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Strain patterns within the late Variscan granitic dome of Velay, French Massif Central
GEOLOGY:TECTONICS & STRUCTURAL GEOLOGY Mountains, south-central Wyoming, reveals that lithological contrasts within the sedimentary section created a mechanical anisotropy that influenced both fault geometry and the relative rates of faultpropagation and fault slip. Two structural lithic units axe identified, a competent lower unit and an overlying incompetent unit. The Black Canyon fault is oriented at a low-angle to bedding in the lower structural lithic unit and is Iayer-parallel near the base of the upper unit. The Red Spring fault is a thin-skinned thrust fault and is interpreted to be linked to the Black Canyon fault to generate a triangle-zone geometry. Similar structures can be identified elsewhere in the Rocky Mountain foreland and this configuration may represent apreviously unrecognized indicator of low-angle basement faults. -from Authors
946016 Displacement transfer at thrust terminations: the Saltville thrust and Sinking Creek anticline, Virginia, USA B. A. Couzens & W. M. Dunne, Journal of Structural Geology, 16(6), 1994, pp 781-793. The Saltville thrust terminates in the core of the Sinking Creek anticline. Changes in the amplitude of the anticline may preserve profile shortening as the thrust displacement decreases towards the fault termination. Consequently, thrust displacement would be transferred into the fold. Newly published maps and new strain and mesostrnctural data from the Tuscarora Sandstone are combined to argue that the Sinking Creek anticline did perform this kinematic role. It is proposed that the anticline developed as a faultpropagation fold that experienced both decollement and anticlinal breakthrough by the Saltville thrust. Thrust displacement was accommodated by the development of the modified fault-propagatlon fold and by transfer to a roof flat from the decollement breakthrough. New strain and mesostructural data indicate no fixed pin lines in either the hinge or bacldimb throughout folding. Consequendy, layer slip occurred through the hinge during folding and the forelimb deformed internally during breakthrough. -from Authors 946017 Strain patterns within the late Variscan grani. tic dome of Velay, F r e n c h Massif Central J.-L, Lagarde, C. Dallain, P. Ledru & G. Courrioux, Journal of Structural Geology, 16(6), 1994, pp 839-852. The Velay granites are intruded as a late Variscan dome in the French Massif Central. Strain patterns within the Velay granites reflect the geometry and-kinematics of late Carboniferous deformauon, at the site of final emplacement. According to the internal strain field two zones can be distinguished in the Velay dome: 1) a central and southern zone where concentric patterns indicate a southward expansion of .granites; and 2) a northern zone where the lateral expansion is limited and combined with top-to-thenorth extensional shearing along the Mont Pilat Detachment zone and high-temperature wrenching. These asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault, during the uplift of a thickened area. Post-thickening combination of extensional and wrench tectonics is inferred. -from Authors 946018 Initiation of folding and boudinage in wrench shear and transpression A. I. James & A. J. Watkinson, Journal of Structural Geology, 16(6), 1994, pp 883-893. A linear stability analysis is performed on a deforming laye rea t.hreeTdnnensional linear viscous system. The stability or.cylindrical perturbations of the form ~ = A(t) cos (fix py) ts examined. It is found that fold-type perturbations are unstable and pinch-and-swell disturbances are unstable in some cases. For the case of wrench shear alone the fastest growing buckling disturbances are oriented at 45" to the positive y-axis, while the fastest growing pinch-and-swell disturbances have positive growth rates and are oriented 90" to the fold axes. Additional shortening parallel to the x-axis (transpression) causes the fold axes to initiate at lower angles to the y-axis. Pinch-and-swell disturbances may or may not be unstable in transpression depending on the magnitude of stretching parallel to the z-axis. -from Authors
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946019 Extensional origin of ductile fabrics in the schist belt, central Brooks Range, Alaska - I. Geologic and structural studies T . A . Little, E. L. Miller, J. Lee & R. D. Law, Journal of Structural Geology, 16(7), 1994, pp 899-918. Data suggest that high-strain tectonites along the southern margin of the Schist Belt did not form during contractional deformation in the Brooks Range orogen, but during midCretaceous crustal extension, which was superimposed on ]~revionsly thickened continental crust of the northernmost Cordillera. Temperatures may have increased during the later stages of extension in the southern Brooks Range and adjacent Yukon-Koyukuk basin region in concert with mid- to Late Cretaceous magmatism, allowing deformation and metamorphism of earlier formed normal faults. Subsequent folding in the latest Cretaceous or Early Tertiary resulted in further exhumation of the schists and modified the geometry of the faults by large-scale folding. -from Authors 946020 Extensional origin of ductile fabrics in the schist belt, central Brooks Range, Alaska - II. Micros. truetural and petrofabric evidence R.D. Law, E. L. Miller,T. A. Little& J. Lee, Journal of Structural Geology, 16(7), 1994, pp 919-940. A regional system of S-dipping faults is exposed in the Florence and Fall Creeks area of the south-central Brooks Range. The structurally highest of these faults places rocks of the oceanic Paleozoic-Mesozoic Angayucham terrane, together with unconformably overlying Cretaceous clastic rocks, on Devonian metagreywacke and phyllite. This metagreywacke-phyllite (MP) unit in turn structurally overlies Devonian (?) and older basement rocks of the Brooks Range Schist Belt along a S-dipping structural contact previously mapped as the Florence Creek fault. The Schist Belt and MP units are both characterized by a regionally developed, S-dipping greenschist facies foliation. Mylonites with strong asymmetric crystal fabrics are well developed in quartz stringers in the NIP unit and in Schist Belt rocks. These asymmetric single- and cross-gixdle c-axis fabrics indicate a top down-to-the south shear sense. This pervasive top-down-to-the-south shear sense is inconsistent with tectonite fabrics in the Schist Belt being related to N-vergent thrust faulting, but accords well with an extensional model for exhumation of high-P rocks of the Schist Belt. -from Authors 946021 The kinematic interpretation of obliquelytransected porphyroblasts: an example from the Trois Seigneurs Massif, F r a n c e C. W. Passchier & P. I. H. R. Speck, Journal of Structural Geology, 16(7), 1994, pp 971-984. Obliquely-transected porphyroblasts (OTPs) contain a straight inclusion pattern that is continuous with the external foliation, but oblique in orientation. OTPs are interpreted to form by porphyroblast growth between two phases of deformation. T h e obliqueness of the inclusion patte.rn has been variously explained by porphyroblast rotation m the kinematic reference frame of bulk flow, or by foliation rotation around a stationary porphyroblast. The two models imply opposite sense-of-vorticity to produce the same geometry and their relevance shouldbe known if OTPs axe to be used as sense-of-vorticity indicators. Andalusite-OTPs in the Trois Seigneurs Massif, French Pyrenees, _have a spool-shape in three dimensions that is interpreted to result from passive overgrowth of andalusite on a spaced S~ cleavage Subseouent non-coaxial D flow led to development of an S 3 crenulauon cleavage and the OTP geometry. Although the porphyroblasts rotated, rotation of the foliation in the kinematic reference frame of bulk flow was mainly responsible for the OTP geometry. -from Authors 946022 Rheological controls on the shapes of single. layer folds P. J. Hudleston & Labao Lan, Journal of Structural Geology, 16(7), 1994, pp 1007-1021. Information about theology can potentially be gained from analyzing the shapes of folds in isolated buckled layers. Two-dimensinnal finite element models of incompressible flow in power-law viscous fluids axe employed to investi-
946016 Displacement transfer at thrust terminations: the Saltville thrust and Sinking Creek anticline, Virginia, USA B. A. Couzens & W. M. Dunne, Journal of Structural Geology, 16(6), 1994, pp 781-793. The Saltville thrust terminates in the core of the Sinking Creek anticline. Changes in the amplitude of the anticline may preserve profile shortening as the thrust displacement decreases towards the fault termination. Consequently, thrust displacement would be transferred into the fold. Newly published maps and new strain and mesostrnctural data from the Tuscarora Sandstone are combined to argue that the Sinking Creek anticline did perform this kinematic role. It is proposed that the anticline developed as a faultpropagation fold that experienced both decollement and anticlinal breakthrough by the Saltville thrust. Thrust displacement was accommodated by the development of the modified fault-propagatlon fold and by transfer to a roof flat from the decollement breakthrough. New strain and mesostructural data indicate no fixed pin lines in either the hinge or bacldimb throughout folding. Consequendy, layer slip occurred through the hinge during folding and the forelimb deformed internally during breakthrough. -from Authors 946017 Strain patterns within the late Variscan grani. tic dome of Velay, F r e n c h Massif Central J.-L, Lagarde, C. Dallain, P. Ledru & G. Courrioux, Journal of Structural Geology, 16(6), 1994, pp 839-852. The Velay granites are intruded as a late Variscan dome in the French Massif Central. Strain patterns within the Velay granites reflect the geometry and-kinematics of late Carboniferous deformauon, at the site of final emplacement. According to the internal strain field two zones can be distinguished in the Velay dome: 1) a central and southern zone where concentric patterns indicate a southward expansion of .granites; and 2) a northern zone where the lateral expansion is limited and combined with top-to-thenorth extensional shearing along the Mont Pilat Detachment zone and high-temperature wrenching. These asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault, during the uplift of a thickened area. Post-thickening combination of extensional and wrench tectonics is inferred. -from Authors 946018 Initiation of folding and boudinage in wrench shear and transpression A. I. James & A. J. Watkinson, Journal of Structural Geology, 16(6), 1994, pp 883-893. A linear stability analysis is performed on a deforming laye rea t.hreeTdnnensional linear viscous system. The stability or.cylindrical perturbations of the form ~ = A(t) cos (fix py) ts examined. It is found that fold-type perturbations are unstable and pinch-and-swell disturbances are unstable in some cases. For the case of wrench shear alone the fastest growing buckling disturbances are oriented at 45" to the positive y-axis, while the fastest growing pinch-and-swell disturbances have positive growth rates and are oriented 90" to the fold axes. Additional shortening parallel to the x-axis (transpression) causes the fold axes to initiate at lower angles to the y-axis. Pinch-and-swell disturbances may or may not be unstable in transpression depending on the magnitude of stretching parallel to the z-axis. -from Authors
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946019 Extensional origin of ductile fabrics in the schist belt, central Brooks Range, Alaska - I. Geologic and structural studies T . A . Little, E. L. Miller, J. Lee & R. D. Law, Journal of Structural Geology, 16(7), 1994, pp 899-918. Data suggest that high-strain tectonites along the southern margin of the Schist Belt did not form during contractional deformation in the Brooks Range orogen, but during midCretaceous crustal extension, which was superimposed on ]~revionsly thickened continental crust of the northernmost Cordillera. Temperatures may have increased during the later stages of extension in the southern Brooks Range and adjacent Yukon-Koyukuk basin region in concert with mid- to Late Cretaceous magmatism, allowing deformation and metamorphism of earlier formed normal faults. Subsequent folding in the latest Cretaceous or Early Tertiary resulted in further exhumation of the schists and modified the geometry of the faults by large-scale folding. -from Authors 946020 Extensional origin of ductile fabrics in the schist belt, central Brooks Range, Alaska - II. Micros. truetural and petrofabric evidence R.D. Law, E. L. Miller,T. A. Little& J. Lee, Journal of Structural Geology, 16(7), 1994, pp 919-940. A regional system of S-dipping faults is exposed in the Florence and Fall Creeks area of the south-central Brooks Range. The structurally highest of these faults places rocks of the oceanic Paleozoic-Mesozoic Angayucham terrane, together with unconformably overlying Cretaceous clastic rocks, on Devonian metagreywacke and phyllite. This metagreywacke-phyllite (MP) unit in turn structurally overlies Devonian (?) and older basement rocks of the Brooks Range Schist Belt along a S-dipping structural contact previously mapped as the Florence Creek fault. The Schist Belt and MP units are both characterized by a regionally developed, S-dipping greenschist facies foliation. Mylonites with strong asymmetric crystal fabrics are well developed in quartz stringers in the NIP unit and in Schist Belt rocks. These asymmetric single- and cross-gixdle c-axis fabrics indicate a top down-to-the south shear sense. This pervasive top-down-to-the-south shear sense is inconsistent with tectonite fabrics in the Schist Belt being related to N-vergent thrust faulting, but accords well with an extensional model for exhumation of high-P rocks of the Schist Belt. -from Authors 946021 The kinematic interpretation of obliquelytransected porphyroblasts: an example from the Trois Seigneurs Massif, F r a n c e C. W. Passchier & P. I. H. R. Speck, Journal of Structural Geology, 16(7), 1994, pp 971-984. Obliquely-transected porphyroblasts (OTPs) contain a straight inclusion pattern that is continuous with the external foliation, but oblique in orientation. OTPs are interpreted to form by porphyroblast growth between two phases of deformation. T h e obliqueness of the inclusion patte.rn has been variously explained by porphyroblast rotation m the kinematic reference frame of bulk flow, or by foliation rotation around a stationary porphyroblast. The two models imply opposite sense-of-vorticity to produce the same geometry and their relevance shouldbe known if OTPs axe to be used as sense-of-vorticity indicators. Andalusite-OTPs in the Trois Seigneurs Massif, French Pyrenees, _have a spool-shape in three dimensions that is interpreted to result from passive overgrowth of andalusite on a spaced S~ cleavage Subseouent non-coaxial D flow led to development of an S 3 crenulauon cleavage and the OTP geometry. Although the porphyroblasts rotated, rotation of the foliation in the kinematic reference frame of bulk flow was mainly responsible for the OTP geometry. -from Authors 946022 Rheological controls on the shapes of single. layer folds P. J. Hudleston & Labao Lan, Journal of Structural Geology, 16(7), 1994, pp 1007-1021. Information about theology can potentially be gained from analyzing the shapes of folds in isolated buckled layers. Two-dimensinnal finite element models of incompressible flow in power-law viscous fluids axe employed to investi-