FAULT-SLIP HISTORY OF THE BRUIN BAY FAULT SYSTEM, INISKIN PENINSULA, COOK INLET, ALASKA

The Bruin Bay fault system (BBFS) is a major northeast-striking structural boundary in southeastern Alaska that is continuous for >450 km from the upper Alaska Peninsula to the northeast terminus of Cook Inlet. The BBFS defines the tectonic boundary between the crystalline batholith and volcanic edifice of the Jurassic Talkeetna arc and Mesozoic and Cenozoic sediments of the Cook Inlet forearc basin that were primarily derived from the arc. Despite its large geographic extent and potential influence on the development of the Cook Inlet forearc basin, relatively little is known about the kinematic history of the BBFS. Kinematic analyses from a population of minor fault surfaces (n=225) that are part of the BBFS near the Iniskin Peninsula describe bulk kinematics of deformation, discern kinematically compatible fault populations, and test for potential genetic relationships between sets of faults. Results indicate that the slip history of the BBFS was dominated by strike-slip deformation with a lesser component of top-southeast reverse motion. 85 percent of the faults measured belong to one of two statistically distinct, kinematically compatible populations of faults. Faults from one set (n=123) include northeast-striking reverse and left-lateral strike-slip faults and northwest-striking right-lateral faults that together record southeast-trending shortening. The second set (n=69) includes northeast- and northwest-striking right- and left-lateral strike-slip faults, respectively, that record east-trending shortening. The remaining faults (15 percent of the population) are classified into two additional kinematically unique subsets of oblique-slip faults (n=25) and normal faults (n=8). Fault slip data and the spatial distribution of the two dominant fault populations indicate that fault reactivation likely occurred along all of the major structures that define the BBFS near the Iniskin Peninsula. We postulate that heterogeneous fault kinematics within the BBFS record two regional deformational events that reflect Paleocene–Eocene oroclinal bending of the Alaska Peninsula and/or temporally variable subduction zone dynamics along the continental margin of North America since the Jurassic.