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Néotectonique et sismotectonique de la région d'El Tocuyo- San Felipe(Vénézuéla centro-occidental)
... Los espesores de sedimentos se calcularon con base en un modelado gravimétrico 3D construido con el programa IGMAS (3D Interactive Gravity Magnetic Application System) (Götze y Lahmeyer, 1988; Schmidt y Götze, 1998) a partir de 43 de secciones bidimensionales (2D) orientadas NO-SE, paralelas hacia el noreste y sureste de la sección mostrada en la figura 2. Se consideró la geología del área (Giraldo, 1985;Betchtold, 2004;Hackley et al, 2005;Rodríguez, 2008), espesores preliminares de sedimentos según resultados de la relación H/V (Rocabado, et al., 2007), información del modelado gravimétrico bidimensional de De Marco et al. (2004) y profundidades del contacto roca-sedimento según modelado de sísmica de refracción (Morales et al., 2008) y pozos. Se tomó información de once (11) perforaciones que alcanzaron el contacto roca-sedimentos; dos de las cuales se realizaron para FUNVISIS (ubicadas en el Decanato de Ingeniería Civil-UCLA en Barquisimeto y en el postgrado de Agronomía-UCLA en Cabudare) (GISCA, 2006), el resto pertenecen a los estudios de prospección por INOS (Figura 1). ...
Using bibliographic data I´ve proposed current stress field pattern in the Caribbean area.
Se reconstruye el campo actual de esfuerzos tectónicos en base a datos geologicos y geofisicos
The Bocono fault is a major NE–SW-trending, dextral fault that extends for about 500 km along the backbone of the Venezuelan Andes. Several large historical earthquakes in this region have been attributed to the Bocono fault, and some of these have been recently associated with specific parts through paleoseismologic investigations. A new trench study has been performed, 60 km to the northeast of Merida in the central Venezuelan Andes, where the fault forms a releasing bend, comprising two conspicuous late Holocene fault strands that are about 1 km apart. The southern and northern strands carry about 70% and 30% (respectively) of the 7–10 mm=yr net slip rate measured in this sector, which is based on a 40 vs. 85–100 m right-lateral offset of the Late Pleistocene Los Zerpa moraines. A trench excavated on the northern strand of the fault (near Morros de los Hoyos, slightly northeast of Apartaderos) across a twin shutter ridge and related sag pond exposed two main fault zones cutting Late Pleistocene alluvial and Holocene peat deposits. Each zone forms a shutter ridge with peat deposits ponded against the uplifted block. The paleoearthquake reconstruction derived from this trench allows us to propose the occurrence of at least 6–8 earthquakes in the past 9000 yr, yielding a maximum average recurrence interval of about 1100–1500 yr. Based on the northern strands average slip rate (2.6 mm=yr), such an earthquake sequence should
have accommodated about 23 m of slip since 9 ka, suggesting that the maximum slip per event ranges between 3 and 4 m. No direct evidence for the large 1812 earthquake has been found in the trench, although this earthquake may have ruptured this section of the fault. Further paleoseismic studies will investigate the possibility that this event occurred on the Bocono fault, but ruptured mainly its southern strand in this region.
Deformation along the southern Caribbean coast, as confirmed by the compilation of stress tensors derived from fault-plane kinematic indicators (microtectonics) and further supported by focal mechanism solutions herein presented, results from a compressive strike-slip (transpressional senso lato) regime characterized by a NNW–SSE maximum horizontal stress (ςH=ς1) and/or an ENE–WSW minimum (ςh=ς3 or ς2) horizontal stress, which is responsible for present activity and kinematics of six sets of brittle features: east–west right-lateral faults, NW–SE right-lateral faults—synthetic Riedel shears, ENE–WSW to east–west dextral faults—P shears, NNW–SSE normal faults, almost north–south left-lateral faults—antithetic Riedel shears, and ENE–WSW reverse faults—sub-parallel to fold axes and mostly in the subsurface; the latter ones being associated to ENE–WSW-trending folding. In this particular region, the brittle deformation obeys the simple shear model, although not all the deformation can be accounted for it since partitioning is also taking place (regional folding and thrusting is essentially due to the normal-to-structure component of the partitioned maximum horizontal stress). Conversely, the maximum horizontal stress on the Maracaibo block and south of the Oca–Ancón fault progressively turns counter-clockwise to become more east–west-oriented, allowing left- and right-lateral slip along the north–south-striking and NE–SW-striking faults, respectively. The orientation and space variation of this regional stress field in western Venezuela results from the superposition of the two major neighboring interplate maximum horizontal stress orientations (ςH): roughly east–west-trending stress across the Nazca–South America type-B subduction along the pacific coast of Colombia and NNW–SSE-oriented one across the southern Caribbean boundary zone.
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