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Two competing models exist for the formation of the Pennsylvania salient, a widely studied area of pronounced curvature in the Appalachian mountain belt. The viability of these models can be tested by compiling and analyzing the patterns of structures within the general hinge zone of the Pennsylvania salient. One end-member model suggests a NW-directed maximum shortening direction and no rotation through time in the culmination. An alternative model requires a two-phase development of the culmination involving NNW-directed maximum shortening overprinted by WNW-directed maximum shortening. Structural analysis at 22 locations throughout the Valley and Ridge and southern Appalachian Plateau Provinces of Pennsylvania are used to constrain orientations of the maximum shortening direction and establish whether these orientations have rotated during progressive deformation in the Pennsylvania salient’s hinge. Outcrops of Paleozoic sedimentary rocks contain several orders of folds, conjugate faults, steep...

We noninvasively detected the characteristics and location of a regional fault in an area of poor bedrock exposure complicated by karst weathering features in the subsurface. Because this regional fault is associated with sinkhole formation, its location is important for hazard avoidance. The bedrock lithologies on either side of the fault trace are similar; hence, we chose an approach that capitalized on the complementary strengths of very low frequency (VLF) electromagnetic, resistivity, and gravity methods. VLF proved most useful as a first-order reconnaissance tool, allowing us to define a narrow target area for further geophysical exploration. Fault-related epikarst was delineated using resistivity. Ultimately, a high-resolution gravity survey and subsequent inverse modeling using the results of the resistivity survey helped to further constrain the location and approximate orientation of the fault. The combined results indicated that the location of the fault trace needed to be adjusted 53 m south of the current published location and was consistent with a north-dipping thrust fault. Additionally, a gravity low south of the fault trace agreed with the location of conductive material from the resistivity and VLF surveys. We interpreted these anomalies to represent enhanced epikarst in the fault footwall. We clearly found that a staged approach involving a progression of methods beginning with a reconnaissance VLF survey, followed by high-resolution gravity and electrical resistivity surveys, can be used to characterize a fault and fault-related karst in an area of poor bedrock surface exposure.

The emerging subdiscipline of hydrogeophysics is underdeveloped in undergraduate curricula relative to its importance in professional engineering/environmental practice. In 2001, the Bucknell Department of Geology initiated efforts to refocus an undergraduate geophysics course on near-surface geophysical methods for hydrologic, environmental, and engineering problems. In addition to offering students practical experience, treatment of hydrogeophysics provides important pedagogical opportunities. Field-based hydrogeophysics labs challenge students to integrate concepts from other geology courses, as well as from physics, math, and chemistry. We faced two challenges in revising our geophysics course: (1) access to wells for field exercises on borehole geophysics; and (2) the costs of acquiring and maintaining equipment. We pursued two strategies to solve these problems. First, we established an on-campus well field, which serves as a field laboratory for downhole and cross-hole experiments. Second, we incor...

Field and microstructural analyses of faults with millimeters to hundreds of meters of displacement within welded tuffs at Yucca Mountain, Nevada, has led us to identify four different fault zone architectures. We designate these fault zones as Classes A–D. We conclude that Classes A, C, and D are genetically related and that observed differences in their morphology and deformation mechanisms are related to differences in displacement magnitude and changes in the rheology of faulted rocks. We show that Class A, C, and D fault zones were formed by progressive cataclasis, leading to the development of a foliated gouge core and wide damage zone at the highest displacement magnitudes. By inference, displacement variations across individual high displacement faults are expected to produce significant lateral changes in fault zone architecture. The broad range of fault rock textures and architectures found on the Class A, C, and D continuum may produce important corresponding variability in hydrologic properties in ignimbrites. By contrast, Class B fault zones have unique mineralogies and microtextures relative to other Yucca Mountain fault zones. Most notable is the occurrence of distinctive jigsaw puzzle texture within fault core breccias, which consists of angular wall rock fragments floating in a coarsely crystalline secondary calcite cement matrix. The calcite cement constitutes more than 65% of the Class B fault zones. Comparisons with other occurrences of secondary minerals within open lithophysal cavities, which appear to have formed in unsaturated conditions, indicate that the secondary mineralization history at Yucca Mountain was complex and likely polygenetic. The age and origin of the secondary calcite minerals in the Class B fault zones remain undetermined.

Studies of progressive deformation aim to identify and establish temporal sequences of structural stages, each characterized by a suite of structures which form under specific rock and environmental conditions. Because of the natural spatial variation in rock type and environmental parameters in an orogen, several structural stages may operate concurrently in different parts of an orogenic wedge. At any given instant in time, rocks experiencing different stages of deformation are bounded by deformation fronts. As intrinsic and extrinsic conditions of deformation vary through time, spatial migration of deformation fronts causes rocks to record temporal overprinting of structural stages. In convergent orogens, deformation fronts, as mapped in their finite state, are typically forelandward-dipping due to the regional orogenic wedge geometry and thermal structure. Depending on the competing deformation processes, deformation rate changes on either side of the front may cause deformation fronts to change orientation, relief and surface area with time. Differences in migration rates of different deformation fronts could cause rearrangement of the sequence of structural stages with depth and laterally across the same mountain belt.

We used the sensitive high-resolution ion microprobe (SHRIMP) to determine the U/Pb ages of zircons within clasts from quartz-pebble conglomerates in two Appalachian clastic wedges in order to further constrain the provenance and tectonic architecture of the central Appalachian basin at key times in its development. The ages of 64 zircons from samples of quartz pebbles in the Lower Silurian Shawangunk Formation and the Middle Pennsylvanian Pottsville Formation were determined using the SHRIMP. Zircons from Shawangunk quartz pebbles are no younger than 900 Ma, and most yield ages of 950–1200 Ma, coincident with the Grenville orogeny. Because multicycle sedimentary sources for the Shawangunk pebbles are not obvious, we conclude that a Grenville terrane, apparently unaffected by significant late Precambrian or early Paleozoic metamorphism, was uplifted, unroofed, and incised during the Taconic orogeny. Zircons from quartz clasts in the Pottsville Formation range in age from 380 Ma to 2100 Ma and also contain a major Grenville component. Some clasts contain zircons in the age range 380–640 Ma, including a significant grouping between about 400 and 450 Ma. These zircons may have been derived from internal terranes affected by the waning stages of the Taconic orogeny. Clasts containing zircons as much as 2.1 Ga are problematic. They may have been derived from the distant interior craton (e.g., Superior Province) or were perhaps derived from the adjacent West African craton during the Pennsylvanian.

Detailed study of selected outcrops in the Pennsylvania Anthracite region has resulted in identification of five, at least partly continuous, stages of Alleghanian progressive deformation. These structural stages are, in sequence: A–layer-parallel shortening and top-to-the-foreland, layer-parallel shear; B—a second episode of layer-parallel shortening; C—flexural-slip and flexural-flow folding; D-fold modification and late, thrust faulting; and E—late veining and fracturing. At each outcrop, an average shortening direction was determined for each observed structural stage. The mean regional shortening direction for each structural stage was calculated and maps showing the spatial distribution of the shortening directions for each stage were constructed. The mean regional shortening directions for the five structural stages successively rotated 25–30° clockwise during the evolution of the fold and thrust belt. Shortening directions for structural stages B and C over the region show a systematic clockwise change in orientation from hinterland to foreland, indicating that these structural stages of progressive deformation were time-transgressive on a regional scale.

In a number of curved fold and thrust belts worldwide, such as the Cantabrian arc and the Wyoming-Idaho salient, paleomagnetic data indicate vertical axis rotations inconsistent with structural findings. This apparent conflict is especially pronounced in the Pennsylvania salient, where the paleomagnetically defined vertical axis rotations are opposite of the rotation sense indicated by structural studies. We resolve this apparent disparity by developing an integrated and kinematically admissible model that has implications for other curved mountain belts. The curvature of the Pennsylvania salient may be explained by deformation partitioning in an initially laterally tapered basin between the Cambrian-Ordovician carbonates and the overlying siliciclastic rocks. Earliest deformation occurred in the lowest strata and progressed toward the foreland and up section with time. Lateral differences in initial layer-parallel shortening in the Cambrian-Ordovician carbonates caused differential translation and rotations about a vertical axis in the cover sequence. Because of initial basin geometry, the orogenic wedge developed a lateral taper. With further shortening in the wedge and involvement of the cover rocks in the fold and thrust deformation, gravitational driving forces became more important, and the paleostress trajectories diverged in opposite directions on the two arms of the salient in response to the lateral taper.

Pedogenic slickensides are convex-concave slip surfaces that form during expansion/contraction in expansive clay soils such as Vertisols. In the central Appalachians, they occur near the tops of fining-upward cycles in Paleozoic redbeds such as the Bloomsburg, Catskill, and Mauch Chunk Formations. Pedogenic slickensides are found in association with other pedogenic (or paleosol) features such as clay-skinned peds, in situ calcareous nodules, and root impressions. Repeated movements along these shear planes during pedogenesis produce strongly aligned clay particles adjacent to pedogenic slickensides; as a result, they are preserved as discrete fractures throughout diagenesis, compaction, and superimposed tectonic deformation. During whole-rock deformation, pedogenic slickensides segregate penetratively deformed rocks into independent, foliated packets and serve as discontinuities that are followed by later structural features. Because the original morphology of pedogenic slickensides is known, they can be used as crude strain markers.