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BELL, J. S., Geological Survey of Canada, Calgary, Alberta, Canada, and P. B. JONES,* International Tectonic Consultants Ltd., Calgary, Alberta, Canada

ABSTRACT: Triangle Zones and Underthrusting Along the Southern Margin of the Cordillera de la Costa, North-Central Venezuela

Along the southern margin of the Cordillera de la Costa, the overthrust foothills give way southwards first to a discontinuous zone of multiple thrusts, then to laterally persistent south-dipping and overturned strata and finally to open folds as the shallow deformation dies out within the upper stratigraphic units of the Eastern Venezuelan Basin. Northwest-southeast trending strike- slip faults testify to right-lateral shearing, as well as south-directed shortening, during a transpressive phase that terminated in latest Tertiary time.

We interpret the Foothills Belt, the Thrust Fault Zone, the Overturned Belt and the Gentle Dips Belt as surface manifestations of the southward rise of a basal detachment surface up stratigraphic ramps. North-dipping thrust faults splay upwards from this detachment surface which terminates in a triangle zone located beneath the south-dipping and overturned strata. The inferred detachment zone must originate north of the overthrust strata, but the site and depth from which it propagates are unknown.

Surface mapping suggests that, beneath much of the northern Foothills Belt, the proposed detachment zone follows a decollement surface within the Upper Cretaceous Querecual formation. Beneath the southern part of the Foothills Belt, it appears to climb into the Paleocene Guarico Formation, from which it rises into the Oligocene Roblecito Formation. Deformation terminates southward in a triangle zone. Balanced cross sections illustrate the kinematics of the overthrusting and highlight the implications for hydrocarbon generation and entrapment.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.