Christopher M. Bailey

General shear deformation in the Pinaleno Mountains metamorphic core complex, Arizona

Abstract Granitic mylonites from the Pinaleno Mountains metamorphic core complex in southeastern Arizona record general shear deformation. The mean kinematic vorticity number ( W m ) was estimated using (1) the strain ratio ( R s ) and the angle ( Θ ) between the long axis of the strain ellipsoid with respect to the high-strain zone boundary and (2) the porphyroclast hyperbolic distribution method. W m for protomylonites and mylonites ranged from 0.6 to 0.9. Ultramylonites record lower W m values (0.1–0.3) suggesting that the incremental vorticity changed during deformation. Three-dimensional strain analysis indicates that deformation in protomylonites and mylonites approximates plane strain and has a monoclinic symmetry. The vorticity path followed by rocks tectonically exhumed in the footwall of major extensional fault systems may evolve from a significant pure shear component early in the deformation towards simple shear as overburden load decreases during uplift.

Strain and vorticity analysis of transpressional high-strain zones from the Virginia Piedmont, USA

Abstract Strain and vorticity analysis of two Late Palaeozoic high-strain zones from the southern Appalachian Piedmont indicates that these zones experienced general shear transpression with a monoclinic to triclinic symmetry. Granitic rocks in the Brookneal high-strain zone from the southwestern Virginia Piedmont were transformed into mylonites under greenschist facies conditions. Sectional strains, estimated from quartz grain shapes, in mylonites range from three to ten and three-dimensional fabrics record flattening strains. The mean vorticity number (Wm) estimated with the Rs/θ method ranges from 0.3 to 0.95. In the central Virginia Piedmont, lower amphibolite facies deformation in the Spotsylvania high-strain zone affected biotite gneisses, amphibolites, and granitic pegmatites. Minimum sectional strains, estimated from folded and boudinaged pegmatite dykes, of 8–20 are common and three-dimensional strains are dominantly constrictional. Porphyroclast hyperbolic distribution analysis of ultramylonites yields Wn values from 0.4 to 0.8. The kinematic significance of these transpressional high-strain zones is threefold: they record tens to hundreds of kilometres of strike-slip offset; 40 to 70% contraction normal to the zone; and significant orogen-parallel material elongation.

Age, origin, and significance of brittle faulting and pseudotachylyte along the Coast shear zone, Prince Rupert, British Columbia

Northwest-striking brittle faults containing cataclasite, fault gouge, and, in one location, pseudotachylyte are common in the vicinity of the Coast shear zone near Prince Rupert, British Columbia. The pseudotachylyte locality, found in a dextral strike-slip fault zone along the Coast shear zone, contains spherulites, alkali feldspar microlites, and amygdules, suggesting that the pseudotachylyte crystallized rapidly from a melt phase within 5 km of the surface. 4 0 Ar/ 3 9 Ar incremental heating of the pseudotachylyte matrix yielded a weighted mean plateau of 29.8 ′ 0.6 Ma and an inverse isochron of 29.8 ′ 1.5 Ma with an 4 0 Ar/ 3 6 Ar intercept of 296.0 ′ 15.2. These results show that pseudotachylyte associated with brittle faulting can be dated precisely, and imply that some dextral coast-parallel displacement occurred across the Coast shear zone in the Oligocene and that the majority of exhumation in the Coast Mountains at the latitude of Prince Rupert (∼54°N) was accomplished by 30 Ma.

Tectonic inversion and basement buttressing: an example from the central Appalachian Blue Ridge province

Abstract The structural geometry of the central Appalachian Blue Ridge province is primarily the result of Paleozoic contractional deformation. However, the Tye River fault zone in the western Blue Ridge of central Virginia preserves an extensional map pattern. These ENE–WSW striking faults place Neoproterozoic to early Cambrian volcanic and sedimentary rocks over Grenvillian granitic basement. Detailed mapping, cross-section restoration, and balancing reveals partially inverted faults, original half grabens, and significantly thicker cover units on hanging walls. The orientation and geometry of the Tye River fault zone, as well as the thickness changes in the cover rocks are inconsistent with extensional reactivation of Paleozoic reverse faults during Mesozoic rifting. The extrusion and deposition of cover rocks was contemporaneous with normal displacement on the Tye River fault zone and the formation of rift basins recording ∼5% crustal extension. Faults were reactivated as Paleozoic contractional structures, but an extensional map pattern is generally still preserved. Regional shortening of 10–20% was accomplished primarily by folding of the cover rocks rather than by reactivation of older faults. The basement formed a rigid buttress that controlled the geometry of folding in the cover sequence.

Glacially influenced sedimentation in the late Neoproterozoic Mechum River Formation, Blue Ridge province, Virginia

The late Neoproterozoic (ca. 700–730 Ma) Mechum River Formation is exposed as a structural inlier within Grenvillian basement in the Blue Ridge province of central Virginia, United States. The southern portion of the Mechum River belt preserves evidence of glaciomarine sedimentation at the margin and basinward of grounding-line fans. Subaqueous glacial till, coarse-grained rhythmites, and dropstones record the incursion of ice into the basin. Marine deposition is evidenced by persistent laminations (cyclopsams) in rhythmically bedded diamictite and possibly by tidally pumped, distal turbidites in laminated mudstones. Glaciomarine sedimentation in the Mechum River Formation may have been coeval with Sturtian-Rapitan glaciations in northwestern Laurentia and Australia, and provides further evidence for a globally cold climate during the late Neoproterozoic.

The Mechum River Formation, Virginia Blue Ridge: A Record of Neoproterozoic and Paleozoic Tectonics in Southeastern Laurentia

Neoproterozoic metasedimentary rocks of the Mechum River Formation crop out in an elongate northeast-southwest trending belt in the central Virginia Blue Ridge province. The northwestern contact of the Mechum River Formation is nonconformable above Mesoproterozoic basement, whereas the southeastern contact is a set of steeply dipping, brittle reverse faults that juxtapose basement against Mechum River rocks. The internal structure of the belt consists of northwest-verging, open to tight, moderately to steeply plunging asymmetric folds. Although previous workers have interpreted the outcrop belt of the Mechum River Formation as a graben formed during Laurentian rifting, structural relations require that its present geometry is a structural inlier related to Paleozoic contractional deformation. Strain analysis reveals that grain-scale deformation processes produced up to 35 percent shortening during foliation development whereas map-scale folds account for an additional 30 to 50 percent shortening. Restoration of the Mechum River Formation to its pre-contractional geometry reveals little about the geometry of the original depositional basin. The bounding reverse faults on the east side of the Mechum River Formation are interpreted as out-of-sequence structures that developed after regional folding, metamorphism and foliation development that may be related to the emplacement of the Blue Ridge thrust sheet over a tectonic ramp. Sediment transport indicators are consistent with a source area to the east, suggesting that the Mechum River Formation was separated from similar units in the eastern Blue Ridge by an asymmetric basement high that may have been produced by block rotation above listric normal faults.

Late Cenozoic Reverse Faulting in the Fall Zone, Southeastern Virginia.

A set of en‐echelon reverse faults cut Paleozoic metamorphosed igneous rocks of the Piedmont and overlying late Cenozoic sediments at the Old Hickory Heavy Mineral Deposit in the Fall Zone of southeastern Virginia. Diorite of the eastern Slate Belt was faulted over nearshore to shore‐face deposits of the Pliocene Yorktown Formation. These NW‐SE‐striking faults experienced oblique dip‐slip movement with a maximum displacement of up to 6 m on individual faults. Faults tip out along strike and are overlain by distinct cobble beds, suggesting that sediment deposition and faulting were contemporaneous. Deformation at Old Hickory may have been formed by reactivation of existing Paleozoic structures under a regionally extensive compressional stress field parallel to the modern one.

Late Neoproterozoic Extension‐Related Magma Emplacement in the Central Appalachians: An Example From the Polly Wright Cove Pluton

The late Neoproterozoic (706 ± 4 Ma) Polly Wright Cove pluton is an elliptical intrusive body composed predominantly of biotite granite that intruded Grenville‐age granitoid crust in the central Virginia Blue Ridge province. Fine‐and medium‐grained biotite granite occurs as multiple tabular sheets near the pluton margins and as finger‐like projections that extend into the country rock. The pronounced sheeting is absent from the pluton interior where medium‐grained biotite granite and a younger biotite leucogranite are massive. Structural elements in and adjacent to the pluton are generally parallel. High‐strain zones in the country rock and pluton record SE‐directed extensional deformation. Country rock fabrics and internal pluton structures appear to be contemporaneous, suggesting that the Polly Wright Cove pluton was emplaced syntectonically into actively extending country rock during the early stages of late Neoproterozoic rifting of Laurentia. Emplacement began with the intrusion of sheeted dikes and was followed by minor late‐stage expansion of the pluton interior. Detailed microstructural analysis, as illustrated by this study, has the potential to aid significantly in the interpretation of the tectonic aspects of magmatism.

Rates of Geologic Processes: Problems for an Introductory Geology Course

Geologic processes occur over a range of time scales. Introductory geology courses should provide students with an understanding of earth processes as well as the rates at which these processes operate. Three quantitative problems involving the rate of radioactive decay, the movement of groundwater pollution, and the rate of erosion in the Himalayas are described. These problems require students to graph data, understand units and orders of magnitude, manipulate equations, and put numerical answers into a broader geologic context.

From the Blue Ridge to the Beach: Geological Field Excursions across Virginia

This volume includes seven field guides that explore the diverse geology of Virginia from its Appalachian highlands to the Atlantic shore. The guides cover an array of topics ranging from cave and karst development in the Valley and Ridge to the exceptional fossil localities at the Carmel Church Quarry and the cliffs near Stratford Hall to Precambrian rocks in the Blue Ridge Mountains. Three guides focus on the Paleozoic to Proterozoic tectonic history of the Blue Ridge and Piedmont provinces, two guides discuss the stratigraphy and fossil assemblages preserved in Cenozoic deposits on the Atlantic Coastal Plain, one guide examines Paleozoic stratigraphy and cave formation in western Virginia, and the final guide explores the relationship between the geology of the Fall Zone and the Civil War during the Petersburg Campaign in 1864–1865.