Mark A. Evans

Fluid inclusion and stable isotope analyses of veins from the central Appalachian Valley and Ridge province: Implications for regional synorogenic hydrologic structure and fluid migration

Fluid inclusion microthermometric analyses and O and C stable isotopic analyses of vein minerals are used to determine the chemistry and trapping conditions of fluids present in the central Appalachian fold-and-thrust belt during the late Paleozoic Alleghanian orogeny. The upper Paleozoic rock section contains three regional hydrostratigraphic systems based on fluid chemistry and temperature. The Ordovician Trenton Formation through the Devonian Helderberg Group was a regional aquitard and was dominated by high-salinity, CH 4 -saturated, in situ fluids. The Devonian Oriskany Formation through the lower portion of the Chemung Formation was a regional aquifer system and underwent an influx of warm migrating fluids. The upper portion of the Devonian Chemung through Pocono Formations was also a regional aquifer, but it was dominated by an influx of meteoric water that mixed with in situ fluids. The migrating fluid was a warm (160 to >220 °C) CH 4 -saturated NaCl-CaCl 2 brine that was stratigraphically restricted to the Oriskany Formation through the lower portion of the Chemung Formation, although there is evidence for infiltration into lower stratigraphic units. Two separate fluid migration events are recorded in the rocks. The first event is either late synfolding to postfolding, and the second event is postfolding. Approximately 2‐4 km of overburden were removed by erosion between the two migration events. The source of the warm migrating fluids is still unknown. However, the most likely source would be fluids that were tectonically driven through the fold-and-thrust belt by large-scale, out of sequence thrusting in the hinterland. The migrating fluids were transported far into the foreland where they may have been directly responsible for (1) flushing of hydrocarbons from the upper Paleozoic of the Valley and Ridge into the Plateau province and (2) elevated thermal maturation indicators in the Valley and Ridge and Plateau provinces.

Strain factorization and partitioning in the North Mountain thrust sheet, central Appalachians, U.S.A.

Abstract By examining finite strain distribution and partitioning strain among deformation mechanisms, a model of thrust sheet deformation is established. The model takes into account kinematic history and environmental conditions. In the North Mountain thrust sheet of northern Virginia, factorizing finite strain into pure and simple shear components shows that the thrust sheet experienced 6–13% pure shear shortening parallel to the transport direction. Thrust-parallel simple shear strains increase slightly from the top of the thrust sheet toward the base. Within 500 m of the floor thrust, however, simple shear values increase markedly toward the basal thrust. This strain pattern occurs throughout the thrust sheet, overprinting earlier imbricate structures. Therefore, the finite strain in the thrust sheet may be modelled as a transport-parallel pure and simple shear applied during the major transport episode, and after thrust sheet imbrication. This transport-related strain overprints a weak layer-parallel shortening that is probably related to earlier passage of the thrust tip. Rocks in the thrust sheet with X / Z strain ratios greater than 1.35 have a slight crystallographic preferred orientation and are therefore tectonites in a strict sense. The distribution of tectonites within the thrust sheet defines a ‘tectonite front’ that is inclined toward the foreland. In the North Mountain thrust sheet the ‘tectonite front’ also generally coincides with the transition to high-temperature deformation mechanisms, and ultimately may parallel a paleo-isotherm within the thrust wedge. Strain partitioning indicates that approximately 70% of the finite strain results from intragranular mechanisms (i.e. dislocation glide, dislocation creep and diffusion mechanisms); 25% results from calcite twinning; and less than 5% resulted from transgranular mechanisms such as pressure solution. In non-tectonites, approximately 50% of the finite strain results from pressure solution and intragranular mechanisms, with the remaining strain due to twinning in calcite.

Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS)

Abstract In order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility (AMS), strain partitioning and AMS analysis was conducted at 35 sites from two stratigraphically adjacent Paleozoic limestone units in the Patterson Creek and Wills Mountain anticlines in the central Appalachian orogen of West Virginia. In addition, anisotropy of anhysteretic remanent magnetization (AARM) was conducted on selected samples to examine the role of preferentially oriented magnetite on the AMS fabric. Strain is partitioned into bed-normal shortening due to compaction solution strain (≤35.0% shortening), bed-parallel shortening due to tectonic solution strain (≤13.3% shortening), calcite twinning strain (≤5.8% shortening), and grain-boundary-sliding (≤26.7% shortening). The AMS fabrics in the rocks were found to be a result of a complex interaction between rock lithology, deformation mechanisms, and strain magnitude. Although all the rocks have experienced the same deformation conditions, six different AMS fabrics are exhibited. Each of the different AMS fabrics is a composite fabric resulting from the overprinting of three components: (1) an inherent primary depositional AMS fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix; (2) a diagenetic and/or compaction AMS fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix; and (3) a tectonic AMS fabric that was imparted on the rocks by layer-parallel-shortening deformation prior to folding, and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix, as well as twinning of ferroan calcite.

The structural geometry and evolution of foreland thrust systems, northern Virginia

Seismic reflection data reveal that the structural geometry of the central Appalachians of northern Virginia consists of three distinct thrust systems. Each thrust system is characterized by a unique internal geometry.The Blue Ridge thrust sheet is a composite thrust sheet composed primarily of imbricated Precambrian crystalline rocks. It over-rode Cambrian-Ordovician carbonates and formed a sheared, basement-cored fault-bend fold. Thrusts within the sheet may be Taconic and earliest Alleghanian, whereas final thrusting and emplacement of the sheet were probably slightly younger but still early Alleghanian. The North Mountain thrust sheet is characterized by imbricated Cambrian-Ordovician carbonates that are deformed into large-amplitude mode II fault-bend folds and fault-propagation folds. Rocks of this sheet were transported more than 60 km across a similar section of carbonates. The leading edge of the North Mountain thrust sheet was deformed into a fold with a mode II fault-bend fold geometry and was juxtaposed against middle Paleozoic rocks. The middle Paleozoic rocks occur in a ramp across which displacement along the North Mountain thrust was transferred to a higher detachment. More than 60 km of cover rocks displaced during the emplacement of the North Mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western Valley and Ridge province or were backthrust above the sheet. The timing for imbrication and emplacement of the North Mountain thrust sheet is probably Main Phase Alleghanian. The Lower Carbonate duplex extends from beneath the Blue Ridge and North Mountain thrust sheets, westward across the western Valley and Ridge province. The Lower Carbonate duplex is characterized by imbricated Cambrian-Ordovician carbonates that form low-amplitude fault-bend folds. This thrust system also probably formed during Main Phase Alleghanian deformation.

Control of mechanical stratigraphy on bed-restricted jointing and normal faulting: Eagle Ford Formation, south-central Texas

Outcrops of the middle Eagle Ford Formation in south-central Texas reveal well-developed joint networks in subhorizontal competent carbonate (chalk) beds and less well developed networks in interlayered incompetent calcareous mudrock beds. Northeast-striking bed-perpendicular joints in competent beds have the longest trace lengths and are abutted by northwest-striking joints. All observed joints terminate vertically in incompetent beds. Normal faults are common but less abundant than joints; dominantly dip north, northwest, or southeast; and are abutted by the joint sets and, thus, predated jointing. The faults cut multiple competent and incompetent beds, providing vertical connectivity across mechanical layering. Products of hybrid and shear failure, the dip of these faults is steep through competent beds and moderate through incompetent beds, resulting in refracted fault profiles with dilation and calcite precipitation along steep segments. Fluid inclusions in fault zone calcite commonly contain liquid hydrocarbons. Rare two-phase fluid inclusions homogenized between about (1) 40 and 58°C, and (2) 90 and 100°C, suggesting trapping of aqueous fluids at elevated temperatures and depths on the order of 2 km (6562 ft). Fluid inclusion and stable isotope geochemistry analyses suggest that faults transmitted externally derived fluids. These faults likely formed at depths equivalent to portions of the present-day oil and gas production from the Eagle Ford play in south Texas. Faults connect across layering and provide pathways for vertical fluid movement within the Eagle Ford Formation, in contrast to vertically restricted joints that produce bed-parallel fracture permeability. These observations elucidate natural fractures and induced hydraulic fracturing within the Eagle Ford Formation.

Examining the relationship between remagnetization and orogenic fluids: central Appalachians

Abstract Preliminary work in the central Appalachians shows that the relationship between orogenic fluids and remagnetization is not as simple as many workers have assumed. Fluid inclusion and stable isotope data from veins show that the Paleozoic section in the central Appalachian Valley and Ridge province contained multiple hydrostratigraphic intervals during the Late Paleozoic Alleghanian orogeny with ‘warm’ migrating fluids restricted to the Middle to Upper Devonian interval. Paleomagnetic core samples from throughout the entire stratigraphic section give a similar syn-folding magnetization with nearly identical paleopoles. Therefore, the relatively homogeneous remagnetization of the rock section does not reflect the stratification of fluids. Consequently, the stratigraphically restricted ‘warm’ migrating fluids are apparently not directly related to the overall syn-folding remagnetization.

Remagnetization signature of Paleozoic sedimentary rocks from the Patterson Creek Mountain anticline in West Virginia

Abstract Paleomagnetic analysis of five adjacent Lower Silurian to Lower Devonian sedimentary units (Oriskany, Helderberg, Tonoloway, Williamsport and McKenzie) plus a carbonate vein, within a single large fold in West Virginia, reveals a secondary, reversed, Permian, magnetization in all rocks. Similar unblocking temperatures (∼350–550 °C) were observed for the characteristic magnetization throughout the study implying similar origins for their magnetization. Unfolding estimates, with 95% confidence intervals, were, Helderberg (60±11%), Tonoloway (69±9%), Williamsport (65±25%), Oriskany (78±11%) and McKenzie (80±4%). Since all units were sampled on a single fold, their burial, temperature and stress histories should be identical. Identification of the mechanism for remagnetization and an explanation of the differences in the fold test results involve awkward interpretations. Hypotheses for remagnetization mechanisms such as fluid migration, clay alteration, partial thermoviscous resetting and strain all appear to be flawed for this data set and true differences in the timing of remagnetization relative to folding would require a very complex multi-stage remagnetization event. The actual mechanism for remagnetization may yet be unidentified. An alternative possibility is that these units have been affected by multiple remagnetizing mechanisms simultaneously and then the magnetization has been modified by minor strain to produce the variation in the results of the fold tests.

Deformation conditions for fracturing in the Middle Devonian sequence of the central Appalachians during the Late Paleozoic Alleghenian orogeny

In the central Appalachians, fluid inclusion microthermometry and oxygen and carbon stable isotope analysis vein minerals from the Middle Devonian shale section show that fluid conditions (pressure, temperature, and composition) are constantly changing during deformation and vary spatially across the fold-thrust belt. The earliest fractures in the region formed prior to folding, early during the Alleghenian orogeny as the rocks were buried into the oil generation window. They contain minerals that contain degraded hydrocarbon inclusions and basinal brine inclusions. During multiple vein reopening events, later mineral stages contain increasingly more mature hydrocarbon fluids. Late quartz mineralization is pervasive and typically contains the high-temperature brine inclusions. The vein opening history is related to changes in fluid connectivity associated with (1) burial by over-thrusting and/or syntectonic depositional loading and/or (2) folding during uplift and erosion. Initial fracture formation and fluid-trapping depths range from 3.5 km (2.2 mi) in the Plateau province and along the Appalachian structural front to 4.5 to 5.0 km (2.8 to 3.1 mi) in the Valley and Ridge province. Late-stage fracturing and fracture reopening is related to the maximum syntectonic burial, which varies from about 4 km (2.5 mi) in the Plateau to over 11 km (6.8 mi) in the Valley and Ridge. Fractures in the Valley and Ridge and western Pennsylvania Plateau provinces cannot be categorized into the simple and classification model. Burial history modeling indicates that fractures forming within and near the end of the oil window were NNW- and NW-striking, not ENE-striking, fractures.

Fracture-controlled palaeohydrology of a secondary salt weld, La Popa Basin, NE Mexico

Abstract Isotopic and fluid inclusion analyses of veins and host rocks constrain the compositions, temperatures and sources of palaeofluids along the La Popa salt weld. Most veins formed after the salt was evacuated from the precursor salt wall; veins are generally more abundant on the downthrown side of the weld and near a significant bend in the trace of the weld. The spatial distribution of fluid types and temperatures suggests the weld served as a vertical fluid conduit and a horizontal baffle. Stable isotopes indicate there was significant fluid–rock interaction and little vertical fluid communication between rock units in areas away from the weld. Fluid temperatures along the weld ranged from 84 to 207 °C, salinities ranged from 4 to 25 wt% NaCl equiv. and methane was abundant in the weld zone and on the downthrown side of the weld. Strontium isotopes suggest that some of the vein-forming fluids were derived from the evaporites that once occupied the weld. Our results suggest the sealing potential of similar welds may be related to the presence of abrupt changes in weld geometry such as cusps or bends, the amount of shortening across the weld and the amount of vertical displacement across the weld.

Abstracts of Other Conference Presentations

Many physical (structural and textural) and compositional (chemical and mineralogical) attributes of volcanic rocks are correlated with techtonic setting. However, only a few casual relations are known. Modern subduction-related volcanic rocks are persistently highly porphyritic and dominantly fragmental, especially if subaerial. These features are expected foe magmas rich in H2O, because ascending magmas become supercooled by effervescence leading to crystallization and explosive eruption. The high H2O is plausibly related to subduction of cool lithosphere rich in hydrous minerals. Has subducting lithophere always been cool and and hydrous? Deep submarine extrusions of identical magmas would be less porphyritic, less vesicular and not so explosive. Variable vesicularity and associated sulfide deposits could, however, help reveal original depths of extrusion. Magnesian olivineis stabilized by H2O in siliceous melts; consequently, olivine in blocky and/or fragmental (highly viscous) volcanic rocks in suggestive of a hydrous magma. Nearly all tholeiitic basalts crystallize cpx before opc at low pressures. Anomalous early opx in them seems best explained by siliceous contamination. But how do we account for Mauna Loa? Granitic contamination may deplete basalt in Na2O with consequent reversal of plagioclase zoning. Little altered oceanic ridge basalts have diagnostic chemical compositions, but most old rocks are strongly altered. Reclosure of the Atlantic would likely preserve parts of Iceland and Azores. How would we interpret them?