Alexander E. Gates

Alleghanian tectono-thermal evolution of the dextral transcurrent Hylas zone, Virginia Piedmont, U.S.A.

Abstract The Hylas zone is the northernmost segment of the Eastern Piedmont fault system and lies in the Grenville Goochland terrane, Virginia. Like the other major segments, the Nutbush Creek zone, the Hollister zone and the Modoc (Irmo) zone, the Hylas zone experienced late Paleozoic dextral transcurrent shearing. Early deformation in the zone was ductile and produced type I and type II S-C mylonites under amphibolite-grade metamorphic conditions. Later deformation was at the brittle-ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons. The feldspar exhibits a strain-dependent sequence of microstructures, including kink-bands, antithetic extensional microfaults (pull-apart), bends in the microfaults by development of transverse fractures and microboudinage. Correlation of mineral mechanical response with a thermal-decay curve based on isotopic mineral ages yields a temporally constrained deformation history. Ductile dextral shearing occurred subsequent to the intrusion of the 330 Ma Petersburg granite and passed into the brittle-ductile transition by approximately 260 Ma. Dextral faulting terminated before 240 Ma. The Appalachian dextral transcurrent faulting event therefore continued through Permian in some areas.

The Role of Toxicological Science in Meeting the Challenges and Opportunities of Hydraulic Fracturing

We briefly describe how toxicology can inform the discussion and debate of the merits of hydraulic fracturing by providing information on the potential toxicity of the chemical and physical agents associated with this process, individually and in combination. We consider upstream activities related to bringing chemical and physical agents to the site, on-site activities including drilling of wells and containment of agents injected into or produced from the well, and downstream activities including the flow/removal of hydrocarbon products and of produced water from the site. A broad variety of chemical and physical agents are involved. As the industry expands this has raised concern about the potential for toxicological effects on ecosystems, workers, and the general public. Response to these concerns requires a concerted and collaborative toxicological assessment. This assessment should take into account the different geology in areas newly subjected to hydraulic fracturing as well as evolving industrial practices that can alter the chemical and physical agents of toxicological interest. The potential for ecosystem or human exposure to mixtures of these agents presents a particular toxicological and public health challenge. These data are essential for developing a reliable assessment of the potential risks to the environment and to human health of the rapidly increasing use of hydraulic fracturing and deep underground horizontal drilling techniques for tightly bound shale gas and other fossil fuels. Input from toxicologists will be most effective when employed early in the process, before there are unwanted consequences to the environment and human health, or economic losses due to the need to abandon or rework costly initiatives.

Late Paleozoic transcurrent tectonic assembly of the central Appalachian Piedmont

Recent investigations in south-eastern Pennsylvania and northern Maryland have demonstrated a major anastomosing strike-slip shear system. The Pleasant Grove-Huntingdon Valley shear system emerges from beneath the coastal plain cover at Trenton, New Jersey, and extends to the area west of Baltimore, Maryland, where it is overlain by the Culpepper Mesozoic rift basin. The sense of offset across this system is dextral. In the Susquehanna River region and north of the shear zone, the rocks of the Octoraro Formation contain evidence for two metamorphisms and deformations prior to strike-slip shearing, whereas south of the shear zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen. Although conclusive ages for the strike-slip deformation do not exist at this time, the timing of deformation is loosely constrained where the shear system crosscuts known Taconian structures in the Piedmont. Comparison of deformation style with other regions in the Appalachian suggests the Pleasant Grove-Huntingdon Valley shear system is related to Alleghanian transcurrent tectonics in the Piedmont. Palinspastic reconstruction of the Pleasant Grove-Huntingdon Valley shear system reveals fundamental problems in current tectonic models for the central Appalachian Piedmont. A minimum of 150 km of dextral offset is proposed for the Pleasant Grove-Huntingdon Valley shear system based on reconstruction of the Cambrian-Ordovician shelf edge between northern Maryland and southeastern New York. Displacement of this magnitude can account for the previously proposed tectonic models that portray a failed Iapetan rift block and microcontinent that contains the Baltimore Grenvillian massifs. Even though a history of early orthogonal collision is preserved within discrete structural blocks, transcurrent shearing has greatly influenced the distribution of those blocks. Models not including the strike-slip component of tectonic assembly need serious reconsideration, as evidence grows that the magnitude of orogen-parallel displacement is equal to or larger than the orthogonal component.

Generation, Segregation, Ascent, and Emplacement of Alleghanian Plutons in the Southern Appalachians

The Carboniferous-Permian (Alleghanian) orogeny in the southern Appalachians produced a distinctive 45 m.y.-long intrusive event that formed a NE-trending band of dispersed, composite plutons covering

Ground-water-simulation apparatus for introductory and advanced courses in environmental geology

10,500 km^{2}

Multiple Reactivations of Rigid Basement Block Margins: Examples in the Northern Reading Prong, USA

. Of these plutons, 75% are metaluminous biotite or amphibole + biotite granitoids with lower crustal sources, and 20% are peraluminous biotite + muscovite ± garnet or cordierite + biotite granitoids with more evolved crustal sources. The remaining 5% are gabbroids, intermediate rocks, and possible alkaline granitoids. The granitoid to gabbroid + intermediate rock area ratio is 180:1. The exposed intrusive rocks were produced at a relatively slow rate of

Three-dimensional shuffling of horses in a strike-slip duplex: an example from the Lambertville sill, New Jersey

2 km^{3}