R.D Elmore

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.

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.

The occurrence of pervasive chemical remanent magnetizations in sedimentary basins: implications for dating burial diagenetic events

Abstract Paleomagnetic results from Jurassic sediment on Skye, Scotland, are consistent with the hypothesis that authigenic magnetite and an associated chemical remanent magnetization (CRM) can form during the conversion of smectite to illite. Sediment in north Skye, where smectite is abundant and the clays are unaltered, contain a weak and unstable magnetization. The same age sediment in south Skye, where the clays have been altered to illite by hydrothermal heating, contain a multi-component CRM residing in magnetite and/or pyrrhotite. The CRM was acquired in the early Tertiary, which is consistent with the timing of the hydrothermal activity that caused the alteration.

An integrated paleomagnetic, rock magnetic, and geochemical study of the Marcellus shale in the Valley and Ridge province in Pennsylvania and West Virginia

An integrated paleomagnetic, rock magnetic, and diagenetic study of the Devonian Marcellus Subgroup from threefolds in the Valley and Ridge province in Pennsylvania and West Virginia indicates that the unit contains an intermediate temperature chemical remanent magnetization (CRM) with south-southeast declinations and shallow negative inclinations residing in pyrrhotite and a CRM with more southerly declinations and shallow positive inclinations residing in magnetite. Rock magnetic results confirm the presence of pyrrhotite and magnetite. Tilt tests indicate that pyrrhotite and magnetite CRMs are syntilting to posttilting, and paleopoles for both CRMs are similar and plot on the Permian part of the apparent polar wander path for North America. The Marcellus Subgroup has a complex paragenetic sequence which includes bitumen deposition and vein formation. The magnetite and pyrrhotite CRMs formed in the Permian, probably due to burial diagenetic processes and not orogenic fluids. The base of the unit contains the highest total organic content (TOC) values and highest magnetic intensities, both of which decrease up section. This connection between TOC and magnetic intensity suggests that the magnetite formed as a result of the maturation of the organic matter although more work is needed to test this hypothesis. The pyrrhotite may have formed as a result of thermochemical sulfate reduction involving organic matter during the production of methane in this gas reservoir. The similar Permian CRM ages may be explained by thrusting which resulted in rapid burial of the Marcellus Subgroup as it passed through the oil window and into the gas window quickly producing magnetite and pyrrhotite, respectively.

Paleomagnetic dating of fluid-flow events in dolomitized Caledonian basement rocks, central Scotland

Abstract Paleomagnetic results suggest that there was a fluid-flow event which caused dolomitization and hematite authigenesis in serpentinite within the Highland Border Group along the Highland Boundary Fault in central Scotland in the late Carboniferous/early Permian. Geochemical data indicate that the fluid was hydrothermal in origin, and in addition to causing hematite authigenesis, the fluid also thermally remagnetized pre-existing magnetite in the serpentinite.

Palaeomagnetic dating of fluid flow events along the Moine Thrust Fault, Scotland

Abstract Palaeomagnetic results suggest that several fluid flow events caused alteration and hematite authigenesis in the MoineThrust Zone, northwest Scotland, between the Late Palaeozoic and Middle Tertiary. Precambrian Torridonian sandstone and Lewisian gneiss in the fault zone on the Isle of Skye contain a Tertiary magnetization interpreted to be the result of hydrothermal fluids associated with Tertiary intrusions. Samples of the Dumess Formation limestone (Cambrian), Torridon Group and quartzites (Cambrian) in and near the fault zone to the north contain a Late Palaeozoic to Triassic magnetization residing in hematite and interpreted to be related to hydrothermal fluids. This magnetization and associated alteration in the Moine Thrust Zone are direct evidence for post-orogenic activity, in which the thrusts vented excess heat during regional crustal extension.

Orogenic fluids and secondary magnetizations: testing the relationship in the South Wales Coalfield foreland basin

Abstract Carbonates that rim the South Wales Coalfield contain a pervasive late Paleozoic CRM residing in magnetite. The CRM in the eastern part of the basin was acquired after deformation, and geochemical results are consistent with a connection between remagnetization and orogenic-type fluids. In the western part of the basin, the CRM is synfolding but the geochemical results do not support a connection with orogenic fluids.