Geometry and seismic properties of the subducting Cocos plate in central Mexico

Abstract

[1] The geometry and properties of the interface of the Cocos plate beneath central Mexico are determined from the receiver functions (RFs) utilizing data from the Meso America Subduction Experiment (MASE). The RF image shows that the subducting oceanic crust is shallowly dipping to the north at 15°for 80 km from Acapulco and then horizontally underplates the continental crust for approximately 200 km to the TransMexican Volcanic Belt (TMVB). The crustal image also shows that there is no continental root associated with the TMVB. The migrated image of the RFs shows that the slab is steeply dipping into the mantle at about 75°beneath the TMVB. Both the continental and oceanic Moho are clearly seen in both images, and modeling of the RF conversion amplitudes and timings of the underplated features reveals a thin low-velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab. By inverting RF amplitudes of the converted phases and their time separations, we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting Cocos plate and its thickness. High Poisson's and Vp/Vs ratios due to anomalously low S wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure. The evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates. This may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust.

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico

“…Main cities along the array are marked with red circles. a low permeability layer directly above the interface in the continental lower crust (Song et al 2009;Kim et al 2010), and that it might be composed of mechanically weak hydrated minerals, such as talc (Kim et al 2010(Kim et al , 2013. The latter is in accordance with numerical experiments of Manea et al (2013), which model the LVL as a remnant of the mantle wedge that experienced significant serpentinization since the onset of flat subduction.…”

“…Our interpretation of these inconsistencies is that the high heterogeneity and structural complexity of this layer is a direct consequence of the fragility of the low strength hydrous minerals that have been proposed to compose it (e.g. talc; Kim et al 2010), which, under a significant amount of stress, could result in a highly heterogeneous fracturing pattern. The lower oceanic crust, on the other hand, appears to be less heterogeneous, as previously suggested by Kim et al (2010).…”

“…talc; Kim et al 2010), which, under a significant amount of stress, could result in a highly heterogeneous fracturing pattern. The lower oceanic crust, on the other hand, appears to be less heterogeneous, as previously suggested by Kim et al (2010). Given the existing high confining pressure at this depth, most of the anisotropy within the lower oceanic crust is likely associated with the presence of foliated serpentine minerals and with the preferred orientation of olivine and pyroxene crystals generated during the formation and spreading of the oceanic plate.…”

“…This layer extends parallel to the trench up to where the Cocos plate changes its geometry, to a dip of 41 • to the north (Dougherty et al 2012) and to a dip of 26 • (Melgar & Pérez-Campos 2011) to the south (Dougherty & Clayton 2014). Based on the null seismicity at the interface in this region and the tensional stress regime in the upper plate (Singh & Pardo 1993), this layer has been proposed to absorb most of the deformation between the subducting and overlying plates, which would explain the horizontal geometry without significant coupling (Kim et al 2010). It has been suggested that this LVL has a high pore pressure that is confined by 360 C The Authors 2017.…”

“…Pardo & Suárez (1995) mapped the change on the slab's dip angle using local earthquake locations. Recently, more precise estimations of this geometry were obtained using teleseismic converted phases (Pérez-Campos et al 2008;Kim et al 2010Kim et al , 2012Song & Kim 2012a,b) and seismic wave tomography (Husker & Davis 2009;Chen & Clayton 2012) using an array of 100 broad-band temporary stations known as the MesoAmerican Subduction Experiment (MASE). MASE crossed Mexico from Acapulco, Guerrero, at the Pacific coast, to Tempoal, * Now at: Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.…”