Description
A simple Bouguer gravity map, based on 775 gravity stations spaced at an average of 1 mile, is presented for the Laguna Salada area, Baja California, Mexico. The area is located in the northwest corner of the Salton Trough-Gulf of California depression. The results confirm that a regional, linear and steep gravity gradient (East Side Anomaly) is oblique to the major faults bounding the east of the Laguna Salada graben. The East Side Anomaly develops about 30 miles north of the international border, passes on the east side of the Laguna Salada basin, and opens to the east around the south end of Sierra del Mayor. It probably closes arounf the northwest end of the Sierra de las Pintas and continues south. The East Side Anomaly is suggested to be a positive effect, due primarily to a near vertical-step discontinuity in the crust-mantle interface. This discontinuity delineates the western limit of thin crust under the Salton Trough. The depth to basement in the Laguna Salada graben cannot be determined as the shape and amplitude of the positive effect are not known. The crust-mantle discontinuity, designated as East Side Anomaly, is inferred to have been originally related to the fault patterns of Cretaceous to Miocene age. The fault patterns consisted of slightly west of north, northwest and northeast trending sets. The slightly west of north set was associated with the major extension and crustal thinning of the Gulf of California depression during the Miocene. This episode gave rise to the development of the East Side Anomaly. The anomaly generally coincided with the general north-south fault trends but locally was off-set by the northwest and northeast faults. The episode of major extension and crustal thinning also resulted in the formation of a zone of horsts and grabens between the Peninsular Range and the Gulf of California depression. Most of the translation of Baja California away from the mainland probably occurred along the strike-slip San Jacinto fault. Right-lateral shear resulted in the development of the Laguna Salada, Cucapa and associated strike-slip faults to the southwest. These later strike-slip faults cut across the original fault patterns and structures, including the north end of the Laguna Salada graben. These faults also distorted and obliquely cut the East Side Anomaly as is observed at the northeast edge of the Laguna Salada graben. The area southwest of the San Jacinto fault including Laguna Salada, does not appear to be one of "spreading" as the magnetic data do not indicate any presence of intrusives in the Laguna Salada fault zone .The observed extension is due to right- lateral shear.
