Carol Simpson

Phase boundary mobility in naturally deformed, high-grade quartzofeldspathic rocks : evidence for diffusional creep

Abstract Grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern Grenville Province, Ontario, Canada, suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep. In these rocks, quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions. In the naturally deformed rocks, however, phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation (i.e. in directions of inferred finite extension) which implies that phase boundary motion and cusp formation occurred during deformation. Optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep. However, the morphology of quartz-feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone. We propose that phase boundary motion resulted from a diffusion-assisted process involving dissolution at foliation-parallel quartz-feldspar phase boundaries, mass transfer over length scales of the order of feldspar domain size (≈200 μm or greater) and precipitation at quartz-feldspar phase boundary cusps. This study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks. It also has important implications for the natural rheological behavior of the mid- and lower-continental crust.

Paleozoic deformation in the Sierras de Cordoba and Sierra de Las Minas, eastern Sierras Pampeanas, Argentina

Basement orthogneisses, paragneisses, and migmatites in the Sierras de Cordoba of the eastern Sierras Pampeanas of central Argentina represent remnants of a Cambrian arc and accretionary prism that initially formed above a subduction zone along the early Cambrian margin of Gondwana. These basement rocks contain many high strain zones that record major events in the tectonic evolution of the western margin of Gondwana during the latest Proterozoic - middle Paleozoic. Initial orthogonal shortening and chevron folding of the accretionary prism rocks occurred prior to a high temperature/low pressure, relatively static metamorphism and migmatization event during 509- 525 Ma, simultaneous with the Pampean orogeny. Localized deformation along a narrow zone of dextral transpression occurred late in the Pampean cycle. After peak metamorphism, the gneisses and migmatites deformed on east-dipping, greenschist-grade, mylonite-, ultramylonite-, and pseudotachylyte-bearing zones. Kinematic data from a selection of these shear zones include field data, microstructural shear sense indicators, and quartz C-axis fabric asymmetry. Almost all show west-directed, dip-slip reverse fault movement, but most do not achieve major crustal shortening. One major ultramylonite zone in western Sierras de Cordoba may represent a major tectonic boundary with the adjacent Sierra de San Luis rocks. This high strain zone is intruded by the Devonian-aged Achala batholith. Other age constraints include a pseudotachylyte vein that has been dated by 40 Ar/ 39 Ar method as mid-Silurian and Ordovician I-type plutonic rocks in the Sierra de Las Minas to the west that are deformed into greenschist-grade protomylonite and pseudotachylyte-bearing zones. Our data are consistent with late Ordovician to mid-Devonian orthogonal deformation throughout the Sierras de Cordoba, related to the emplacement of the Precordillera and Chilenia terranes to the west. q 2002 Elsevier Science B.V. All rights reserved.

Volume loss and tectonic flattening strain in granitic mylonites from the Blue Ridge province, central Appalachians

Abstract Granitic mylonites from the Blue Ridge province in the central Appalachians demonstrate the contribution of various deformation mechanisms to the strain geometry that developed during greenschist facies deformation in thrust-related shear zones. Strain accumulated through cataclasis in feldspar, crystal plastic processes in quartz, and a significant contribution of pressure solution in both feldspar and quartz. Strain analysis was performed using R f / φ f techniques on quartz grain shapes and by measuring stretched and boudinaged feldspars. Petrographic and geochemical evidence indicates that deformation ranged from isovolumetric to upwards of 60% bulk-rock volume loss. Quartz and feldspar strain values plot within the field of apparent flattening. Feldspars in both XZ and YZ sections are boudinaged and separated by transverse veins and record a true tectonic flattening strain. Strain data indicate that the bulk deformation path in shear zones throughout the Blue Ridge province significantly deviated from plane strain.

Practical analysis of general shear zones using the porphyroclast hyperbolic distribution method: An Example from the Scandinavian Caledonides

The porphyroclast hyperbolic distribution (PHD) method of kinematic vorticity analysis draws heavily upon the pioneering work of Ghosh and Ramberg (1976), among others. In general shear zones, rigid ellipsoidal grains may rotate either forwards (with the sense of shear) or backwards (counter to the sense of shear) according to their axial ratios and orientations. Grains of a critical shape have their forward motion exactly countered by backward motion in response to the pure shear component of deformation. When plotted on the hyperbolic net, the axial ratios and orientations of these easily recognizable ‘stable end’ grains define a hyperbola asymptotic to the eigenvectors of flow.

Deformation of Gondwana margin turbidites during the Pampean orogeny, north-central Argentina

Basement uplifts in the Eastern Sierras Pampeanas of north-central Argentina expose sedimentary rocks that originated on the Neoproterozoic to early Cambrian margin of Gondwana for over 1000 km along strike. A series of transects through unmetamorphosed sections of the Vendian-aged Puncoviscana Formation and metasedimentary equivalent rocks from deeper structural levels demonstrates that an early stage compaction cleavage has been folded into chevron structures, with a concomitant axial planar tectonic cleavage, prior to erosion and deposition of the overlying Middle to Late Cambrian Meson Group. Changes in structural style are shown to be gradational; all mesoscopic ductile fold structures in the region can be explained with one major deformation episode. Late-stage Andean brittle folds and thrusts are more noticeable in the northern sections, but do not significantly alter the overall structural style or geometry of the basement rocks. A static metamorphic overprint in the Ordovician with associated granitic intrusions only affects fabrics and mesoscale structures adjacent to plutons. Published age constraints indicate an overlap between the timing of sedimentary deposition and that of structural and metamorphic events at depth. We develop a tectonic model for the Pampean orogeny that involves the buttressing of off-scraped sedimentary rocks above a subducting slab beneath the Gondwana margin. The collision of outboard terranes did not occur in this area until at least the mid-Ordovician Famatinian orogeny.

Dynamic recrystallization and chemical evolution of clinoamphibole from Senja, Norway

Clinoamphibole from a mylonitic amphibolite exhibits microstructures characteristic of dynamic recrystallization, including porphyroclasts in a finer grained matrix of needle-shaped amphibole. The matrix amphibole defines an LS fabric and porphyroclasts have core and mantle structures with a core containing undulose to patchy extinction and (100) deformation twinning surrounded by a mantle of recrystallized grains. In addition intragranular grains also occur within the cores. TEM analyses of the porphyroclasts reveal that they contain a wide variety of lattice defects including high densities (5 × 108cm−2) of free dislocations and dislocation arrays, dissociated dislocations, stacking faults, and (100) micro-twins. TEM also shows that matrix grains and intragranular grains have relatively low defect densities, and that the intragranular new grains occur at localities in the porphyroclasts characterized by high densities of dislocations. These observations along with the chemical and orientation relationships between the recrystallized grains and porphyroclasts indicate that the new grains may have formed by heterogeneous nucleation and that further growth probably occurred by both strain assisted and chemically induced grain boundary migration or liquid film migration. This recrystallization event is interpreted to be synkinematic based on the fact that no recrystallization textures are present in the matrix grains and that the matrix grains define an LS fabric. However, the low defect densities in the matrix grains and the lack of intracrystalline strain in other phases indicate that post-kinematic recovery processes were active.

Rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the Funeral Mountains, Death Valley, California

The Funeral Mountains in eastern California preserve a record of Early Cretaceous (?) metamorphism followed by ductile deformation, uplift, and low-angle normal (detachment) faulting. 40Ar/39Ar age spectra indicate that cooling and uplift of the lower plate began in Cretaceous time. Uplift was accommodated by normal-sense movement along a wide northwest dipping shear zone. Mylonitic fabrics, some of which have been dated as Late Cretaceous, deformed older high-temperature metamorphic textures. Analyses of shear bands, mica fish, σ and δ porphyroclasts, grain shape fabrics, and folds indicate that the upper surfaces moved toward 299° ±12 (top to the northwest) relative to lower surfaces. Uplift continued until the near present, the youngest phase being accommodated by top-to-the-northwest movement along the detachment fault, which formed subparallel to lower-plate mylonitic fabrics. Fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 Ma. Reconstruction along the movement vector places the Grapevine Mountains over the Funeral Mountains, having been displaced at least 40 km. Isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest. The maximum temperature and pressure was determined on a sample from Monarch Canyon using thermobarometry, 700°C at a depth of 32 km. At Chloride Cliff, 5 km southeast of Monarch Canyon, 4 samples yielded 575°–600°C at depths of 19–27 km. At Indian Pass, 17 km southeast of Monarch Canyon, a temperature of 490°C was determined. In the southern Funeral Mountains, about 50 km southeast of Monarch Canyon, conodont color alteration indexes indicate temperatures of 325°–425°C. These data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism. Thermochronologic data (K-Ar on muscovite, biotite, and hornblende, 40Ar/39Ar on hornblende, and fission track on apatite, titanite, and zircon) indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined to the interval 21–6 Ma; during the latter part of this interval (11–6 Ma), rapid uplift and movement along the detachment fault are documented. The findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt.

Sequential ductile to brittle reactivation of major fault zones along the accretionary margin of Gondwana in Central Argentina

Abstract Metamorphic and plutonic basement rocks and cover sequences of the Eastern Sierras Pampeans, Argentina, have undergone multiple episodes of fault reactivation. Faults take advantage of mid- to late Cambrian, NW-SE-striking, steeply east-dipping foliations in Vendian-aged accretionary prism metasedimentary rocks. Foliations in peraluminous schists, paragneisses and migmatites are deflected into late Cambrian amphibolite-grade high-strain zones. Greenschist-grade mylonite zones and thick retrogressed ultramylonite zones with mainly NNW strikes, easterly dips, and east-over-west movement, affect the metasedimentary rocks and Ordovician-aged intrusive rocks and are presumably related to early Devonian accretion of terranes to the west of Gondwana. pseudotachylyte veins occur in nearly all mylonite zones. Brittle deformation during Carboniferous to Triassic time produced major pull-apart basins located above terrane boundaries. Outcrop patterns of Triassic to Cretaceous sedimentary rocks are consistent with transtensional pull-apart basins followed by Andean transpressional deformation. The theoretical basis for fault reactivation and production of ‘short cuts’ is examined in the context of Tertiary to Recent basin inversion faults. The inversion faults follow the Palaeozoic trends and produce the present-day NNW-oriented, deep sedimentary basins and intervening ranges of basement rocks.

Extensional and contractional deformation in the Blue Ridge Province, Virginia

Basement rocks of the Blue Ridge anticlinorium in central and northern Virginia were involved in two temporally distinct deformation episodes. The older event is characterized by a locally pervasive low-strain fabric and high-strain shear zones. A preferred grain shape orientation in recovered quartz, feldspar, and biotite, combined with a poorly developed symmetric quartz c -axis pattern, defines the low-strain fabric. Kinematic indicators are typically symmetric, but where asymmetric, they consistently indicate top down-to-the-southeast extensional motion. Discrete shear zones are within and close to Late Proterozoic plutons; kinematic evidence fron these zones records large noncoaxial strains also with a down-to-the-southeast extensional shear sense. Feldspar microstructures range from cataclastic fractures to complete recovery, suggesting that deformation occurred at greenschist to amphibolite grade. Both low-strain foliations and high-strain shear zones are cut by Paleozoic thrust-related tectonites. On the basis of kinematic evidence and spatial association with Late Proterozoic plutons, cross cutting relations with thrust-related structures, and regional geologic relations, we suggest that extensional deformation is related to Late Proterozoic rifting of the Laurentian margin. The kinematic histories of the Rockfish Valley fault zone and related shear zones appear to be relatively simple and consistent with Paleozoic west- to northwest-directed thrusting. The Rockfish Valley fault zone does not separate distinctly different basement terranes, and thus its displacement is thought to be small. Mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars, under lower to middle greenschist-facies conditions. Solution transfer processes were an important deformation mechanism in these tectonites. Thrust- related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands, suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust. The Rockfish Valley fault zone appears to predate the formation of the South Mountain cleavage in the Valley and Ridge province, but it may be part of a lengthy, progressive, thrust-related deformation.

The role of solution in the formation of boudinage and transverse veins in carbonate rocks at Rheems, Pennsylvania

The interaction of pressure solution with extensional strain is the most important factor influencing boudinage and vein structures in a mesoscopically folded sequence of subgreenschist-grade carbonate rocks from the overturned limb of the Lebanon Valley fold nappe at Rheems, Pennsylvania. The formation of stylolites and slickolites in association with boudins and veins indicates that extreme bed-normal contraction is intimately connected with bed-parallel extension. A variety of boudin shapes in dolomitic beds, from rectilinear to pinch-and-swell and fish-mouth structures, are formed by a combination of cataclastic flow and heterogeneous pressure solution. Many veins record catastrophic failure and collapse of wallrock prior to or during crystallization of the vein-fill. At the earliest stage of boudinage, veins in boudin necks are straight and perpendicular to bedding. With continued extension, they adopt a bow-tie form in cross section as a result of inward flow of the surrounding matrix and pressure solution of the fragment corners and vein margins. With continued stylolitization and separation of boudins, bow-tie veins are converted into transverse veins of extraordinary dimension. Failure to recognize the role of pressure solution in the formation of boudinage structure may result in inaccurate estimates of extension, erroneous assumptions regarding volume change, and incorrect interpretations of the changes in stress regime with time.