Exploring the physics of earthquakes and landscapes

Exploring Yucaipa Ridge

I’ve always been intrigued by the rapid uplift rate of Yucaipa Ridge. This ridge is a sliver of crystalline rock, almost 3 km high, exhuming at a rate of over 2mm/yr (2 km per million years) between two strands of the San Andreas fault in southern California. Such rates of uplift are quite rare here. I am starting a new project with my Ph.D. student, Alex Morelan, to try to figure out how this uplift works, and how it relates to earthquakes along the San Andreas fault through the San Gorgonio Pass. It is likely that the uplift has something to do with the complicated faulting in the pass, but it’s not clear yet quite how this works.

Earlier this month Alex and I spent a week together clambering around the steep terrain of Yucaipa Ridge and the Mill Creek (north branch of the San Andreas) fault zone. The asymmetry of debris shed from each side of Mill Creek canyon is a clear testament to the rapid rate of uplift of the ridge relative to the San Gorgonio block to the north (despite that this block is higher in elevation). There is also a clear signal of headward migration of Mill Creek that takes advantage of the ~100 meter-wide damage zone along the fault. This culminates in a spectacular headwall, known as the Mill Creek jump off, which forms the drainage divide with the Whitewater River. The top of the jump off is occupied by a several meter-high sand dune, semi-stabilized with manzanita and whitethorn bushes. This appears to form here due westerly winds that blow up the canyon and then are forced up over the jumpoff.

View downstream from headwaters of Mill Creek
This photo shows a view downstream from headwaters area of Mill Creek. Mill Creek fault visible in middle of image as smooth, bare slopes formed in damaged crystalline rocks. Near part of image shows a bouldery debris fan shed from Yucaipa Ridge to the south (left) This contrasts sharply with steep, but relatively sediment-starved catchments draining San Gorgonio block to the north (right).

Mill Creek fault, Whitewater River
Alex and I searched for evidence of activity of the Mill Creek fault in a number of places. We found the fault cutting gravel deposits at the Mill Creek jumpoff, and within the Whitewater river drainage (shown above is the Mill Creek fault cutting and folding gravel along the east fork Whitewater river, south side up). We need to know something about the age of these deposits to say something definitive about the rate of activity of the Mill Creek fault. But for now we can be confident that the Mill Creek fault is active. We also found some exciting evidence for the mechanism of uplift of Yucaipa Ride. However that story needs to wait for more work to be completed.