Neotectonic and paleoseismicity studies on the Urumaco Fault, northern Falcon Basin, northwestern Venezuela

The northern Falcon Basin in northwestern Venezuela is affected by several small active faults, subordinated to the major right-lateral east-west-trending Oca-Ancon Fault System. A set of prominent NW-SE right-lateral faults - synthetic shears - such as the Urumaco, Rio Seco, Lagarto and La Soledad faults, stands out among those. The Urumaco Fault, located between the Lagarto and Mitare rivers (in the Urumaco Trough, west of Coro), presents a rather complex active fault trace that comprises two NW-SE fault segments linked by an ENE-WSW reverse echelon, all showing a restraining stepover geometry. Its western segment seems to continue to the north at sea. Conversely, the eastern one dies out on land and its northern tip ends in a transtensive horse-tail structure, that disrupts an Early Pleistocene conglomerate. This same unit is flexed and upheaved some 30 m at the restraining overlap. The kinematics and present stress tensor, the latest activity and the seismogenic potential of the eastern segment of the Urumaco Fault, have been assessed at a set of three river cuts of an ephemeral tributary stream of the Urumaco River, 3 km north of the Urumaco village, where the Urumaco Formation is truncated by a Late Pleistocene terrace (C-14 date of 20, 700 +/- 950 yr BP at the base) of the Urumaco River. On the one hand, one of these outcrops features the Urumaco Fault affecting the Late Miocene Urumaco Formation, which comprises two prominent fault planes disposed as a wedge. The southwestern bounding plane juxtaposes two different sequences whereas the northeastern one does not, implying different slip behavior. In fact, the northeastern plane shows oblique-slip striations (29 degrees N, normal-dextral), whereas the other one shows perfectly horizontal striations (right-lateral). On the other hand, both updip plane prolongations in the overlying alluvial unit are not so sharp, if the 17-cm throw of the erosive bottom of such terrace measured at the lowermost part of the southwestern plane is regarded as an artifact. However, a mudflat deposit within this unit is bent with a 14-cm throw right above the northeastern fault plane clearly affecting the underlying Miocene unit. The estimated total offset per event allows to infer the occurrence of two individual events of magnitude ranging between M-s 5.8 and 6.4 on this strand of the eastern segment of the Urumaco Fault in the last 20,000 yr. (C) 1999 Elsevier Science B.V. All rights reserved.