Gravity modelling of regional crustal and upper mantle structure of the Guerrero terrane - 1. Colima graben and southern Sierra Madre Occidental, western Mexico.

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J. Urrutia Fucugauchi
R. S. Molina-Garza

Abstract

Quantitative modelling of the Bouguer and Free-Air composite gravity anomaly map for western-central Mexico and of a long, -400 km north-south, gravity profile is used to investigate the crust and upper mantle structure beneath the region. Crustal thickness increases from the Pacific ocean margin towards the continental interior, presenting higher values beneath the plateaus of the Trans-Mexican volcanic belt (TMVB) and the Sierra Madre Occidental (SMO) (in the range of -40.46 km thick). The region appears to be under regional isostatic equilibrium with shorth wavelengtll local departures. Results of the gravity study do not support interpretations of an igneous Mesozoic island-arc assemblage over oceanic lithosphere for the western Guerrero terrane. Instead, gravity modelling suggests a crust of continental (or transitional) character, with a thick lower crust of metamorphic or igneous nature. This may correspond to a high density granulitic lower crust, probably consisting of crystallized mafic liquids, cumulates and/or residues from removal of a partial melt, and garnet-rich metasediments, or alternatively, an igneous (dioritic or gabbroic) crust. Two-dimensional ¢rustal models are compatible with the occurrence of a low-to intermediate-density layer at the base of the crust which has: been proposed as responsible for the uplift mechanism of the volcanic plateaus. Geotherms constructed from travel time analysis of surface Raleigh and Love waves for central Mexico indicate a high thermal regime with partial molten material at the base of the crust. If this interpretation is correct, it provides a simple mechanism for close-to-trench volcanic activity and regional plateau uplift that incorporates a high thermal regime associated with low angle rapid subduction of a very young lithospheric plate.

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Urrutia Fucugauchi, J., & Molina-Garza, R. S. (1992). Gravity modelling of regional crustal and upper mantle structure of the Guerrero terrane - 1. Colima graben and southern Sierra Madre Occidental, western Mexico. Geofisica Internacional, 31(4), 493–507. https://doi.org/10.22201/igeof.00167169p.1992.31.4.1370
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