Electronic Theses and Dissertations

Identifier

254

Date

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Earth Sciences

Concentration

Geophysics

Committee Chair

Jer-Ming Chiu

Committee Member

Mitch Withers

Committee Member

Robert Smalley

Committee Member

Oliver Boyd

Abstract

Three-dimensional velocity structures and seismicity patterns have been studied across the Andean Foreland of San Juan Argentina using data acquired by PANDA deployment. Distinct velocity variations are revealed between Precordillera in the west and Pie de Palo in the east. The low velocity anomaly beneath Precordillera is associated with the presence of thick sedimentary rock and thick sediment cover of Matagusanos valley. Similarly, the high velocity anomaly east of Eastern Precordillera is correlated with the presence of basement rocks. These anomalies are observed from the station corrections of Joint Hypocentral Determination (JHD) analysis. A northeast trending west dipping high velocity anomaly is imaged beneath the southern half of Pie de Palo. This anomaly represents a Grenvillian suture zone formed when Pie de Palo collided with the Precordillera. Relocated seismicity using 3-D Vp and Vs models obtained in this study revealed crustal scale buried faults beneath the Eastern Precordillera and Sierra de Pie de Palo. The fault defined by the seismicity extend down to a depth of ~40 km and ~35 km beneath Precordillera and Pie de Palo, respectively, defining the lower bound of the brittle to ductile transition of the crust. These results confirm that present day active crustal thickening involves the entire crust in the tectonic process and results in thick-skinned deformation beneath both the Eastern Precordillera and Pie de Palo. Based on the seismicity pattern, geomorphology, and velocity structures, Sierra Pie de Palo, a basement uplift block, can be divided into two separate semi-blocks separated by a northeast trending fracture zone. The northern block is characterized by a well-defined west dipping fault and low Vp/Vsration particularly at a depth of 12 to 16 km, while the southern block shows poorly-defined east dipping fault with high Vp/Vs ratio at a depth of 20 to 26 km. Spatial distribution of the well-relocated crustal earthquakes along these faults reveals that major deformation of the thick Andean crust in the region is mainly concentrated along localized regions.The geometry of the subducted Nazca plate defined by the relocation of 1040 intermediate depth events is flat at a depth of 100 ± 3 km until it resumes its normal subduction further to the east and south of the study area. The thickness of the seismogenic zone at intermediate depth is narrowly defined to 10 km within the flat subducting slab, a result that is a significant improvement from previous studies. Previous focal mechanism studies show dominant normal mechanism with tensional axis perpendicular to the trench axis indicating the likely mechanism of deformation is slab-pull (Stauder, 1973; Reta, 1992; and Anderson et al., 2007). Thus, the geometry of the subducting slab Nazca plate and their corresponding tectonic process can be better understood.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.

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