PROJECT SUMMARY: Active faulting, sedimentation, and landscape evolution along a rift in a tropical monsoonal setting: Cabo fault and basin, Baja California Sur

Arrowsmith, Whipple, Heimsath – ASU; Umhoefer – NAU; Martínez Gutiérrez - UABCS

Intellectual Merit: We propose to study the largest active fault and sedimentary basin exposed on land along the southwestern margin of the Gulf of California: the San Jose del Cabo Fault, adjacent footwall range, and the upper part of the associated hanging wall San Jose del Cabo Basin in order to better understand the tectonic, surficial, and sedimentary processes active during the rift to drift transition along oblique-divergent plate margins. The fault system of the La Paz-Los Cabos region is an active left-stepping normal fault array with major offshore components. This project is the second part of a complete transect across the SW plate margin from the rift escarpment to the shallow offshore – the first part along the latitude of La Paz is being completed this year. Both projects have offshore research, with a La Paz Bay CHIRP cruise planned for late summer 2008, and a companion cruise to this proposed project on the offshore parts of the Cabo fault and basin to be planned once the La Paz Bay cruise concludes.

The Gulf of California is one of the best places in the world to study rifting: the plate boundary is actively forming so a complete transect across the rift margin is possible, rift segmentation is documented, major ongoing NSF-MARGINS projects have crustal to upper mantle foci, and it is a safe and accessible place to work. Our work will help resolve how upper crustal processes contribute to plate margin development in a rift. The project provides, for the first time that we are aware of, an integrated study of the topographic and basin development and sedimentary systems associated with active rifting in a setting of highly variable tropical storm-driven precipitation. Invoking the “source to sink” MARGINS concept, we can link the structure and tectonics on the rift margin to erosion and the subsequent sediment dispersal into both the terrestrial basin and the adjacent Gulf using recent high-quality bathymetry. Because this project completes an across-margin transect, we will also model the geodynamics of the deformation given the evolving topography and the steep gradient from escarpment to ocean ridges.

This project and completion of the across-margin transect is built upon testing three hypotheses that have global implications for rupturing of continental lithosphere: (1) Faulting has continued along the plate margin after sea-floor spreading initiated, but at reduced rates driven mainly by the 3-5 km relief in topography formed during earlier rifting from the rift escarpment (Cabo fault footwall) to the spreading ridge. (2) The early rift basin sedimentary record is driven by tectonics (faulting & isostasy), because footwall uplift enhances erosion that is further enhanced by tropical storms, which results in an overfilled basin and bypass to the offshore marine. (3) Slowing of faulting at initiation of sea floor spreading dampens the positive feedback of uplift-erosion-storms, but slowing subsidence keeps basin filled; this system and sedimentary record are climate driven with minor tectonic modifications.

We have assembled a new team for this project that includes the main PI’s on the present MARGINS project near La Paz (Umhoefer & Arrowsmith and colleague Martínez), and new members Whipple and Heimsath who bring added expertise in geomorphology and linked climate-landscape-tectonic processes. We will continue our working relation with colleagues studying near-offshore faults and sedimentation, thermochronology of footwall and basin rocks, and GPS and paleomagnetic rotations.

Broader Impacts: This project will have broader impacts in at least three areas: 1) Natural hazard understanding in the actively urbanizing tourist destination of the La Paz-Los Cabos region. The region is cut by low slip rate but active faults, which are sufficiently long and show paleoseismic evidence for late Pleistocene-Holocene >M7 earthquakes. In addition, approximately 10 hurricanes per decade pass within 250 km and produce significant flooding. 2) International educational and scientific exchange. Building on the strong ties with Professor Genaro Martínez-Gutiérrez at Universidad de Baja California Sur in La Paz that developed during our current MARGINS project, students and faculty from both the US and Mexico will spend significant time in academic exchange between the two universities. 3) We will make every attempt to involve women and minority students in our project. In our current project, 4 of 5 graduate students are women.