IMAGING THE RIVERA-COCOS SLAB AND DEFINITION OF THE RIVERA-COCOS BOUNDARY

The boundary between the Rivera and the Cocos plates in western Mexico has been debated for about three decades. Geologic and geophysical studies have described the northern and southern sections of the Colima rift. The northern portion of the rift ends in the vicinity of the Colima volcanoes and is well developed morphologically. The southern portion lacks a similar definition, since no trace of a graben, or rifted section, has been found. We carried out a gravimetric survey inland comprising 754 stations that cover the region from the Colima volcanoes to the coast. Offshore we obtained Free Air anomalies at 1650 points from satellite derived measurements. The Bouguer anomaly map of the joint sea and land measurements was constructed from those measurements. The general trend was computed and subtracted from the total Bouguer anomaly in order to obtain the residual Bouguer anomaly of the region. Six trench-parallel and three trench-perpendicular sections were modeled gravimetrically from the residual Bouguer anomaly. The geometry of the oceanic slab was adjusted in order to fit the gravimetric values observed at each station. The models show that the westernmost of the latter sections presents a dip angle of 55º while the other two lines show dip angles of 32º. This result suggested to us that the former line samples the Rivera plate, while the other two lay along the Cocos plate. This in turn allowed for the location of a well defined boundary between the plates. From the nine 2-D gravimetric models of the above lines we extracted points corresponding to the upper slab surface of each line. By minimum curvature interpolation of that set of points we constructed the 3-D surface of the upper slab which shows a fragmented topography in the offshore region, smoothing out as the slab penetrates deeper. We attribute the disrupted topography of the slab to transpression in the offshore and Southern Colima rift regions. The boundary line runs along a depression of the slab in the offshore region, penetrates inland and continues up to the Colima volcanoes.