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Características de la deformación en las rocas cristalinas cretácicas de la región E-SE de la ciudad de La Paz, B.C.S., México.
Characteristics of the deformation in the Cretacic crystalline rocks at the E-SE of the La Paz city, B.C.S., Mexico
Ernesto Ramos Velázquez
John Mackrain Fletcher
Acceso Abierto
Rocas cataclásticas - México - La Paz (Baja California Sur)
El extremo sur de la península de Baja California está compuesto principalmente por rocas cristalinas Cretácicas, que se encuentran en las partes montañosas que exceden las 2000 m. de elevación. Estas contrastan marcadamente con las regiones relativamente bajas, de estratos volcanogénicos Terciarios, que dominan la geología de la península hacia el WNW. La falla La Paz ha sido propuesta para separar estos terrenos con diferencias litológicas y topográficas, y se ha reconocido coma una estructura dominante en el extrema sur de la Provincia Extensional del Golfo. El área de estudio de esta tesis esta inmediatamente al sureste de la ciudad de La Paz y contiene un prominente lineamiento que puede ser visto en fotografías aéreas e imágenes de satélite, y ha sido definida coma la traza de la falla La Paz. Todos los afloramientos del lineamiento revelan que este es un contacto intrusivo Mesozoico y no una falla Neogénica.
The southern tip of the Baja California peninsula is composed mainly of Cretaceous crystalline rocks that are found in mountain ranges that exceed 2,000 m in elevation. This contrasts markedly with the relatively low lying ranges of Tertiary volcanogenic strata that dominate the geology of the peninsula toward the WNW. The La Paz Fault has been proposed to separate these topographically and lithologically different terranes and has become recognized as a dominant structure in the southern Gulf Extensional Province. The study area of this thesis lies immediately southeast of La Paz and contains a prominent lineament that can be seen in air photos and satellite images and has been defined as the trace of the La Paz Fault. Nearly all exposures of the lineament reveals that it is a Mesozoic intrusive contact and not a Neogene fault. Rocks in the study area were grouped into three lithostratigraphic units: a) Cretaceous granitoids and metasedimentary rocks crystalline, which are the most abundant units, b) Tertiary volcanogenic rocks that include subvolcanic dykes and volcanogenic sedimentary units, c) Quaternary sedimentary rocks that are dominated by alluvial fans and sheetwash sand deposits. Mesozoic deformation is divided into three generations of structures and fabrics. D1 is characterized by magmatic and high-grade solid-state fabrics defined predominantly by a steep east-dipping foliation and down-dip lineation. D2 structures include solid-state ductile shear zones that are generally less than 15 cm thick. D3 structures are characterized by west­dipping shear zones that display transitional brittle-ductile deformation mechanisms. The D1 foliation (S1) ios penetratively developed in the study area, whereas, D2 and D3 are restricted to scarce localities.We infer that D1 and D2 are associated with the emplacement and early stages of crystallization of the granitoid rocks, whereas, D3 was formed at lower temperatures during the final phase of cooling and unroofing of the batholith. The Cenozoic deformation (D4) consists of joints and faults that formed after deposition of the Tertiary volcanogenic strata and are associated with modern seismic activity. Joints are mainly found in granitoid rocks, and typically develop as three suborthogonal sets. The San Juan de Los Planes fault is the dominant neogene structure in the study area and has a cataclasis zone of about 200 m width, with breccia and foliated gouge. This zone strikes NNW, dips steeply to the east, and records nearly pure normal displacement. A second group of faults strikes west-northwest, has a characteristic spacing of approximately 1-3 km and commonly shows sinistral separation of steeply dipping mesozoic contacts. A comparison whole-rock geochemistry of fault rocks with their protoliths typically shows an enrichment of chemically mobile elements (alkalis, Si) and a depletion in the inmobile elements (Ti, Ni, Sr). This relation is interpreted to reflect an increase of volume in the rocks, that likely occurred at low-moderate temperature conditions and at shallow depths in the crust. Analysis of apatite fission track thermochronology, in the southern tip of the Baja California peninsula, reveals a radical gradient in ages from west to east. Three samples from the footwall of the proposed La Paz Fault, along the western margin of the cristalline basement, block yield Cretaceous ages (74-79 Ma). In contrast, three samples from the footwall of the San Jose del Caba Fault, along the eastern margin of the crystalline basement, block yield Miocene ages (9-11 Ma). We infer that this gradient reflects an regional westward tilt of the crystalline basement and that the dominant topographic expression of the area was produced by uplift in the footwall of the east-dipping San Jose del Caba fault. In contrast we infer Neogene uplift in the footwall of the proposed La Paz Fault was very limited and if such a structure exists it lies beneath alluvial fans west of the range front.
Tesis de maestría
Ramos Velázquez, E. 1998.Características de la deformación en las rocas cristalinas cretácicas de la región E-SE de la ciudad de La Paz, B.C.S., México.. Tesis de Maestría en Ciencias. Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California. 122 pp.
Appears in Collections:Tesis - Ciencias de la Tierra

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