Non Washington Post: Unconformity in the Mecca Hills

I took a trip to visit friends and they joined me on a little geology adventure to the Mecca Hills. The Mecca Hills are a low range of hills on the southern end of the San Andreas Fault zone. The southwest side of the hills has the main trace of the fault and a second fault is located along the northeast side of the hills. The landscape along and between these fault strands is spectacularly contorted. Sylvester and Smith (1987) describe the unique structures that evolved along the faults. The canyons cutting through the hills are a structural delight and a stopping place for many new geologists to map strike and dips and folds and get a three dimensional sense of rock units.  

 

One feature that is tucked up in Painted Canyon is that basement rocks have been uplifted and the unconformity contact is extremely well exposed.   

 

The older basement rocks themselves tell a very complicated story that is variously interpreted and requires a broader context to even begin to appreciate. The short story is that these rocks were thrust under the North American margin, were penetrated by magma, and later unroofed. See Jacobson and others (2007) for a more detailed introduction and an appreciation of working out very complex tectonics. The exposures of these rocks along the canyon walls of upper Painted Canyon provide an opportunity to see a deep crustal section. 

Getting to these features is a fun adventure and has been greatly improved since I once frequented these canyons (we lived near here during another era).

The Mecca Hills are accessed via Painted Canyon - county park and along Box Canyon, a paved road that connects from the Highway 111 and Interstate 10. The upper Painted Canyon and most of the hills are BLM managed Wilderness. A volunteer group maintains the ladders that get one past the various cliffs and drop offs in the canyons.