Abdolali Babaei

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.

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.

Two-decked nature of the Ouachita Mountains, Arkansas

The Ouachita Mountains of Arkansas and Oklahoma are made up of two structural decks. The lower deck of tight to isoclinal folds in pre-Middle Mississippian strata records multiple folding and low-grade metamorphism. The upper deck of open folds in Carboniferous rocks shows no evidence of the multiple folding and metamorphism. Dips of fold limbs in the lower deck are typically more than 60°; dips of fold limbs in the upper deck are generally less than 45°; fold wavelengths in the lower deck are in the range of 0.5 to 3.5 km; fold wavelengths in the upper deck are generally in the range of 12 to 15 km. Estimates of shortening of the folds in the lower deck are five times greater than those of shortening of the upper deck. The change from tight folding to broad folding takes place in the middle part of the Mississippian Stanley Group. The difference in fold style has been attributed to disharmonic folding of a stiff upper deck and a ductile lower deck. However, the boundary between harmonic and disharmonic folds shows no apparent relation to the fold wavelength or the stratigraphic spacing of stiff beds. We hypothesize that the difference in structural style reflects the unconformable deposition of younger folded and faulted foreland-basin strata (the upper deck) over the older lower deck strata, which were stacked in an accretionary wedge.

Evolution of antivergent folds on a Paleozoic accretionary prism, Arkansas: An alternative view

Rocks around the western plunge of the Benton uplift in the Ouachita Mountains of western Arkansas show multiple periods of deformation during the Ouachita orogeny. Seismic-reflection interpretations and surface geology are consistent with a thick section of highly deformed Paleozoic rocks that are separated as thrust sheets by north-vergent regional-scale thrust faults. North-vergent folds develop in such a setting; however, south-vergent folds with the axial planes dipping opposite to the direction of underthrusting are also observed on the Benton uplift. Development of such folds has been explained by models such as mechanical decoupling along zones of low shear strength in trenches, backthrusting, and backfolding, but none explains the south-vergent folds of the Benton uplift, mostly because of lack of adequate field data. Geometrical analyses show that reactivation of thrust faults during a secondary phase of deformation tightened and reoriented open folds of an initial phase and, as a result, developed the macroscopic and mesoscopic antivergent folds in the Benton uplift. Curvilinear map traces of the thrust faults and broad open folds that refold earlier structures indicate that there was continuous deformation after the development of antivergent folds.