Hazardous Landscape at the Fluvio- Alluvial Lower Colorado River Basin, in the Cerro Prieto Area, Mexico

Landscape evolution studies are needed to improve our understanding of the erosion-deposition and deformational process in tectonically active zones. The Cerro Prieto area in Baja California, Mexico, is an excellent location for such a case study because it displays characteristic geomorphic and sedimentary features resulting from an active segment of the San Andreas Fault System. The Cerro Prieto area is a plio-quaternary sedimentary basin mainly filled by the Colorado River discharges and lying in an extensional bend between two major NW-SE trending right strike-slip faults known as the Imperial and the Cerro Prieto faults.

Ongoing field-based studies and satellite imagery analysis reveal that the Cerro Prieto area exhibits mud pots, mud volcanoes, fumaroles, geysers, thermal springs, load casts, mud dykes, sand blow structures, minor fault scarps, terraces, benches, deflected streams, and episodes of cutting and filling occurred in the drainage. The observed surficial deposits derived from adjoining desert ranges but mostly from the Colorado River show a distribution influenced by the Cerro Prieto fault and several subsidiary oblique and/or orthogonal trending faults. These faults are all active and form a pull-apart network style that is correlative with a regional thermal anomaly and frequent seismic activity, usually accompanied by subsidence (although subsidence occurs aseismically as well).

The youngest stratigraphic record in this arid landscape contains evidence of several unconformities and a large-scale southeastward shift of the Colorado River in order to reach the Gulf of California. The shifting of the Colorado River is clearly suggestive of a syn-tectonic deposition and attests a genetic relationship to the Cerro Prieto Fault that controls the deformation of the Cerro Prieto area.

The geomorphological and sedimentological features are the result of tectonic subsidence, natural thermal fluid activity, earthquake activity and fault movements in a hazardous scenery during late quaternary times.