Subsurface constraints of an active detachment fault in Laguna Salada Basin, Baja California, México, from interpretation of seismic-reflection profiles

The Laguna Salada Basin in northeastern Baja California, Mexico, is an active half-graben with subsidence principally controlled by two major faults along the eastern basin margin—the Canada David detachment fault and the dextral oblique Laguna Salada fault. Active-source, seismic-reflection data constrain the geometry of the active detachment fault and indicate two structural domains. The south domain is a supradetachment basin controlled by the Canada David detachment fault. Two seismic profiles indicate the detachment fault dips 17°–20° west, has a minimum of 10.1 km of slip, and accumulates a sedimentary wedge more than 2.5 km thick in the west-central part of this basin domain. This estimation indicates that the subsurface portion of the Canada David detachment accommodates 24% of extension in the western main plate boundary zone. The north domain is a dilatational stepover (or pull apart) controlled by the northwest-trending, west-dipping, dextral-oblique Laguna Salada fault and the north-trending, dip-slip Canon Rojo fault, which defines the south boundary of the pull-apart basin domain. The Canon Rojo fault accumulates more than 2 km of subsidence, but geometric considerations indicate that the basement in the hanging wall of the Laguna Salada fault projects to a depth of ∼3.8 km and intersects the 70° west-dipping Laguna Salada fault. Several faults cut the west margin of the floodplain lagoon and the hanging wall of both the Canada David detachment and the Laguna Salada fault. The largest fault is west dipping and produces ∼500 m of vertical offset. Its location projects south of the Canon Rojo fault, and we speculate these two faults may correlate. Seismic facies reflect its sedimentary environment and processes. Seismic facies 1 is high-amplitude, laterally continuous reflectors that represent flooding and prolonged lacustrine conditions. Seismic facies 2 is high- to low-amplitude, laterally discontinuous reflectors also representing flooding conditions. Seismic facies 3 is low-amplitude, poorly contrasted continuous to discontinuous reflectors interpreted as subaerial distal fan sandstone deposits. Seismic facies 4 is high-amplitude, discontinuous, imbricated to a chaotic pattern of reflectors. We interpret facies 4 as coarse-grained, high-energy alluvial fan deposits prograding over the basin floor from the west in the range front of Sierra Juarez. Seismic facies 1 and 2 predominate in the east and central portions of seismic profiles where the depocenter accumulates thicker sedimentary sequences.