentiated and, to a smaller degree, contaminated, co- genetic suites, as indicated by negative Eu anomalies that reflect plagioclase fractionation in rhyolites. Ion microprobe ages of zircons from Roca Consag are heterogeneous. The youngest ages are ~120 ka and several pre-Quaternary xenocrysts were observed, but the data define a dominant peak at ~1 Ma. The isotope data suggest recent differentiation of dominantly mantle-derived young crust. The preponderance of intermediate to felsic volcanism in the northern Gulf of California suggests that only low- density magmas can reach shallow levels where they may form a significant volume of new crust within the active rift basins.
T11A-1842
Preliminary Pliocene-Pleistocene Stable-Isotope and Paleosol Data From the Fish Creek- Vallecito Basin, Southern California: Insights Into Paleoclimate From Pedogenic Carbonate
In this study we use detailed measurements and isotopic analyses of paleosols in the Fish Creek-Vallecito basin (FCVB), southern California, to interpret changes in Pliocene-Pleistocene paleoclimate in the area. The FCVB currently lies in a hyperarid rain shadow (MAP = 15-17 cm) formed by the Peninsular Ranges. The timing of Peninsular Range uplift is not known, although recent work suggests it could have occurred as recently as early Pleistocene (Mueller et al., 2006). In the FCVB, abundant paleosols are exposed in a thick, tilted stratigraphic section that accumulated in the hanging wall of the West Salton detachment fault. New high-resolution magnetostratigraphic dating allows us to determine the age of paleosol horizons to within an average of ∓ 0.06 m.y.. Pedogenic carbonate nodules from 23 horizons ranging in age from 3.7 to 1.0 Ma, spanning a thickness of 2.5 km, were analyzed for oxygen and carbon isotopic compositions on a Gasbench and MAT 253 mass spectrometer. The data reveal an increase in carbonate δ18O values at about 2.5-3.0 Ma, from -10.5 ∓ 0.1 ‰ to -9.2 ‰ ∓ 0.2 ‰ (VPDB). Pedogenic carbonate δ13C values vary between -10.4 ‰ and -3.8 ‰ (VPDB) with no apparent trend. A total of forty-nine paleosols were described in the study interval. Most paleosols have shallow carbonate (Bk) horizons and thin, poorly-developed A horizons. Our finding of an increase in δ18O corresponds broadly to a previous study of fossil horse teeth (Brogenski, 2001), which recorded a 2 ‰ increase in δ18O in meteoric water at about 2 Ma. Preliminary recalculation of Brogenski's fossil site ages suggests that the change in δ18O occurred earlier than previously reported, around 2.4 Ma. The increase in δ18O at 2.5-3.0 Ma coincides with a global climate change caused by the onset of northern hemisphere glaciation, and may reflect (1) an increase in enriched Pacific Ocean-derived storms and decrease in the concentration of isotopically depleted monsoonal sources, (2) a change in the source of atmospheric water vapor within the Pacific Ocean, or (3) an increase in soil water evaporation driven by an increase in local temperature or of summer precipitation. The observed increase in δ18O is opposite of the change that would be produced by the onset of a rain shadow in the FCVB. This suggests that uplift of the Peninsular Ranges occurred before 3.7 Ma or after 1 Ma, or perhaps took place in two stages before 3.7 Ma and after 1 Ma. We interpret the majority of paleosols as Aridisols that formed under arid to semi-arid conditions. Measurements of depth to the soil carbonate (Bk) horizon show an average decompacted depth to Bk of 19.7 ∓ 1 cm, which corresponds to a mean annual precipitation of approximately 25 cm (Retallack, 2005). This is similar to modern annual rainfall in coastal San Diego and is 8-10 cm more than in the present-day FCVB. While there is considerable scatter in depth-to-Bk measurements, clear trends are not apparent, suggesting that aridity was the dominant climate condition in the basin between 3.7 and 1.0 Ma. Other indicators of climate change in the area, including rise and fall of lake levels and inferences from faunal assemblages, may reflect external factors (i.e. fluvial inflow) rather than local climate conditions.
T11A-1843
Spatial properties of the San Andreas Fault plate boundary surface trace between Desert Hot Springs and the Bombay Beach based on satellite and B4 Imagery.
We present preliminary results of a hyper-accurate (~ few m) inventory of SAF features between Bombay Beach and Desert Hot Springs based on overhead imagery. The ultimate goal is to identify fault features that can be used to determine offset distance and slip rates. Many fault structures have been previously reported and about two dozen new ones have been found in this study (vegetation lineaments, offset channels, soil color changes, scarps, pressure ridges, etc.). Using approximately one hundred fault components we defined a map view piecewise continuous trace of features (and interpolations when no structure could be discerned) that we are calling the provisional plate boundary (PPB). The resulting PPB trace closely matches the faults reported by Clark (1984) and those in the qfaults data base. Using a variety of techniques including radius-of-curvature retrieval and wavelet analysis, we analyzed the PPB and present evidence of spatial structure on scales of a few hundred meters.
T11A-1844
Morphology and surface dynamics of mud volcanoes in the Salton Trough transtensional basin, southern California
Over one hundred active mud pots and mud volcanoes are present in the northern Imperial Valley of southern California. These features are part of the Salton Sea Geothermal System (SSGS), which is located within a releasing step-over between the southern end of the San Andreas Fault and the Imperial Fault. Although these features have been observed as a curiosity for over 200 years, little is known about what controls the morphology and dynamics of these features or how they relate to local tectonic activity. Despite the fact that most mud volcanic features in the world are associated with areas of compressional tectonics, here they are found in an area characterized by transtension. Studying the morphology of the mud seeps in the Davis-Schrimpf field can provide new insights into the near-surface dynamics of mud volcanism worldwide. In one location within the SSGS, over fifty mud volcanic features including gryphons, salses, gas vents and springs are concentrated in an area of 1000m2 known as the Davis-Schrimpf field. Detailed surveying, measurements of physical properties of the mud, and field observations were used to evaluate the spatial and temporal variations in activity and morphology of features at the Davis-Schrimpf field. Unlike other areas of mud volcanism in the world, this seep field lacks a larger-scale build up of erupted material around the area that would be expected based on the constant activity and daily eruption of mud. This may be due to rapid removal of erupted material by erosion or localized re-cycling of material within the subsiding caldera- like ring fractures that surround the individual gryphon clusters. Detailed surveying of the caldera structures show an average of 5 ±3 cm of subsidence over a three-month period. The height of the individual gryphons is loosely linked to the viscosity of the mud extruded although other factors such as variations in hydraulic head, gas amounts, and activity levels may also influence the growth of individual gryphons. Large variations in the temperature and viscosity of the mud exist over distances of less than a meter and water levels in the pools are not consistent and also vary over short distances. Both of these observations suggest that a complex plumbing system exists and that the individual vents may not be inter-connected at shallow depths.
T11A-1845
Non-characteristic Slip and Earthquake Clustering on the Imperial Fault, Mesquite Basin, Imperial Valley, California
We excavated a series of trenches across the Mesquite Basin section of the Imperial fault, the primary plate- boundary fault in the southern Imperial Valley, to explore its late Holocene rupture history and to document slip per event. We identified several channels that cross the fault at a high angle and are displaced in the subsurface by the fault. These channels are incised into or embedded within lacustrine strata that are associated with major filling events of Lake Cahuilla which are dated by 14C combined with historical observations. 3D excavation of these channels has yielded information on timing and slip for the past 6 surface ruptures. Displacement is well documented for the 1979 and 1940 events, with ~0.2 m of coseismic right-lateral (RL) slip occurring in each event. A small rill, which also corresponds to the feeder channel for older beheaded channels, is deflected by about 0.6 m, which we attribute to the 1979 and 1940 events plus creep and afterslip. This rill, which incises the lake sediments from the ca. AD 1700 lake, appears to record slip from only these two historical earthquakes, which is consistent with earlier paleoseismic studies farther south. Earlier events occurred in ~AD 1700 (~1.4 m RL) and ~AD 1650 (~0.2 m RL), with two additional large events between ~AD 1500 and 1600 (1.4 m and 1.5 m of RL slip, respectively). The lack of rounding or deflection of most paleo-channels as they approach the fault suggests that the inferred larger prehistorical displacements are not composite slips from multiple events. These observations suggest that: (1) the 1940 and 1979 events were not "characteristic" for the northern Imperial fault, but rather, that the 1940 event sustained an incomplete rupture which was followed by the 1979 "make-up" event, perhaps due to stress loading by high slip in the border region; (2) the Imperial fault appears to be more typified by end-to-end ruptures, assuming that the larger displacements represent more complete and evenly distributed stress release; (3) the slip rate for the Imperial fault in the Mesquite Basin is about 1 cm/yr for the past five centuries; (4) slip per event ranges from about 0.15 to 1.5 m; and (5) scaling slip per event in Mesquite Basin with maximum slip near the International Border, and assuming that most end-to-end ruptures sustain 5-6 m along the central portion of the fault, suggests that the Imperial fault accommodates a slip rate of about 3-4 cm/yr, close to that estimated by GPS.
T11A-1846
Designing Shots for the 2010 Seismic Refraction and Reflection Survey in the Salton Trough, Southern California
The NSF and the USGS have funded a large seismic refraction and reflection survey of the Salton Trough in southern California. The goals of the project are to study earthquake hazards on the San Andreas and Imperial Faults as well as rifting processes in the northernmost Gulf of California extensional province. Seismic velocity models and reflection images of the basins, whole crust, and upper mantle will constrain the structure and petrology of the rift valley, adjacent ranges and bounding faults. Fault and sedimentary basin geometry and 3-D seismic velocity will aid in the construction of seismic hazard models in an area considered at high risk. The obliquely rifted continent appears to be producing new lithosphere in the central trough by magmatic underplating and sedimentation. Interactions between extension, magmatism, sedimentation, and faulting will be constrained by the seismic images. Fieldwork is tentatively scheduled to take place in January 2010. To prepare for the upcoming fieldwork, shot design was investigated based on data from a 1979 USGS seismic survey, when 41 shots of varying sizes were detonated at 9 locations throughout the Imperial Valley. Picking errors in the first arrival travel times were correlated with source-receiver offset, shot size, and shot- hole geology. As expected, shots in the irrigated valley farmland in lake sediments produced much better shot coupling than those located in unsorted desert stream wash. This is likely due to better packing of the lake sediments and/or drilling through a shallower water table. The goals of the 2010 survey include two 250- km whole-crust refraction profiles. This would require shot sizes of 1000-1500 kg to record the entire length of these lines. Several shorter upper-crust refraction lines would require ~500-kg shots to record to distances greater than 50 km. Dense refraction and low-fold reflection shots spaced at 2-4 km in the valley floor need 100-200 kg to image both whole-crust low-fold reflectivity and refraction to distances greater than 20 km.
T11A-1847
Young Saucer-Shaped Sills Within Rapidly Accumulated Sediments of the Central Gulf of California
Multi-channel seismic reflection profiles collected recently in the central Gulf of California display saucer- shaped igneous sills and associated overburden deformations, associated fluid migration pathways, and possible feeder networks; many sills 1-2 km in diameter have a characteristic concave-upwards profile. The shallow intrusions occur (I) within sediment ponded in the axial troughs of the oceanic Guaymas, Carmen, and Farallon Basins, (II) off-axis in the basin floors, and (III) within the sediment cover of subsided continental crust at the basin margins. In all three settings, some of the sills are very young, uplifting the present seafloor and disrupting turbidite deposition patterns creating onlap of the youngest strata. Seismic 'chimneys' or 'pipes' leading up from the inclined tips of the sills are probably fluid or gas migration pathways away from the super-heated magma-sediment contact zone. Directly above the sills, low-amplitude seismic zones may be lenses of homogenized sediments or gas filled, highly fractured cavities. Semblance velocity analysis of these blank zones shows a substantial decrease in stacking velocities (~1375 m/s), supporting the lower-velocity gas-filled hypothesis. The seismic profiles also reveal sub-vertical, cross-cutting reflectors below the sills. These features differ from seismic artifacts and sideswips, in that they do not resemble the unique diffractions caused by migration and are only found underlying areas of intrusions. Some of these 'feeders' can be traced directly below a sill and probably represent the inclined tips of older sills, feeding the overlying younger sills. All ROV-sampled sills at fault-scarp outcrops are doleritic or gabbroic textured tholeiites; plainly, tholeiitic melts are being delivered to the uppermost crust over a broad area, not just to the spreading centers.
T11A-1848
Subsidence History of the Laguna Salada Basin in Northeastern Baja California, Mexico
The Salton Trough region in southern California and the Mexicali valley in northwestern Mexico are areas of (i) rapid subsidence due to trans-tension along the San Andreas-Imperial fault system, and (ii) high flux of sediments transported by the Colorado River, all of which confer this region with a high potential to preserve a complete record of climatic and tectonic activity information. Here we present the subsidence history of the Laguna Salada basin, and the history of activity of the master bounding faults on its eastern side. The Laguna Salada is a lacustrine basin located west of the Mexicali valley and to the south of the Salton Trough. Sedimentological as well as time series analyses performed on two 42 m-long cores drilled in the center of the basin, estimated to span the past 50 and 70KaBP, indicate a modulation of the late Quaternary stratigraphy by cyclic variations in lake level driven by Milankovitch forcing. Based on these results we derive the long-term history of the basin from a gamma-ray log recovered from a 2.8 km-deep geothermal borehole drilled by the Mexican Power Company adjacent to the Laguna Salada fault. The stratigraphy of the deep borehole reveals a history of activity pulses related to the initial breakage of the Laguna Salada fault and its interaction with neighboring faults. A first pulse started at 1.5 Ma and records the initiation of the Laguna Salada fault and rapid uplift of the crystalline block of the Sierra Cucapa. A second pulse started around 1 Ma, and is very likely related to the hard linking of the Laguna Salada fault with the Cañada David detachment by the Cañon Rojo fault. The onset of the Laguna Salada fault at 1.5 Ma appears to be synchronous with an early Pleistocene regional fault reorganization among the San Jacinto, San Andreas and Elsinore fault systems in southern California, suggesting that this reorganization may have affected a large area from San Gorgonio pass to the northern Gulf of California.
T11A-1849
Seismic velocity in the Gulf of California from the analysis of Rayleigh wave group velocities
In this study we develop a broad-scale velocity model for the crust and upper mantle of the Gulf of California from seismic data recorded by the broadband NARS-Baja (14 stations) and RESBAN (5 stations) networks. Regional constraints on crust and upper mantle velocities are important for studies of the lithospheric structure and deformation. This is particularly true in target areas like the Gulf of California, where the scientific community hopes to gain a fundamental understanding of the lithospheric rupture. We measure Rayleigh waves dispersion curves of 134 earthquakes to create a 2D map of the group velocities in the region, which can then be used to determine the 3D structure. Our results show noticeable differences in the group velocities of different paths. Due to the fact that the NARS-Baja array straddles the plate boundary, we are able to make additional first-order comparisons of the crust on opposite sides of the conjugate margins.
T11A-1850
Effects of crust and mantle variations on seismic waves within the Gulf of California
Previous studies have demonstrated that the crust and mantle from the Pacific Ocean to mainland Mexico changes dramatically from Baja to the Gulf of California and into mainland Mexico. Utilizing these previous results we construct a 3D model for the Gulf of California region; extending this model west to the Pacific Ocean and north to southern California. This 3D model is used to identify the importance of variations in crustal structure with the Gulf region through a comparison to local and regional seismic data recorded by the temporary NARS array and permanent Mexican seismic stations. Synthetic seismograms are calculated through this new 3D model and compared to local and regional seismic data. The importance of specific features and transition length scales between provinces are investigated. Variations of crustal thickness, mantle seismic velocity, and the basin, i.e. sediment cover, thickness are examined and constraints placed on each regionally. Within the new model crustal thicknesses are defined by seismic reflection experiments and receiver function studies, along with nominal oceanic crustal thickness where appropriate. Basin thicknesses are isolated to the northern half of the Gulf, north of 27 N, and extend into the Imperial Valley of southern California. Sediments are constrained by surface wave studies and seismic reflection experiments. Within the borderland region, the crustal thickness is inversely proportional to the water depth and has a transition into oceanic crust within 50 km. Mantle structure is defined by the tomographic two-plane wave methodology using teleseismic arrivals that pass through the region. Construction of the model is accomplished using a set of 1D models placed at strategic locations and triangulated then interpolated within each triangle to produced the 3D model. Discontinuities are honored and material-parameter interpolation is only done with within layers. This method permits the dipping interfaces and velocity contrasts across them.
T11A-1851
Preliminary Basin Analysis of Latest Miocene Conglomerate Near Bahía Kino, Coastal Sonora: A New Record of Crustal Deformation During Initial Opening of the Northern Gulf of California
In coastal Sonora northwest of Bahía Kino, we have initiated a basin-analysis study of nonmarine
conglomerate (Tcg) that overlies and is locally interbedded with the 6.4-Ma Tuff of Mesa Cuadrada (Tmc).
Tcg conglomerate is deeply eroded and cut by a complex set of syn- to post-basinal, N- to NNE-striking
normal faults and NW-striking dextral strike-slip faults. We carried out detailed mapping and field
descriptions, measured stratigraphic sections, and collected paleocurrent and clast-count data in Tcg.
Zircons from the newly discovered Seri tuff near the base of the section, 160 m below Tmc, were dated using
U-Pb SHRIMP geochronology at Stanford University. We obtained a 206Pb/238U weighted mean age of
6.53+/-0.18 Ma (n=14; MSWD=1.3), which yields a sediment-accumulation rate of 0.8+/-0.2 mm/yr for lower
Tcg. Tcg is dominated by debris-flow facies consisting of massive, poorly sorted, weakly bedded, matrix-rich,
pebble, cobble, to small-boulder conglomerate. A subsidiary sheet-flood facies is somewhat better sorted,
generally lacks boulders, and contains weak but pervasive horizontal stratification. Both facies of Tcg record
deposition in an alluvial fan environment. Well sorted and stratified stream deposits are conspicuously
lacking. Preserved sections of Tcg range up to 170 to 240 meters thick in the southern part of the study
area, and increase to at least 470 meters in the north where the Punta Chueca fault, a large NW-directed
low-angle normal fault, apparently produced more extension and subsidence than smaller faults in the south.
Clasts in Tcg are dominated by middle Miocene andesite, the 12.6-Ma Tuff of San Felipe, minor amounts of
Tmc, and variable amounts of older basement rocks including tonalite, granodiorite, quartzite, metavolcanic
rock, and a megacrystic K-spar-bearing granite that we have not seen in the local bedrock. After correcting
for bedding dips and vertical-axis clockwise rotations determined from paleomagnetic analysis, restored
paleocurrents reveal consistent overall transport to the SSW.
We interpret Tcg to record deposition in a 10- to 20-km wide coastal belt of coalesced alluvial fans (bajadas)
that formed on the margin of the nascent northern Gulf of California. Alluvial fans were likely derived from a
large uplands source area NE of the Sacrificio fault that is now submerged beneath broad alluvial plains of
the Rio Sonora. Prior to this time, between 12.5 and 6.5 Ma, extension caused tilting and erosion but did not
produce significant crustal thinning or subsidence. The abrupt change to rapid subsidence and
sedimentation in a belt of coastal alluvial fans records a major change in deformation style and crustal
dynamics at about 6.5 Ma. We infer that this change coincides with the transition from pre-6.5 Ma diffuse
regional extension (late Miocene proto-gulf stage) to rapid, localized latest Miocene oblique dextral shear that
resulted from initiation or acceleration of plate-boundary strain in the northern Gulf of California. If this
interpretation is correct, the onset of northern Gulf opening is thus dated at ca. 6.5 Ma.
http://www.uoregon.edu/~rdorsey/Sonora.html
T11A-1852
A New High-Precision Paleomagnetic Reference Vector From Mesa El Burro, Mesa Cartabón, and Mesa El Pinole, Baja California for the Tuff of San Felipe, a Miocene Ignimbrite Marker Bed Exposed in Baja California and Sonora, México
High precision paleomagnetic vectors from regional ignimbrite markers are valuable for quantifying distributed shear deformation in plate boundary zones. In central Baja California, we measured a new high- precision paleomagnetic reference vector for the 12.50 Ma Tuff of San Felipe (Tsf), a regionally-extensive Miocene ignimbrite deposit which blanketed >4000km2 of present day northeastern Baja California and western Sonora (Oskin and Stock, 2003) prior to the opening of the northern Gulf of California. This new reference location is west of both the main Gulf topographic escarpment and the San Pedro Martír fault that marks the western edge of the Gulf Extensional Province. Because this reference location has remained tectonically stable since the eruption and deposition of Tsf, the paleomagnetic vector measured here is a good estimate of the magnetic field at the time Tsf cooled below the Curie temperature. We selected three mesas capped by Tsf for paleomagnetic sampling: Mesa El Burro 1 km south of El Metate, Mesa Cartabón 9 km southwest of El Metate, and Mesa El Pinole 20 km northwest of El Metate. These mesas are capped by 10-30 m of the Tuff of San Felipe and are amongst the most western exposures of this ignimbrite documented in Baja California. In the area of these mesas, Tsf was deposited on a generally flat landscape with minor paleo-topography incised into 15-110 m of coarse sedimentary rocks. These strata nonconformably overlie a variety of plutonic and metamorphic basement rocks. Locally, Tsf infills westward- draining paleo-drainages carved into the still-horizontal underlying conglomerates. 50 randomly-oriented cores were drilled in Tsf at these three mesas and 48 of these cores together yield a mean direction of 212.4° declination, -3.0° inclination with an α-95 confidence of 1.33°. This direction lies well off of the expected Miocene paleo-pole position and records an apparent geomagnetic excursion during reversed polarity subchron C5Ar.2r (Stock et al., 1999). This unique magnetic signature strengthens the utility of this ignimbrite as a regional tectonic marker. The declination measured at these mesas is ~6° counter-clockwise of the Mesa Cuadrada Tsf reference site in the Sierra San Felipe. This finding increases the amount of distributed dextral shear recorded by rotation of Tsf on the margins of the Gulf of California.
T11A-1853
NET SLIP ACROSS THE BALLENAS TRANSFORM FAULT MEASURED FROM OFFSET IGNIMBRITE DEPOSITS
The Ballenas Transform Fault in the Gulf of California separates the Baja California Peninsula (to the W on the Pacific Plate) from Isla Angel de la Guarda (to the E on the North America plate). This active right-lateral fault originated in a Pliocene plate boundary reorganization when part of the rift margin jumped westward into the Baja California peninsula, transferring Isla Angel de la Guarda from the Pacific plate to the North America plate. The net slip along the Ballenas transform fault system is expected to be less than the 250-300 km of post-Miocene opening of the northern Gulf of California basins. Here we constrain the net slip using geological similarities between the west coast of the island and the east coast of Baja California. Reconnaissance geological mapping on Isla Angel de la Guarda reveals the presence of a middle Miocene high-silica rhyolite ignimbrite with an Ar/Ar age of 11.8±0.2 Ma on sanidine. This deposit is preserved in a paleo-low in Cretaceous (?) granitic rocks in the central part of the island. The major and trace element compositions are similar to those of the Tuff of San Felipe, identified in Sonora and Baja California, but not previously recognized on Isla Angel de la Guarda. The deposit from the island exhibits a paleomagnetic remanence vector toward the SW and nearly horizontal, similar to that of the Tuff of San Felipe, and far outside of the expected paleomagnetic remanence direction. The ignimbrite on Isla Angel de la Guarda has an age younger than that previously reported for the Tuff of San Felipe, but within the range of ages from other outcrops of the Tuff of San Felipe in Sonora (Vidal-Solano et al., 2008). We correlate this ignimbrite to a similar unit preserved in isolated locations northwest of Cataviña on the Baja California peninsula, for which paleomagnetic work shows a very similar remanence direction. Using the SE limit of these outcrops against the granitic rocks in both locations yields an estimate of ~130 km right-lateral strike-slip offset along the Ballenas channel.
T11A-1854
Neotectonics across an active oblique-divergent plate margin, SW Gulf of California
Onshore and offshore paleoseismology provides new constraints on late Quaternary to present deformation rates across the SW margin of the Gulf of California plate boundary at the latitude of La Paz, Baja California Sur, Mexico. The paleoseismology is being conducted in an innovative approach using traditional onshore techniques and CHIRP seismic data in the immediate offshore area on the same fault systems; the CHIRP survey was completed in August, 2008. From west to east the Carrizal, San Juan de los Planes (SJP), and La Gata faults are being studied in detail, and to date the San Jose del Cabo (SJC) fault is being studied in reconnaissance. GPS results suggest rates of motion across the whole array (including the offshore Espiritu Santo and Cerralvo faults) of 1-2 mm/year. Estimated slip rates in the late Quaternary on the Carrizal fault from uplifted marine terraces, mapping, dating units, and two trenches are 0.1 – 0.2 mm/yr. Estimated slip rates from the Los Planes fault is 0.1 to possibly as much as 1 mm/yr. Modern bathymetric data and earthquakes in 1969 (M=5.6) and 1995 (M=6.2) on the Cerralvo and Espiritu Santo faults, respectively, suggest that those faults are much more active than the Carrizal and Los Planes faults. Reconnaissance on the Cabo fault suggests that it was, and possibly remains, a more active fault, perhaps in the range of 0.5 mm/yr. We conclude from the ongoing project that the faulting pattern across the SW margin of the Gulf of California is dominated by a large step from the major Cerralvo and Cabo faults in the south to the NW to the Espiritu Santo fault on the east side of Espiritu Santo Island; these faults define the eastern edge of the narrow shallow marine shelf and the highest onshore mountain range in the southern Baja California peninsula suggesting that they were the major faults that produced the edge of the oblique rift for most or all of its history. The Los Planes – La Gata faults would be the southern splays of the Espiritu Santo fault and part of the larger step over in this interpretation. The Carrizal fault would be a lesser fault that defines the boundary of the main plate boundary zone and runs along the topographic rift escarpment; this relationship of a lesser slip boundary fault and large slip offshore fault is similar to relations on the eastern side of the Gulf of Suez and other rifts.
T11A-1855
Deformation and rotation in southern Baja California, MX
We use velocity data from 16 campaign GPS stations to study the motion and deformation in southern Baja California. The GPS timeseries span a total time of eight years and have four data collection periods with each more than two days. Due to the campaign mode GPS uncertainties are relatively large with respect to the estimated deformation signal of ~2 mm/yr. Therefore we choose robust analysis of the regional and local strain. We calculate the rigid block motion, its residuals, and relative motions. We use triangulation for local estimates of rotations and strain rates. To better estimate the deformations within the Southern Baja rigid block, we calculate a local GPS reference frame. The residual motion within this block is of the order of 0.5 mm/yr. Due to the small extent and small number of station within the rigid Southern Baja block, the errors associated with the regional velocity field is comparable with studied signal. Nevertheless, the direction of the residual w.r.t the local reference frame is in good agreement with the observed slip motion of the local known faults (Busch et al., 2006; 2007; and P. Umhoefer, personal communications) with significant extension between the Baja California peninsula and islands in the Gulf of California. In accordance to results from geological mapping, a strain analysis of the data shows significant deformation in the southeastern region of Baja California. The rotations induced by this deformation are also compared with long-term paleomagnetic reconstructions to evaluate the history of deformation of the region.