Search

The Pescadero Basin Complex (PBC) comprises three distinctive rhomb-shaped pull-apart basins separated by short and highly overlapped transform faults. Multibeam bathymetric data collected from ship at 40-m resolution, combined with the interpretation of three 2D high-resolution multichannel seismic reflection profiles, were used to establish the architecture of the PBC. Detailed mapping and cross-sectional kinematic modeling based on the seismic images of the North Pescadero Basin reveal a highly evolved pull-part geometry, characterized by a well defined ∼1.8 km-wide axial graben extending ∼32 km in a NNE-SSW direction. Among the fundamental elements controlling basin architecture and evolution of the PBC are the geometry of the initial configuration of the master strike-slip fault step-over and fault dynamics, which may cause transients in fault system activity and basin reconfigurations. Structural analyses carried out in this study point out the PBC pull-apart basins developed under sustained transtensional deformation, where the relative motion of the crustal blocks is oblique and divergent to the transforms or principal displacement zones. Cross-cutting relationships between the main fault systems controlling basin's subsidence and evolution, indicate that underdeveloped basin-crossing faults terminate against basin bounding normal faults, suggesting that ongoing pull-apart rifting continues to dominate basin evolution of the PBC. Furthermore, we propose that the undeveloped cross-basin faults of the PBC initiated as synthetic Riedel faults that, with progressive deformation along the divergent-wrench fault zone, rotated clockwise around a vertical axis to acquire their present orientation oblique to the master bounding transforms. Basin-crossing faults with lesser obliquities control the subsidence along the basin-side faulted segments of the narrow graben systems that characterize the plate boundary at the corners of the PBC pull-apart basins. These narrow transtensional synforms may have served as connections facilitating marine waters to flood the PBC during the early stages of formation of the Gulf of California.

Author(s): Negrete-Aranda, R; Neumann, F; Contreras, J; Harris, RN; Spelz, RM; Zierenberg, R; Caress, DW | Abstract: Heat flow measurements collected throughout the Auka and JaichMaa Ja'ag' hydrothermal vent fields in the central graben of the Southern Pescadero Basin, southern Gulf of California, indicate upflow of hydrothermal fluids associated with rifting dissipate heat in excess of 10nW/m2 around faults that have a few kilometers in length. Paradoxically, longer faults do not show signs of venting. Heat flow anomalies slowly decay to background values of ∼2nW/m2 at distances of ∼1nkm from these faults following an inverse square-root distance law. We develop a near-fault model of heat transport in steady state for the Auka vent field based on the fundamental Green's function solution of the heat equation. The model includes the effects of circulation in fracture networks, and the lateral seepage of geothermal brines to surrounding hemipelagic sediments. We use an optimal fitting method to estimate the reservoir depth, permeability, and circulation rate. Independently derived constraints for the model, indicate the heat source is at a depth of ∼5.7nkm; from the model, permeability and flow rates in the fracture system are ∼10−14nm2 and 10−6nm/s, respectively, and ∼10−16nm2 and 10−8nm/s in the basin aquitards, respectively. Model results point to the importance of fault scaling laws in controlling sediment-hosted vent fields and slow circulation throughout low permeability sediments in controlling the brine's chemistry. Although the fault model seems appropriate and straightforward for the Pescadero vents, it does seem to be the exception to the other known sediment-hosted vent fields in the Pacific.

The Wagner basin, located in the northernmost Gulf of California, hosts a large reservoir with geothermal potential documented in recent heat flow surveys. Although there is evidence of heat generation above the average value for an oceanic crust in the Wagner basin, it is unclear what the heat source is. To better understand the thermal structure in the northern Gulf of California, we acquired four 2D multichannel seismic reflection profiles and two systematic heat flow profiles across the Wagner basin. The survey targeted a geothermal anomaly, which we denominated “Devil's Hole” in a clear reference to pockmarks on the seafloor, and very high heat flow values. The heat flow profiles are ∼12 and ∼9 km long, are sub perpendicular to each other, have a nominal measurement spacing of ∼1 km, and were located along the transects of two of the four seismic reflection profiles. The two remaining seismic profiles are ∼34 and ∼40 km long, respectively, and are oriented SWW-NEE. To obtain seismic images, we used a conventional seismic processing workflow. After heat flow data correction, the minimum, maximum, and standard deviation of heat flow values for the Wagner basin are 295±42 and 10,894±114 mW/m2. We interpret the relatively high heat flow and large variability in the central part of the acquired profiles is caused by upward fluid flow at the Wagner fault zone. In contrast, the lower heat flow in the western flank of the basin suggest conductive heat transfer given the absence of faults. Based on these observations, we develop a geological model in which venting is not the product of a nascent spreading center as other authors have suggested. Instead, hydrothermal activity appears to result from isolated thermal plumes, rising from the base of the sedimentary column, captured by permeable fracture systems.

The Pescadero Basin Complex (PBC) in the southern Gulf of California comprises three distinctive stretched rhomboid pull-apart basins separated by several short transforms. Multibeam and Autonomous underwater vehicle (AUV) bathymetry data collected at 40-m and 1-m resolution, respectively, combined with the processing and interpretation of three 2-D high-resolution multichannel seismic reflection profiles, were used to characterize the architecture of the entire PBC, as well as the internal structure of the northern Pescadero basin. Detailed mapping and cross-sectional kinematic modeling based on multichannel seismic images of the northern Pescadero basin reveals a highly evolved pull-part geometry, characterized by a well-defined ~1.8 km wide axial graben stretching ~32 km in an NNE-SSW direction. Both finite and incremental strain analyses carried out in this study point out that the PBC developed under sustained transtensional deformation, where the relative motion of the crustal blocks is oblique and divergent to the transforms or principal displacement zones (PDZ's), and subsidence is likely being accommodated by one of more décollement layers located at the bottom of a broad negative flower structure. We also present new geochemical data of lava flows with a N-MORB composition outcropping on the NE segment of the northern Pescadero axial graben, and lava-flow samples of E-MORB composition from an uplifted sediment hill on the western margin of the southern Pescadero basin. MORB samples from the PBC represent the northernmost surface flows known in the Gulf of California, highlighting that the PBC has evolved beyond being a pull-apart complex to having initiated seafloor spreading with new oceanic crust formation in response to the opening of the Gulf of California.

New heat flow measurements collected throughout the Auka and JaichMaa Ja' ag' hydrothermal vent fields in the central graben of the Southern Pescadero Basin, southern Gulf of California, indicate that upflow of hydrothermal fluids associated with active rifting dissipate heat in excess of 10 W/m2 around faults that have a few tens-of-meters of displacement. Heat flow anomalies slowly decay to background values of ~2 W/m2 at distances of ~1 km from these faults following an inverse square-root distance law. We develop a physical model of the Auka vent field based on the fundamental Green's function solution of the heat equation. The model includes the effects of circulation in the porous networks of faults and the lateral seepage of geothermal brines through the fault walls to surrounding hemipelagic sediments. We use an optimal fitting method to estimate the reservoir depth, permeability, and circulation rate. Our model indicates the heat source is at a depth of ~5.7 km; permeability and flow rates in the fracture system are ~10-14 m2 and 10-7 m/s, respectively, and ~10-16 m2 and 10-8 m/s in the basin aquitards, respectively. Model scaling laws point to the importance of faults in controlling sediment-hosted vent fields and slow circulation throughout low permeability sediments in controlling the brine's chemistry. Although the fault model seems appropriate and straightforward for the Pescadero vents, it does seem to be the exception to the other known sediment-hosted vent fields in the Pacific.

Benthic foraminifera are excellent environmental indicators; CaCO3 test records the isotopic concentration of their surroundings and can be used to analyze environmental changes that occur during a certain time period. Stable isotopes, particularly those of oxygen (δ18O), are useful for interpreting ancient environments, given that they are used as “proxy” environmental variables (temperature and salinity). In this study, we provide ranges of isotopic variation in benthic foraminifera communities from the Colorado River delta. Four sampling campaigns were conducted in one year (2009-2010) in the adjacent subtidal zone of Baja California. Four transects with a total of sixteen sample station were drawn perpendicular to the coastline. Here, we recorded the following in situ: sand-type, salinity, and temperature. In the laboratory, 300 individuals per sample were separated, and sub-samples were taken for isotopic analysis. Data was processed using the software’s R 2.12.2, PAST 1.81 and Arc Map 9.3. Forty species were identified in the dead assemblages (Thanatocoenosis), whereas thirteen species were found in the living assemblages (Biocoenosis). The most abundant species in both communities belong to the genera Ammonia and Cribroelphidium. In the living assemblages, isotopic variation −2.15% to 5.94% within a temperature interval of 11°C, indicated antiestuarine conditions. In the dead assemblages, isotopic composition −3.04 to −0.74 served as a sign of estuarine conditions prior to damming.

We present the results of detailed mapping and paleoseismic investigations along a section of the complex fault scarp array associated with the Canada David detachment, a low-angle oblique normal fault that controls the southwestern flank of the Sierra El Mayor, Baja California, Mexico. We used high-resolution unmanned aerial vehicle imagery to map portions of a sequence of alluvial fans that comprise a bajada that flanks the southwestern range front. Their ages are distinguished based on a regional soil chronosequence. Three paleoseismic trenches were excavated across active surface faults that root into the Canada David detachment at depth. Fine-grained stratigraphic units in the trenches were dated by optically stimulated luminescence techniques. The three trenches in this study show strong evidence for four large ( M w ≥ 7 ) Holocene earthquakes at or just prior to about 1.7, 4.3–6.1, 7.7–7.6 and 8.4–10.7 ka, as well as weak to moderate evidence for two additional late Pleistocene earthquakes at or just prior to 12.7–14.1 ka and 15.2 ka, indicating an average recurrence of such events of ∼3 ka. This earthquake recurrence interval is slightly longer than that for the Laguna Salada fault to the north that flanks the Sierra Cucapah. The regular recurrence of major surface ruptures strongly suggests that severely-misoriented low-angle normal faults like the Canada David detachment have a mechanical behavior that is very similar to other more optimally oriented faults suggesting that earthquakes on such faults should be common and may be difficult to recognize from seismologic methods due rolling-hinge ramps and/or multifault ruptures.

The Baja California Peninsula is located within a complex geodynamic setting, marked by continental rifting immediately to the east in the Gulf of California, and abundant geothermal manifestations. We report helium and carbon isotopic and relative abundance data sampled from gas and fluid localities (n = 12) across northern Baja California, Mexico. Samples were collected in three distinct regions of Baja: Pacific Borderlands (PB) in northwestern Baja, Peninsula Ranges and Inland Valleys (PRIV) in central Baja, and the Gulf Extensional Province (GEP) in northeastern Baja. Helium isotopes (3He/4He), carbon isotopes (δ13CO2), and CO2/3He values all are highest in GEP samples, and lower in samples from the PB and PRIV. In the GEP, helium isotopes reach moderately mantle-like values (1.74 RA) (where RA = air 3He/4He), while in the PRIV and PB provinces values are more radiogenic (~0.11 RA). 3He/4He values suggest mixing between mantle-derived and crustal-derived radiogenic components, with a maximum mantle contribution of ~21% in the GEP samples. High He isotopes correlate with low mantle velocity zones at a depth of 25–40 km, suggesting a mantle source of He in the GEP, due to extensional crustal thinning. Carbon isotopes (δ13CO2) vary widely between −17.6 and 4.5‰ (vs. PDVB) and CO2/3He values vary over several orders of magnitude (2.0 × 105 to 1.1 × 1013). Variable δ13CO2 and CO2/3He values in the PRIV and PB localities are consistent with calcite precipitation in shallow-level (upper 5 km) hydrothermal systems. The bimodal nature of the He-CO2 data suggests fundamentally different transfer mechanisms in each of the three distinct regions, indicating complex interplay between regional mantle degassing, faulting-assisted migration of crustal fluids, and volatile sequestration via secondary processes (e.g., calcite precipitation). These results, along with literature data, allow for systematic assessment of both mantle sources and secondary processes, providing new insights into how volatiles are transported through the Baja California peninsula.

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (

The 4 April 2010 moment magnitude (M_w) 7.2 El Mayor–Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.–Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific–North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of east-dipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10–30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast–striking cross-faults with minor offset (

Volcanic activity continued to occur along the length of the Baja California Peninsula (northwestern Mexico) even after the cessation of subduction during the middle Miocene. This volcanism occurred mainly in monogenetic volcanic fields, erupting lavas with a wide variety of compositions, including: adakites, niobium-enriched basalts, high-niobium basalts, and high-magnesian andesites. The chemical compositions of these magmas suggest an origin in partially melted basaltic oceanic crust that was subsequently subducted below the peninsula. Several attempts have been made to explain the origin and compositional diversity of post-subduction volcanism in Baja California. Many of these attempts rely on the hypothesis that the magmas were formed through adiabatic decompression of upwelling asthenosphere in direct response to the formation of a window or tear in the subducted slab. This process, however, cannot offer a satisfactory explanation for all existing observations, particularly the lithospheric structure, of Baja California. Here, we present a physical model that sheds light onto the origin of the post-subduction volcanism in Baja California. The model calls upon viscous dissipation as the causative agent of volcanism. Our starting conjecture is that shearing along a low-viscosity channel confined between the stalled oceanic slab and continental crust of Baja California peninsula caused partial melting at moderate depths following cessation of subduction. Our modeling results show that it is indeed possible for rocks to reach their solidus temperature by means of this mechanism. Numerical results indicate that shear heating could lead to a temperature increase of close to 200 °C at a depth of 30 km, sufficient to produce more than 30% melt by volume.

Because of its long-lived (Late Miocene to Pleistocene) post-subduction volcanic activity and location, the San Borja volcanic field (SBVF) is a key area for understanding the physical mechanisms controlling the spatial distribution of post-subduction volcanism on the Peninsula of Baja California. In this paper, we study the distribution and general characteristics of individual eruptive centers in the SBVF, aiming to provide insight into the changing physical nature of post-subduction magmatism and the control on vent location in a very unique tectonic setting. Volcanic activity has built more than 227 high-magnesium andesites (HMA) monogenetic scoria cones and thick lava flows capping large mesas during the last 12.5 Ma. The average (mean) eruptive center in the SBVF has a height of 85 m, a basal diameter of 452 m and a slope angle of 30°. Our volcanic alignment analysis of this field is based on field data, satellite images, and a quantitative method for detecting volcanic center alignments. The morphologic data, together with new 40Ar/39Ar geochronology data have been used to refine our alignment analysis and to better delineate the temporal evolution of post-subduction volcanic activity in this field. The available ages vary from 3.53 ± 0.18 to 10 ± 0.23 Ma suggesting that the long-lived HMA volcanism occurred almost continuously (with some concentration of pulses) throughout Late Miocene and extending to the Pleistocene and even into the Quaternary, replacing the arc-related activity of the Comondu arc which vanished in this area around 12.5 Ma. Alignment analysis confirms a NW–SE direction as the preferred orientation of volcanic alignments and shows that vent elongations do not display a strong parallelism in any particular direction, marking the absence of matches between lineaments defined by multiple vents and orientations of vent elongation. The results of the volcanic alignment analysis allowed us to infer the direction of the maximum instantaneous extension (S1_local) at SBVF from Late Miocene to Quaternary. We compared this value to the direction of maximum instantaneous extension for the Baja California Peninsula at a regional scale (S1_regional), which can be estimated from regional tectonic reconstruction. The comparison of S1_regional and S1_local allows us to quantify the influence of tectonic stresses in controlling the localization of monogenetic volcanism at the SBVF.

The Laguna Salada rift basin is within the zone of shearing between the Pacific and North American plates and is an asymmetric half-graben controlled on its eastern margin by the Laguna Salada fault and the Canada David detachment. Both faults dip west, have accommodated >10 km of offset since the middle-late Miocene, and are associated with an extensive late Quaternary fault array. The Laguna Salada fault is a high-angle fault that strikes northwest and has an oblique normal-dextral sense of shear. The Canada David detachment is a low-angle normal fault with a curvilinear trace that extends ~55–60 km and contains two prominent megamullion antiform-synform pairs. The late Quaternary scarp array that extends along the entire mountain front shows remarkable variations with antiformal and synformal megamullions. In antiformal domains, the scarp array is generally wider, closer to the mountain front (

ARTICLE I NFO Late Quaternary slip across the Canada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q1 (youngest) to Q8 (oldest)) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide 10 Be concentrations from individual boulders on alluvial- fan surfaces Q4 and Q7 yield surface exposure ages of 15.5±2.2 ka and 204±11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial- fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets b 4 m is 0.051 and 0.066 m 2 /ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada.