A geophysical and geologic analysis of the development, structure, and activity of the Eastern Sierras Pampeanas, Argentina

Tonya J Richardson, Purdue University

Abstract

The Eastern Sierras Pampeanas located >600 km east of the Andean Cordillera is interpreted to be a response to lithospheric shortening related to flat subduction of the Nazca plate beneath the South American plate. This region consists of a series of basement-involved uplifts with elevations up to ∼2 km separated by topographically low areas of 500-1000 m elevation. The western margins of these ranges are bounded by east-dipping reverse faults. Uplift of the ranges occurred during Neogene time, but several questions remain regarding the timing of deformation, amount of seismicity, and subsurface structure of the Eastern Sierras Pampeanas. We address these unknowns at multiple scales using fault observations, preliminary exhumation data, local seismicity, and receiver functions and integrate the results into a geologic model of the region. Our results from these analyses are the following: (1) From observations at major fault exposures and the most recent dating of footwall strata, we determine that range-bounding faults thrust late Paleozoic schist and gneiss over Pliocene to Pleistocene alluvial sequences. The timing of fault displacement and age of footwall strata suggest that deformation of the region began in Pliocene time and youngs to the west. (2) To clarify the low-temperature exhumation history of the region, we conducted a preliminary study of the amount and timing of uplift of a single range using apatite and zircon (U-Th)/He thermochronometry. This analysis yields cooling ages ranging from Permian to Early Jurassic time for the upper 9 km of the crust, from which we infer 1-3 km of exhumation for the range in late Neogene time. (3) To determine the modern deformation and structure of the Eastern Sierras Pampeanas, we deployed the first local seismic array, and from recorded data, we located local seismicity and calculated receiver functions. From these crustal scale observations, we interpret the position of the Moho at 37 km depth, the transition from brittle to ductile deformation between 20-25 km depth, and a mid-crustal detachment zone between 15-20 km depth. The majority of seismicity is located away from range-bounding faults and defines a vertical boundary between the craton and terranes that we propose as a right-lateral, strike-slip fault zone. From these interpretations, we construct a model of the Eastern Sierras Pampeanas in which the craton acts as a rigid backstop to the eastward propagation of stresses from the subducting slab. Deformation then is forced to propagate back to the west along a detachment near the brittle-ductile transition. This is the first study to integrate surface and subsurface data to determine the structure and Pliocene to modern deformation of the Eastern Sierras Pampeanas in response to active flat-slab subduction.

Degree

M.S.

Advisors

Ridgway, Purdue University.

Subject Area

Geology|Geophysics|Latin American Studies

COinS