The University of Arizona

An integrated geophysical and geological study of the Monturaqui impact crater, Chile

H. UGALDE, M. VALENZUELA, B. MILKEREIT

Abstract


The Monturaqui impact crater (350-370 m in diameter and 0.1 Ma old), located in a remote area in northern Chile, was surveyed in December 2003 with detailed geophysics (gravity and magnetics), topography, petrophysics, and geology. The geology of the Monturaqui area is characterized by a basement of Paleozoic granites overlain by Pliocene ignimbrite units. No impact breccia was found in the area. The granites are the main lithology affected by the impact. Although the granite samples analyzed did not show evidence of shock metamorphism, quartz, and to a lesser extent feldspar and biotite grains from impactite samples exhibit different degrees of shock, ranging from planar microdeformation and cleavage to the development of intense planar deformation features (PDFs) and diaplectic glasses in some grains. The differential GPS survey allowed the creation of a detailed digital elevation model of the crater. Its dimensions are 370 m along the eastwest direction, 350 m along the north-south direction, and ~34 m deep. The crater exhibits a circular morphology with a preferred northwest-southeast elongation that coincides with the steepest slopes (~35°) on the southeast edge. The newly acquired gravity data shows a negative anomaly of ~1 mGal at the center and allowed the creation of a 3-D model with a RMS error of <0.1 mGal, which supports the predictions of a fracturing-induced low-density granitic layer on top of the unfractured basement.

Keywords


Monturaqui;geophysical signature;impact structures

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