A receiver function study of Southern Costa Rica : indications of steep Cocos Ridge subduction.

Dzierma, Yvonne (2009) A receiver function study of Southern Costa Rica : indications of steep Cocos Ridge subduction. (Doctoral thesis/PhD), Christian-Albrechts-Universität, Kiel, 231 pp.

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Abstract

Central America is characterized by the subduction of the oceanic Cocos Plate underneath the Caribbean Plate and Panama Microplate along the Middle America Trench, in a setting that is transitional between oceanic island arc and continental active margin subduction. In addition to providing an interesting comparison with purely oceanic or continental subduction zones, the Central American subduction zone is particularly fascinating owing to its pronounced lateral variability regarding the nature of the overriding plate, subducting plate, seismicity, volcanism and geochemistry. To explore the deep structure of the subduction zone, a transect of broadband seismometers across the Talamanca Range (Southern Costa Rica) was operated from spring 2005 to spring 2007. This region is characterized by the subduction of the aseismic Cocos Ridge, which has been observed to dip at an angle of 18 deg down to depths of about 40 km. Based on this shallow subduction angle, it has been speculated that shallow underplating of the Cocos Ridge is responsible for the gap in active volcanism and strong uplift in the Cordillera de Talamanca region. A receiver function analysis of 522 teleseismic earthquakes was performed, yielding 1777 receiver functions. The data from the Talamanca Transect stations was complemented by short-period data recorded at the Quepos network land-stations, adding another 100 receiver functions for 39 events. The Moho of the overriding plate is imaged around 35 km depth, and an intracrustal discontinuity is observed at about 20 km depth, in good agreement with previous active seismic studies. Contrary to expectations, the migrated sections clearly show a steeply dipping (ca. 60 deg) conversion down to more than 150 km depth, interpreted as the subducting slab. 3d-representations based on the migrated profiles do not indicate significant shallowing of the subducting plate towards the Cordillera de Talamanca. These results indicate for the first time that the Cocos Ridge may be subducting beneath Southern Costa Rica at a considerably larger angle and to greater depths than previously thought, opening new questions about the mechanism causing the gap in active volcanism and strong uplift in the Cordillera de Talamanca. Possible explanations include a compressional regime hindering the ascent of magmas to the surface or an anhydrous part of the subducting slab, preventing fluid-induced melting in the mantle wedge. Alternatives may be combined subduction/underplating scenarios or a transitional regime. Modelling studies confirm that the dipping feature observed in the migrated receiver functions can be caused by a slab dipping at approximately the same angle. Possible anisotropy scenarios, such as corner-flow and the resulting olivine lattice-preferred orientation, are also investigated. The results point to the existence of mantle-wedge corner-flow, with possible additional arc-parallel flow.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: UNSPECIFIED
Keywords: Geodynamics; seismology, receiver functions, subduction, Central America
Research affiliation: OceanRep > SFB 574
OceanRep > SFB 574 > A2
Open Access Journal?: Yes
Date Deposited: 15 Jul 2010 11:04
Last Modified: 06 Jul 2012 14:56
URI: http://oceanrep.geomar.de/id/eprint/8667

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