Gulf of California 2015, Leg 7 – Seafloor Faults

The research goal on this leg is to investigate seafloor deformation associated with tectonic motion along potentially young active faults on the continental margin off northern Baja California.

The faults on the continental margin off northern Baja California are the offshore extensions of tectonic faults that cross the northern Baja California peninsula or run parallel to the peninsula. Geologic evidence shows that these faults have been active in recent times onshore, but little is known abouit the seismic activity of the offshore extensives. The faults are believed to be part of a system of faults that extends northward, crossing the U.S.-Mexico border west of Tijuana and continuing offshore past Los Angeles, California. The faults in this system may constitute a significant seismic and tsunamic hazard extending from Ensenada to Los Angeles.

Data and samples collected by this team will be used to determine the age of fault slippage and to investigate the processes that are modifying the seafloor in these fault zones.

Logbook

The Western Flyer at sunset in the Gulf of California, March 2015. This photo was taken by a drone flown by Andrew McKee and Ben Erwin.

A well-oiled machine

Over the last 10 days, we got into a very efficient groove that required coordination of everyone onboard the Western Flyer. We conducted two ROV dives each day, collecting as many cores as we can, given the time we have on the seafloor.
We don’t yet know what minerals make up these mounds. They extended over a vast area along the fault.

Mysterious rock mounds

Today we explored the San Clemente Fault Bank and were intrigued by what we found. Along the fault, we found a vast expanse of rock mounds made of some kind of precipitated mineral.
Vesicomyid clam siphons (in pink) peek out of the mud. The red dots are lasers spaces 29 centimeters apart as a measurement tool.

Fault system off northern Baja California

Today we explored another section of the fault system off northern Baja California that had been previously mapped with MBARI’s AUV. The AUV mapping surveys were conducted in areas where Chief Scientist Charlie Paull suspected that faults are present.
Lauren Shumaker watches as the ROV pilot uses the manipulator arm to open the drawer on the ROV sled. The vibracorer is mounted on the front of the vehicle, which you can see on the main monitor.

Understanding offshore faults

Offshore faults are important sources of earthquakes and associated tsunamis. To investigate the occurrence of offshore faults, marine geologists look for two kinds of evidence.
Lauren Shumaker takes a photo of a core that has been split in half. The sediment will then be bagged and refrigerated until it can be analyzed by Mary McGann at the U.S. Geological Survey.

Sampling sediment at Coronado Canyon

On this expedition we are exploring the offshore extensions of tectonic faults that cross the northern Baja California peninsula or run offshore parallel to the Baja peninsula and coastline of Southern California.