DEFORMATIONAL HISTORY AND FRACTURE INTENSITY OF THE INISKIN PENINSULA, COOK INLET, ALASKA

This study investigates the intensely fractured Triassic? – Late Jurassic section of the Iniskin Peninsula, Cook Inlet, Alaska, to determine paleostresses, deformational history, and normalized fracture intensity (the number of fractures per unit length of a certain size or larger). The orientations, opening modes and crosscutting relationships of a population of fractures (n=850) were characterized at 18 field locations. These measurements reveal three primary fracture sets: 308°/78° (set A); 196°/83° (set B); and 248°/82° (set C) [mean strike/ dip]. Fracture sets B and A cross cut one another, the average angle between the two sets is ^_~70°, and they have mixed opening modes. Fracture set C strikes NE, sub-parallel to the trend of regional fold axes, and commonly abuts fracture sets A and B. Unfolding of fractures sets A and B by local bedding orientations results in more densely clustered fracture orientations, indicating that these fractures predate folding. We suggest that sets A and B form a conjugate pair that preserves a SSE-NNW principal compressive stress direction (sigma 1) at the time of deformation and that set C is the product of later strike-normal extension resulting from the bending of layers along the limbs and axes of regional folds. Fracture intensity was quantified by measuring 31 scanlines in 20 beds in 9 locations, and normalized to size (aperture) by using power law regressions to fit cumulative frequency fracture-size distributions. Fracture intensity correlates most strongly with grain size, with finer grained units having higher fracture intensities. A 52.0 ± 0.9 Ma ⁴⁰Ar/³⁹Ar whole rock age from a dike intruding fracture set A suggests SSE-NNW regional shortening occurred in the Eocene. The dike displays chilled margins parallel to the fracture wall, suggesting multiple episodes of dike emplacement during the opening of set A. Set C is the only fracture set that cross cuts the dike, indicating that it postdates dike emplacement. We postulate two episodes of deformation: 1) initial opening of sets A and B during SSE shortening ca. 52 Ma followed by 2) regional folding and the opening of fracture set C. We explore the hypothesis that SSE-trending shortening resulting from the subduction of the Kula ridge (Resurrection plate – Kula plate spreading center) during the Eocene caused the regional deformation.