HR: 1340h
AN: T13C-1390
TI: Measurements of Active Tectonic Deformation on the Guerrero Coast, Mexico
AU: * Ramirez, T
EM: maria_teresa_ramirez@yahoo.com
AF: WLA, 1777 Botelho Drive, Suite 262, Walnut Creek, CA 94596 United States
AU: Cundy, A
AF: University of Sussex, School of Life Sciences, Falmer, Brighton, BN1 9QJ United Kingdom
AU: Carranza-Edwards, A
AF: Instituto De Ciencias del Mar, Universidad Nacional Autonoma de Mexico, Circuito de la
investigacion, Mexico D.F., 041000 Mexico
AU: Morales, E
AF: Instituto De Ciencias del Mar, Universidad Nacional Autonoma de Mexico, Circuito de la
investigacion, Mexico D.F., 041000 Mexico
AU: Kostoglodov, V
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Circuito de la
Investigacion, Mexico D.F., 04100 Mexico
AU: Urrutia-Fucugauchi, J
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Circuito de la
Investigacion, Mexico D.F., 04100 Mexico
AB: The study of tectonic deformation rates using displaced shoreline features is relatively wellestablished, and has provided much useful information on seismic hazard. Such studies have
frequently been complemented by analysis of the coastal sedimentary record, where past marine
to terrestrial environmental changes (and vice versa) may be recorded by clear changes in
stratigraphy. Studies of this type are particularly valuable for tectonically-active areas where the
preservation of former shoreline features is poor, or where long-term subsidence has resulted in
their erosion, drowning or burial. The specific objective of this study is to derive rates of tectonic
deformation from geomorphic and stratigraphic studies of the Guerrero coastal area, and to
examine the feasibility of this stratigraphic approach in the coastal lagoons of the Mexican Pacific
coast, in the Guerrero gap. The Guerrero gap coastal area, where a major earthquake is
expected to occur, parallels the Cocos plate subduction zone. Here convergence rates vary from
5.2 cm/yr to 5.8 cm/yr. The Guerrero gap has experienced several historical earthquakes, notably
the 1911 (7.8 Ms). However, no large magnitude events since the 1911 earthquake and only a
few Ms~6 events have occurred near the Guerrero gap edges. It is expected that a major
interplate earthquake of estimated magnitude Mw=8.1 to 8.4 has a high probability to occur.
Landforms within the Guerrero gap indicate that the coast is subsiding. A series of key indicators
such as elongated islands reminiscent of ancient barriers, submerged barriers island, extensive
marshy environments, increased depths in the lagoons, and submerged anthropogenic features
(shell mounds), among others, suggest active tectonic subsidence of the coast. In contrast, the
adjacent northwest area off the Guerrero gap exhibits landforms characteristic of tectonic uplift
(marine terraces and uplifted beach ridges), indicating a different seismo-tectonic regime
northwest of the gap. Core samples from nine study sites show distinct stratigraphic changes with
depth, indicating clear rapid change in depositional environments over time. Preliminary 14C data
indicate that the sediment sequences cored are up to ca. 4,626+/- 37 yr BP, and suggest an
estimated subsidence rate of about 1mm/yr. Geochemical characterization of the cores collected
show the potential for geochemical discrimination of terrestrial-marine and marine-terrestrial
environmental changes. Preliminary detailed linking of the geochemistry to the core stratigraphies
show downcore variations in sediment sequences that match sedimentary horizons considerably
enriched in S, Cl, and Br concentrations, that indicate possible marine inwash events (e.g.
tsunami).
DE: 7221 Paleoseismology
DE: 1824 Geomorphology (1625)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting