Faulting, fluid flow and geochemical alteration in the Matanuska Valley and Cook Inlet, Alaska

Faulting, fluid flow and geochemical alteration in the Matanuska Valley and Cook Inlet, Alaska
Bunds, M. P.
University of Utah, Salt Lake City
162 p., Illus., Maps
Rasmuson Library: ALASKA QE606.5.A4 B86 2001a
The shear strength of faults is integral to earthquake mechanics and tectonics, and pore fluids strongly influence rock strength. Deformation can be partitioned onto low strength faults. Knowing the shear stress required for seismic slip is also crucial to earthquake forecasting. Elevated pore pressure in a fault lessens its frictional strength. Chemical reactions between fluids and rock can produce cements that impede escape of over-pressured fluids from faults and sedimentary rocks. This dissertation investigates the shear strength of the Castle Mountain fault (CMF), and the origin of cements along the fault and in Cook Inlet basin forearc strata, which the fault cuts. The CMF is a seismically active, orogen-parallel strike slip fault in the southern Alaskan margin. Structures along the CMF indicate that it is weak, with σ 1 at 70° to 80° to the fault. The fault's bulk friction coefficient is ∼0.5, based on comparison of the mineralogy of CMF fault gouge and published rock friction data. The fault must be further weakened to explain its angle to σ1 , possibly by elevated pore pressure along it. The development of forearc folds, faults and dikes along the CMF reveal that E. Tertiary clockwise vertical-axis block rotations ceased by late Oligocene time, possibly in response to the partitioning of shearing onto the CMF as it weakened. Mixing of two fluids in the Cook Inlet subsurface has produced local aquitards. The amount of cementation from fluid mixing is limited by advection of solutes. Cementation was probably particularly important in conduits formed by deformation related to faulting, because faults formed fluid flow pathways that locally breached widespread stratigraphic and compaction-driven aquitards. Cementation from advection of the fluid mixture blocked these conduits to maintain the continuity of the widespread stratigraphic seals. Along the CMF, juxtaposition of rocks containing the two chemically distinct pore fluids present in the Cook Inlet basin subsurface led to fluid mixing and cementation of fracture systems. In contrast, in the highly cataclasized core of the fault reactions between the pore fluids and rocks have dominated and produced low-friction clays. These alteration styles impart spatial variation in brittle strength of at least 2x on the fault. The distribution of alteration mineralogy in the fault is controlled by pore fluid chemistry, the interplay between structural and geochemical controls on flow paths, and the protolith.
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)