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Fletcher J.M.,Research Center Cientifi Ca Educacion Superior Of Ensenada | Teran O.J.,Research Center Cientifi Ca Educacion Superior Of Ensenada | Rockwell T.K.,San Diego State University | Oskin M.E.,University of California at Davis | And 17 more authors.
Geosphere | Year: 2014

The 4 April 2010 moment magnitude (Mw) 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentifi ed 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 eastdipping 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 defi nes 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 (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ̃20°. Net surface offsets in the Sierra Cucapah average ̃200 cm, with some reaching 300-400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ̃085° and the dominant slip direction is ̃310°, which is slightly (̃10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region. © 2014 Geological Society of America.

Mark C.,Imperial College London | Mark C.,Trinity College Dublin | Gupta S.,Imperial College London | Carter A.,Birkbeck, University of London | And 3 more authors.
Geology | Year: 2014

Continental rifts are commonly flanked by zones of high elevation, but the cause of uplift remains controversial. Proposed uplift mechanisms include active and induced asthenospheric upwelling, and isostatically driven lithospheric flexure. Discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension. Here, we focus on the wellpreserved Neogene Gulf of California rift. The western rift margin is characterized by a prominent east-facing kilometer-scale escarpment, which bounds a west-tilted, topographically asymmetric rift flank. We exploit west-draining canyons incised into the rift flank to constrain the timing of uplift to between ca. 5.6 and 3.2 Ma using 40Ar/39Ar dating of lavas, which show cut-and-fill relationships to the canyons. Rift flank uplift closely followed the onset of slip on the principal fault of the Loreto rift segment at ca. 8-6 Ma, the age of which we obtain from apatite (U-Th)/He and fission-track thermochronologic analysis of rift escarpment exhumation. Uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca. 6 and 3 Ma, but post-dates a proposed asthenospheric upwelling event by ~8-10 Ma. The timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms, and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading. © 2014 Geological Society of America.

Aguirre-Diaz G.J.,National Autonomous University of Mexico | Aguillon-Robles A.,Autonomous University of San Luis Potosi | Tristan-Gonzalez M.,Autonomous University of San Luis Potosi | Labarthe-Hernandez G.,Autonomous University of San Luis Potosi | And 3 more authors.
Geosphere | Year: 2013

Peña de Bernal is a natural monument located near the town of Bernal, in Querétaro State, central Mexico. It is one of the tallest monoliths of the world, with a maximum height of 433 m. Peña de Bernal was recently declared Intangible Cultural Heritage of Humanity Patrimony by United Nations Educational, Scientifi c, and Cultural Organization (UNESCO). In spite of being both a natural and cultural monument, little is known about its origin, physical characteristics, and chemical composition. It is a leucocratic-igneous rock intruding marine Mesozoic sedimentary rocks and has been misinterpreted as a pluton of Eocene or older age. However, this study shows that Peña de Bernal is a dacitic dome with SiO2 = 67 wt% and an age of 8.7 ± 0.2 Ma. The complete Peña de Bernal body includes three plugs that crop out in an 3.5 × 1.5 km area elongated N40°E. Texture of the rock is porphyritic, nearly holocrystalline (80 vol% crystals and 20 vol% glass),with a mineral assemblage of pyroxene, hornblende, biotite, plagioclase, and quartz, plus accessory apatite and zircon. Peña de Bernal dacite is a spine-type endogenous dome that was forcefully intruded through the Mesozoic sequence practically as a solid plug. © 2013 Geological Society of America.

Negrete-Aranda R.,Research Center Cientifi Ca Educacion Superior Of Ensenada | Negrete-Aranda R.,University of California at San Diego | Contreras J.,Research Center Cientifi Ca Educacion Superior Of Ensenada | Spelz R.M.,Autonomous University of Baja California
Geosphere | Year: 2013

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 fi elds, 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 confi ned 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. ©2013 Geological Society of America.

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