The Loreto basin, in Baja California Sur (Mexico), is a Pliocene to Quaternary transtensional basin that formed in response to oblique rifting along the western margin of the Gulf of California (see maps, below). My work in the Loreto basin has been carried out over the past 4 years in collaboration with Paul Umhoefer (NAU), Paul Renne (Berkely Geochronology Lab), and Larry Mayer (Miami University, Ohio). Graduate students Andy Stone and Peter Falk (recently finished) and Darius Semmens (work in progress) have also played key roles in our Loreto research activities. We now have a good understanding of the structure, stratigraphy, and geochronology of the Pliocene part of the basin's evolution, and we recently initiated an expanded new study to better understand older (Miocene proto-Gulf) and younger (Quaternary) tectonic evolution and structural segmentation of this active region.
The Loreto basin evolved as an oblique pullapart that formed during oblique, dextral-normal slip on the Loreto fault and rapid westward tilting between about 3.5 and 2.0 Ma. Syn-extyensional strata are over 1000 meters thick and consist of coarse-grained nonmarine, deltaic and marine facies. Volcaniclastic and bioclastic Gilbert-type fan deltas comprise a substantial volume of these deposits. We recently developed a controversial new model for temporal clustering of earthquakes to explain the distinctive vertical stacking of Gilbert-delta parasequences in the Loreto basin (see recent abstracts below). The southeastern part of the basin was cut by a complex array of antithetic normal, oblique-slip and strike-slip faults between about 2.4 and 2.3 Ma. Since 2.0 Ma the southern basin has been uplifted in a poorly understood accommodation zone, while the northern basin has continued to subside forming a modern alluvial plain.
Here is a list of Papers, Theses, and Abstracts resulting from our Loreto basin research.
The peninsula of Baja California formed by oblique rifting away from mainland Mexico since about 5 Ma. The structural and tectonic evolution of eastern Baja California has been controlled by dynamic processes of oblique continental rifting that are still poorly understood. Geologic structures, geomorphic features, and sedimentary basins all preserve a critical record of these processes. The Loreto basin (L.B.) is an ideal area in which to study the tectonic controls on faulting, subsidence, sedimentation, uplift, and erosion.
We have initiated a new project to study the Miocene to modern tectonic evolution of the area from Puerto Escondido (PE) north to Bahia Concepcion. We are integrating studies in structural geology, stratigraphy, and geomorphology, to learn more about the regional tectonic controls on development of major structures, basins, fault segments, and segment boundaries through time. The core of this research group consists of Paul Umhoefer (NAU), Larry Mayer (Miami University, Ohio), and myself. We recently initiated a new collaboration with Arturo Martin (CICESE, Ensenada) on this project, and we also interact regularly with other geologists working in the region: Jorge Ledesma (UABC, Ensenada), Markes Johnson (Williams College), Sue Kidwell (Univ. of Ghicago), Keith Meldahl (Oberlin College), and others.
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