... derived from deforma tion experiments, suggest that under a wide variety of geological condit... more ... derived from deforma tion experiments, suggest that under a wide variety of geological conditions, deformation of finegrained lime stones may ... The contact between the hanging wall Eldon Formation limestones and footwall Belly River Forma tion shale and sandstones is locally ...
Fine-grained gouge (< 5 microns) is generated along shear zones during the emplacement of daci... more Fine-grained gouge (< 5 microns) is generated along shear zones during the emplacement of dacite domes during the most recent volcanic activity at Mount Saint Helens. Here, we present grain size data from both the natural fault rocks from Mount Saint Helen's and experimentally generated gouge. Our triaxial rock deformation experiments were run at confining pressures (Pc) of 0, 25, 50, and 75MPa, at room temperature and strain rates of ~1 x 10-4/s. We performed experiments that were terminated after failure and experiments on frictional sliding of the gouge. Our starting material has low (6-8%) porosities, a uniform bulk composition (65 wt% SiO2) and is highly. The dacite experiments show a progressive increase in peak strength (100-700 MPa) with increasing Pc and all cores show brittle behavior, characterized by a rapid stress drop. Run products contain macroscopic fractures with deformation extremely localized around the shear fractures. Experimentally deformed dacites show extreme grain size reduction and the production of gouge. Gouge material can reach < 1 micron in diameter. Frictional sliding experiments generated more gouge than experiments stopped after failure. However, there is little modification of the finest grain sizes (<20 microns) because of frictional sliding. We propose that the large stress drop at failure generates the fine-grained gouge and subsequent sliding does not further reduce the grain size. The amount of energy required to produce the fine-grained material is calculated and we compare this to the energy required to generate volcanic ash from fragmentation.
Oxygen isotope data from syntectonic veins, thrust faults, and wall rocks suggest that fluids inf... more Oxygen isotope data from syntectonic veins, thrust faults, and wall rocks suggest that fluids infiltrated the Western Ranges of the Rocky Mountain foreland from deeper rocks of the Dogtooth Range during Mesozoic contraction. This signifies the first such evidence for kilometer-scale fluid migration at the hinterland-foreland transition of the Canadian Cordillera. Fluid infiltration resulted in isotopic depletion in wall rocks and isotopic disequilibrium between veins (fluid) and their host rocks downstream of a lithologic, structural, and isotopic shift between predominantly siliciclastic (Dogtooth Range) and calcareous (Western Ranges) sequences. The shift corresponds with an average δ18O that is 3.0‰ higher in the Western Ranges. In the Dogtooth Range, atypically low carbonate values (15.6‰ +/- 0.8‰ (VSMOW)), and the consistency with which vein signatures approach the average bulk rock δ18O of individual outcrops indicate fluid buffering by a siliciclastic reservoir and outcrop-scale (10-100 m) fluid circulation. In contrast, rocks and veins in the Western Ranges exhibit a gradational increase in δ18O that extends >=14 km eastward and up section from the isotopic shift and possess values that are below or at the low end of typical carbonate compositions (16.2-20.9‰ (VSMOW)). Furthermore, veins systematically exhibit lower oxygen values than their host rocks; this implies that the fluids that entered the Western Ranges were in modest disequilibrium with the rocks through which they flowed. We use a one-dimensional reactive transport model to infer that rocks in the Western Ranges experienced a time-integrated fluid flux of 1.1 × 105 mol H2O cm-2 (~2.4 × 106 cm3 cm-2 ) and sluggish reaction kinetics during the flow event.
Core retrieved from SAFOD Phase 3 drilling has been examined in three locations: (1) with the Sal... more Core retrieved from SAFOD Phase 3 drilling has been examined in three locations: (1) with the Salinian Terrane near its contact with the presumed Great Valley sequence (Hole E-Run 1-Section 4 & 6); (2) proximal to the ‘10480' fault zone with which are associated casing deformation and seismic aftershocks indicative of active faulting (Hole G-Run-1-Section 2 & Hole G-Run 2-Section 3); and (3) adjacent to the ‘10830' fault zone in the centre of the damage zone identified in Phase 2 drilling (Hole G-Run 4-Section 2). The sampling locations translate to an across-strike distance from outside the damage to its centre of approximately 125 metres, and a change in current depth from 2610 m to 2685m. The Salinian Terrane material (E14, E16) comprises coarse-grained quartz and perthitic feldspar clasts that locally form slightly foliated cataclasite. The matrix is commonly chloritic with very fine-grained aggregates and zones of quartz and/or feldspar. There are both corroded clasts, particularly of quartz, and globular infillings of calcite with sutured contacts. Foliated siltstone-shale cataclasites (G12, G23) at the edge of the damage zone close to the ‘10480' fault zone exhibit brecciation and cataclasis at different scales; deformation is episodic as there are distinct overprinting relationships. The fine-grained matrix exhibits a strong SPO of phyllosilicates and cryptocrystalline quartz (<5μm). The quartz is introduced as fine stringer veins that are progressively incorporated into the overall fabric. Similar thing calcite veins form parallel to the cataclastic foliation, suggestive of fault parallel hydraulic fracture. Coarser grained phyllosilicate zones develop C-S type fabrics with dextral displacement sense. Oxidation within deformation bands is variable, though very well developed in the late, coarse fragment cataclasites. The latter zones can exhibit well-rounded clasts separated by thin foliae of a pressure solution foliation. Sheared siltstone/sandstone from within central portion of the damage zone approximately 7m across strike from the ‘10830' fault zone extensive evidence of fluid-rock interaction. Grains commonly have overgrowths, and there are well-developed pressure solution foliae. Quartz grains commonly ‘float' in a calcite matrix. The fine-grained matrix itself has a strong foliation. The optical microstructures described here are examined in detail by electron microscopy.
Limestones from the Tuscan Nappe sequence, northern Apennines, Italy, have been deformed during c... more Limestones from the Tuscan Nappe sequence, northern Apennines, Italy, have been deformed during crustal thickening at temperatures less than 250°C. As part of this deformation, bedding-parallel shear zones have formed within two distinct protoliths: (1) very fine-grained (< 5μm) micritic calcite (with some veins) that comprises the host material and (2) initially coarse-grained vein calcite. Evolution of the shear zones produced extreme grain size reduction in the vein calcite such that at high strains, the grain size distribution and microstructure of host micrite and deformed vein calcite is essentially the same. Although vein calcite in the micrite experiences similar microstructural changes to that of the vein hosted shear zone, indicative of similar deformation conditions, the grain size of the deformed micrite is comparable or slightly larger than the starting material. In terms of the mechanical and microstructural changes, the variation in initial grain size would, at first blush, be the dominating factor. Although, in the end-state, the two initially distinct calcites are microstructurally similar, they none the less exhibit contrasting crystallographic fabrics and chemical signatures. The micrite shear zone, despite a well-developed SPO, exhibits no CPO, while the vein-hosted shear zone has an intense CPO irrespective of SPO development. Likewise, micrite shear zones exhibit extreme Sr depletion relative to adjacent undeformed layers. Throughout the evolution of the shear zones, grain boundaries in each calcite type exhibit, and maintain, contrasting structures. Micrite grain boundaries are heavily decorated with voids and tubules indicative of fluid transport and interaction; this can explain the selective chemical depletion in these shear zones, and inferred activity of grain-size-sensitive flow processes. In contradistinction to the micrite, recrystallized vein calcite has ‘tight', clean grain boundaries effectively bereft of inclusions. The latter is consistent with a more quasi-uniform deformation in which macroscopic and grain-scale dislocation-mediated dominates. Examination of these two calcite types demonstrates that microstructural similitude at high strain may not be sufficient to infer comparable rheological response; instead, differences in initial defect substructure, in this case grain boundary structure, constrain the micromechanical behaviour as deformation accumulates.
Episodic brittle-ductile interactions reflect a complex interplay of micromechanical hardening an... more Episodic brittle-ductile interactions reflect a complex interplay of micromechanical hardening and softening that typical involves some type of fluid pressure transition in combination with introduction of new material that acts as the switch from coseismic to interseismic response. Three crustal levels reflecting distinct temperature-pressure conditions are used to demonstrate the independence of this cyclic behaviour on depth, though it is best prescribed by certain combinations of crustal parameters and host lithologies. The situations examined comprise: (1) upper crustal faulting of limestone/shale units 3-5 km depth; (2) mid-crustal faulting developed at 520°C and 20km depth; and (3) brittle shear and cataclasis in mafic granulites deforming at 750-800°C and 30km. Each regime exhibits variations in fluid pressure represented, respectively, by carbonate saturated water, shear-induced melt (pseudotachylyte) and anatectic melts and/or pseudotachylyte. In terms of the rock record, evidence of seismic events are embedded as the new or reconstituted material introduced to the deforming host as a consequence of brittle deformation; for example, calcite veins and pseudotachylyte. This new material acts as an important sink for strain energy whereby brittle responses are suppressed until such time as a new critical state is reached. In turn, the strain rate softening abetted by the new material provides a ductile overprint of their own syn-fracture origin. A common microstructural aspect of the otherwise distinct materials is the intense development of glide-mediated deformation, as opposed to creep, in the latter stages of a ductile (interseismic) period. This raises the issues of whether all seismic events are nucleated as classic pressure-dependent brittle failure, or if plastic instabilities can initiate the same macroscopic response.
... derived from deforma tion experiments, suggest that under a wide variety of geological condit... more ... derived from deforma tion experiments, suggest that under a wide variety of geological conditions, deformation of finegrained lime stones may ... The contact between the hanging wall Eldon Formation limestones and footwall Belly River Forma tion shale and sandstones is locally ...
Fine-grained gouge (< 5 microns) is generated along shear zones during the emplacement of daci... more Fine-grained gouge (< 5 microns) is generated along shear zones during the emplacement of dacite domes during the most recent volcanic activity at Mount Saint Helens. Here, we present grain size data from both the natural fault rocks from Mount Saint Helen's and experimentally generated gouge. Our triaxial rock deformation experiments were run at confining pressures (Pc) of 0, 25, 50, and 75MPa, at room temperature and strain rates of ~1 x 10-4/s. We performed experiments that were terminated after failure and experiments on frictional sliding of the gouge. Our starting material has low (6-8%) porosities, a uniform bulk composition (65 wt% SiO2) and is highly. The dacite experiments show a progressive increase in peak strength (100-700 MPa) with increasing Pc and all cores show brittle behavior, characterized by a rapid stress drop. Run products contain macroscopic fractures with deformation extremely localized around the shear fractures. Experimentally deformed dacites show extreme grain size reduction and the production of gouge. Gouge material can reach < 1 micron in diameter. Frictional sliding experiments generated more gouge than experiments stopped after failure. However, there is little modification of the finest grain sizes (<20 microns) because of frictional sliding. We propose that the large stress drop at failure generates the fine-grained gouge and subsequent sliding does not further reduce the grain size. The amount of energy required to produce the fine-grained material is calculated and we compare this to the energy required to generate volcanic ash from fragmentation.
Oxygen isotope data from syntectonic veins, thrust faults, and wall rocks suggest that fluids inf... more Oxygen isotope data from syntectonic veins, thrust faults, and wall rocks suggest that fluids infiltrated the Western Ranges of the Rocky Mountain foreland from deeper rocks of the Dogtooth Range during Mesozoic contraction. This signifies the first such evidence for kilometer-scale fluid migration at the hinterland-foreland transition of the Canadian Cordillera. Fluid infiltration resulted in isotopic depletion in wall rocks and isotopic disequilibrium between veins (fluid) and their host rocks downstream of a lithologic, structural, and isotopic shift between predominantly siliciclastic (Dogtooth Range) and calcareous (Western Ranges) sequences. The shift corresponds with an average δ18O that is 3.0‰ higher in the Western Ranges. In the Dogtooth Range, atypically low carbonate values (15.6‰ +/- 0.8‰ (VSMOW)), and the consistency with which vein signatures approach the average bulk rock δ18O of individual outcrops indicate fluid buffering by a siliciclastic reservoir and outcrop-scale (10-100 m) fluid circulation. In contrast, rocks and veins in the Western Ranges exhibit a gradational increase in δ18O that extends >=14 km eastward and up section from the isotopic shift and possess values that are below or at the low end of typical carbonate compositions (16.2-20.9‰ (VSMOW)). Furthermore, veins systematically exhibit lower oxygen values than their host rocks; this implies that the fluids that entered the Western Ranges were in modest disequilibrium with the rocks through which they flowed. We use a one-dimensional reactive transport model to infer that rocks in the Western Ranges experienced a time-integrated fluid flux of 1.1 × 105 mol H2O cm-2 (~2.4 × 106 cm3 cm-2 ) and sluggish reaction kinetics during the flow event.
Core retrieved from SAFOD Phase 3 drilling has been examined in three locations: (1) with the Sal... more Core retrieved from SAFOD Phase 3 drilling has been examined in three locations: (1) with the Salinian Terrane near its contact with the presumed Great Valley sequence (Hole E-Run 1-Section 4 & 6); (2) proximal to the ‘10480' fault zone with which are associated casing deformation and seismic aftershocks indicative of active faulting (Hole G-Run-1-Section 2 & Hole G-Run 2-Section 3); and (3) adjacent to the ‘10830' fault zone in the centre of the damage zone identified in Phase 2 drilling (Hole G-Run 4-Section 2). The sampling locations translate to an across-strike distance from outside the damage to its centre of approximately 125 metres, and a change in current depth from 2610 m to 2685m. The Salinian Terrane material (E14, E16) comprises coarse-grained quartz and perthitic feldspar clasts that locally form slightly foliated cataclasite. The matrix is commonly chloritic with very fine-grained aggregates and zones of quartz and/or feldspar. There are both corroded clasts, particularly of quartz, and globular infillings of calcite with sutured contacts. Foliated siltstone-shale cataclasites (G12, G23) at the edge of the damage zone close to the ‘10480' fault zone exhibit brecciation and cataclasis at different scales; deformation is episodic as there are distinct overprinting relationships. The fine-grained matrix exhibits a strong SPO of phyllosilicates and cryptocrystalline quartz (<5μm). The quartz is introduced as fine stringer veins that are progressively incorporated into the overall fabric. Similar thing calcite veins form parallel to the cataclastic foliation, suggestive of fault parallel hydraulic fracture. Coarser grained phyllosilicate zones develop C-S type fabrics with dextral displacement sense. Oxidation within deformation bands is variable, though very well developed in the late, coarse fragment cataclasites. The latter zones can exhibit well-rounded clasts separated by thin foliae of a pressure solution foliation. Sheared siltstone/sandstone from within central portion of the damage zone approximately 7m across strike from the ‘10830' fault zone extensive evidence of fluid-rock interaction. Grains commonly have overgrowths, and there are well-developed pressure solution foliae. Quartz grains commonly ‘float' in a calcite matrix. The fine-grained matrix itself has a strong foliation. The optical microstructures described here are examined in detail by electron microscopy.
Limestones from the Tuscan Nappe sequence, northern Apennines, Italy, have been deformed during c... more Limestones from the Tuscan Nappe sequence, northern Apennines, Italy, have been deformed during crustal thickening at temperatures less than 250°C. As part of this deformation, bedding-parallel shear zones have formed within two distinct protoliths: (1) very fine-grained (< 5μm) micritic calcite (with some veins) that comprises the host material and (2) initially coarse-grained vein calcite. Evolution of the shear zones produced extreme grain size reduction in the vein calcite such that at high strains, the grain size distribution and microstructure of host micrite and deformed vein calcite is essentially the same. Although vein calcite in the micrite experiences similar microstructural changes to that of the vein hosted shear zone, indicative of similar deformation conditions, the grain size of the deformed micrite is comparable or slightly larger than the starting material. In terms of the mechanical and microstructural changes, the variation in initial grain size would, at first blush, be the dominating factor. Although, in the end-state, the two initially distinct calcites are microstructurally similar, they none the less exhibit contrasting crystallographic fabrics and chemical signatures. The micrite shear zone, despite a well-developed SPO, exhibits no CPO, while the vein-hosted shear zone has an intense CPO irrespective of SPO development. Likewise, micrite shear zones exhibit extreme Sr depletion relative to adjacent undeformed layers. Throughout the evolution of the shear zones, grain boundaries in each calcite type exhibit, and maintain, contrasting structures. Micrite grain boundaries are heavily decorated with voids and tubules indicative of fluid transport and interaction; this can explain the selective chemical depletion in these shear zones, and inferred activity of grain-size-sensitive flow processes. In contradistinction to the micrite, recrystallized vein calcite has ‘tight', clean grain boundaries effectively bereft of inclusions. The latter is consistent with a more quasi-uniform deformation in which macroscopic and grain-scale dislocation-mediated dominates. Examination of these two calcite types demonstrates that microstructural similitude at high strain may not be sufficient to infer comparable rheological response; instead, differences in initial defect substructure, in this case grain boundary structure, constrain the micromechanical behaviour as deformation accumulates.
Episodic brittle-ductile interactions reflect a complex interplay of micromechanical hardening an... more Episodic brittle-ductile interactions reflect a complex interplay of micromechanical hardening and softening that typical involves some type of fluid pressure transition in combination with introduction of new material that acts as the switch from coseismic to interseismic response. Three crustal levels reflecting distinct temperature-pressure conditions are used to demonstrate the independence of this cyclic behaviour on depth, though it is best prescribed by certain combinations of crustal parameters and host lithologies. The situations examined comprise: (1) upper crustal faulting of limestone/shale units 3-5 km depth; (2) mid-crustal faulting developed at 520°C and 20km depth; and (3) brittle shear and cataclasis in mafic granulites deforming at 750-800°C and 30km. Each regime exhibits variations in fluid pressure represented, respectively, by carbonate saturated water, shear-induced melt (pseudotachylyte) and anatectic melts and/or pseudotachylyte. In terms of the rock record, evidence of seismic events are embedded as the new or reconstituted material introduced to the deforming host as a consequence of brittle deformation; for example, calcite veins and pseudotachylyte. This new material acts as an important sink for strain energy whereby brittle responses are suppressed until such time as a new critical state is reached. In turn, the strain rate softening abetted by the new material provides a ductile overprint of their own syn-fracture origin. A common microstructural aspect of the otherwise distinct materials is the intense development of glide-mediated deformation, as opposed to creep, in the latter stages of a ductile (interseismic) period. This raises the issues of whether all seismic events are nucleated as classic pressure-dependent brittle failure, or if plastic instabilities can initiate the same macroscopic response.
Uploads
Papers by lori kennedy