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San Andreas Fault System in Southern California (SAFSOC)

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TASK 3 - Fault Interactions in the Southwestern San Gabriel Mountains

Task Leader: Robert E. Powell

Statement of Problem:
Through the last 20±2 Ma, the San Gabriel Mountains block has been displaced 10s to 100s of km along various rightslip strands of the San Andreas Fault system from blocks that were once contiguous to the east and west. During the last 2 Ma or so, the southern and southwestern margins of the San Gabriel Mountains block have undergone great contraction, accompanied by sinistral shear. As a result of this complex history, at least four fault sets have interacted along the range front: (1) right lateral faults of the San Gabriel and Vasquez Creek fault zones; (2) left-lateral and leftoblique faults, including the Raymond, Clamshell-Sawpit, and San Antonio Canyon faults; (3) reverse faults of the Sierra Madre-Cucamonga fault zone; and (4) as yet steeply southward-dipping normal faults. This last set is newly recognized and not documented on existing maps.

On existing published maps, the relative significance of various fault strands is commonly obscured by the sheer number of mapped breaks. The distribution of bedrock and surficial units and of degradational and aggradational landforms offers opportunities to better document sense, magnitude, and timing of fault displacements; to distinguish major from minor breaks; and to establish kinematic and sequencing relations among the various fault sets. Deciphering temporal and spatial relations among these fault sets is essential to understanding the tectonic and slope-failure history of the rangefront and key to assessing seismic and landslide hazards. Both the USGS Earthquake Hazards Program and the CGS Seismic Hazards Mapping Program have identified as a high-priority need an improved understanding of the geologic and structural framework of the Sierra Madre fault zone. This task is a key complementary effort to seismicity- related, crustal imaging, and Quaternary studies underway within the Earthquake Hazards Program and the California Geological Survey, and it will provide a regional geologic framework for helping to understand hazards related to seismic events, slope-failure, and mass movement.

Objectives:
Develop a geologic and geomorphic framework as a base for understanding the evolution of the range front and for assessing hazards along. This objective embraces the following goals:

(1) Generate new and revised mapping to distinguish and accurately represent the various fault sets (right- and leftlateral, reverse, normal) and the geologic units, structures, and landforms that help demonstrate sense, magnitude, and timing of displacement on the faults;
(2) Document relative and numerical dates for surficial deposits and geomorphic surfaces;
(3) Develop models that show kinematic and dynamic interactions among the various fault sets;
(4) Begin to define the role of tectonic segmentation along the range-front;
(5) Ascertain the lithologic, structural, and slope controls on past and present slope-failure events along the range-front.

This undertaking complements USGS and CGS efforts in assessing hazards and provides a useful geologic base for evaluating the effects of fires, road-building, rainfall, vegetation, and seismic events on slope-failure susceptibility.

Methodology:
By mapping relations among crystalline basement units, overlapping late Cenozoic strata, surficial deposits, and remnant in situ regoliths, we hope to develop a new geologic and geomorphic framework for documenting and understanding the evolution of kinematic and dynamic segmentation of faulting along the range-front. (1) Revise existing mapping and conduct new mapping to: (a) Map lithologic units in the crystalline basement to improve our understanding of the distribution of Proterozoic and Mesozoic igneous and metamorphic rocks, Cenozoic dike swarms, and Lower Tertiary strata, and their relations to Late Mesozoic and Cenozoic structures. (b) Map Upper Tertiary and Quaternary sedimentary strata deposited on the piedmont and caught up in deformation along the range-front; distinguish units based on lithology, stratigraphy, and clast provenance. (c) Identify and map remnants of degradational surfaces capped by relict in situ regoliths on the crystalline basement; attempt to establish sequencing relations between these features and aggradational and degradational events recorded in the piedmont deposits. (2) Employ stratigraphic relations, pedogenic soil descriptions, magnetostratigraphy, and cosmogenic nuclide and luminescence dating to establish ages of surficial deposits and landforms. (3) Use findings of (1) & (2)to document direction, magnitude, and timing of fault displacements.
 

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