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Terry Engelder - One of the best experts on this subject based on the ideXlab platform.

  • jointing within the outer arc of a forebulge at the onset of the Alleghanian orogeny
    Journal of Structural Geology, 2007
    Co-Authors: Gary G Lash, Terry Engelder
    Abstract:

    The oldest joint set in Devonian rocks of western New York state has an atypical NS strike and predates regionally more abundant NW-striking and ENE-striking joints driven by hydrocarbon-related fluid decompression. The NS joints originated in higher modulus diagenetic carbonate and were driven initially by a different mechanism, either joint-normal stretching and/or thermoelastic contraction. The origin of these joints in higher modulus carbonate concretions indicates the presence of a tensile stress produced by uniform regional extensional strain. Upper Devonian shale hosting the NS joints crops out in that area of the Appalachian Basin where a Morrowan erosional unconformity marks the region of maximum upward lithospheric flexure of an early Alleghanian forebulge. The NS strike of these early joints points to a forebulge stretching axis oriented approximately east-west and associated in time and space with crustal loading that drove both the Northfieldian Orogeny and the underplating of the Bronson Hill Anticlinorium in New England. Ultimately, subsidence of the Morrowan forebulge buried the Upper Devonian shale succession to the oil window during the latter part of the Alleghanian tectonic cycle resulting in the propagation of fluid driven NW- and ENE-trending joints in black shale.

  • early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the Alleghanian orogeny
    Geology, 2006
    Co-Authors: Terry Engelder, Amy E Whitaker
    Abstract:

    Early ENE-striking joints (present coordinates) within both Pennsylvanian coal and Devonian black shale of the Central and Southern Appalachians reflect an approximately rectilinear stress field with a dimension > 1500 km. This Appalachian-wide stress field (AWSF) dates from the time of joint propagation, when both the coal and shale were buried to the oil window during the 10-15 m.y. period straddling the Pennsylvanian-Permian boundary. The AWSF was generated during the final assembly of Pangea as a consequence of plate-boundary tractions arising from late-stage oblique convergence, where maximum horizontal stress, S-H, of the AWSF was parallel to the direction of closure between Gondwana and Laurentia. After closure, the AWSF persisted during dextral slip of peri-Gondwanan microcontinents, when SH appears to have crosscut plate-scale trans-current faults at around 30{sup o}. Following > 10 m.y. of dextral slip during tightening of Gondwana against Laurentia, the AWSF was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline-shaped Alleghanian map pattern seen today.

  • horizontal slip along Alleghanian joints of the appalachian plateau evidence showing that mild penetrative strain does little to change the pristine appearance of early joints
    Tectonophysics, 2001
    Co-Authors: Terry Engelder, Benjamin F Haith, Amgad Younes
    Abstract:

    Abstract Some Alleghanian joints in black shales of the Geneseo and Middlesex Formations of the Catskill Delta complex, Finger Lakes district, New York, slipped horizontally up to 8 cm. Horizontal slip is measured by the offset of ENE-striking joints. Alleghanian joints striking 330–350° display a right-lateral slip with an average value of 1.9 cm, while joints striking 004–010° slip in the left-lateral sense with an average value of 1.3 cm. The maximum horizontal stress (SH) driving this slip falls between 350° and 004°, the orientation of local Alleghanian layer-parallel shortening as indicated by both disjunctive and pencil cleavage. By commonality of orientation, we infer that slip on Alleghanian joints is driven contemporaneously with layer-parallel shortening. If so, the offset ENE-striking joints predate the Alleghanian stress field. These observations mean that both pre-Alleghanian and early Alleghanian joints persist through a period of penetrative strain.

  • fringe cracks key structures for the interpretation of the progressive Alleghanian deformation of the appalachian plateau
    Geological Society of America Bulletin, 1999
    Co-Authors: Amgad I Younes, Terry Engelder
    Abstract:

    Vertical joints in Devonian clastic sedimentary rocks of the Finger Lakes area of New York State are ornamented with arrays of fringe cracks that reveal the complex deformational history of the Appalachian plateau detachment sheet during the Alleghanian orogeny. Three types of fringe cracks were mapped: gradual twist hackles, abrupt twist hackles, and kinks. Gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet. Abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints. Kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds. The breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field, internal fluid pressure, and the elastic properties of the bed. The twist angle of gradual twist hackles is larger in coarser clastic beds, indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown. Assuming that the vertical stress axis (S v ) equals 78 MPa at 3 km burial depth, the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as S H ‐ S h ∪ 2.5 MPa when S H ‐ S h ∪ 12 MPa in sandstone beds. The twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during Alleghanian tectonics. These parent joints indicate a regional clockwise stress rotation of Alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area. A counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet. The clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the Pennsylvania Valley and Ridge, but is opposite to the sense of rotation in the southwestern portion of the detachment sheet (western Pennsylvania and West Virginia). The two regional rotation domains are separated by the Juniata culmination.

  • Veins in the Lockport dolostone: evidence for an Acadian fluid circulation system
    Geology, 1992
    Co-Authors: M. R. Gross, L. K. Hutton, Egill Hauksson, Jim Mori, Terry Engelder, Thomas H. Heaton, Hiroo Kanamori, Neil H. Kenyon, Simon R. Poulson, K T Pickering, J.d. Clark, Lucile M Jones
    Abstract:

    The orientation and spatial distribution of veins in the Lockport dolostone attest to a fluid circulation system active during the Acadian orogeny in western New York and southern Ontario. Outcrops east of the Clarendon-Linden fault zone are cut by a prominent east-northeast systematic calcite-filled vein set, whereas these systematic veins are absent west of the fault zone, except in two quarries. Systematic veins display distinct characteristics: the mean vein orientation rotates clockwise from 067° in the east to 086° farther west, veins do not propagate into the basal Lockport Group, and calcite vein δ18OPDB values are significantly lighter to the east of the Clarendon- Linden fault zone. Maximum horizontal stress (SH) trend lines drawn parallel to the strike of the systematic veins are incompatible with SH inferred from Alleghanian plateau and other post-Paleozoic structures. However, because east-west SH trend lines are compatible with an Acadian tectonic event in western New England, our interpretation is that systematic veins in the Lockport Group are a cratonward signature of the Acadian orogeny.

Charles M Onasch - One of the best experts on this subject based on the ideXlab platform.

  • development of a dilatant damage zone along a thrust relay in a low porosity quartz arenite
    Journal of Structural Geology, 2006
    Co-Authors: Jennie E Cook, William M Dunne, Charles M Onasch
    Abstract:

    A damage zone along a backthrust fault system in well-cemented quartz arenite in the Alleghanian foreland thrust system consists of a network of NW-dipping thrusts that are linked by multiple higher-order faults and bound a zone of intense extensional fractures and breccias. The damage zone developed at an extensional step-over between two independent, laterally propagating backthrusts. The zone is unusual because it preserves porous brittle fabrics despite formation at O5 km depth. The presence of pervasive, late-stage fault-normal joints in a fault-bounded horse in the northwestern damage zone indicates formation between two near-frictionless faults. This decrease in frictional resistance was likely a result of increased fluid pressure. In addition to physical effects, chemical effects offluid also influenced damage zone development. Quartz cements, fluid inclusion data, and Fourier Transform Infrared analysis indicate that both aqueous and methane-rich fluids were present within the damage zone at different times. The backthrust network likely acted as a fluid conduit system, bringing methane-rich fluids up from the underlying unit and displacing resident aqueous fluids. The presence of methane not only enhanced the effects offluid pressure, which facilitated brittle fracturing, but

  • quantitative assessment of low temperature deformation mechanisms in a folded quartz arenite valley and ridge province west virginia
    Tectonophysics, 2000
    Co-Authors: Michael J Harrison, Charles M Onasch
    Abstract:

    Abstract Analysis of the grain-scale deformation mechanisms in folded Tuscarora Sandstone from the western margin of the Appalachian foreland near Keyser, WV, reveals that deformation was accomplished by a combination of pressure solution, microfracturing, and crystal plastic mechanisms. The finite strain was assessed using the normalized Fry method and showed a predominant apparent flattening strain regime resulting from diagenetic compaction. Partitioning of the strain between operative deformation mechanisms revealed that layer-normal shortening (25%) and layer-parallel shortening (10%) by pressure solution were the greatest contributors to the finite strain, followed by microfracturing and crystal plastic strains respectively. Although the crystal plastic contribution to the finite strain is small (1.0% layer-normal shortening), strain partitioning allowed detection of layer-parallel extension, consistent with the structural position of the samples on the outer arc of a fold. The variation in inferred stress directions throughout the progressive deformation indicates that local stratigraphic and structural control on the stress field is important in the development of certain microstructures. Only the earliest stages of deformation record far-field Alleghanian compression, whereas later stages reflect local stress fields controlled by bedding orientation and position on developing folds. Similarities in the microstructure evolution between the western and eastern margins of the Appalachian foreland indicate that the entire belt underwent a similar deformation history.

  • variation in quartz arenite deformation mechanisms between a roof sequence and duplexes
    Journal of Structural Geology, 1993
    Co-Authors: Charles M Onasch, William M Dunne
    Abstract:

    Abstract Microstructural abundances and histories in quartz arenite of the Lower Silurian Tuscarora Sandstone were used to determine the nature and role of microscale deformation in a cover sequence and underlying thrust system, and to assess the degree to which the cover sequence accommodated emplacement of the thrust system. Sandstone samples are located across the transition from the central to southern Appalachian foreland thrust system where the thrusts change from blind to emergent southwards and where southern deformation of early Alleghanian age has been previously shown to be overprinted by central deformation of Alleghanian age. Microstructures observed with transmitted light and cathodoluminescence microscopy indicate that grain-scale deformation occurred by dislocation flow, pressure solution, and microfracturing, with the last being generally the most important. The sequence of deformation mechanisms is the same for the cover sequence and the thrust system: pressure solution during sedimentary compaction; dislocation flow during layer-parallel shortening; and localized microfracturing with limited pressure solution near major thrust ramps and in steep fold limbs. A greater abundance of dislocation flow microstructures in the cover sequence from layer-parallel shortening indicates grain-scale accommodation in the Tuscarora Sandstone of some shortening associated with emplacement of the thrust system. The transition zone between the central and southern Appalachians contains the greatest occurrence of every microstructure which is consistent with the area having been affected by diachronous central and southern Alleghanian deformations.

Michael J Kunk - One of the best experts on this subject based on the ideXlab platform.

  • p t t paths and differential Alleghanian loading and uplift of the bronson hill terrane south central new england
    American Journal of Science, 2003
    Co-Authors: R P Wintsch, Michael J Kunk, J L Boyd, John N Aleinikoff
    Abstract:

    Late Paleozoic U-Pb ages of sphene and 4 0 Ar/ 3 9 Ar cooling ages of amphibole and muscovite from rocks of the Bronson Hill terrane in Connecticut and central Massachusetts reflect a late Paleozoic (Alleghanian) overprinton Acadian metamorphic rocks. Prograde Alleghanian sphenes crystallized during the Late Pennsylvanian, and eliminate the possibility that amphibole ages reflect delayed Permian cooling from Devonian Acadian metamorphism. Fourteen new amphibole ages from Connecticut form a north-to-south trend of decreasing age from 294 to 245 Ma, while in Massachusetts four new amphibole ages together with three others from the literature produce a random Carboniferous pattern. Seven new muscovite ages support existing data indicating uniform cooling throughout the Bronson Hill terrane through ∼350°C in the Early Triassic. The rate of Permian cooling defined by amphibole-muscovite pairs increases from ∼4°C/my in northern Connecticut to ∼50°C/my near Long Island Sound. Hinged loading and hinged but delayed exhumation in the southern part of the Bronson Hill terrane (with the hinge in central Connecticut) explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient. One-dimensional thermal modeling indicates that loading of Bronson Hill rocks must have begun by the Late Mississippian. The time of peak Alleghanian metamorphic temperature decreases southward from Early Permian in northern Connecticut to Late Permian to the south. These results demonstrate that the metamorphic effects of the Alleghanian orogeny are not restricted to the Avalon terrane of southeastern New England. On the contrary, the Alleghanian orogeny reset 4 0 Ar/ 3 9 Ar mineral ages, recrystallized minerals, partially melted felsic rocks, and transposed fabrics at least as far west as the Bronson Hill terrane in south-central New England.

  • 40ar 39ar thermochronology and Alleghanian development of the southernmost appalachian piedmont alabama and southwest georgia
    Geological Society of America Bulletin, 1993
    Co-Authors: Mark G Steltenpohl, Michael J Kunk
    Abstract:

    40Ar/39Ar age spectra of hornblende, muscovite, and microcline, and total fusion ages of biotite from metamorphic rocks of the Inner Piedmont, Pine Mountain, and Uchee belts are reported. Mineral cooling ages from the eastern part of the Inner Piedmont are as follows: hornblende, 320 Ma; muscovite, 296 Ma; biotite, 293 Ma; and microcline (diffusional release patterns) Tmax = 267 Ma, Tmin = 234 Ma. A 347 Ma hornblende spectrum from the highest Inner Piedmont structural level sampled is the oldest date determined and implies earlier passage of this level through the 500 °C isotherm. Most release spectra from Pine Mountain belt units are discordant with little or no apparent geologic meaning. Modified saddle-shaped release patterns for hornblende indicate extraneous argon with a maximum age of ∼358 Ma. Muscovite from the Pine Mountain belt cover sequence is 286 Ma (plateau age), and one from the underlying Grenville basement is 277 Ma (correlation age), indicating cooling below the 350 °C isotherm. Plateau ages on Uchee belt rocks are as follows: hornblende, from 297 to 288 Ma; muscovite, 285 Ma; biotite, 276 Ma; and microcline Tmax = 261 Ma, Tmin 230 Ma. Muscovite fish from a Bartletts Ferry fault zone phyllonite have a plateau age of 283 Ma. The 40Ar/39Ar results combined with other geologic data indicate that (1) a large part of the southern and Inner Piedmonts of Alabama and southwest Georgia experienced a late Paleozoic amphibolite-facies thermal and deformational event contemporaneous with the Alleghanian orogeny observed in the foreland; (2) the tectonic development of this event, characterized by initial crustal thickening followed by right-slip and normal-slip movements, is grossly similar to that described for the amphibolite-facies Alleghanian belt in the eastern Piedmont of South Carolina and Georgia; and (3) extensional movements along the flanks of the Pine Mountain window occurred between ca. 277 Ma and the Late Triassic-Early Jurassic and thus may reflect latest Alleghanian extensional collapse or Mesozoic rifting.

  • Alleghanian development of the goat rock fault zone southernmost appalachians temporal compatibility with the master decollement
    Geology, 1992
    Co-Authors: Mark G Steltenpohl, Steven A Goldberg, Thomas B Hanley, Michael J Kunk
    Abstract:

    The Goat Rock and associated Bartletts Ferry fault zones, which mark the eastern margin of the Pine Mountain Grenville basement massif, are controversial due to the suggestion that they are rare exposed segments of the late Paleozoic southern Appalachian master decollement. The controversy in part stems from reported middle Paleozoic (Acadian) radiometric dates postulated as the time of movement along these fault zones. Ultramylonite samples from the type area at Goat Rock Dam yield a 287 [plus minus] 15 Ma Rb-Sr isochron interpreted as the time of Sr isotopic rehomgenization during mylonitization. This date is corroborated by Late Pennsylvanian-Early Permian [sup 40]Ar/[sup 39]Ar mineral ages on hornblende (297-288 Ma) and muscovite (285-278 Ma) from neomineralized and dynamically recrystallized rocks within and straddling the fault zone. These Late Pennsylvanian-Early Permian dates indicate the time of right-slip movement (Alleghenian) along the Goat Rock fault zone, which is compatible with the timing suggested by COCORP for thrusting along the southern Appalachian master decollement.

Nathan R Miller - One of the best experts on this subject based on the ideXlab platform.

  • ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for Alleghanian tectonics american journal of science v 316 n 5 p 470 503 doi 10 2475 05 2016 03
    American Journal of Science, 2020
    Co-Authors: E J Catlos, Nathan R Miller
    Abstract:

    The original version of the article inadvertently contained a mistake in the title, where 208Th should be 232Th. The corrected title is “Ion microprobe 232Th-208Pb ages from high common Pb monazite, Morefield Mine, Amelia County, Virginia: Implications for Alleghanian tectonics

  • ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for Alleghanian tectonics
    American Journal of Science, 2016
    Co-Authors: E J Catlos, Nathan R Miller
    Abstract:

    Monazite [(Ce,Th)PO4] from a pegmatite in the Morefield Mine of the eastern Piedmont of central Virginia has unusually high and variable amounts of common Pb, leading to problematic interpretations of its U-Th-Pb ages and how the monazite relates to nearby granite intrusions and faults. To address these issues, we analyze a single large monazite grain from the pegmatite using electron microprobe analysis (EMPA, n = 64), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS, n = 58), and secondary ion mass spectrometry (SIMS, n = 59). The monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization. Although the compositions of these regions fit the ideal stoichiometry of monazite, they have lower Si, Th, U, and Y, and higher P, rare earth element (REE), and Ca concentrations compared to visibly unaltered portions of the grain. LA-ICP-MS and SIMS analyses demonstrate that common Pb, as proxied by 204Pb, is enriched in proximity to microcrack regions and correlates with 137Ba. SIMS 232Th-208Pb analysis from grain regions with lowest contents of common Pb (208Pb comprises >99% of Pb isotopes) yields two sets of ages: 263.5±3.0 Ma (±1σ; MSWD = 1.7; n = 11) and 234.1±3.3 Ma (±1σ; MSWD = 0.4; n = 13). Regionally, the ages are similar to the youngest Appalachian pegmatite bodies emplaced during the terminal (Alleghanian) Laurentia-Africa collision. However, the monazite ages are younger than locally surrounding intrusions. The closest intrusive in distance (∼30 km) and age is the Petersburg granite (296.33±0.11 Ma, zircon 238U-206Pb), the emplacement of which coincided with activity along the Hylas Fault. Because the fault experienced a brittle-ductile transition in the Late Permian (∼262 Ma), coeval with the older 232Th-208Pb monazite age group, we postulate that the Hylas Fault may have operated as a fluid migration system between the Petersburg granite and Morefield Mine pegmatite. The younger monazite age coincides with Triassic normal and/or sinistral faulting linked to the development and deformation of local rift basins.

G.c. Hobbs - One of the best experts on this subject based on the ideXlab platform.

  • Fate of ‘warm’ migrating fluids in the central Appalachians during the Late Paleozoic Alleghanian orogeny
    Journal of Geochemical Exploration, 2003
    Co-Authors: Mark A. Evans, G.c. Hobbs
    Abstract:

    Abstract Middle to Upper Devonian elastic rocks throughout the Valley and Ridge province in the central Appalachians contain evidence for fluid migration during the Late Paleozoic Alleghanian orogeny. Fluid inclusion microthermometry of vein minerals indicate that the migrating fluid was a ‘warm’ (160 to >220 °C) CH 4 -saturated NaCl-CaCl 2 brine. However, rocks in the same stratigraphic interval immediately forelandward of the Valley and Ridge contain veins that have significantly different fluid inclusions, indicating that the ‘warm’ fluids did not significantly migrate into the rocks of the Appalachian Plateau province. The ‘warm’ fluids are interpreted to have migrated to the paleosurface as hot springs during the Late Paleozoic. Stable and strontium isotope chemistry of the vein minerals suggests that some infiltration of the 'warm' fluids into the easternmost Plateau province did occur. This indicates that wholesale flushing of a foreland during an orogeny may not be as common as originally thought.