Graduate Thesis Or Dissertation
 

Seismotectonics in the eastern Precordillera, San Juan, Argentina : reconciling earthquakes and structural geology in the vicinity of the 1944 earthquake for a new model of crustal-scale deformation

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  • This thesis synthesizes surficial structural data, a detailed analysis of an earthquake in 1944, and historical seismicity into a new model of crustal scale deformation in the eastern Precordillera in northwestern Argentina in the vicinity of San Juan. The eastern Precordillera was uplifted during previous thin-skinned deformation episodes, and is currently being overprinted by active structures in the basement. Therefore there is a disconnect between the active basement faults and surficial structures because movement is filtered through previous deformations. The 1944 earthquake is an example of this disconnect: it occurred at 25 km depth on a north-south striking thrust fault, and resulted in northeast-southwest flexural slip related to fold growth at the surface. We used teleseismic long period body waves to invert for parameters such as centroid depth, the source time function, the moment, the strike, dip, and rake of the nodal planes, and type of mechanism. The centroid depth is located at 24 km, the source time function shows one sharp pulse indicating a simple event, the moment equates to a body wave magnitude of 6.76, and the nodal planes are north-south striking, either steeply east- or west-dipping thrust solutions with a small component of right lateral strike slip motion. We prefer the east-dipping plane in the context of isoseismals, other published epicentral locations, microseismicity in the area, and regional structural geology. A suite of progressively abandoned Pleistocene-Holocene fluvial strath terraces has recorded recent deformation, whereas the underlying bedrock of Neogene redbeds and Paleozoic carbonate thrust sheets making up the ranges of the eastern Precordillera has recorded earlier deformational phases. The combination of these three structural levels in space in time show how deformation has progressed from thin-skinned back-thrusting associated with the development of a foreland fold and thrust belt, to fold-growth related deformation associated with movement on underlying thin-skinned thrust sheets, to block uplift and fold-growth associated with basement faults uplifting and deforming previously folded and faulted strata. The source parameters of the 1944 earthquake support the notion of thin-skinned structures being refolded by basement faults, and a crustal-scale model is constructed using structural geology to help constrain the preferred epicentral location of the 1944 earthquake.
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