GRAVITY STUDY IN THE MANZANILLO AREA, COLIMA MEXICO

The Manzanillo area, located in the state of Colima México, is a complex tectonic zone situated in the southern Colima rift. It is located ~150 km from Guadalajara and ~80 km from the active Colima volcano.

The offshore area comprises an active convergent zone (the Middle America Trench) where the Rivera and Cocos plates are subducting beneath the North American plate. The lithospheric fracture between the two subducting plates is not clearly defined (e. g., DeMets and Wilson, 1997). Some authors have proposed that the rupture is a consequence of a divergent and oblique movement of the Rivera plate relative to the Cocos plate (e. g., Bandy et al., 1995). This process has fragmented and deformed the overriding North American plate producing different tectonic structures in the onshore area (e.g. Ochoa et al., 2015) like the Colima rift and the Jalisco and Michoacán crustal blocks.

Recent studies have shown anomalous geophysical characteristics in the region of the southern Colima rift; this includes low body and surface seismic wave velocities (e. g., Escudero and Bandy, 2017; Brudzinski et al., 2016) and gravity anomalies (Álvarez and Yutsis, 2015).

To better define the source of these anomalies, we acquired 99 new relative gravity and GPS measurements during March and April 2017. The gravity data were tied to the global Latin American Gravity Standardization Network LAGSN77 via the absolute base (Ness, 1984) located in the Naval Base of Manzanillo (previously called the “Instituto Oceanográfico de Manzanillo”).

The gravity data has been corrected based on the new standards proposed by Hinze et al. (2005) for the North American region. A compilation of available gravity data (e.g., Skidmore, 1988; Serpa et al., 1982) in this area has been used to model the regional complete Bouguer trend. Once this trend is subtracted from our local data, we obtain at least two important negative gravity residual anomalies (~26 mGal). These lie near the active Tamazula fault zone where the March 2000, M_w = 5.3, normal earthquake occurred (Pacheco et al., 2003). Based on the anomaly wavelength and spectral analysis, we have found a middle to upper crust source causing these residual gravity anomalies probably related to fracturing due to the regional NW-SE extensional stresses.