Ridge Province Research Articles

Periglacial resurfacing of hillslopes and channels with large boulders in the Virginia Appalachians

AbstractLarge, resistant, quartz‐rich boulders deposited on hillslopes and in channels armour the landscape, trap sediment and influence hillslope angle and erodibility. In the Virginia Appalachians, such boulders are a significant component of hillslopes and channels. Establishing the timing of and processes responsible for bedrock fracture and boulder deposition is a critical piece of understanding the landscape as a system. In this study, we use cosmogenic 10Be exposure dating to resolve the timing of boulder deposition at three sites in the Virginia Valley and Ridge province: Gap Mountain, Brush Mountain and Little Stony Creek, and at one site in the Virginia Blue Ridge: Devil's Marbleyard. The correlation between measured boulder exposure ages (101.7 ± 6.9 ka to 10.8 ± 0.8 ka; n = 23) and the Wisconsin Glacial Stage and subsequent Laurentide Ice Sheet (LIS) deglaciation (~115–11.7 ka) suggests a periglacial origin for deposition of large hillslope and channel boulders in the Virginia Appalachians. The lack of boulder exposure ages corresponding to the Last Interglacial Stage or following Wisconsin LIS retreat suggests interglacial non‐deposition and stability. The absence of exposure ages from the penultimate Illinoian or older Quaternary Glacial Stages suggests that periglacial hillslope processes allow the landscape to be resurfaced with large boulders during each return to cold climate conditions. This cyclic resurfacing of hillslopes and channels is an example of how climatic oscillations insert disequilibrium into the landscape cycle and contributes to our appreciation of the timescales over which contemporary climate change may impact boulder dominated landscapes in rapidly warming alpine and arctic environments.

3D Seismic Anatomy of a Watershed Reveals Climate‐Topography Coupling That Drives Water Flowpaths and Bedrock Weathering

AbstractTo investigate how bedrock transforms to soil, we mapped the topography of the interface demarcating onset of weathering under an east‐west trending shale watershed in the Valley and Ridge province in the USA Using wave equation travel‐time tomography from a seismic array of >4,000 geophones, we obtained a 3D P‐wave velocity (Vp) model that resolves structures ∼20 m below land surface (mbls). The depth of mobile soil and the onset of dissolution of chlorite roughly match Vp = 600 m/s and Vp = 2,700 m/s, respectively. Chlorite dissolution initiates porosity growth in the shale matrix. Depth to the 2,700 m/s contour is greater under the N‐ as compared to S‐facing hillslopes and under sub‐planar as compared to concave‐up land surfaces. Broadly, the geometries of the ‘soil’ and ‘chlorite’ Vp contours are consistent with the calculated potential for shear fracture opening under weak regional compression. However, this calculated fracture potential does not consistently explain observations related to N‐ versus S‐facing aspect nor fracture density observed by borehole televiewer. Apparently, regional compression is only a secondary influence on Vp: the primary driver of P‐wave slowing in the upper layers of this catchment is topographic control of reactive water flowpaths and their integrated effects on weathering. The Vp result is best explained as the long‐term integrated effect of groundwater flow‐induced geochemical weathering of shale in response to climate‐driven patterns of micro‐ and macro‐topography.

Detrital zircon geothermochronology reveals pre-Alleghanian exhumation of regional Mississippian sediment sources in the southern Appalachian Valley and Ridge Province

AbstractThe Black Warrior foreland basin records sedimentation associated with the development of intersecting Ouachita and Alleghanian thrust belts along the southern margin of Laurentia. Mississippian–Pennsylvanian units in the Black Warrior basin are interpreted to be sourced from either the northern Appalachians and mid-continent or more regionally from the southern Appalachians or nearby Ouachita thrust belt. We present detrital zircon U-Pb ages and Th/U values from Paleozoic units that indicate zircon from the Mississippian Hartselle Sandstone are temporally and chemically compatible with being sourced from the southern Appalachians. Zircon mixing models suggest sediment was primarily recycled from Cambrian, Ordovician, and Devonian strata in the Appalachian Valley and Ridge, with minor influx from Piedmont units. A ca. 415 Ma zircon population requires additional input from the Maya Block of the Yucatan Peninsula or similar outboard terranes. We present zircon (U-Th)/He analysis and thermal history modeling of Paleozoic units, which detail pre-Alleghanian exhumation in the Appalachian Valley and Ridge. Both the Cambrian Chilhowee Group and Pennsylvanian Pottsville Formation exhibit (U-Th)/He dates ranging from 507 to 263 Ma with a Mississippian subset (353–329 Ma, n = 4), which indicates rapid cooling and inferred exhumation during Late Devonian–Early Mississippian Neoacadian tectonism. We propose a Mississippian drainage system that transported material along southern Appalachian structural fabrics to the juncture between Appalachian and Ouachita thrust belts followed by a sediment-routing rotation toward the Black Warrior foreland. This interpretation honors chemical-age zircon data, accounts for metamorphic grains in thin section petrography, and matches Mississippian–Pennsylvanian Black Warrior foreland lithostratigraphic relationships.

Seismic Imaging of a Shale Landscape Under Compression Shows Limited Influence of Topography‐Induced Fracturing

AbstractWe used seismic refraction to image the P‐wave velocity structure of a shale watershed experiencing regional compression in the Valley and Ridge Province (USA). From estimates showing strong compressional stress, we expected the depth to unweathered bedrock to mirror the hill‐valley‐hill topography (“bowtie pattern”) by analogy to seismic velocity patterns in crystalline bedrock in the North American Piedmont that also experience compression. Previous researchers used failure potentials calculated for strong compression in the Piedmont to suggest fractures are open deeper under hills than valleys to explain the “bowtie” pattern. Seismic images of the shale watershed, however, show little evidence of such a “bowtie.” Instead, they are consistent with weak (not strong) compression. This contradiction could be explained by the greater importance of infiltration‐driven weathering than fracturing in determining seismic velocities in shale compared to crystalline bedrock, or to local perturbations of the regional stress field due to lithology or structures.

How the capacity of bedrock to collect dust and produce soil affects phosphorus bioavailability in the northern Appalachian Mountains of Pennsylvania

AbstractMore above‐ground biomass (kg m−2) grows in the northern Appalachian Mountains (USA) in forests on shale than on sandstone at all landscape positions other than ridgetops. This has been tentatively attributed to physical (rather than chemical) attributes of the substrates, such as elevation, particle size, and water capacity. However, shales have generally similar phosphorus (P) concentrations to sandstones and, in the Valley and Ridge province, they erode more quickly. This led us to hypothesize that faster replenishment of the lithogenic nutrient P in shale soils through erosion + soil production could instead control the differences in biomass. To test this, soils and foliage from 10 sites on shales and sandstones in the northern Appalachians from roughly the same elevation and aspect were analysed. We discovered that, when controlling for location, concentrations of bioavailable P in soils and P in foliage were higher and P resorbed from senescing red oak leaves was lower on slower‐eroding sandstone than on faster‐eroding shale. Lower resorption generally can be attributed to lower P limitation for trees. Further investigation of weathering and erosion on one of the sandstone–shale pairs within a larger, paired watershed study revealed that the differences in P concentrations in biomass and foliage between lithologies likely developed because sandstones act as ‘collectors’ that trap nutrients from residual and exogenous sources, while shales erode quickly and thus promote production of soil from bedrock that releases P to ecosystems. We concluded that the combined effects of differential rates of dust collection and erosion results in roughly equal biomass growing on sandstone and shale ridgetops. This work emphasizes the balance between a landscape's capacity to collect dust versus produce soil in controlling bioavailability of nutrients.

Spatially variable syn- and post-Alleghanian exhumation of the central Appalachian Mountains from zircon (U-Th)/He thermochronology

Abstract To assess spatial and temporal patterns of Phanerozoic orogenic burial and subsequent exhumation in the central Appalachian Mountains, we present mid-temperature zircon (U-Th)/He (ZHe; closure temperature [TC] = 140–200 °C) dates for 10 samples along a 225 km, strike-perpendicular transect spanning the Appalachian Plateau, Valley and Ridge, Blue Ridge, and Piedmont physiographic provinces in West Virginia and western Virginia. Ranges of single-grain ZHe dates exhibit an eastward younging trend from 455–358 Ma in the Pennsylvanian Appalachian Plateau to 336–209 Ma in the Valley and Ridge, 298–217 Ma in the Blue Ridge, and 186–121 Ma in the Piedmont. Within the Pennsylvanian Appalachian Plateau, detrital ZHe dates are older than corresponding depositional ages, thus limiting postdepositional burial temperatures to less than 160 °C. These ZHe dates capture predepositional mid-Paleozoic cooling signatures, indicating provenance from either recycled Taconic or Acadian basin strata or mid-Paleozoic Appalachian terranes. Across the Valley and Ridge and western Blue Ridge provinces, reset Permian detrital ZHe dates feature flat date-effective uranium correlations that suggest rapid Alleghanian cooling initiating prior to 270 Ma. ZHe dates within the Valley and Ridge are more than 100 m.y. older than previously reported regional apatite fission-track dates, reflecting a protracted period of stable post-Alleghanian thermal conditions within the foreland. By contrast, post-Triassic single-grain ZHe dates in the interior Piedmont document rapid postrift cooling, likely resulting from both the relaxation of an elevated geothermal gradient and exhumation from rift-flank uplift. The spatial discontinuity between stable synrift thermal conditions in the Valley and Ridge and rapid cooling in the Piedmont suggests that rift-flank uplift and cooling were concentrated outboard of the foreland within the Piedmont province.

Soil Chemistry following Single-Entry, Dormant Season Prescribed Fires in the Ridge and Valley Province of Virginia, USA

ABSTRACT In this study, mineral soil (0–10 cm depth) chemistry [pH, calcium (Ca), carbon (C), magnesium (Mg), nitrogen (N), phosphorus (P), potassium (K)] was measured pre – and post-fire (6 and 14 months post-fire) at two prescribed fire locations near Blacksburg, Virginia, USA. A similar, adjacent, unburned (control) forest was sampled during this time period, as well. Combinations of treatment and time did not indicate significant differences between the control and burned locations for any of the soil chemical variables when burned and control plots were evaluated within each, individual, sampling period (pre-fire, 6 months post-fire, and 14 months post-fire). Differences did exist, however, for some variables when compared across all treatment and time combinations. Differences were also noted for Ca, K, Mg, P, C, and N within the burned plots alone as a result of sampling time. These results agreed with those from other studies in this region that suggest 1) soil chemistry is highly variable, with and without prescribed fire, and 2) short-term changes in soil chemistry following singular, dormant season, prescribed fires are often negligible, particularly when compared to adjacent, unburned, control units. Increased fire intensity, severity, or fire frequency may enhance both short- and long-term post-fire soil responses.

The Western Limit of Iapetan Rifting in the Eastern United States: A New Assessment

AbstractSpecifying the extent and location of rifted, crystalline Precambrian crust in the eastern United States is important for seismic hazard evaluation and for models that relate upper-mantle structure to ancient tectonic features and ongoing tectonism. As currently depicted in the National Seismic Hazard Maps (NSHM), the western limit of Iapetan rifted crust is beneath the Appalachian plateau physiographic province, west of the Valley and Ridge province. New estimates of crustal thickness using EarthScope Transportable Array and other data do not support the presence of rifted crust beneath the Blue Ridge, Valley and Ridge, and Appalachian plateau physiographic provinces. Crustal thicknesses exceed 45 km throughout most of this region. The crust thins to the southeast beneath the southeastern part of the Piedmont physiographic province and is only 36 km thick near the edge of the Atlantic coastal plain. We suggest that the western limit of Iapetan rift-extended crust is east of the Blue Ridge province and is associated with the prominent Appalachian gravity gradient. This location coincides with palinspastic reconstructions based on geologic data for the Iapetan rifted margin. Recognition of thick crust beneath the Blue Ridge and Valley and Ridge provinces, unextended by Iapetan rifting, will support more robust modeling of the effects of mantle structure (such as delamination and abrupt changes in lithospheric thickness) on ongoing tectonism and earthquake activity in the eastern United States and will provide more accurate seismotectonic zonation in the NSHM.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of >300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Common Spring Types in the Valley and Ridge Province: There Is More than Karst

ABSTRACT The Valley and Ridge Province (V&R) of the central Appalachians is rich in springs that support ecosystems, provide local water resources, and export water from the region. Although there has been extensive research on springs in the province, the focus has been on chemically variable karst springs. The purpose of this work is to identify common spring types found in the V&R based on an analysis of three regions. Three types of V&R springs are included in this comparison, and their relationship to more general classification systems is included. Headwater springs, located near ridge tops and along ridge flanks, are typically small, may be ephemeral, have localized flow paths, and are associated with siliciclastic units. Karst springs, generally located in the valleys, include both the more chemically variable limestone springs and the more stable dolomite springs. Thermal warm springs, with temperatures higher than the mean annual air temperature, are less common than the other spring types; they may be large and are typically associated with major thrust faults. The temperature, chemistry, and locations of the springs are controlled by the structural geology and topography as well as the formations and lithologies through which the recharge water travels. There is overlap in the water chemistry and storm responses of the spring groups, but some general trends can be identified, such as lower pH in the headwater springs. The V&R springs are critical resources, but their sustainability, chemistry, and hydrology need to be considered within the local geologic framework.

Preservation of Valley and Ridge topography via delivery of resistant, ridge-sourced boulders to hillslopes and channels, Southern Appalachian Mountains, U.S.A.

Discrete bands of resistant strata found along ridgelines in lithologically controlled, geologically heterogeneous landscapes are generally credited with maintaining high topography in these settings. However, the mechanisms by which these “ridge-formers” prevent erosion and preserve topography are not well described. Here we propose a new way to conceptualize the landscape-scale geomorphic function of ridge-forming lithologies, not only as units whose intermittent resistant outcrops slow erosion of ridgelines themselves, but as producers of large, durable boulders that litter adjacent hillslopes and channels, extending the caprock's resistant qualities to weaker underlying units and inhibiting channel incision over laterally extensive portions of the landscape. This conceptual model is based on observations of boulder distributions in the slowly-eroding Valley and Ridge province of the south-central Appalachian Mountains, and is in agreement with recent numerical models exploring the role of large boulders in channel and hillslope evolution.Though boulders are widely observed in the Valley and Ridge, boulder distributions have not generally been mapped, nor has their presence been linked to topographic persistence. To examine the potential link between resistant, ridge-derived boulders and relief preservation in the Valley and Ridge, we use UAV imagery and field observations to create detailed maps of boulder distributions along representative hillslopes and channels in SW Virginia, and document boulder appearance, size, and strength to make inferences about longevity and geomorphic function. We find that boulders cover significant portions of hillslopes (up to 9%) and are concentrated in channels (up to 90%). Boulders are very large (average size >1 m and up to 6 m diameter), indicating fluvial immobility and channel armoring, and often trap sediment upslope, indicating hillslope shielding. Boulders are entirely sourced from durable ridgeline lithologies, and exhibit features indicating longevity (e.g., fracturing, extensive lichen coverage, rounding). We interpret boulders to be relict from Pleistocene periglacial conditions, when intense freeze-thaw cycles likely enhanced boulder production. Our results are consistent with processes described in numerical models of boulder function and suggest that ridge-sourced boulders in this region play an important role in topographic preservation by stabilizing hillslopes and channels, challenging the idea that ridge-forming units primarily maintain relief by stabilizing the ridgelines themselves. In addition, the linkage of boulder production to a colder Pleistocene climate highlights a mechanism by which past climatic conditions may continue to impact landscape evolution after the transition to a new climatic regime.

Vulnerability of high-elevation endemic salamanders to climate change: A case study with the Cow Knob Salamander (Plethodon punctatus)

Rapid contemporary climate change is a potential threat to long-term persistence of montane wildlife species because they often have narrow thermal tolerances and have limited potential to shift their distributions. The Appalachian Mountain region in the eastern United States is a global biodiversity hotspot for woodland salamanders (genus Plethodon), many of which are high-elevation endemic species. Robust assessments of the vulnerability of high-elevation endemic salamanders to climate change, including delineation of future potential climate refugia, are needed to guide climate change adaptations strategies. The Cow Knob Salamander (Plethodon punctatus) is a species of conservation concern found at high elevations in the Valley and Ridge Province of western Virginia and eastern West Virginia. We used habitat suitability models to examine relationships between landscape characteristics, climate variables, and P. punctatus occurrence, and estimated effects of future climate scenarios on the species’ climatic niche. We found that elevation, slope, aspect, and hillshade were influential landscape predictors of species occurrence, and that mean annual temperature was the most influential climate variable. Future climate projections indicated this species will likely lose most of its climatic niche by mid-century, and that amount of suitable habitat will continue to decline through 2100. We identified several pockets of habitat that may represent climate change refugia for P. punctatus due to cooler microclimates from greater hillshade and aspects that receive less direct solar radiation; however, we found these refugia exist in small, isolated habitat patches. Our study provides quantitative estimates that support the general concern that high-elevation endemic salamanders are particularly vulnerable to climate change. Our models can be used by natural resource managers to guide current P. punctatus monitoring and habitat conservation efforts, as well as to identify focal areas that will likely serve as refugia for the species as the climate continues to change over this century.

A new species of stygobitic snail in the genus Antrorbis Hershler & Thompson, 1990 (Gastropoda, Cochliopidae) from the Appalachian Valley and Ridge of eastern Tennessee, USA.

A new species of cave snail (Littorinimorpha: Cochliopidae) in the genus Antrorbis is described from the dark zone of two caves in the Appalachian Valley and Ridge province in eastern Tennessee, United States. The Tennessee Cavesnail, Antrorbis tennesseensis Perez, Shoobs, Gladstone, & Niemiller, sp. nov. is distinguished from its only known congener, Antrorbis breweri, by the absence of raised tubercles on its finely spirally striate protoconch, and its unique radular formula. Moreover, A. tennesseensis is genetically distinct from A. breweri based on substantial divergence at the mitochondrial CO1 locus. This is the first cavesnail to be described from the Appalachian Valley and Ridge (AVR) physiographic province in the state of Tennessee, which previously represented a substantial gap in the distribution of stygobitic (i.e., aquatic, subterranean-obligate) gastropods.

Thermal State of the Blake Ridge Gas Hydrate Stability Zone (GHSZ)—Insights on Gas Hydrate Dynamics from a New Multi-Phase Numerical Model

Marine sediments of the Blake Ridge province exhibit clearly defined geophysical indications for the presence of gas hydrates and a free gas phase. Despite being one of the world’s best-studied gas hydrate provinces and having been drilled during Ocean Drilling Program (ODP) Leg 164, discrepancies between previous model predictions and reported chemical profiles as well as hydrate concentrations result in uncertainty regarding methane sources and a possible co-existence between hydrates and free gas near the base of the gas hydrate stability zone (GHSZ). Here, by using a new multi-phase finite element (FE) numerical model, we investigate different scenarios of gas hydrate formation from both single and mixed methane sources (in-situ biogenic formation and a deep methane flux). Moreover, we explore the evolution of the GHSZ base for the past 10 Myr using reconstructed sedimentation rates and non-steady-state P-T solutions. We conclude that (1) the present-day base of the GHSZ predicted by our model is located at the depth of ~450 mbsf, thereby resolving a previously reported inconsistency between the location of the BSR at ODP Site 997 and the theoretical base of the GHSZ in the Blake Ridge region, (2) a single in-situ methane source results in a good fit between the simulated and measured geochemical profiles including the anaerobic oxidation of methane (AOM) zone, and (3) previously suggested 4 vol.%–7 vol.% gas hydrate concentrations would require a deep methane flux of ~170 mM (corresponds to the mass of methane flux of 1.6 × 10−11 kg s−1 m−2) in addition to methane generated in-situ by organic carbon (POC) degradation at the cost of deteriorating the fit between observed and modelled geochemical profiles.

Tufa Deposition Dynamics in a Freshwater Karstic Stream Influenced by Warm Springs

Sweet Springs Creek, located in the Valley and Ridge Province of the Appalachian Mountains in southeastern West Virginia and southwestern Virginia, USA, contains major fluvial tufa formations at the sites of localized fractures and faults. Sweet Springs Creek receives input from higher-temperature thermal springs of lower pH and higher sulfate concentration that differ significantly in chemical composition due to differences in the underlying geology. In this study, theoretical rates of tufa accumulation were compared with those measured on travertine tiles left in situ for 30 days during periods of high and low stream flow above and below the sites of major fluvial tufa formations. Consistent with the chemistry of the spring waters, observed and predicted tufa accumulation rates in the stream were low compared to others reported worldwide. Tufa formation rate estimates were consistently higher during seasonal conditions of low flow, warm temperatures, and higher pH that occurred in late summer, but net annual accumulation may still be zero or less due to formation erosion during periodic flooding events. Computer tomography analysis determined that the natural porosity of travertine tiles results in a total surface area 32% greater than that calculated based solely on tile dimensions, which may overestimate initial tufa accumulation rates in situ. Measured rates of carbonate deposition on travertine tiles were 1.6–82 × lower than rates predicted based on theoretical models, consistent with the hypothesis of rate reduction due to variable diffusional boundary layer limitations and variability in stream hydrology. The generation of loose, platy, and unconsolidated precipitate on tiles under geochemical conditions predicted to be the greatest for optimal tufa formation suggested that the precipitation of particulate calcite in the stream system may predominantly result in the formation of unconsolidated marl deposits.

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane, central Appalachian Piedmont Province

ABSTRACTThe Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.

A Review of the Geology of the Rio Del Rey Basin, Offshore Cameroon

A review of the tectonic evolution, stratigraphic and structural framework, petroleum systems and hydrocarbon potential of the offshore portion of Rio Del Rey (RDR) Basin in Cameroon is presented in this paper. The RDR basin is a passive margin basin present in the Gulf of Guinea and it is located southeast of Niger Delta over an area of about 7,000 km2 offshore. Its stratigraphic components, like those of Niger Delta, comprise of a diachronous succession of prodelta shales (Akata Formation), overlain by delta front sands and shales (Agbada Formation), in turn, overlain by fluvial sands (Benin Formation). The age of this basin ranges from Eocene to Recent and it consists of four structural provinces namely; the growth fault province in the north, the Cretaceous Onshore province in the southeast, the Shale Ridge province in the southwest, and the Delta Toe-thrust in the south-central region. The RDR Basin features three stages of tectonic development: Pre-rift phase (Late Proterozoic to Late Jurassic); Syn-rift phase (Late Jurassic to Early Cretaceous) and Post-rift phase (Late Cretaceous to Holocene). It presently stands out as the major contributor of the petroleum in the country.

Acadian hinterland-vergent detachment structures in the southwestern Appalachian Plateau: Implications for the Marcellus Shale gas exploration and production

Three-dimensional seismic data in southwestern Pennsylvania in the Appalachian Plateau demonstrate that the structural style in the Devonian section is dominated by east-vergent folds and reverse faults, which contrasts with that in the Valley and Ridge Province where west-vergent folds and thrusts dominate. Vertical (cross-stratal) variations in fold curvature and fault throw indicate that the intensity of shortening increases from the Salina (Upper Silurian) to the Onondaga (Middle Devonian) and then decreases from the Onondaga to the Elk (Upper Devonian). Lateral (along-stratal) variations in fold curvature and fault throw indicate that the folds and faults tend to propagate in the cross-strike and along-strike directions. Isochron thickness below the Onondaga increases on the anticlinal, up-thrown side of the faults, whereas isochron thickness above the Onondaga increases to the synclinal, down-thrown side of the faults. In concert with seismic structure and isochron thickness, seismic facies see vertical and lateral variations that are spatially and temporally related to folds and faults. Four years of gas production data from the Middle Devonian Marcellus Shale show that the gas productivity drops near the regional reverse faults, whereas regional drilling patterns from a broader perspective of the Plateau reveal operational gaps near major cross-regional wrench faults. These observations are indicative of the dynamic interplay among hinterland-vergent detachment deformation, syntectonic sedimentation, and shale gas preservation during the Acadian (Middle Devonian–Early Mississippian).

Constraints on the timing and duration of orogenic events by combined Lu–Hf and Sm–Nd geochronology: An example from the Grenville orogeny

Grenville age basement rocks of the Virginia Blue Ridge Province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent Rodinia. Although the igneous origins of these rocks have been the subject of much previous work, few studies have examined the complex metamorphic history of these rocks during the Grenville orogeny.To better constrain the timing and duration of metamorphism, we determined Lu–Hf and Sm–Nd garnet and U–Pb zircon rim dates of garnet-bearing rocks exposed in the Virginia Blue Ridge. Multiple periods of Mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before ∼1065 Ma and subsequent local episodes occurring at least until 1000 Ma. Zircon overgrowths and monazite dates suggest metamorphic events between ∼1050 and 960 Ma.Our new Lu–Hf garnet dates range from 1032 ± 3 to 1019 ± 3 Ma. New Sm–Nd dates, determined on the same garnet dissolutions, define a younger range, from 965 ± 6 to 949 ± 6 Ma. Our zircon dates are consistent with previously measured ages (∼1090–940 Ma) and often overlap with the Lu–Hf garnet dates from the same samples. There is a large and systematic difference (60–80 m.y.) between the garnet dates from the two isotopic systems. Although Sm–Nd dates slightly younger than those recorded by Lu–Hf are not uncommon, the large discrepancies between the two systems that we see here are unusual. The dated garnets are unzoned and yield Lu–Hf dates that coincide with the zircon overgrowth dates; Sm–Nd ages overlap with the younger monazite dates. We interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems. Further, these data require slow cooling following large-scale crustal thickening after the assembly of Rodinia during Grenville orogenesis. These data illustrate that the use of the coupled Lu–Hf and Sm–Nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events.

Characterization of the seismically imaged Tuscarora fold system and implications for layer parallel shortening in the Pennsylvania salient

The Tuscarora fold system (TFS) is located in the Pennsylvania salient in the foreland of the Valley and Ridge province. The TFS is imaged in high quality 3D seismic data and comprises a system of small-scale folds within relatively flat-lying Lower Silurian Tuscarora Formation strata. We characterize the TFS structures and infer layer parallel shortening (LPS) directions and magnitudes associated with deformation during the Alleghany Orogeny. Previously reported LPS data in our study area are from shallow Devonian and Carboniferous strata (based on outcrop and core analyses) above the shallowest of three major detachments recognized in the region. Seismic data allows us to characterize LPS at depth in strata beneath the shallow detachment. Our LPS data (orientations and inferred magnitudes) are consistent with the shallow data leading us to surmise that LPS during Alleghanian deformation fanned around the salient and was distributed throughout the stratigraphic section – and not isolated to strata above the shallow detachment. We propose that a NW-SE oriented Alleghanian maximum principal stress was perturbed by deep structure associated with the non-linear margin of Laurentia resulting in fanning of shortening directions within the salient.