Late Pleistocene-Holocene major earthquakes along the Boconò Fault (Mérida Andes, Venezuela): sedimentary record in the Los Zerpa moraine-dammed paleo-lake.
by
Christian Beck
Laboratoire de Géodynamique des Chaînes Alpines, U.M.R. C.N.R.S. 5025, Université de Savoie, Campus Savoie-Technolac, F-73 376 Le Bourget du Lac
Coauthors: Franck AUDEMARD (Fundaciòn Venezolana de Investigaciones Sismologicas/FUNVISIS, El Llanito, Caracas 1070, Vénézuela), Christian BECK, Eduardo CARRILLO (Instituto de Ciencias de la Tierra, Universidad Central de Venezuela, Caracas 1070, Vénézuela), Michel COUSIN (Laboratoire de Géodynamique des Chaînes Alpines, U.M.R. C.N.R.S. 5025, Université de Savoie, Campus Savoie-Technolac, F-73 376 Le Bourget du Lac

The Boconò Fault – a major active tectonic feature in northern South-America - roughly follows major axial valleys within the Mérida Andes. The latters (between 8° to 10°N, culminating at 5 004 m at Pico Bolivar) presents remnants of the last climatic event (Salgado-Sabouriau et al, 1977): a system of moraines and terraces is well-perserved close to the Fault, and often upon the Fault, and cutted and displaced (Schubert & Vivas, 1993).

There, seismological chronicles and integrated instrumental survey, were recently completed by paleosismological investigations, especially trenches across the Boconò fault (Audemard et al, 1997). During the last deglaciation, most of the frontal part of the morainic complexes acted as natural dams, and small lakes developed. Several are still present (Mucubaji, La Victoria, Negra) while most of these lakes desappeared and abandonned lacustrine sediments outcrops. Among the different sites, we selected for a detailed study the Los Zerpa complex, close to the mouth of Santo Domingo river.

The preserved Los Zerpa lacustrine fill, at the foot of two lateral moraines, has a general morphology of apparently « imbricated » small and low terraces. The sediments outcropping were deposited during Late glacial and beginning of the Holocene (14C datations are in process). They consist of siliciclastics, with one-meter thick finely laminated intervals and coarse sandy episodes ; within the first, a seasonal varved sensu scticto system has been established.

Synsedimentary deformations are frequent at centimetric to decimetric scale, and affect both clayey and sandy material. Different behavior are observed: liquefaction and flow, brittle-like and ductile-like disturbances. Faulting, slumping, and associated local high coarse supply, are tentatively related to major sismic events. Some of the latters may have induced instantaneous emptying of the lake ; thus, the « imbrications » of the different terraces should be considered as mainly a seismo-tectonic record.

Audemard F. A..- (1997) J. Geodynamics, vol. 24, 1-4 :155-167.

Salgado-Sabouriau M. L., Schubert C, & Valastro S. Jr..- (1977) J. Biogeogr., 4 :313-325.

Schubert C. & Vivas L..- (1993) Universidad de Los Andes, 345 p.., ISBN 980 221 707 7.

Date received: March 8, 2002