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Association of folds and veins in shear zones
GEOMECHANICS ABSTRACTS
903O04 Association of folds and veins in shear zones Hudleston, P J J Struct Geol VII, N8, 1989, P949-957
Geology Tectonic processes and structural geology 9O3001 Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau Wong. I G; Humphrey, J R Bull Geoi Soc Am VIOl, N9. Sept 1989. P1127-1146 Results of seismic monitoring over the past 30 years for the Colorado Plateau have been analysed. Seismic activity is of small to moderate magnitude, with a low to moderate rate of occurrence with earthquakes widely distributed. Some concentrations are seen, notably induced activity related to coal mining. Results are consistent with a highly faulted basement and upper crust and a moderate level of differential tectonic stress, Focal mechanisms indicate contemporary state of stress is characterised by approximately northeast-trending extension.
903002 Neotectonic joints Hancock, P L; Engelder, T Bull Geol Soc Am VIOl, NIO, Oct 1989, P1197-1208 Neotectonic joints are the most recent joint systems to form within a region subject to uplift and erosion. Shallow tectonic joints propagate within the upper 0.5km of the crust where minimum principal stress is essentially horizontal and tensile, and not greatly different from maximum principal stress. Unloading due to denudation and uplift are prerequisite to their formation. They show promise for tracking the contemporary stress fields in regions where in situ measurements are not available. Outcrop studies from the USA provide evidence to support this.
9O3O03 Mechanics of graben formation in crustal rocks: a finite element analysis Melosh. H J: Williams. C A J Geophys Res V94, NBIO, Oct 1989, P13961-13973 Graben occurrence is characteristic of extensional terrains. The state of stress in a crustal layer subject to gravity and horizontal extensional stress is first examined. Stress perturbation caused by introduction of an initial normal fault is considered. Purely elastic interactions favour the formation of a pair of faults over a single fault. Strain energy is lower if the secondary fault is in an antithetic (graben) formation rather than a synthetic configuration. Graben formation is thus favoured over synthetic fault formation. Graben width is largely controlled by depth of the initial fault.
Paired, hook shaped asymmetric folds, with shared limbs cut by axial planar veins, dykes or fractures, are seen in glaciers and rocks. Offset across the vein is characteristically opposite to that expected from sense of fold symmetry. Kinematics of development of these structures is demonstrated by simple clay experiments or consideration of the nature of glacial flow. In zones of simple shear, dilational fractures form rapidly at about 45 deg to shear zone boundaries. Continuous deformation leads to rotation of fractures and simultaneous formation of folds. Folding is a consequence of fracture, not vice versa. 9O3005 Boudinage in homogeneous foliated rocks Mandal, N; Karmakar, S Tectonophysics VITO, NI/2, 10 Dec 1989, P151-158 Platt and Vissers suggestions that boudinage structures can develop within foliated rocks with no lithological contrast by deformation following fracture initiation are confirmed by theoretical and laboratory modelling studies. Conditions for symmetrical and asymmetrical foliation boundinage to develop are further evaluated.
Environmental effects, weathering and soil formation 903006 Rates and patterns of bedrock denudation by coastal salt spray weathering: a seven-year record Mottershead, D N Earth Surf Process Land VI4, N5, Aug 1989, P383-398 Denudation of greenschist in coastal Devon was studied. Mean annual lowering rate was 0.625mm/year. Spatial variation of this rate within sites was large, but between-site variation was not significant. Surface lowering rate was strongly correlated with mean monthly temperature. Spatially and temporally episodic swelling of rock was seen, but this did not correlate with any climatic variable and was assumed to be a largely stochastic phenomenon, due to rock bursting on the granular scale by haloclasty. Crystallisation and thermal expansion of halite, which are enhanced by high summer temperatures, are considered the main denudation mechanisms. 903007 Enhanced boulder weathering under late-lying snowpatches. Short communication Ballantyne, C K; Black, N M; Finlay, D P Earth Surf Process Land VI4. N8, Dec 1989. P745-750 Schmidt hammer tests were used to characterise aggregate surface hardness of granite, gneiss, and schist rocks from Norway, Scotland, and Switzerland. Results from boulders dug out from late lying snowpatches are compared with those from
© 1990 Pergamon Press plc. Reproduction not permitted 131A
903O04 Association of folds and veins in shear zones Hudleston, P J J Struct Geol VII, N8, 1989, P949-957
Geology Tectonic processes and structural geology 9O3001 Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau Wong. I G; Humphrey, J R Bull Geoi Soc Am VIOl, N9. Sept 1989. P1127-1146 Results of seismic monitoring over the past 30 years for the Colorado Plateau have been analysed. Seismic activity is of small to moderate magnitude, with a low to moderate rate of occurrence with earthquakes widely distributed. Some concentrations are seen, notably induced activity related to coal mining. Results are consistent with a highly faulted basement and upper crust and a moderate level of differential tectonic stress, Focal mechanisms indicate contemporary state of stress is characterised by approximately northeast-trending extension.
903002 Neotectonic joints Hancock, P L; Engelder, T Bull Geol Soc Am VIOl, NIO, Oct 1989, P1197-1208 Neotectonic joints are the most recent joint systems to form within a region subject to uplift and erosion. Shallow tectonic joints propagate within the upper 0.5km of the crust where minimum principal stress is essentially horizontal and tensile, and not greatly different from maximum principal stress. Unloading due to denudation and uplift are prerequisite to their formation. They show promise for tracking the contemporary stress fields in regions where in situ measurements are not available. Outcrop studies from the USA provide evidence to support this.
9O3O03 Mechanics of graben formation in crustal rocks: a finite element analysis Melosh. H J: Williams. C A J Geophys Res V94, NBIO, Oct 1989, P13961-13973 Graben occurrence is characteristic of extensional terrains. The state of stress in a crustal layer subject to gravity and horizontal extensional stress is first examined. Stress perturbation caused by introduction of an initial normal fault is considered. Purely elastic interactions favour the formation of a pair of faults over a single fault. Strain energy is lower if the secondary fault is in an antithetic (graben) formation rather than a synthetic configuration. Graben formation is thus favoured over synthetic fault formation. Graben width is largely controlled by depth of the initial fault.
Paired, hook shaped asymmetric folds, with shared limbs cut by axial planar veins, dykes or fractures, are seen in glaciers and rocks. Offset across the vein is characteristically opposite to that expected from sense of fold symmetry. Kinematics of development of these structures is demonstrated by simple clay experiments or consideration of the nature of glacial flow. In zones of simple shear, dilational fractures form rapidly at about 45 deg to shear zone boundaries. Continuous deformation leads to rotation of fractures and simultaneous formation of folds. Folding is a consequence of fracture, not vice versa. 9O3005 Boudinage in homogeneous foliated rocks Mandal, N; Karmakar, S Tectonophysics VITO, NI/2, 10 Dec 1989, P151-158 Platt and Vissers suggestions that boudinage structures can develop within foliated rocks with no lithological contrast by deformation following fracture initiation are confirmed by theoretical and laboratory modelling studies. Conditions for symmetrical and asymmetrical foliation boundinage to develop are further evaluated.
Environmental effects, weathering and soil formation 903006 Rates and patterns of bedrock denudation by coastal salt spray weathering: a seven-year record Mottershead, D N Earth Surf Process Land VI4, N5, Aug 1989, P383-398 Denudation of greenschist in coastal Devon was studied. Mean annual lowering rate was 0.625mm/year. Spatial variation of this rate within sites was large, but between-site variation was not significant. Surface lowering rate was strongly correlated with mean monthly temperature. Spatially and temporally episodic swelling of rock was seen, but this did not correlate with any climatic variable and was assumed to be a largely stochastic phenomenon, due to rock bursting on the granular scale by haloclasty. Crystallisation and thermal expansion of halite, which are enhanced by high summer temperatures, are considered the main denudation mechanisms. 903007 Enhanced boulder weathering under late-lying snowpatches. Short communication Ballantyne, C K; Black, N M; Finlay, D P Earth Surf Process Land VI4. N8, Dec 1989. P745-750 Schmidt hammer tests were used to characterise aggregate surface hardness of granite, gneiss, and schist rocks from Norway, Scotland, and Switzerland. Results from boulders dug out from late lying snowpatches are compared with those from
© 1990 Pergamon Press plc. Reproduction not permitted 131A