Bouguer Gravity Anomalies and Regional Crustal Structure in Central Mexico

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Abstract
Analysis of the Bouguer gravity anomalies for central Mexico, between 18° and 21° N and 96° and 106° W, is used to document the regional crustal structure beneath the active magmatic arc, the continental margin, and the high-elevation provinces north of the arc. A variogram analysis was performed first to investigate the statistical properties of the gravity anomaly map and to define an optimum sampling scheme for application of spectral and iterative modeling methods. A sampling rate of 6′ (11.11 km) was selected. Bouguer gravity is characterized by large wavelength negative anomalies of −200 to −250 mgal over the magmatic arc and a smooth increase of about 2 mgal/km to positive values toward the Pacific and Gulf margins. Short-wavelength anomalies 20 to 50 km in size correlate with major structural features. Crustal models were derived for 11 N-S profiles spaced one degree apart and three long E-W profiles, from spectral analysis and two different iterative modeling schemes that model the crust-mantle interface by a series of prisms or Talwani-type polygons. Crustal thickness increases from the Pacific and Gulf of Mexico margins toward the continental interior, up to about 46 to 50 km beneath the east-central sector of the magmatic arc, the Central Altiplano and central Michoacan. This regional pattern can be observed in the upward analytical continuations at 16 km and 32 km above reference level, and it correlates well with available seismic refraction and surface-wave data. There is an inverse correlation between the crust-mantle interface and topography, which is compatible with crustal roots in Airy-type isostatic models. The area is under regional isostatic equilibrium, with short wavelength departures over the high-altitude volcanic terrains.