Determining the Geometry and Former Extent of the North Mountain Thrust from Fluid Inclusion and Microstructural Analysis
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DETERMINING THE GEOMETRY AND FORMER EXTENT OF THE NORTH MOUNTAIN THRUST FROM FLUID INCLUSION AND MICROSTRUCTURAL ANALYSES Megan Castles A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE May 2010 Committee: Charles Onasch, Advisor James Evans John Farver ii ABSTRACT Charles Onasch, Advisor Fluid inclusion microthermometry, microstructural analysis, and vitrinite reflectance measurements were used to determine pressure and temperature conditions in several Paleozoic rock units in the Valley and Ridge province in order to determine the former extent and geometry of the North Mountain thrust sheet. Overburden thicknesses determined for the Devonian Hampshire/Catskill Formation, Mississippian Pocono Sandstone, and Pennsylvanian Pottsville Sandstone were found to be 5.5-6.0, 4.5-10.5, and 4.8-8.0 km, respectively. Differences in salinity and CH4 content between the Hampshire/Catskill Formation and the Pocono Sandstone and Pottsville Formation indicate that the Hampshire/Catskill Formation was affected by fluids from a different source than the younger units. These fluids were also cooler than those in overlying units, which explains why the older Hampshire/Catskill Formation yielded lower overburden thicknesses. All units experienced considerably greater overburdens than can be explained stratigraphically, which supports the model of Evans (1989) that the North Mountain thrust once extended over these rocks so that the large overburdens are of tectonic origin. iii This paper is dedicated to my family and loved ones. Especially my parents Kevin and Mary Castles who have always had faith in my abilities and pride in my accomplishments, I could not have completed this project without their love and support. I would also like to dedicate this paper to my grandfather William Sheehan who sadly passed away while I was in graduate school, I love and miss you Grampy and wish you could be here with me to celebrate my accomplishment. iv ACKNOWLEDGMENTS I would first like to thank my advisor Charles Onasch, without whose patience, dedication and guidance I would never have completed this project. I would also like to thank John Farver and Jim Evans for taking time in their busy schedules to be on my committee. I would like to thank the Geology Department at BGSU and the Geological Society of America for funding my research. Special thanks to James Hower for analyzing and measuring my vitrinite samples. A final thank you to my fiancé Mark Humphrey and friends Laura Webb, Erin Sullivan,Will Emery, Colleen O’Shea, Jessica Lawrence, Kelsey Garner, Gary Michelfelder and Aaryn Goldbaum for their love, friendship, support and most importantly comic relief though this long process. v TABLE OF CONTENTS Page CHAPTER I. INTRODUCTION .......................................................................................... 1 CHAPTER II. BACKGROUND .......................................................................................... 2 CHAPTER III. METHODS .................................................................................................. 21 CHAPTER IV. RESULTS .................................................................................................... 27 CHAPTER V. DISCUSSION ............................................................................................... 41 CHAPTER VI. SUMMARY ................................................................................................ 52 CHAPTER VII. CONCLUSIONS ........................................................................................ 53 REFERENCES ...................................................................................................................... 54 APPENDIX A. SAMPLE LOCATIONS AND TYPE OF ANALYSIS PERFORMED ..... 56 APPENDIX B. HISTOGRAMS OF FLUID INCLUSION DATA...................................... 58 APPENDIX C. PRESSURE/TEMPERATURE DIAGRAMS ............................................. 75 vi LIST OF FIGURES Figure Page 1 Geologic map of the study area showing sample locations and type of analysis performed on samples at each location. ..................................................................... 4 2 Partial stratigraphic column for study area showing detachment zones (large horizontal arrows) and hydrostratigraphic units. ....................................................... 7 3 Geometry of the NMT interpreted from Evans (1989). ........................................... 11 4 Geometry of the NMT interpreted from Kulander and Dean (1986) ......................... 13 5 Geometry of NMT interpreted from Mitra (1986). .................................................... 13 6 Photomicrograph of quartz vein from Pocono Sandstone taken with CL microscope 24 7 Examples of primary fluid inclusions found in the Pocono Sandstone ..................... 26 8 VX plot of CH4-rich inclusions showing that inclusions are >90% methane ........... 29 9 Relationship between two-phase aqueous inclusions (with bubble) and single- phase methane inclusions (grey, no bubble) at room temperature as observed in a sample from the Pocono Sandstone (Sample TH 5). ................................................. 31 10 Example of Pressure/temperature diagram showing trapping pressures determined from the ThA-isochore intersection method for Pocono Sandstone, in central portion of study area. ................................................................................................. 32 11 Example of Pressure/Temperature diagram showing range of trapping pressures and temperatures as determined from the Geothermal Gradient methodfor the Hampshire/Catskill Formation, cental part of study area. ......................................... 33 12 Example of pressure/temperature diagram showing trapping pressures as determined from ThH isochore method for the Pocono Sandstone, in eastern portion vii of study area ............................................................................................................ 34 13 Common quartz microstructures found in the Pocono Sandstone (TH 48) ............... 36 14 Abundance of deformation mechanisms as a function of location relative to the NMT trace (east), Broadtop synclinorium/Sideling Hill (central) and the Alleghany front (west). .............................................................................................. 37 15 Abundance of each deformation mechanism as a function of rock unit within the Broadtop synclinorium/Sideling Hill region .............................................................. 38 16 Correlation between %R (mean) and LOM (Level of Metamorphism) for Pocono Sandstone samples collected in eastern portion of study area ................................... 40 17 Relationship between LOM, burial time in millions of years and maxium temperature in °C for vitrinite samples collected from the Pocono Sandstone the eastern portion of the study area. ............................................................................... 40 viii LIST OF TABLES Table Page 1 Summary of abbreviations used for fluid inclusions measurements ................... 15 2 Summary of modal homogenization temperatures determined from fluid inclusion microthermometry ................................................................................ 28 3 Summary of TmA and corresponding wt. % NaCl equivalent, as well as Te values of primary fluid inclusions measured from each rock unit in its given locality. ............................................................................................................ 30 4 Summary of trapping pressure/temperature determinations by region, rock type, and method .................................................................................................. 35 5 Summary of overburden thicknesses for each rock unit and region determined from fluid inclusion data using each of the three methods (see text for descriptions) assuming a hydrostatic pressure gradient ....................................... 43 6 Maximum burial temperatures and overburden thicknesses from vitrinite reflectance measurements from the Pocono Sandstone ....................................... 48 7 Summary of the overburden determinations in km derived from each method used in this study for each rock unit in central and eastern portions of the study area ............................................................................................................ 49 1 CHAPTER I. INTRODUCTION The central and southern Appalachians are dominated by a series of imbricated thrust sheets consisting of Paleozoic sedimentary rocks that have been displaced northwestward over a gently sloping basement of Precambrian crystalline rocks (Evans, 1989). Of the major sheets, the North Mountain thrust (NMT), is the most significant in terms of extent and displacement (Evans, 1989). Despite its importance, its geometry, history, and relationship to other thrusts in the region have been the subject of considerable debate (Evans, 1989). Thrust sheets can advance either by bodily displacing hanging wall rocks over footwall rocks or by transferring hanging wall displacement in the rear of the sheet to homogeneous shortening in the front of the sheet (Davis and Reynolds, 1984). In the case of the NMT, both models have been proposed for the displacement history. Evans (1989) proposed that a significant amount of displacement took place up the upper ramp, which placed a portion of the sheet on top of Paleozoic rocks