The active trace of Boconó fault, across Llano Corredor and Mucubají passes

Highlands of the cordillera de Mérida, Venezuela: tectonic landforms and implications

Abstract

Two Boconó fault (BF) segments, at the crossing of the Mucubají and Llano Corredor passes (Cordillera de Mérida), have been characterized: the southern portion of Boc-C and its relay with Boc-B. Tectonic landforms were mapped following geomorphologic criteria, specifically for strike-slip faults. This geomorphologic-geologic approach provides reliable and precise location of the (potentially) active tectonic structures in a given region, but also characterizes its tectonic style parameters: type of faulting, sense of slip, slip rate, lateral extent, simplicity/complexity of the fault trace(s), damage zone and fault-zone width.

Segment Boc-C in the study area has been subdivided into 4 distinct sections: 2 of them (1 and 4) cut across or bound Last Glacial Maximum (LGM) glacial landscapes typical of Páramo conditions, while the others run along the southern margin of large valleys excavated by the Santo Domingo and Aracay rivers, with valley bottom profiles exhibiting 7 to 12% gradients. Each of these sections shows its particularity: section 1 mainly cuts across incompetent LGM moraine complexes, developing an “en echelon” fault pattern with transpressional (pressure ridges) and transtensional jogs or relay (pull-apart basins; PAB), while the BF at other sections is preserved in competent basement rocks. Section 4 juxtaposes LGM deposits resting onto the BF scarp, which are displaced some 200 m dextrally and 40 normally. While the slip rate of Boc-C inside the Apartaderos PAB is still ranging between 7 to 8 mm/a, it jumps to 11.9 mm/a outside the PAB, very comparable to the geodetic rate. On the other hand, the slip rate of Boc-B, at the Apartaderos PAB, remains in the order of 2.3-3.0 mm/a.