Teleseismic measurements of upper mantle shear wave anisotropy in the Isthmus of Tehuantepec, Mexico

Abstract

Summary Shear wave splitting measurements in the Isthmus of Tehuantepec (IT), southern Mexico, inferred from teleseismic core phases are presented. Measurements were made along a south-to-north profile across the IT. The results show a predominantly trench-normal pattern of fast polarization orientations with averaged delay times up to 2.2 s. Fast orientations near the trench suggest a corner flow in the mantle wedge and an entrained flow in the subslab region. Away the trench, fast orientations are parallel to the Absolute plate Motion, suggesting that the anisotropy in that region is driven by a simple asthenospheric flow. A comparison with splitting measurements made in the Mexican subduction zone shows a 17º clockwise rotation of the fast orientations of between east and west Mexico. This is consistent with the observed change in orientation of 19º clockwise in the Middle America Trench (MAT). This suggests that the rotation of the fast orientations is controlled by the change of orientation in the MAT.

New Constraints on the Complex Subduction and Tearing Model of the Cocos Plate Using Anisotropic Pn Tomography

“…We are not sure whether this is limited by the resolution or because its results contain structural information of different depths. Based on our velocity and anisotropic imaging results, (Bernal-Lopez et al, 2016;Castellanos et al, 2017;Soto et al, 2021;van Benthem, Valenzuela, & Ponce, 2013) we support the model that the flat-slab subduction zone of the Cocos plate has a retreat process from the Miocene; the low-velocity to the north of TMVB is due to the high temperature at the uppermost mantle, which is caused by subduction and dehydration in the northern region before the Central Cocos plate retreats to the further southern position (Figure 4). Despite the southward retreat of the Cocos plate, the uppermost mantle in the northern region has not yet cooled, leading to low velocity.…”

“…The short black bar in (b), whose length is proportional to the anisotropy magnitude, represents the fastest Pn direction. White bars with pink core denote shear‐wave splitting measurements (Bernal‐Lopez et al., 2016; Castellanos et al., 2017; Soto et al., 2021; van Benthem, Valenzuela, & Ponce, 2013). EC, El Chichon volcano; GM, Gulf of Mexico; IT, Isthmus of Tuxtlas; LTVF, Los Tuxtlas Volcanic Field; MAT, Middle America Trench; MCVA, Modern Chiapanecan Volcanic Arc; OGFZ, O'Gorman Fracture Zone; OFZ, Orozco Fracture Zone; SMDS, Sierra Madre Del Sur; SMOC, Sierra Madre Occidental; SMOR, Sierra Madre Oriental; TMVB, Trans Mexican Volcanic Belt; TR, Teuhantepec Ridge; YUB, Yucatan Block.…”

“…In some relatively new imaging results, the detailed information on the velocity structure directly related to the possible tearing of the Cocos plate is also not provided (Amaru, 2007; Hosseini et al., 2020; Li et al., 2008; Lu et al., 2019; van Benthem, Govers, et al., 2013; van der Hilst et al., 1997; Zhu et al., 2020). The existing shear‐wave splitting research in this region can provide the overall anisotropic effect under the station (Bernal‐Lopez et al., 2016; Castellanos et al., 2017; Celis et al., 2022; Soto et al., 2021; van Benthem, Valenzuela, & Ponce, 2013), but the spatial coverage and depth constraint are insufficient. Given the ongoing debate regarding the presence and location of slab tears in southern Mexico, we emphasized the need for new high‐accuracy and high‐resolution velocity and anisotropy tomography studies for further investigation.…”