Charles M. Onasch

Disequilibrium partitioning of oxygen isotopes associated with sector zoning in quartz

Fractionation of oxygen isotopes during crystal growth is commonly assumed to be an equilibrium process. We present evidence here that large variations in delta(18)O values in a zoned vein quartz crystal are the result of disequilibrium partitioning between fluid and crystal. Within a single crystal,vith well-developed concentric and sector zoning, delta(18)O values determined with laser extraction techniques on milligram- to submilligram-sized samples range from 4.7 parts per thousand to 7.3 parts per thousand. Core to rim variations within single growth sectors fluctuate by 2.6 parts per thousand, whereas differences between adjacent sectors along the same growth zone can exceed 2.0 parts per thousand. In contrast, fluid-inclusion homogenization temperatures and salinities are relatively constant and show no effect of grow th or sector zoning. Differences in delta(18)O values between sectors along the same growth zone cannot result from variations in the oxygen isotope composition of the fluid and strongly suggest disequilibrium partitioning of oxygen isotopes between fluid and the growing crystal. The cause of this disequilibrium is believed to be due to differences in surface structure and/or growth mechanisms between nonequivalent faces.

Microfractures and their role in deformation of a quartz arenite from the central Appalachian foreland

Abstract Microfractures are pervasive in the Massanutten Sandstone, a quartz arenite exposed in a doubly-plunging synclinorium in the central Appalachian foreland. They are present as fluid inclusion planes (FIPS) and microveins parallel the FIPS. FIPS occur in three orthogonal sets: horizontal; vertical and normal to the fold axis; and vertical and parallel to the fold axis. The geometry, distribution and age of FIPS relative to other microstructures shows that microfractures developed during folding. Microfracturing followed layer-parallel shortening by dislocation flow and was coeval with the peak of pressure solution that marked the final stages of folding. Microfracture densities are extremely high (> 160 mm −1 ) in many samples. Rapid closure of microfractures suppressed brecciation and prevented microfractures from propagating unstably to form joints. Microfracture opening accounts for significant finite strain with extensional strains of up to 11.1% and associated volume increases of over 16.0%. The general lack of shortening microstructures indicates that a net volume increase is likely and that the Massanutten Sandstone may have served as a sink for dissolved phases from adjacent units undergoing volume-loss deformation.

The problem of strain-marker centers and the fry method

Abstract The Fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale, despite homogeneous behavior at a population scale. This difference results from the post-deformation centers not coinciding with the original pre-deformation centers. The amount of error may rapidly approach 50% for small strains in ideal cases, but is likely to be in the range of 20–30%.

Assessing brittle volume-gain and pressure solution volume-loss processes in quartz arenite

Abstract Quartz arenite of the Tuscarora Sandstone has been deformed by dislocation flow, pressure solution and microfracturing of which the last two are most important. Pressure solution involved shortening with no extension. Most samples have approximately 20% shortening normal to bedding interpreted to be compaction, and a few have up to 20% layer-parallel shortening of tectonic origin. Because there is no extension, these shortenings resulted in an equivalent amount of bulk volume loss. Multiple sets of microfractures in the form of fluid inclusion planes and microveins account for up to 10% extension and bulk volume increases up to 17%. When both microfracture volume gains and pressure solution volume losses are considered together, all samples show a net bulk volume loss ranging from 14 to 35%. Changes in material volume are different than bulk volume because of the presence of porosity at the time of pressure solution. Whereas all samples show bulk volume losses, most samples show net material volume gains of up to 16% with only those having experienced both compactional and tectonic pressure solution showing material volume losses. Pressure solution during compaction can account for most but not all cement. Compactional pressure solution surfaces do not extend into pore-filling cement suggesting that much of the cement was externally derived after compaction. Therefore, the flux of material may have been considerably greater than the material volume changes would indicate.

Variation in quartz arenite deformation mechanisms between a roof sequence and duplexes

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.

STRUCTURE AND TECTONIC EVOLUTION OF THE TRANSITIONAL REGION BETWEEN THE CENTRAL APPALACHIAN FORELAND AND INTERIOR CRATONIC BASINS

Abstract The transitional region between the central Appalachian foreland basin and Michigan and Illinois interior cratonic basins is characterized by first and second order structures related to plate convergent processes at the Laurentian plate margin. The first order structures are the Waverly and Cincinnati arches believed to be forebulges associated with the Taconic and Alleghanian orogenies, respectively. The second order structures are basement faults that exhibit a protracted history of recurrent displacements. First and second order structures are linked in that the basement faults have had a major effect in controlling the location and uplift history of the arches. The Waverly and Cincinnati arches share many similarities that suggest a common process of arch development. The Waverly arch developed over the low density rocks in the Central Metasedimentary belt of the Grenville province near its structural contact with the Central Gneiss belt. Uplift on the arch commenced during passive margin development in the Cambrian, but reached its maximum development at the end of the Middle Ordovician Taconic orogeny. Reactivation of faults along the arch continued as late as the Pennsylvanian. The Cincinnati arch as expressed in the Precambrian basement surface is coincident with the underlying Grenville front on a regional scale. The front is a major crustal suture which served to localize and control the history of the Cincinnati arch. Local deviations between the location of the arch and front are explained by the presence of Proterozoic rift basins filled with low density sedimentary rocks, by interactions between the developing Appalachian, Michigan, and Illinois basins, and by a northward decrease in Alleghanian tectonic loading. In a pattern similar to the Waverly arch, the Cincinnati arch began its history with a period of regional uplift as early as the Middle to Late Ordovician followed by maximum arch development in the Pennsylvanian in response to the Alleghanian orogeny. Neither arch showed significant migration following initial development as predicted by general models of forebulge development. We attribute this to the effect of preexisting basement anisotropies, such as lithotectonic boundaries, which served to control the initial location of the forebulge and affect its subsequent history. A third major arch, the Kankakee arch, occurs in the western part of the transitional region and is oriented at right angles to the other arches. Unlike the Waverly and Cincinnati arches, this arch owes its development to subsidence in the adjacent Michigan and Illinois basins rather than orogenic activity in the Appalachians.

Strain variations and three-dimensional strain factorization at the transition from the southern to the central Appalachians

Abstract The transition between the southern and the central Appalachian foreland thrust system is a region subjected to at least two non-coaxial deformations, where typical two-dimensional analyses such as cross-section balancing or plane-strain retro-deformation cannot be applied. Three-dimensional factorization can simulate the non-coaxial strain history. Simulation results for compaction and layer-parallel shortening (LPS) were compared to finite strains of 16 quartz arenite samples from the Tuscarora Sandstone. Successful simulations indicate: in the southern Appalachians—30–35% total compaction by volume loss including a 4.5–9% E-W horizontal shortening during lithification, and 5% LPS along 150–330° by plane strain or axial symmetric flattening; in the central Appalachians—30–35% total compaction by volume loss including a 3–7% N-S horizontal shortening during lithification, and 10% LPS along 120–300° by plane strain or axial symmetric flattening; in the transition zone—30% compaction by volume loss including a 3% E-W horizontal shortening during lithification, 5% ‘southern’ LPS along 150–330° by axial symmetric flattening, and 5–10% ‘central’ shortening along 130° or 140° by plane strain or axial symmetric flattening at 10° to the normal ‘central’ trend. From the simulations, horizontal shortening in the southern Appalachians during lithification in the Mississippian was greater than the later Alleghanian deformation. This shortening probably represents a very early LPS developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation. Also, the oblique and weaker central tectonic LPS in the transition zone probably represents behavior at the limits of central Appalachian deformation. Finally, the LPS simulations indicate that axial symmetric flattening (oblate strain) is more successful than plane strain as the representative behavior, so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands.

Recurrent tectonics in a cratonic setting: An example from northwestern Ohio

Structural, stratigraphic, and sedimentologic relations in northwest Ohio indicate that this part of the craton was the site of recurring tectonic activity throughout much of the Paleozoic. The locus of much of this activity was the Bowling Green fault, a complex north-trending zone of diverse fault types. At least six episodes of displacement can be documented, most of which involved vertical displacement along steeply dipping faults. The youngest structures in the fault zone are southwest-directed thrusts. Faulting began as early as the Ordovician Period and continued possibly as late as the Cenozoic Era. Abrupt thickness changes, isopach and structure-contour trends, intraformational unconformities, and soft-sediment deformation structures in Ordovician and Silurian units are attributed to syn-depositional deformation associated with the Bowling Green and related faults. Silurian paleogeography in the region also was affected by ongoing tectonic activity. The location of the Bowling Green fault and the cause for its long history of displacement appear to be related to the underlying Grenville front. Stresses associated with orogenic activity in the Appalachians, lithospheric flow, or forebulge migration were localized by the front and resulted in displacements in the overlying Paleozoic strata along and adjacent to the Bowling Green fault.

Detecting grain boundaries in deformed rocks using a cellular automata approach

Cellular automata (CA) are widely used in geospatial dynamic modeling and image processing. Here, we explore the application of two-dimensional cellular automata to the problem of grain boundary detection and extraction in digital images of thin sections from deformed rocks. The automated extraction of boundaries, which contain rich sources of information such as shape, orientation, and spatial distribution of grains, involves a CA Moores neighborhood-based rules approach. The Moores neighborhood is a 3x3 matrix that is used for changing states by comparing differences between a central pixel and its neighbors. In this dynamic approach, the future state of a pixel depends upon its current state and that of its neighbors. The rules that are defined determine the future state of each cell (i.e., on or off) while the number of iterations to simulate boundaries detection are specified by the user. Each iteration outputs different detection scenarios of grain boundaries that can be evaluated and assessed for accuracy. For a deformed quartz arenite, an r^2 of 0.724 was obtained by comparing manually digitized grains to model derived grains. The value of this proposed method is compared against a traditional manual digitization approach and a recent GIS-based method developed for this purpose by Li et al. (2007).

Determination of pressure solution shortening in sandstones

Abstract A new method for the determination of pressure solution shortening in sandstones uses the geometry of grain-to-grain interpenetrations and grains truncated against solution surfaces. These features are used to construct plots from which the magnitude and direction of the pressure solution shortening can be determined. Using simulated pressure solution deformation of artificial and natural grain populations, the new method is shown to correctly assess a variety of coaxial and non-coaxial shortenings. Although primarily intended to determine shortening, the method can also quantify extension related to growth of beards or overgrowths during pressure solution. Application of the method to naturally deformed quartz arenite samples shows that pressure solution shortening of up to 26% occurred during compaction and 22% during layer-parallel shortening.