Forest Geology - Structure, folds, faults

Geology of Los Padres National Forest

by Thomas W. Dibblee - July 2001

Geologic Structure

Thick sedimentary accumulations 100 to 38 million years old (described previously under Rock Types) were originally deposited horizontally on flat basement rock platforms, and then subsequently on top of each other. As the land that is now coastal California subsided under the eastern Pacific Ocean, the sediments accumulated to great thickness. This subsidence was in large part counteracted by episodes of uplift and emergence from shear and compressive tectonic forces. The earliest episode occurred in Paleocene / early Eocene time, which emerged much of what are now the coastal mountains. It was followed by an episode in Oligocene time (described in Rock Types), which affected about the same area. Subsidence of this area in Miocene time was followed by the latest and most widespread episode of emergence and uplift beginning about 6 million years ago. The uplift has occurred in several stages during Pliocene, and Pleistocene times and through to the present. Thus the present Coast Ranges and western Transverse Ranges evolved and are still evolving.

Faults

Previously described crustal movements occurred mostly along major faults that formed west of what is now the Great Central Valley. The largest and most active is the San Andreas fault, one of the world's largest vertical faults, that cuts southeast up Santiago Canyon, Mill Potrero, Cuddy, Frazier, and Gorman Valleys, and beyond, as a straight rift zone through the San Emigdio Mountains and vicinity. Movement along this great fault is right lateral: horizontal motion, along which the terrain on the southwest side, shifts northwest with respect to the northeast side. This shear movement has prevailed at least since Miocene time, possible since Oligocene, Eocene, or even Cretaceous time.

The San Andreas Fault is thought to be the present boundary between the North American continental plate and the Pacific oceanic plate, along which the Pacific plate is shifting north with respect to the North American plate. This lateral shear movement on the San Andreas fault is designated as dextral shear movement. This motion is combined with compressive movement as each block on opposite sides of the fault is being pushed against the other. This compressive movement caused the uplift of the San Emigdio Mountains.

There are other vertical faults within the Coast Ranges on the Forest, which run parallel to the San Andreas, but Southwest of it; they accommodate similar, but lesser movement. Such faults include the Sur-Nacimiento fault in the northwest Santa Lucia Mountains and the related Rinconada fault in the Garcia and Sierra Madre Mountains. Dextral shear movements prevailed on these faults mostly in Pliocene time, but the faults have been inactive since. Although these faults are hidden by dense chaparral, they can be detected and mapped from the different rock formations they juxtapose and by the fold structure of those rocks.

Associated with these faults as well as the San Andreas fault are compressive thrust faults that bound the mountain ranges uplifted on those faults. The La Panza, South Cuyama, Ozena, San Guillermo, Frazier Mountain, Pine Mountain, Little Pine and San Cuyetano faults are all thrust faults related the aforementioned vertical faults.

In the southern part of the Forest are two major near vertical faults that trend nearly east-west: 1) the Big Pine fault, which intersects the San Andreas fault near Frazier Park and extends west to Big Pine Mountain, and 2) the Santa Ynez fault along the north side of the Santa Ynez - Topatopa transverse range. Movement along these two faults was mostly left lateral, where the block on the north side shifted west with respect to the block on the south side.

The Big Pine and eastern Santa Ynez faults are in the higher elevation terrain where the northwest-trending Coast Ranges and east-trending Transverse Ranges converge. This may be the effect of conflict between dextral shear movement on the northwest-trending San Andreas and its parallel, related faults, and sinistral shear movements on the east-trending Big Pine and Santa Ynez faults. This conflict of shear movements may have converted itself into compressive movements, which greatly elevated the mountainous terrain through compressive folding and slip along the associated thrust faults.

Fold Structures

Compressive folding along and between the major faults described above has been so intense, especially since Miocene time, that the once deeply buried crustal basement complexes have been squeezed up to the surface. The overlying, once flat lying, sedimentary rocks have been complexly tilted at high angles and folded, to generate the mountain ranges. This great fold structure is especially evident in the mountainous terrain where the Coast and Transverse Ranges converge, and in the San Rafael and Santa Lucia Ranges. These mountain ranges are major fold belts, with fold axes that trend generally west-northwest.

[Forest Geology - Minerals]