The crust at northwestern Mexico interpreted from Magsat anomalies: Implications for the existence of the Mojave-Sonora megashear

Abstract

In an effort to characterize the crustal structure of northwestern Mexico (and constrain the Mojave-Sonora megashear) we studied the Magsat magnetic anomalies from that area. Published anomaly maps covering this area include an extensive positive anomaly covering the southern United States, a positive anomaly over the southern half of the Baja California peninsula, and a magnetic low in between. We interpreted a magnetic profi le over these anomalies, focusing on its tectonostratigraphic terrane nature. The profi le was further constrained by crustal thicknesses from seismological studies and heat fl ow data. In our model the Cochimi terrane and the North American craton (Colorado Plateau and southern Basin and Range) are characterized by high magnetic susceptibility in agreement with the mafi c nature of their corresponding crusts. The Yuma and the Seri terranes have lower magnetic susceptibilities as expected from their felsic to basic crustal nature. The oceanic crust from the Gulf of California is modeled with a low magnetic susceptibility value due to the high heat fl ow observed at the extensional basins. Our model satisfi es the presence of a subvertical contact between crusts with contrasting magnetic signatures of the Seri terrane (comprising the Caborca subterrane) and the southwestern sector of the North American craton. The Mojave-Sonora megashear itself is below the resolution of the Magsat data. Nevertheless, our model implies that because of their different magnetic signatures, the crystalline basements from the southern United States and northern Mexico (Seri terrane) are of different nature, which does not support the continuity of the North American craton into northwestern Mexico.

Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

“…9) correlate with a minor high in the Magsat data around the same location ( fig. 3 in Campos-Enríquez et al 2005). This lowvelocity feature is discontinuous below ∼45-50-km depth where it instead becomes a velocity high (bottom two panels in Fig.…”

“…9, left-central panel) may be associated with mafic intrusions at lower crustal depths that were emplaced in the wide core complex belt post 30 Ma and have since solidified. Alternatively, they may reflect differences in lower crustal composition of separate terranes at California Institute of Technology on January 5, 2015 http://gji.oxfordjournals.org/ Downloaded from or suture zones as they appear to be aligned with the second vertical derivative of Magsat anomalies used to map terrane boundaries in the region (Campos-Enríquez et al 2005). We note that the lower resolution of the Magsat data (150-300 km) relative to our shear wave velocity model does not allow all components of the lows and highs in that data set to be distinguished.…”