Laurel B. Goodwin

Internal architecture, permeability structure, and hydrologic significance of contrasting fault-zone types

The Sand Hill fault is a steeply dipping, large-displacement normal fault that cuts poorly lithified Tertiary sediments of the Albuquerque basin, New Mexico, United States. The fault zone does not contain macroscopic fractures; the basic structural element is the deformation band. The fault core is composed of foliated clay flanked by structurally and lithologically heterogeneous mixed zones, in turn flanked by damage zones. Structures present within these fault-zone architectural elements are different from those in brittle faults formed in lithified sedimentary and crystalline rocks that do contain fractures. These differences are reflected in the permeability structure of the Sand Hill fault. Equivalent permeability calculations indicate that large-displacement faults in poorly lithified sediments have little potential to act as vertical-flow conduits and have a much greater effect on horizontal flow than faults with fractures.

Cataclasis and particulate flow in faulted, poorly lithified sediments

Microscopic observations of normal faults in poorly lithified sediments from the Rio Grande rift, New Mexico, USA, reveal that the mode of grain fracture within the fault zones is controlled by mineralogy and relative grain strength. Transgranular fracturing of quartz is rarely observed—quartz typically deforms by flaking of grain edges, feldspar by transgranular fracture facilitated by easy cleavage, and lithic fragments by transgranular fracture or distributed microcracking. Particle size measurements indicate that progressive deformation produces a particle size distribution that can be described by a power-law model, characterized by low D values (1.7–2.1). This indicates a preponderance of large particles with respect to cataclasite produced by constrained comminution (D∼2.6). We interpret these results in terms of cataclastic deformation by controlled particulate flow under low confining pressure, in which extensive transgranular fracturing is not necessary for strain accumulation. This style of cataclastic deformation is different from that observed in crystalline and well lithified sedimentary rocks. It is in part responsible for the characteristic internal structure and hydrologic properties of normal faults in poorly lithified sediments, and thus has implications for diagenetic processes and interpreting fault zone deformation history.

Competency contrast, kinematics, and the development of foliations and lineations in the crust

Similar foliation patterns are observed in faults in poorly lithified sediments, fault gouge and cataclasite, and greenschist to granulite facies shear zones. We suggest that this similarity in form reflects similar controls on development. We propose that these patterns are a fundamental consequence of deformation of heterogeneous media, in which material heterogeneity is manifest as competency contrast. Strain incompatibilities along competence domain boundaries can produce mechanical instabilities, resulting in the nucleation of shear bands and/or C-surfaces. Mechanical segregation of incompetent material into these foliations produces compositional bands. Competency contrast promotes strain partitioning between compositional bands, accommodated in part by domain-boundary sliding. In three-dimensional general shear, incompetent domains and domain boundaries will preferentially accommodate the non-coaxial component of flow, and strain rates will be higher than in competent domains. Consequently, lineations in incompetent domains are different from, and may be orthogonal to, those in competent domains. Lineations that record tectonic transport will form preferentially in incompetent domains; lineations in more competent domains may (if not steady-state) approximate the long axis of the finite strain ellipsoid of the competent domains. The number and orientations of foliations and lineations, and microstructures that record strain rate, thus document both the flow field and the magnitude of competency contrast.

Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations

The Sand Hill fault is a steeply dipping normal fault that cuts poorly consolidated sediments of the Albuquerque basin, New Mexico. The fault zone, which varies in width from ∼1 to 6 m, is selectively cemented by calcite. The margins of cemented areas are characterized locally by striking patterns of elongate cementation that have a strong subvertical orientation. We have considered a variety of possible mechanisms for the formation of these elongate cements, including deformation, weathering, rotation of adjacent cemented beds, and precipitation from flowing ground water. All but the latter can be ruled out. In addition, the elongate cements closely resemble elongate concretions that form from flowing ground water in sedimentary rocks. We conclude that the cements precipitated from flowing ground water, and are elongate parallel to the flow direction at the time of precipitation. Thus, these elongate cements provide an important constraint on models of fluid flow along faults in poorly consolidated sediments; i.e., the orientation of outcrop-scale flow.

Deformation path partitioning within a transpressive shear zone, Marble Cove, Newfoundland

Detailed study of the structures in Marble Cove, Newfoundland, indicates that they formed during a single transpressional event, during which deformation was partitioned between dominantly strike-slip and dominantly dip-slip domains. Marble Cove exhibits steeply dipping foliations, steeply to shallowly plunging stretching lineations, greenschist facies mineral assemblages, and abundant quartz veins. Within the broad domain where stretching lineations plunge steeply, sense-of-shear indicators and asymmetry of quartz-c-axis distributions in mylonitized quartz veins indicate dominantly reverse motion with a component of dextral shear. Adjacent greenschists exhibit orthorhombic symmetry and apparently accommodated pure shear. The abrupt transition to a narrow, high strain domain of shallowly plunging stretching lineations is accommodated by a rheologically distinct serpentinite body. Shear bands in this domain indicate dextral strike-slip movement with a reverse component. Smaller, conjugate shear bands are evident in thin section, indicating a component of pure shear. Sense-of-shear indicators and asymmetry of c-axis distributions in mylonitized quartz veins also record dextral strike-slip and reverse shear with a component of pure shear. Crenulations and asymmetric folds in foliation overprint all other fabrics in Marble Cove and record a final phase of reverse shear with a dextral component. With partitioning of this kind, structures recording strike-slip movement may be limited in areal extent, and therefore less likely to be exposed, than structures recording dip-slip motion. Such partitioning within transpressional shear zones may therefore impede recognition of strike-parallel motions within orogenic belts.

Development of phyllonite from granodiorite; mechanisms of grain-size reduction in the Santa Rosa mylonite zone, California

Abstract Field and laboratory investigations indicate that phyllonite within the Santa Rosa mylonite zone developed from granodiorite without hydrous alteration of the original mineral assemblage. Initial grain-size reduction within zones of phyllonite was accomplished through whole-rock cataclasis; subsequent deformation occurred through ductile flow. The original cataclastic failure is recorded by sharp boundaries with the surrounding rock, which are retained through the ductile overprint. Biotite in phyllonite has a mean grain size of ~3 μm and exhibits straight boundaries and a strong crystallographic preferred orientation. Trace analyses of HVEM images indicate that the largest biotite grains are most strongly oriented, and the grains are elongate parallel to lineation. TEM observations indicate that typical grains have irregularly spaced stacking faults and twins. Evidence for intracrystalline folding and cataclasis, present in protomylonite and mylonite, is absent; bending or kinking of grains is rare. These observations suggest that biotite in phyllonite accommodated deformation through dynamic recrystallization in concert with intracrystalline slip and mechanical rotation. Grain-size reduction to form phyllonite is accompanied by macroscopically and microscopically visible changes in the character of foliations in the rock. We suggest that these changes are linked to the deformation mechanisms operating within the fine-grained phyllonite.

Deformation bands in nonwelded ignimbrites: Petrophysical controls on fault-zone deformation and evidence of preferential fluid flow

The impact of faults on fluid flow and transport through thick vadose zones depends in part on the nature of fault-zone deformation. Both fractures and deformation bands occur in ignimbrite sequences at Los Alamos, New Mexico, and Busted Butte, Nevada. The primary controls on mode of failure are grain-contact area and strength, which are directly related to degree of welding and crystallization and inversely proportional to porosity. Low-porosity welded units deform by transgranular fracture; high-porosity, glassy, nonwelded units deform by cataclasis within deformation bands. Moderately high porosity, nonwelded units that have undergone devitrification and/or vapor-phase crystallization form either deformation bands or fractures, depending on local variations in the degree and nature of crystallization. Grain- and pore-size reduction in deformation bands commonly produces indurated, tabular zones of clay-sized fault material. Many of these bands are locally rich in smectite and/or cemented by carbonate. Preferential wetting of deformation bands is inferred to promote alteration and cementation. We therefore interpret variably altered fault-zone material as evidence of preferential fluid flow in the vadose zone, which we infer to result from enhanced unsaturated permeability due to pore-size reduction in deformation bands.

Fracture and fault patterns associated with basement-cored anticlines: The example of Teapot Dome, Wyoming

Teapot Dome is an asymmetric, doubly plunging, basement-cored, Laramide-age anticline. Most of the fractures, deformation bands, and faults at Teapot Dome are interpreted to have formed during contemporaneous longitudinal and transverse stretching of the sedimentary cover over a basement-involved thrust. Strain was accommodated by fractures, deformation bands, and normal and normal-oblique faults that strike both parallel and perpendicular to the fold hinge. The fracture and fault patterns at Teapot Dome are distinctly different from those formed within anticlines associated with thin-skinned thrust systems. The inferred fracture-influenced permeability anisotropy of thick-skinned systems is therefore distinct from that of thin-skinned systems. We propose that Teapot Dome is a good analog for similar basement-involved, thrust-generated anticlines.

Systematic diagenetic changes in the grain-scale morphology and permeability of a quartz-cemented quartz arenite

The material properties of sedimentary rocks are controlled by a range of parameters, including grain size, sorting, and modification of the original sediment through the diagenetic processes of compaction and cementation. To isolate the effects of diagenesis and explore how they modify permeability, we quantified changes in grain and pore morphology accompanying progressive diagenesis of a simple system: a well-sorted, variably quartz-cemented quartz arenite of relatively uniform grain size. The most common type of authigenic cement in sandstones, quartz overgrowths, is responsible for significant porosity and permeability reduction. The distribution of overgrowths is controlled by available pore space and the crystallographic orientations of individual quartz grains. We show that progressive quartz cementation modifies the grain framework in consistent, predictable ways. Detailed microstructural characterization and multiple regression analyses demonstrate that both the number and length of grain contacts increase as the number of pores increases and the number of large well-connected pores decreases with progressive diagenesis. The aforementioned changes progressively alter pore shape and reduce pore-size variability and bulk permeability. These systematic variations in the pore network correlate with changes in permeability, such that we can use our data to calibrate the Kozeny-Carmen relation, demonstrating that it is possible to refine predictions of permeability based on knowledge of the sedimentary system.

Intracrystalline folding and cataclasis in biotite of the Santa Rosa mylonite zone: HVEM and TEM observations

Abstract High voltage electron microscope (HVEM) and transmission electron microscope (TEM) analyses indicate that grain size reduction of biotite in deformed granodiorite of the Santa Rosa mylonite zone, southern California, was in part accomplished by Intracrystalline folding and cataclasis. Through this process, deformation takes place within discrete parallel-sided or lens-shaped zones up to 6 μm wide. Within the zones, deformation begins with the opening of cavities along cleavage planes. Further deformation causes layers parallel to (001) to become detached from the surrounding crystal along cleavage planes or ruptured along micro-shear zones at an angle to (001). The layers may be strongly folded, branching, or completely fragmented, the latter suggesting cataclastic behavior. The smallest measured lens of cataclasite is 0.05 μm in width. High resolution TEM images show that (001) lattice fringes fold into these regions. Zones of Intracrystalline folding and cataclasis are visible petrographically. Because of the many orientations of biotite within these regions, the optical properties are effectively averaged. The zones are characterized by little or no change in color with rotation of the optical microscope stage and, in most orientations, by higher refractive indices than the surrounding crystal.