HR: 12:05h
AN: S12A-08    [Abstracts]
TI: Using Paleoseismic Investigations to Test Hypothesized Fold Growth Mechanisms: A Case Study From the La Laja Fault System, San Juan Province, Argentina
AU: * Schultz, E S
EM: eschultz@geo.oregonstate.edu
AF: Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Meigs, A J
EM: meigsa@geo.oregonstate.edu
AF: Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Ragona, D
S12A-08 AF: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography University of California-San Diego, La Jolla, CA 92093 United States
AU: Rockwell, T K
S12A-08 AF: San Diego State University, 5500 Campanile Dr, San Diego, CA 92182 United States
AU: Costa, C
S12A-08 AF: Universidad Nacional de San Luis, Ejercito de los Andes 950, San Luis, 5700 Argentina
AU: Ahumada, E
S12A-08 AF: Universidad Nacional de San Luis, Ejercito de los Andes 950, San Luis, 5700 Argentina
AU: Middleton, T
S12A-08 AF: San Diego State University, 5500 Campanile Dr, San Diego, CA 92182 United States
AU: Verdugo, D
S12A-08 AF: San Diego State University, 5500 Campanile Dr, San Diego, CA 92182 United States
AB: In 1944, a devastating earthquake (Mw 7.4) near the city of San Juan, Argentina created a 6-km-long fault scarp along an east-dipping secondary flexural-slip fault (La Laja fault). Both the La Laja fault and a monoclinal backlimb fold within the hangingwall to the east are growing above a blind thrust fault. Earlier studies of deformed fluvial terraces in the La Laja area postulated that fold growth accompanying increasing blind thrust displacement occurred by kink-band migration. This current collaborative study tested the validity of this fold growth mechanism via paleoseismic investigations. Two large (approx. 50m x 7m x 6m) trenches were excavated on the backlimb fold of the La Laja fault. The geometry of the strata exposed in the trenches provides new data to test existing models of fold growth on shorter scales than those represented by the deformed terraces. Growth strata revealed in the trenches provide evidence for two competing fold growth models. In the north wall of the southern trench (Trench MT), western and eastern domains in which strata have nearly horizontal dips are separated from a larger central domain characterized by relatively uniform east dips of 13-17 degrees by discrete axial traces. Fold growth by constant limb-dip kink-band migration is consistent with these observations. In contrast, folded strata exposed on the south wall of MT are more analogous to a model of flexural-slip folding via limb rotation. Strata in the central domain thicken downdip and dips fan from 21 degrees near the trench bottom to 4 degrees eastward near the ground surface. Secondary faulting and folding have modified the fold limbs in a northern trench (Trench MTJ) located approximately 150m north of MT. A keystone pop-up structure superimposed a tread-riser surficial morphology on tilted, older terrace deposits. Like the south wall of MT, stratal dips decrease with depth towards the surface. Fold growth apparently occurs by mixed-mode kink-band migration and limb rotation. Whether individual stratal packages or unconformities record discrete paleoearthquakes depends upon the outcome of ongoing excavations on the La Laja fault, age determinations of stratal packages, and closer scrutiny of the trench logs.
DE: 8002 Continental neotectonics (8107)
DE: 8036 Paleoseismology (7221)
DE: 8038 Regional crustal structure
DE: 8094 Instruments and techniques
DE: 8108 Continental tectonics: compressional
SC: Seismology [S]
MN: Fall Meeting 2005