V34B-02
Multi-sensor geophysical constraints on crustal melt in the central Andes: the PLUTONS project
Wednesday, 16 December 2015: 16:15
308 (Moscone South)
Matthew E Pritchard1, Matthew Joseph Comeau2, Michael Edwin West3, Douglas H Christensen3, Heather L Mcfarlin4, Alexandra Kathryn Farrell5, Rodrigo Del Potro6, Joachim Gottsmann7, Stephen Russel McNutt5, Gary Michelfelder8, Mikel Diez9, Julie Elliott10, Scott T Henderson1, Laura Keyson11, Francisco Delgado12 and Martyn Jonathan Unsworth2, (1)Cornell University, Ithaca, NY, United States, (2)University of Alberta, Edmonton, AB, Canada, (3)University of Alaska Fairbanks, Fairbanks, AK, United States, (4)Univ South Florida, Tampa, FL, United States, (5)University of South Florida Tampa, Tampa, FL, United States, (6)University of Bristol, Bristol, United Kingdom, (7)University of Bristol, School of Earth Sciences, Bristol, United Kingdom, (8)Missouri State University, Springfield, MO, United States, (9)University of Bristol, Bristol, BS8, United Kingdom, (10)Purdue University, West Lafayette, IN, United States, (11)Incorporated Research Institutions for Seismology, Seattle, WA, United States, (12)Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY, United States
Abstract:
The central Andes is a key global location to quantify storage, transport, and volumes of magma in the Earth’s crust as it is home to the world’s largest zone of partial melt (the Altiplano-Puna Magma or Mush Body, APMB) as well as the more recently documented Southern Puna Magma Body (SPMB). We describe results from the recently completed international PLUTONS project that focused inter-disciplinary study on two sites of large-scale surface uplift that presumably represent ongoing magmatic intrusions in the mid to upper crust – Uturuncu, Bolivia (in the center of the APMB) and Lazufre on the Chile-Argentina border (on the edge of the SPMB). In particular, a suite of geophysical techniques (seismology, gravity, surface deformation, and electro-magnetic methods) have been used to infer the current subsurface distribution and quantity of partial melts in combination with geochemical and lab studies on samples from the area. Both Uturuncu and Lazufre show separate geophysical anomalies in the upper and mid/lower crust (e.g., low seismic velocity, low resistivity, etc.) indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used – reflecting the different sensitivity of each method to subsurface melt of different compositions, connectivity, and volatile content. For example, the depth to the top of the APMB is shallower in a joint ambient noise tomography and receiver function analysis compared to a 3D magnetotelluric inversion. One possibility is that the seismic methods are detecting brines above the APMB that do not have a large electromagnetic signature. Comparison of the geophysical measurements with laboratory experiments at the APMB indicate a minimum of 4-25% melt averaged over the region is needed -- higher melt volumes are permitted by the gravity and MT data and may exist in small regions. However, bulk melt values above 40-50% are considered unlikely as they would not be physically stable.
