4-D Evolution of the Hat Creek Fault, Northern California: An Outcrop Analog for Seismic-Scale, Polyphase, Segmented Normal Faults

Abstract

The 50-km-long Hat Creek fault (HCF) is located 20 km NNE of Lassen Peak volcano in northern California. This active, west-dipping normal fault is easily accessible, well preserved in Late Pleistocene lavas, and can be viewed in the field from multiple perspectives that maximize 3D cognition. The HCF is highly segmented with three stages of fault growth reflected in three systems of scarps of different ages, total throw, and orientation, with a cumulative maximum throw of ∼570 m. Faulted surface lavas indicate that the HCF developed in less than 1 Myr. Unraveling the multi-stage history of the HCF requires recognition of distinct fault segments, their relative timing, and relation to regional tectonics. Fortunately, segmented normal faults in volcanic rocks preserve fine details of fault geometry and kinematics that simplify fault system analysis. The oldest segment orientations and kinematic indicators suggest initial NE-SW extension followed by reactivation during E-W extension, consistent with the documented stress state of the Cascades backarc. An intermediate aged set of scarps developed during this E-W extension, with local magma-induced stress heterogeneity near a small shield volcano (Cinder Butte) that resulted in variable fault segment orientations. Recent dextral-oblique kinematics along the youngest set of scarps in ∼24 ka lavas imply WNW-ESE extension: possibly from transfer of dextral shear into the system from the Walker Lane Belt in western Nevada. Hence, a gradual ∼50-60° clockwise rotation of the horizontal principal stresses occurred in the HCF region, resulting in a complex fault geometry and kinematic history despite a relatively short time frame (∼1 Myr). Faults of similar style, scale, and complexity as the HCF are common in sub-surface petroleum reservoirs. As with the HCF, many of these faults show multiple fault orientations with complex interactions that may create compartmentalization, changing orientations along-strike of individual fault segments, a hierarchy of fault throw magnitudes dependent on fault set, and complex distributions of throw along individual faults that speak to throw partitioning between simultaneously active fault sets. The HCF reveals that such complexity can develop over very short periods of geologic time. Hence, the HCF may be a useful analog for capturing the evolution of similar polyphase faults and for delineating and risking exploration targets in complex normal fault systems.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016