Don,
Your reference to the San Gabriel mountains and what it going on there would be my guess in regards to the Basin and Range.

One thought is that the Basin and Range is all part of the plate boundary between the Pacific and North American plates and is made up of smaller pieces that had been basically welded (compressed) together. Based on this it would seem that this area isn't all that stable and can easily be torn apart again and is being done because of the difference in direction of movement of the two plates.

Happy holidays,
Dennis