LAUSR

Abstract This work aims to characterize the crustal architecture and structure in an area of the Mexicali Valley between the southwestern part of the Cerro Prieto pull-apart basin, which contains the geothermal field of same name, and the southeastern edge of the Sierra Cucapah, Baja California, Mexico. Although the region surrounding the geothermal field has been extensively studied, the structural features in the vicinity are poorly known. Interpretations based on processed crooked-line 2D seismic reflection images made here reveal evidence of structural deformation and seismostratigraphic features in the study area. Two structural domains were identified adjacent to the Cerro Prieto fault: the northeastern domain, which belongs to the pull-apart basin, and the southwestern domain, which exhibits deformation associated with the Sierra Cucapah uplift. Subsurface images indicate the presence of the Cerro Prieto fault through lateral discontinuities in the reflectors; the fault is sub-vertically oriented and dips ∼80° to the east. Two seismic reflectors were identified to west of the Cerro Prieto fault, one of which corresponds to the acoustic basement. The other is associated with the top of an alluvial and deltaic facies and is described here for the first time. Three other seismic reflectors to the east of the Cerro Prieto fault correspond to gray shale sequences with interspersed sandstones in the Cerro Prieto geothermal field that were reported previously by other authors. The presence of two previously unknown structures are reported for the first time. The Sonora and Nayarit faults, with azimuths of ∼305° and ∼307° and lengths of ∼12 km and ∼9 km, respectively, are both located within the block bounded by the Cerro Prieto fault to the northeast and the Indiviso fault to the southwest. The La Puerta fault is also described for the first time, which exhibits reverse fault characteristics and lies transverse to the fault system of the Sierra Cucapah. The acoustic basement between the Sierra Cucapah and the Cerro Prieto fault is sub-horizontal and dipping slightly westward, and differs from the eastward-dipping acoustic basement observed to the south between the Sierra Las Pintas and the deltaic plain. Subsidence in the sequences within the Cerro Prieto basin can be more clearly observed after 500 m, with evidence in profile L-25 suggesting greater vertical displacement towards the southeast. Two areas of opportunity are reported for geothermal interest purposes. Despite the additional knowledge about regional tectonics and strucural background provided by this work, further studies are needed. The fault geometry remains poorly defined and due to its location, much of the fault is buried by sediments that prevent a full understanding of the potential danger in the region. Here, previously unknown structures are reported that represent geological and seismic risks. In addition to their scientific value, the findings have a potentially large societal impact due to the presence of several cities in the study area. The results should therefore be incorporated into future regional risk assessments.