HR: 0800h
AN: H31A-1262    [Abstracts]
TI: River Captures and Erosional Disequilibrium Along Strike-slip Faults
AU: * Brocard, G Y
EM: broca002@umn.edu
AF: Department of Geology and Geophysics - University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AU: Fayon, A K
EM: fayon001@umn.edu
AF: Department of Geology and Geophysics - University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AU: Perg, L A
EM: lperg@umn.edu
AF: Department of Geology and Geophysics - University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AU: Paola, C
EM: cpaola@umn.edu
AF: National Center for Earth surface Dynamics, St. Anthony Falls Laboratory - 3rd Avenue SE & Mississippi River, Minneapolis, MN 55414 United States
AU: Teyssier, C
EM: teyssier@umn.edu
AF: Department of Geology and Geophysics - University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: Department of Geology and Geophysics - University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AU: Mota, M
EM: mmota_gt@yahoo.com
AF: Insivumeh, 7 Avenida 14-57, Zona 13, Guatemala, 01013 Guatemala
AU: Moran-Ical, S
EM: usacoban@usac.edu.gt
AF: Cunor - Universidad de San Carlos de Guatemala, Apartado 55, Coban, AV 16001 Guatemala
AB: River captures are internal instabilities of erosion systems and are inherently promoted by strike-slip faulting. A capture event can generate a wave of incision that propagates from the capture site upstream and/or downstream, resulting in an increased bulk erosion rate around the capture site. Thus, under steady boundary conditions, drainage diversions trigger pulses of erosion, sediment production, rock exhumation and isostatic rebound. Therefore, a significant part of the erosion in oblique tectonics can be achieved in a state of significant departure from short-term dynamic equilibrium. The frequency, intensity, and duration of these events set the timescale over which their integrated effects can be regarded as the expression of a long-term dynamic equilibrium. We are investigating the effects of a large river capture on the oblique collision between the North American and Caribbean plates in Guatemala. Several thousands of kilometers of strike-slip displacement have been accommodated along this boundary during the Tertiary. The deformation is now concentrated mostly along the E-W Motagua strike-slip fault. Oblique tectonics is discernable within a 50 km wide topographic belt, north of this fault (Sierra de las Minas - Sierra de Chuacus range). On the northern flank of this range, deformation includes 130 km offset across the Polochic strike-slip fault, documented by both geological structures and drainage patterns. Numerous elbows and dry valleys show the progressive transformation of the initial transverse (S-N) drainage crossing the fault into a transverse-parallel (E-W) system that developed during increasing displacement along the fault. The drainage reorganization operates by river lengthening, captures, and avulsions. One of the latest capture sites is surrounded by a large (110x30 km) zone of deeply (1500 m) dissected landscape that coincides with the captured catchment. This zone sharply contrasts with the surrounding areas where large fragments of a very subdued topography are preserved on the highlands. The capture has been interpreted as the diversion of the former headwaters of a westward flowing river located south of the fault (Rio Selegua) into a northward flowing river located north of the fault (Rio Chixoy), based on drainage pattern and preserved conglomerates. The capture event, the dissection of the landscape, and the uplift of the summit paleosurface are closely related, and likely Miocene in age. Newly discovered conglomerates confirm that the captured basin was drained by the Selegua River before being drained into the Chixoy River. Other newly discovered paleovalleys and conglomerates further document the expansion of the dissected captured watershed at the expense of surrounding catchments. Since the capture event, both the captured stream paleovalley and the subdued topography have been displaced by large normal faults, many of them striking parallel to the Polochic strike-slip fault. Recent faulted sediments on the Polochic Fault trace also display a significant vertical component of slip. The inception of this tectonic activity after the abandonment of the paleovalleys suggests that the faults may have accommodated the isostatic uplift that followed unloading of the captured drainage basin by erosion.
DE: 1130 Geomorphological geochronology
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8175 Tectonics and landscape evolution
SC: Hydrology [H]
MN: Fall Meeting 2005