1. How were the San Gabriel Mountains Uplifted?
Kim Gloersen
One mountain said to the other across the rift "Hey, it's not my fault!"

2. Plate Movements
30 Ma. , subductive tectonic movements occurred between the Farallon, North American, and Pacific plates.
20 Ma. the relative movements of the North American and the Pacific Plate changed from a head on head contact with the Farallon Plate to a lateral slipping against each other.
About 20 million years ago, the relative movements of the North American and the Pacific Plate changed from a head on head contact with the Farallon Plate to a lateral slipping against each other. Eventually, this created a zone of slippage, extending nearly the length of the state is known as the San Andres fault system. Along this zone, folding of the sea floor along the margin of the North American plate resulted in the creation of the coastal and the transverse mountains ranges, more specifically the San Gabriel Mountains. From its junction with the Garlock Fault, the San Andreas Fault makes a marked bend to the southeast for about 120 km. This segment is appropriately referred to as the Big Bend

3. The Big Bend
The Big Bend is known as a Restraining bend. When a strike-slip fault is offset along strike such that the resulting bend in the fault hinders easy movement occurs, a local shortening or transpression transpires. Because of the significant component of compression in the region, rocks are actively being squeezed and uplifted. As a result of the compression, spectacular mountain ranges have been thrown up along the margins of the fault, such as the San Gabriel Mountains. The steep mountain slopes have shed enormous quantities of debris that are spread across the range fronts in large alluvial fans. The fans, increasingly occupied by high-density housing and commercial developments, are in many places cut by reverse faults caused by compression in this area. The right-lateral motion along the San Andreas system is taken up by a number of different number of fault strands in the Big Bend segement. The San Andreas itself defines the eastern end of the Big Bend segment, and at present is the most active fault strand.
. In addition to the San Andreas Fault system, another active fault system, known as the Sierra Madre fault system, also contributes to the formation of the San Gabriel Mountains. The Sierra Madre fault is a reverse fault that is currently thought to have formed around 5 to 7 million years ago. The Sierra Madre fault is a reverse fault that is thought to have contributed to geologically recent uplift of the San Gabriel Mountains.
The “big bend” of the San Andreas fault causes compression that pushes up the Transverse Ranges.
Perspective of Big Bend compressional areas, with the addition of thrust faulting from the Sierra Madre.

4. Thrust Faulting
North-south cross section of the San Gabriel Mountains
Another active fault system involved with recent geologic uplift is the Cucamonga fault zone. The Cucamonga Fault zone is located along the southern margin of the eastern San Gabriel Mountains, and marks the eastern end of the frontal-fault system of the San Gabriel Mountains. The Cucamonga fault is a zone of Quaternary reverse and thrust faults. The fault zone consists of numerous inter-twining, east-striking, north-dipping thrusts that separate crystalline basement rocks of the eastern San Gabriel Mountains from alluvium of upper Santa Ana valley to the south. Some thrust faults of the zone lie entirely within alluvium (Morton and Matti, 1987). Slickensides in the basement rocks are consistently oriented down-dip, indicating the most recent displacements along the Cucamonga Fault zone have been pure thrust. The pre-Quaternary history of the Cucamonga fault is obscure, but its latest Pleistocene and Holocene history reflects convergence between the Perris block and the San Gabriel Mountains. Individual faulting events are estimated at about 6.7 M with a recurrence of about 625 years for the past 13,000 years (Matti and others, 1982; Morton and Matti, 1987). The average north-south convergence across the Cucamonga Fault zone is estimated to have been in the range of 3 mm/yr (Weldon, 1986) to 5 mm/yr (Matti and others, 1982b; Morton and Matti, 1987). This fault is considered to be a compressional fault zone. Since around 7 million years ago, the geologically active mountains have been thrust up quickly -- and have been cut back by erosion almost as fast. Much of the Los Angeles metropolis, in fact, sits atop the accumulated sediments washed off the slopes of the San Gabriels.
Compressional zones and thrust faulting.

5. References
Atwater, Tanya. "Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America." Geological Society of America Bulletin (1970): Print.
"Geology of the San Gabriel Mountains, Transverse Ranges Province." Western Region Geology and Geophysics Science Center. Western Earth Surface Processes Team, 26 May Web. 15 June 2013.
Masters, Nathan. "Rise of the Sierra Madre: A Brief History of the San Gabriel Mountains | LA as Subject | SoCal Focus | KCET." KCET. KCET, 30 May Web. 15 June 2013.
Phillips, Donna E. "Geology of the San Gabriel Mountains, Transverse Ranges Province." Western Region Geology and Geophysics Science Center. USGS, 1 Jan Web. 15 June 2013.