Factors
Controlling the Development and Maintenance of Fault Seals in Heterogeneous
Sedimentary Rocks: A Case Study from the Moab Fault, Utah
Solum, John G.1,
Nicholas C. Davatzes2,
David
A. Lockner2 (1) Sam Houston
State University, Huntsville, TX (2) U.S. Geological Survey, Menlo Park, CA
The Moab normal fault, located
in central eastern Utah, cuts a heterogeneous
sequence of siliciclastics. This study examines the lithologic, mineralogic, and
mechanical properties at the point of maximum throw to quantify the impact of authigenesis on fault zone evolution. The fault consists of
two zones of clay gouge, with 60 and 900 m of throw respectively, bounded by
shale smear and fractured sandstone grading into undeformed
protolith. The heterogeneous protolith
has coefficient of friction, µ, ~0.4-0.6 (all values of µ are with distilled
water as a pore fluid). Shale smear is dominated by protolithic
clays but also contains faulting-related illite-smectite
resulting in µ ~0.4-0.5. Gouge from the minor fault contains significant neoformed smectitic clays
resulting in µ ~0.25-0.43. Gouge in the major zone is largely composed of neoformed illite resulting in µ
~0.35-0.4. Clay neoformation suggests significant
fluid-rock interaction throughout the fault zone. However, bleaching patterns
indicate that reducing fluids (related to hydrocarbons) were largely confined
to fractured sandstone between the two gouge zones. These results indicate
spatiotemporal variations in fluid chemistry and fault zone permeability
structure. In addition to reductions in mu and
permeability resulting from clay growth, the slip behavior of smectites is sensitive to fluid chemistry. The coefficient
of friction varies by a factor of ~2 between a K-rich brine and distilled water
and the rate dependence varies from neutral to slightly negative suggesting a
transition from creep in fresher water to stick slip in brine. Persistent seals
in fault zones depend on clays and these results demonstrate these clays evolve
due to strain, neomineralization, permeability
structure, and fluid chemistry.