J. Hadizadeh

On the influence of porosity on the low-temperature brittle—ductile transition in siliciclastic rocks

Abstract A compilation of available experimental data, coupled with new results on Tennessee sandstone, shows that the low-temperature brittle faulting to cataclastic flow transition in siliciclastic rocks takes place at progressively higher confining pressures as porosity is reduced. The collapse of initial porosity compensates for the tendency for brittle deformation to be dilatant. According to a stability criterion, this in turn favours spreading of the cataclastic deformation throughout the rock volume instead of fault localization. At sufficiently high strains, dilatation, and hence fault localization, supervenes. From microstructural observations on Oughtibridge Ganister (7% porosity), deformed under conditions of the faulting to flow transition, the mechanism of pore collapse involves crushing and sliding on shear-oriented grain boundaries, with accumulation of wear products in the pore spaces. This explains the enhancement of pore collapse by differential stress relative to purely hydrostatic compression.

Interaction of cataclasis and pressure solution in a low-temperature carbonate shear zone

The mineralogical and elemental variations across the main shear zone of the Saltville thrust at Sharp Gap in Knoxville, Tennessee, U.S.A., were studied in a suite of deformed and undefromed carbonate rock samples using X-ray diffraction and electron microprobe methods. An examination of the samples for deformation effects at mesoscopic scale and under the optical microscope reveals familiar cataclastic deformation features including foliated cataclasites and microbreccias occurring in a well-defined, 1–2 m wide zone of intense deformation, plus evidence of hydrofracturing and extensive syndeformational pressure solution. There exists a clear correlation between the observed cataclastic deformation and mineral and elemental distribution which we interpret to result from a deformation-induced dolomite to calcite transformation in the shear zone. The transformation has resulted in removal of Mg from the shear zone, selective deposition of calcite as an intergranular cement in cataclasite/microbreccia units and a relative increase in the concentration of detrital quartz and feldspars.The compositional difference between the shear zone and wall rocks is explained in connection with cataclastic deformation features in terms of a model in which a dual pressure-solution/cataclastic flow mechanism leads to a gradual cementation-hardening of segments of the shear zone. Instabilities could occur via permeability reduction and increased pore pressure within these segments. Hydrofracturing of the hardened segments along with high strain rate reordering of the shear zone materials reset the ruptured zone back to the dual deformation mechanism regime. As a long-term effect, the compositional transformation of the shear zone is expected to prolong periods of creep and cause smaller coseismic stress drops since under the imposed conditions calcite is more ductile and soluble than dolomite.

Estimating local strain due to comminution in experimental cataclastic textures

Abstract Cataclastic textures were produced by deforming porous quartz sandstone samples at room temperature, 15–200 MPa confining pressure, and 13–57% strain ( γ ∼2–20). Particle size distribution (PSD) and fractal dimension ( D ) values were measured on different digital images of each sample. Over the range of applied axial strain the detectable effects of comminution (particle-size reduction through fracture) on PSD were limited to the population of particles ≥7 μm. Textural variable ψ , named comminution intensity, is defined in terms of total sectional area of particle population greater than a critical size S c . For the Massillon sandstone the S c is close to four times the suggested value for quartz grinding limit. The comminution intensity vs. axial strain data due to the entire range of applied pressure and strain fits a power function of the form ϵ 11 = ψ m , with 0.25≤ m ≤0.5. The local strain due to constrained comminution is then predictable by model function ϵ L = ψ n , where n =1/ m , with empirical n =2.97 for the studied sandstone. The model predicts relatively monotonous textures for very small and very large strains, while the highest variety of particle size occurs at between 2 and 13% strain. Applications of the method include millimeter to meter scale mapping of the relative degree of comminution in natural fault gouge and the local strain values in experimental samples.

Cataclastic flow and semi-brittle deformation of anorthosite

Abstract A series of experiments on dry Bushveldt anorthosite (An 75 , 350 μm grain size, 5% impurities) is carried out at 25–700°C, 500–1500 MPa pressures and 10 −5 s −1 strain rate. The strength of the Bushveldt anorthosite remains pressure-sensitive throughout the applied P–T range while weakening occurs with increased temperature at a given pressure. A transition from stick-slip to stable sliding occurs at 200–300°C and pressures of ≥1500 MPa. Reduction in strength at 200–300°C, localized cataclastic flow, and lack of microscopic evidence of crystal-plastic deformation raises the possibility that the transition may be coincident with the propagation of a critical number of randomly distributed thermal cracks. Uniform ductile flow occurs only at temperatures of 600 and 700°C, and 1000–1500 MPa pressures, coincident with the development of mechanical twinning, patchy and banded undulatory extinction and is likely to be due to semi-brittle deformation. Under these conditions, the deformation remains distributed to strains as high as 44%.

Development of interlaced mylonites, cataclasites and breccias: example from the Towaliga fault, south central Appalachians

Abstract A variety of cataclastic rocks from crush breccias to cataclasites and silicified breccias are associated with retrograde mylonites along the Towaliga fault zone of south central Appalachians in Georgia. A zone of alternating breccias and quartz ultra-mylonites, bordered by quartz mylonites that are roughly laminated with mm-scale mica-rich bands of cataclasites, occur subparallel to the mylonitic foliation. Elsewhere along the fault zone, evidence for plastically deformed microfractures is found. In discussing the mechanical evolution of these rocks two possibilities are considered: (a) the cataclasis is an overprint representing deformation in an entirely brittle regime; and (b) cataclasis and mylonitization occurred at T > 300° C in a predominantly plastic regime. Within the framework of the latter model it is suggested that the cataclasite bands are either due to the relaxation of dilational stresses following downward propagation of seismic ruptures or represent strain-induced seismic instabilities during plastic shearing.

Self-similar cataclasis in the Saltville thrust zone, Knoxville, Tennessee

Fault rocks from the Saltville thrust zone, Knoxville, Tennessee, display a fractal geometry of clast size over 3 orders of magnitude. The cataclastic fractal geometry occurs at each magnification in different clast size classes and at combined magnifications. The mean of the fractal dimension (D) measured at each of the optical photomicrographs is generally smaller than that of the scanning electron microscopy images because of the smaller clast density in the optical sections. The fractal dimensions measured on randomly selected areas of the sections cut parallel to the thrust and normal to the thrust along the dip and strike, show a normal distribution with its mean, median, and mode that correlate closely with the dimension of ideal, fractal cataclasis (DI) based on the Sierpinski carpet model. The cataclasis was a statistical random, isotropic, and homogeneous fractal process that deformed the carbonates similarly parallel and normal to the thrust plane and in different parts of the thrust zone.

Feasibility of estimating cementation rates in a brittle fault zone using Sr/Ca partition coefficients for sedimentary diagenesis

Abstract Cement phases such as calcite or quartz often incorporate trace elements from the parent fluids as they crystallize. Experimental sedimentary diagenesis indicates that trace element partition coefficients reflect rates of cementation. The applicability of these findings to fault zone cementation is examined as we make a preliminary attempt to estimate calcite cementation rate in a brittle fault zone directly from the fault-rock composition data. Samples for this study were collected from the Knoxville outcrop of the Saltville fault in Tennessee. The cementation rates for the fault rock samples range from 1×10 −12 to 3×10 −13 m 3 / h per m, in agreement with some experimental rates and the rates reported for samples from the DSDP sites. When applied to a non-responsive pore-system model, these rates result in rapid precipitation sealing indicating the influence exerted by the surface-area/volume ratio of the pore network. We find it feasible to obtain a reasonable range of values for the cementation rate using the trace element partition method. However, the study also indicates the need for relatively accurate values for the trace/carrier element ratio in the fault zone syntectonic pore fluid, and exhumed cement.

Initiation of cataclastic flow and development of cataclastic foliation in nonporous quartzites from a natural fault zone

Abstract Microstructural study of foliated quartz cataclasites of a segment of the Towaliga fault zone, Georgia, USA, indicates that rock strain by cataclastic deformation occurred through microscopic displacements along a distributed system of shear bands. The localized cataclastic flow in the bands was in turn the result of frictional sliding along a network of mesoscopic shear fractures that accommodated initial strains and were in part created by high fluid pressures in the fault zone. The shear fractures at both the microscopic and mesoscopic scale were formed in the Riedel R1, P, and Y orientations. Simultaneous displacements along these Riedel shears produced the cataclastic foliation, rhomb-shaped and tabular quartz grain fabrics, and straight, stepped grain boundaries. Crystallization by fluids accompanied fracturing and possibly aided the cataclastic flow by forming muscovite and quartz crystals along fractures. Most fluid inclusion trails and quartz veins cut across the cataclastic features, indicating that fluids which assisted the deformation later healed the fractures. A right-lateral sense of displacement is suggested for the cataclastic flow, consistent with the kinematics of the mylonitization along the fault zone.

Timing and temperature of cataclastic deformation along segments of the Towaliga fault zone, western Georgia, U.S.A.

Abstract Foliated cataclasite and breccia exposed along a segment of the Towaliga fault zone (TFZ) in western Georgia, U.S.A., formed during two episodes of cataclastic deformation. Conventional KAr crystallization ages of muscovite collected along fractures in the foliated cataclasite at Dixon Mountain, suggest that following mylonitization, at about 296 Ma (Pennsylvanian), rocks along the TFZ were elevated upward across the 300°C isothermal surface and ductile deformation via cataclastic flow was initiated. Fluid inclusion microthermometry and quartz microfabric in the cataclasite suggest that the temperature during this first cataclastic deformation, which probably occurred at a depth greater than 8 km, was between 140 and 300°C. At around 237 Ma (Late Permian), cataclastic deformation was dominated by brecciation and extensive vein sealing and formed the silicified breccias at Dixon Mountain. Muscovites from another cataclastic segment of the TFZ give a relatively low apparent age of around 269 Ma, probably because of loss of argon or mixing with younger neocrystallized muscovite during cataclasis.

AN INTERDISCIPLINARY COURSE CENTERED ON STUDENT DEBATE OF CURRENT ENVIRONMENTAL ISSUES

This paper presents an active-learning approach applied to an upper level elective course titled Human Societies & Environment, which draws its subject matter from a variety of disciplines in the social and natural sciences. The paper recognizes that the desire to discuss current environmental issues is a significant learning motive, and experiments with a coursework design centered on a set of regularly convened student debate sessions. A student moderator conducts the debates between two teams of students. Lectures, Internet searches, numerical analysis, and research projects form supporting activities that serve to organize and inform the debates as well as to consolidate pedagogic value of the course.