Structural Evolution of the Mcdowell Mountains Maricopa County
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Structural Evolution of the McDowell Mountains Maricopa County, Arizona by Brad Vance A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science Approved November 2012 by the Graduate Supervisory Committee: Stephen Reynolds, Chair Steven Semken Edmund Stump ARIZONA STATE UNIVERSITY December 2012 ABSTRACT The accretion of juvenile island-arc lithosphere by convergent tectonism during the Paleoproterozoic, in conjunction with felsic volcanism, resulted in the assembly, ductile to partial brittle deformation, uplift, and northwest-directed thrusting of rocks in the McDowell Mountains region and adjacent areas in the Mazatzal Orogenic belt. Utilizing lithologic characteristics and petrographic analysis of the Proterozoic bedrock, a correlation to the Alder series was established, revising the stratigraphic sequences described by earlier works. The central fold belt, composed of an open, asymmetric syncline and an overturned, isoclinal anticline, is cut by an axial-plane parallel reactivated thrust zone that is intruded by a deformed Paleoproterozoic mafic dike. Finite strain analyses of fold geometries, shear fabrics, foliations, fold vergence, and strained clasts point to Paleoproterozoic northwest-directed thrusting associated with the Mazatzal orogen at approximately 1650 million years ago. Previous studies constrained the regional P-T conditions to at least the upper andalusite-kyanite boundary at peak metamorphic conditions, which ranged from 4-6 kilobars and 350-450⁰ Celsius, although the plasticity of deformation in a large anticlinal core suggests that this represents the low end of the P-T conditions. Subsequent to deformation, the rocks were intruded by several granitoid plutons, likely of Mesoproterozoic age (1300-1400 Ma). A detailed analysis of Proterozoic strain solidly places the structure of the McDowell Mountains within the confines of the Mazatzal Orogeny, pending any contradictory geochronological data. i ACKNOWLEDGMENTS I extend a heartfelt thank you to my advisor and mentor, Stephen J. Reynolds, for his good-natured hands-off approach that allowed me to sort things out on my own, and for stepping in with his extensive expertise at the perfect times when I most needed it. Many thanks to my committee members, Steve Semken and Ed Stump, for their invaluable input along the way; to Paul Knauth for his open-door policy and insight into microstructures; to Julia Johnson, the perfect commiserative cohort and ear to the ground, who afforded me not only the first opportunity to teach, but great insight as well; to Dr. William M. Jacobsen, MD, whose flexibility and support over these years were instrumental in achieving my goals; and of course to all my professors for preparing me for such an undertaking. My appreciation also goes to Steven J. Skotnicki for his prior and current work in the McDowell Mountains, and for his enthusiasm for understanding the history of these elegant mountains. My mother Billie, brother Sean, and sister Shelly deserve special recognition not only for their encouragement, but simply for whom they are and what they have always meant to me. You are my inspiration. Finally, my most sincere gratitude to you, Daniel Wright, partner and best friend, for your unfailing support and unflinching criticism, without which I could not have succeeded. ii TABLE OF CONTENTS Page LIST OF TABLES ....................................................................................... vi LIST OF FIGURES ..................................................................................... vii INTRODUCTION......................................................................................... 1 Purpose and Scope .............................................................................. 1 Geographic Setting ............................................................................. 1 Physiography....................................................................................... 2 Previous Work .................................................................................... 7 Methods of Investigation .................................................................... 9 Conventions ........................................................................................ 9 Abbreviations and Nomenclature ..................................................... 10 GEOLOGY AND STRATIGRAPHY ....................................................... 11 General Bedrock Geology ................................................................ 11 Surficial Geology .............................................................................. 12 Stratigraphy ....................................................................................... 12 Paleoproterozoic Rocks .....................................................16 Mesoproterozoic Rocks .....................................................29 Tertiary Rocks ...................................................................32 STRUCTURE .............................................................................................. 34 General Statement ............................................................................. 34 iii Page Bedding ............................................................................................. 34 Foliation ............................................................................................ 35 Lineations .......................................................................................... 42 Faults ................................................................................................. 45 Hydrothermal Veins.......................................................................... 45 Joints .............................................................................................46 Strained Clasts .................................................................................. 48 Shear Fabrics ..................................................................................... 55 Central Fold Belt ............................................................................... 55 Horseshoe Ridge syncline ..................................................55 Dixie Mine anticline ..........................................................58 Dixie Mine .........................................................................63 Microstructure ................................................................................... 66 DISCUSSION ............................................................................................. 74 General Statement ............................................................................. 74 Proterozoic Accretion ....................................................................... 74 Volcanism and Sedimentation .......................................................... 79 Metamorphism .................................................................................. 80 Mazatzal Orogeny ............................................................................. 82 Fold Relationships .............................................................86 Shear Bands and Heterogeneous Softening .......................91 iv Page Intrusives ........................................................................................... 92 Unconstrained Structures .................................................................. 93 Tertiary Structure .............................................................................. 93 CONCLUSIONS ......................................................................................... 96 REFERENCES ............................................................................................ 98 APPENDIX A ........................................................................................... 102 FOLIATION AND BEDDING DATA TABLES ......................... 103 APPENDIX B............................................................................................ 112 PETROGRAPHIC THIN SECTIONS........................................... 113 APPENDIX C............................................................................................ 114 GEOLOGIC MAP OF THE MCDOWELL MOUNTAINS ........ 115 v LIST OF TABLES Table Page 1. Summary of Foliation and Fold Orientations ..................................... 47 2. Ratios of Strain Axes in Deformed Clasts .......................................... 52 3. Ri and Rf Strain Axes Ratios of the Metaconglomerate Clasts ......... 52 4. Sample Locations in the metasedimentary, metavolcanic, and felsite units ..................................................................................................... 69 vi LIST OF FIGURES Figure Page 1. Regional map and study area ................................................................ 3 2. Shaded relief map of elevation ............................................................... 4 3. Facing NW from the Dixie Mine ridge ................................................... 5 4. View of Thompson Peak ........................................................................ 6 5. Geographic features map ........................................................................ 7 6. Generalized stratigraphy ..................................................................... 13 7. Red quartzite . ....................................................................................... 24 8. Red and gray quartzite contact ............................................................ 25 9. Felsic member of