Arc‐parallel shear deformation and escape flow in the mantle wedge of the Central America subduction zone: Evidence from P wave anisotropy

Abstract

[1] The P wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of Central America covering offshore and onshore parts of southern Nicaragua and northern and central Costa Rica. Our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half-year intervals between 2002 and 2006. The P wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic P wave velocity distribution. In the upper crust beneath Costa Rica, the seismic anisotropy is of the order of 2%-5% and locally strongly variable in orientation corresponding to the complicated tectonic structure. In contrast, the upper mantle beneath both study regions shows stronger anisotropy of the order of 5%-10% and a more coherent orientation of the axis of fast P wave velocity that varies systematically on regional scales: The upper mantle of the incoming oceanic plate is characterized by a trench-normal orientation of the fast P wave velocity axis presumably corresponding to mineral alignment (lattice preferred orientation LPO) in transport direction. This pattern is corroded in the uppermost part of the subduction zone, possibly by the influence of bending-related trench-parallel faults and serpentinization that can overprint or annihilate the LPO anisotropy. The upper mantle of the overriding plate and mantle wedge shows a clear trench-parallel orientation of the fast P wave velocity axis. The anisotropy is stronger in Nicaragua than in southern central Costa Rica (factor of 2 orders of magnitude). The development of a stronger anisotropy in the Nicaragua could be driven by a change in the stress regime from compressional in southern Costa Rica to transpressional in Nicaragua corresponding to a change from near-orthogonal subduction in the SE to oblique subduction and slab retreat in the NW. From a comparison with S/SKS wave, GPS, and geochemical observations follows that the most likely explanation for the observed pattern of P wave anisotropy in the mantle wedge is LPO caused by a trench-parallel shear deformation and/or NW oriented escape flow originating in the compressional zone near the Cocos Ridge collision area.

Seismic anisotropy tomography: New insight into subduction dynamics

“…The tectonic setting of Central America is characterized by the subduction of the Cocos plate beneath the Caribbean plate (e.g., Rabbel et al, 2011). The Cocos plate shows strong structural variations within relatively short distances.…”

“…The Cocos plate shows strong structural variations within relatively short distances. Lateral changes along the volcanic arc of the overriding Caribbean plate are a consequence of lateral changes in the structure, age and composition of the incoming oceanic Cocos plate (e.g., Carr et al, 2007;Rabbel et al, 2011). The incoming plate consists of three segments (von Huene et al, 1995): (1) a smooth segment of regular oceanic lithosphere with long trench-parallel fractures off northern Costa Rica (Nicoya Peninsula segment) continuing to Nicaragua, (2) a topographically rough segment covered with seamounts off central Costa Rica (Jaco-Quepos segment), and (3) the thickened and rough oceanic lithosphere of the Cocos Ridge subducting beneath southern Costa Rica (Osa Peninsula segment).…”

Subduction Zone Tomography

“…3.17 but for P-wave radial anisotropy tomography under SW Japan oblique subduction and slab retreat in the NW. The obtained P-wave anisotropy in the mantle wedge reflects LPO associated with a trench-parallel shear deformation and/or NW oriented escape flow produced in the compressional zone near the Cocos Ridge collision area (Rabbel et al 2011).…”

“…Long (2013) has made a detailed review of seismic anisotropy studies of subduction zones; in particular, studies using shear-wave splitting measurements and receiver functions. So far, P-wave anisotropy tomography has been applied to investigate the 3-D anisotropic structure in a number of subduction zones, including New Zealand (Eberhart-Philips and Henderson 2004; Eberhart-Philips and Reyners 2009), Northeast Japan (Ishise and Oda 2005;Wang and Zhao 2008Cheng et al 2011;Huang et al 2011a;Tian and Liu 2013), Southwest Japan (Ishise and Oda 2008;Zhao 2012, 2013), South Kuril Liu et al 2013a); Java (Koulakov et al 2009), Central America (Rabbel et al 2011), Alaska (Tian and Zhao 2012), Eastern Tibet (Wei et al 2013), North China Craton above the stagnant Pacific slab (Wang et al , 2014Tian and Zhao 2013), and western USA (Huang and Zhao 2013c).…”

“…Applying the method of Koulakov et al (2009) to local earthquake arrival-time data, Rabbel et al (2011) obtained a 3-D P-wave anisotropy tomography of the Central America subduction zone (Fig. 3.20).…”