EVIDENCE OF THE EXISTENCE OF AN UPLIFTED AND A STATIC DOMAIN IN THE JALISCO BLOCK

The limits between the Jalisco Block (JB) in western Mexico and the NAM plate are identified with graben structures; to the NE it is bounded by the Tepic-Zacoalco (TZ) graben, to the E by the Colima graben, and to the NW by a graben that extends from the TZ graben down to Bahía de Banderas. The JB is also described as an uplifted domain of exposed crystalline basement; the Puerto Vallarta batholith is exposed at 1000 to 2500 m of elevation, indicating a significant uplift since its emplacement. Most of the uplift occurred in the Paleogene; fission track findings indicate that it was at a depth greater than 1000 m by Eocene times. There is a sharp increase of ~1000 m in average altitude from the lowlands to the plateau; the drastic physiographic difference suggests that this may be the western limit of the uplifted domain of the JB. The JB is herein divided in two domains: the continental platform-coastal plains, and the uplifted domain proper. The aeromagnetic response shows that although the uplifted portion of the JB has scattered, positive anomalies, a negative anomaly background permeates the area. Upper continuations to 4, 6, and 10 km in the JB (~35,000 km²), show that in the uplifted portion the scattered positive anomalies tend to disappear, leaving a wide magnetic low, with values of the residual field at 10 km ranging from -140 to -50 nT. Magmatism is also confined to the uplifted domain. In the lowlands, magnetic highs dominate the response in a triangular area whose vertices are Cabo Corrientes, Autlán and Manzanillo; the upward continued data show residual values at 10 km ranging from 50 to 180 nT. The limit from Autlán to Cabo Corrientes trends NW, and roughly approximates the limit of the uplifted domain. The region of positive anomalies is the region of the lowlands, which either was not uplifted, or the uplift was not as large as in the former region. We will call this the static domain of the JB. These observations suggest that, originally emplaced as a batholith, the JB was modified by tectonic forces that have acted differently on the pluton, creating two domains, at least since the Paleogene. Although the whole of the batholith can be described as an assemblage of granitoid rocks, the tectonic alterations in time apparently have induced physiographic, magmatic, and geophysical variations between the two domains.