Arthur Goldstein

Factors affecting the kinematic interpretation of asymmetric boudinage in shear zones

Abstract Based on work along a major mylonite zone in the northern Appalachians and scale model studies, a new mechanism for the origin of asymmetric boudins in shear zones is proposed. Along the Honey Hill Fault in southern Connecticut, granitic sills intruded into calc-silicate gneisses and schists were oblique to the boundaries of the mylonite zone and experienced the following sequence in the production of sigmoidal boudins: (1) as the calc-silicate schists experienced mylonitization and flow, the more competent, coarse-grained granitic sills deformed by extensional fracturing and quartz veining; (2) continued extension of granitic sills was accommodated by ‘normal’ shear on early-formed quartz veins; (3) continuing extension of sills and ductile modification of the corners of boudins resulted in granitic ‘fish’ with tails which stream from the top of the boudin in the ‘down-dip’ direction and from the bottom in the ‘up-dip’ direction. Based on a variety of kinematic indicators, the sense of asymmetry of the tails is identical to that expected for recrystallization tails on sheared augen (σ structure). Models composed of silicone putty and Plasticine were created to investigate the effect of pre-shearing geometry on boudin evolution, and were deformed in a simple shear device. The models reproduce the kinematics deduced from field relations and suggest that one of the primary factors in controlling the amount of extension is the angle which early veins make with the shear zone boundary. Varying the angular relationships in the models suggests that other pre-shearing geometric factors may affect the geometry of boudins formed in this way. Boudins formed through this mechanism appear very similar to Hanmers type II asymmetric boudins. Because the pre-shearing geometry can exert a control on boudin asymmetry, caution should be used when attempting to deduce shear sense or shear strain values from asymmetric boudinage.

Magnetic susceptibility anisotropy of mylonites from the Brevard Zone, North Carolina, U.S.A.

Abstract This study presents the results of magnetic susceptibility anistropy measurements on mylonites from the Brevard Zone of the southern Appalachians. The Brevard Zone is a major crustal mylonite zone and the samples analyzed for this study represent the progressive increase of mylonitization (decrease in grain size) of the Henderson Gneiss with increasing proximity to the Brevard Zone. Our results suggest that magnetic fabric measurements in mylonites are at least qualitatively significant with respect to the strain history of mylonites. The orientations of the principal susceptibilities are parallel to rock fabric elements indicating that the magnetic fabric is a deformationally induced phenomenon. Although the Henderson Gneiss appears to undergo a smooth progressive mylonitization with increasing proximity to the Brevard fault zone, our magnetic measurements suggest that strain states do not increase smoothly as the Brevard Zone is approached. However, the magnetic measurements do suggest that at some sites the strain is very high. The highest site mean anisotropy [(max-min)/mean × 100] we have measured is 120% (cgs) which could, based on strain to magnetic susceptibility anisotropy (MSA) correlations by other workers, indicate strain ratios (1 +e 1 ) (1 +e 3 ) of between 186 and 1.4 × 10 17 . Because we cannot correlate our magnetic fabric measurements with strain in these rocks, we cannot prove that such high strain states in fact exist in the Brevard mylonites. However, these appear to the highest magnetic anisotropies yet recorded. Our data also indicate that the susceptibility ellipsoid is severely flattened with k values commonly lower than than 0.1. We also observe an inverse relationship between k values and anistropy. Similar relationships have been observed in other shear zones and suggest that mylonites follow a strain path of increasing departure from plane strain ( k = 1).

Pressure, temperature, and composition history of syntectonic fluids in a low-grade metamorphic terrane

Analysis of stable isotopes and fluid inclusions in centimeter-scale fibrous quartz-calcite strain fringes from the Taconic thrust belt in Vermont and New York allows reconstruc- tion of fluid conditions and sources over the course of strain-fringe growth. Oxygen isotope ratios in quartz show regular variations in d 18 O, ranging between 19.2‰ and 20.0‰ in different parts of strain fringes. Fluid inclusions have low salinities (2.4% NaCl equivalent) in all parts of strain fringes, and we propose that fluids were derived from clay dehydra- tion. Variations in fluid temperature and pressure derived from stable isotope and fluid- inclusion analysis mimic changes in the orientations of fibers, indicating a strong corre- lation between tectonics and fluids. Earliest fluids had temperatures of ;240 8C; temperatures declined over time to ;200 8C and rose again to ;260 8C. Fluid pressures began at ;1.6 kbar (160 MPa) and fell to ;0.8 kbar (80 MPa) before rising to ;2 kbar (200 MPa). We interpret these results in the context of thrust faulting and the dynamics of thrust wedges. Our model begins with thrusting along the Bird Mountain fault, placing rocks from depth on shallower ones and raising the temperature of the footwall. Increasing temperatures would have stimulated clay dehydration, raising fluid pressures and weak- ening the rocks within the orogen. Reduced strength of the Taconic accretionary wedge would have required lower critical taper angles, stopping motion on the Bird Mountain fault and stimulating horizontal elongation within the Taconic thrust wedge. As fluid pressures and temperatures declined, increasing strength of the rocks would have de- manded higher critical taper angles, stimulating renewed Bird Mountain thrusting and increases in temperature and fluid pressure. We cannot uniquely determine the depth of burial during strain-fringe growth, but we can constrain the depth for different geother- mal gradients and evaluate how close fluid pressures were to lithostatic. The highest fluid pressures were ;70% of lithostatic, and could have been higher. Similar mechanisms tying fluid conditions to tectonics in a feedback loop may be active in accretionary prisms and thrust belts worldwide.

DEFORMED GRAPTOLITES, FINITE STRAIN AND VOLUME LOSS DURING CLEAVAGE FORMATION IN ROCKS OF THE TACONIC SLATE BELT, NEW YORK AND VERMONT, U.S.A.

Abstract An analysis of graptolites in the Taconic slate belt of eastern New York and western Vermont (U.S.A.) shows that they are nearly ideal strain markers. For the three species used in this study, Orthograptus whitfieldii , Orthograptus calcaratus and Climacograptus bicornis , the spacing of thecae is constant except for the first five or so thecae in the proximal part of the fossil rhabdosome. Further, the thecal apertures are perpendicular to the long axis of the stipe. Observations of the thecal spacing in deformed rocks leads to a determination of extension ( e =( l f − l 0 )/ l 0 ) and a measure of the angle between thecal aperture and stipe axis yields a direct determination of angular shear strain. In practice, we find it is most straightforward to use length changes to determine the magnitude of principal strains. In Taconic slates, e 1 ranges from 1.0 to 0.24, e 2 ranges from 0.23 to −0.43 and e 3 ranges from −0.56 to −0.74. Thus, we find that the absolute finite strain in these slates is constrictional at three sites, plane strain at another and a true flattening at only one site. An examination of volume changes based on strain results in determinations of between 81% volume loss and 7% volume gain, with volume losses between 28% and 81% in 9 out of 10 calculations. These conclusions are in accord with previous determinations of volume loss based on reduction spot analyses and are consistent with the observation that pressure dissolution was a common grain scale deformation process in cleavage formation but that these slates lack abundant veins, fibrous overgrowths or other identifiable sites of reprecipitation.

Finite strain heterogeneity and volume loss in slates of the Taconic Allochthon, Vermont, U.S.A.

Abstract Slates of the Taconic Allochthon have ellipsoidal reduction spots which have been used to determine the finite state of strain at seven sites. The strain is highly heterogeneous at all scales, ranging from several centimeters to many kilometers. Variations in X Y ratios is small but variations in Y Z ratios is large and this pattern is seen at all scales. The heterogeneity can be used to determine a strain path, which is nearly horizontal on a Flinn diagram. We propose that uniaxial flattening associated with late stage cleavage development was accommodated by volume loss, and that the most highly strained sites have experienced an average 55% volume loss, at a minimum. Explaining our results without volume loss requires a subhorizontal extension and creates strain compatibility problems. Consideration of the strain history of these rocks suggests that folding-related strains have been modified by later cleavage-related events, the last of which was uniaxial flattening accommodated by volume loss. The finite strains are reflective almost exclusively of the latest, cleavage producing strains.

Reorientation of remanent magnetism during tectonic fabric development: an example from the Waynesboro Formation, Pennsylvania, U.S.A.

Abstract A paleomagnetic study of the late Early-early Middle Cambrian Waynesboro Formation in south, central Pennsylvania reveals a pre-folding magnetization which is close to the expected Cambrian field at three sites, while other site mean directions are rotated by as much as 82° from the expected Cambrian field. These results are based on detailed thermal demagnetization analysis of 84 red siltstone and sandstone samples from nine sites collected over a 163 km 2 area. Local faulting, fault block rotation, and remanence acquisition during rapid apparent polar wander do not appear to be responsible for the observed remanence deviations. Total strain at the sites, as indicated semiquantitatively by the anisotropy of magnetic susceptibility (AMS) of 28 samples, has been studied to test the hypothesis that deformation has rotated the remanence directions. AMS data vary considerably both between sites and within sites and show a path of progressive deformation on a Flinn diagram. There is a strong, positive correlation between the deviation of a samples remanence from the expected Cambrian field and its position along the deformation path, suggesting that finite strain/fabric development has rotated the magnetizations. The study area is structurally contained in the South Mountain Anticlinorium whose southern margin forms the displacement transfer zone to the southern Appalachian Alleghenian thrust sheets (Elliott, 1976). If thrusting had continued and incorporated the Waynesboro rocks into these thrust sheets, the rotated magnetizations could have been incorrectly interpreted to result from rigid thrust sheet rotation.

Deformation of Permian strata overlying a zone of salt dissolution and collapse in the Texas Panhandle

Brittle deformation of strata overlying salt dissolution zones has been identified in Caprock Canyons State Park, Texas Panhandle. The geometry and distribution of the structures indicate that systematic regional joints that predated dissolution collapse influenced salt dissolution. Collapse has resulted in a sequence of deformations including normal faulting, reverse faulting, folding, and veining. The sequence of structural events suggests horizontal extension prior to major collapse. It is proposed that dissolution fronts are complex features with re-entrants controlled by the locations of joint zones.

A shear zone origin for Alleghanian (Permian) multiple deformation in eastern Massachusetts

The area around Worcester, Massachusetts, has been used to determine which deformational and metamorphic features are due to the Alleghanian (Permian) orogeny and which are pre-Alleghanian. Isolated, fault-bounded inliers of Carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages. The first cleavage formed synchronously with metamorphism. Pre-Carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages. The initial cleavage formed during the first metamorphism and the second cleavage formed during the second, retrogressive metamorphism. Thus the second two cleavages and the second metamorphism in pre-Carboniferous rocks are interpreted as Alleghanian. Normal displacements on two distinct faults are bracketed by the formation of the first and second Alleghanian cleavages. The initial faulting, along the newly defined Wachusett mylonite zone (WMZ), formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest. The second faulting, along the Clinton-Newbury fault (CNF) occurred along a steeply inclined plane which cuts the WMZ mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west. Alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined. This zone is interpreted as a ductile shear zone which moved twice, forming the first and second Alleghanian cleavages. Early-formed, pre-Alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates, and new hydrous minerals formed in the shear zone, indicating that it was a pathway for fluid flow. The initial motion was left-lateral with a thrust component. The second cleavage formed during top-to-the-northwest normal displacements. Thus the history of Alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones. This suggests that the strain history included two distinct episodes, with the second experiencing slightly different strain orientations at different times, resulting in first WMZ normal motion, then CNF normal motion and finally normal displacements related to the second Alleghanian cleavages. This history agrees well with other work on the nature of the Alleghanian orogeny in the northeast United States.