Seismic Anisotropy Across the Appalachian Mountains

The Pennsylvania State University The Graduate School College of Earth and Mineral Sciences SEISMIC ANISOTROPY ACROSS THE APPALACHIAN MOUNTAINS AND PLATEAU A Thesis in Geosciences by Austin White-Gaynor c 2015 Austin White-Gaynor Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2015 ii" " " " " " " The thesis of Austin White-Gaynor was reviewed and approved* by the following: Andrew Nyblade Professor in Geosciences Thesis Adviser Charles J. Ammon Professor in Geosciences Sridhar Anandakrishnan Professor in Geosciences Demian Saffer Professor in Geosciences Interim Associate Head of Graduate Programs and Research *Signatures on file in the Graduate School iii Abstract Few modern mountain ranges have as extensive of a deformational history as the Appalachian Mountains. Running nearly the entire length of the eastern coast of North America, the Appalachian Mountains are the result of a roughly 500 Ma Wilson Cycle. Geochemical and geophysical evidence points toward a diachronous orogenic evolution of northern and southern segments of the mountain belt. I characterize seismic anisotropy in the mantle throughout the central Appalachian region, the boundary between the northern and southern segments, in order to identify any first-order lithospheric changes between the two segments. Shear-wave splitting measurements are made throughout the region, and while delay times are similar everywhere (average = 0.87 seconds), fast azimuth directions (φ) change from roughly E-W in the eastern part of the study area rotating smoothly to NE-SW in the western part of the study region, across the Allegheny Front. Extended analysis was performed at three permanent stations located in the eastern, western and central part of the study area. These results show complex, or multilayered, anisotropy in the eastern portion of the study area and less complex, likely single layered anisotropy in the western portion of the study area. The top layer of the multilayered case has a φ of 60◦, roughly normal to the continental margin, which can be associated with frozen-in anisotropy from Triassic rifting. This top layer does not extend into the western most part of the study area, but instead pinches-out or diminishes in magnitude to zero. The bottom layer, present at all three permanent stations, is modeled with φ = -60◦, which is sub parallel to apparent plate motion and likely produced by iv finite strain in the asthenosphere. Fast azimuth directions in the northern portion of the study show less change along longitude and remain mostly E-W oriented. These results are attributed to the large E-W shear-zone in southern New York that formed during the Alleghenian orogeny and could represent a dividing boundary between northern and southern segments of the Appalachian Mountains. v Table of Contents List of Tables ...................................... vivii List of Figures ..................................... viiviii Acknowledgments ................................... ixx Chapter 1. ....................................... 1 1.1 Introduction................................ 1 1.2 Tectonic Background . 4 1.2.1 TheGrenvilleProvince...................... 4 1.2.2 The Taconic Orogeny . 5 1.2.3 The Acadian Orogeny . 7 1.2.4 The Alleghanian Orogeny . 9 1.2.5 Continental Break Up . 11 1.3 Anisotropy . 12 1.3.1 Seismic Anisotropy . 12 1.3.2 Mantle Anisotropy . 14 1.3.3 Shear-wave Splitting . 15 1.3.4 Global and Regional Studies . 17 Chapter 2. ....................................... 20 2.1 Data and Methods . 20 vi 2.1.1 Data . 20 2.1.2 Transverse Component Energy Minimization Method . 22 2.1.3 Multiple Layer Modeling . 28 Chapter 3. ....................................... 29 3.1 Results................................... 29 3.2 Discussion................................. 34 3.3 Summary and Conclusions . 48 Bibliography ...................................... 49 Appendices . 59 Appendix A List of individual events for each station . 59 Appendix B Summed Transverse Component Energy Grid for each station . 76 Appendix C Individual results for each station . 149 Appendix D Plots of individual results for stations MVL, SSPA, and ALLY . 166 viivi List of Tables 1:#List#of#stacked#results#for#each#station#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.##32 viii List of Figures 1.1 Location of study area. 3 1.2 Appalachian promontories and embayments. 4 1.3 MapshowingextentofGrenvilleProvince.. 6 1.4 Cross-sectional diagram of the Taconic Orogeny. 7 1.5 Map showing the major elements of the Taconic Orogeny. 8 1.6 The Appalachian peri-Gondwanan realm and its major elements. 8 1.7 Schematic cross-section of the Acadian Orogeny. 9 1.8 Schematic cross-section of the Alleghanian Orogeny. 10 1.9 Map showing the major elements of the Alleghanian Orogeny. 11 1.10 Map showing the extent and location of Triassic rift basins in eastern North America. 13 1.11 P-wave and S-wave velocities (in km/s) in a single olivine crystal. 14 1.12 Schematic diagram of an incident S-wave traveling through an anisotropic medium(gold). ................................ 16 1.13 SKS raypaths at epicentral distances of 95 - 140 degrees. 17 1.14 Global shear-wave splitting results. 18 1.15 Shear-wave splitting results for central and southeastern North America. 19 2.1 Location of stations used in this investigation. 21 2.2 Event (Mw 5.5) distribution for station MVL. 23 ≥ ix 2.3 Example waveform and particle motion before and after splitting correc- tions....................................... 25 2.4 Final stack for station L60A. 26 2.5 Example waveform and particle motion for a null result. 27 3.1 Map showing splitting results for individual stations. 30 3.2 Consistency check for stations WRPS and SSPA with previously pub- lishedresults. ................................. 31 3.3 Fast Azimuth Direction (φ) vs longitude (a) and latitude (b) for all high- qualityresults. ................................ 35 3.4 Map of all shear-wave splitting results in eastern North America. 36 3.5 Fast Azimuth Direction (φ) vs. Distance from the Allegheny Front. 38 3.6 Inversion results from Levin et al. [1999]. 40 3.7 Observed vs Synthetic results at station MVL. 41 3.8 Observed vs Synthetic results at station ALLY. 42 3.9 Observed vs Synthetic results at station SSPA. 43 3.10 Developed cross-section from modeling results. 44 3.11 Previous model, new model, and observed results at station MVL. 45 3.12 Previous model, new model, and observed results at station SSPA. 45 3.13 Previous model, new model, and observed results at station ALLY. 46 3.14 Moment tensor solutions for study area taken from Herrmann et al. [2011]. 47 x Acknowledgments For this work I received much support from my faculty mentors, including Andy Nyblade, family members and friends to whom I owe a big Thank You. xi The mountains are the soul of the region. To understand the mountains is to know ourselves. – Sandra H.B. Clark 1 Chapter 1 1.1 Introduction The Appalachian Mountains exhibit remnant features from a deep tectonic history. Mountain building began in the Mesoproterozoic with the formation of the Grenville Mountains. After subsequent delamination, these mountains collapsed and formed the underlying basement rock for eastern North America [Rivers, 1997]. Atop the east- ward dipping metamorphic basement now lie the Appalachian Mountains, the result of a roughly 500 Ma Wilson Cycle comprised of three distinct orogenic phases, the Taconic, the Acadian and the Alleghenian [Stanley and Ratcli↵e, 1985]. Figure 1.1 shows the study area for this investigation, including the Allegheny Front, an escarpment dividing the Valley and Ridge Province to the east from the undeformed Allegheny Plateau to the west. The Grenville Front is a demarcation of the western extent of the Grenville Province. Substantial evidence points toward a diachronous evolution of northern and southern segments of the Appalachian Mountains which contain both throughgoing and nonthoughgoing elements (Figure 1.2). While the Taconic Suture extends the entire length of the orogen, geochronology shows a major distinction between the two segments with the northern segments incurring the oldest deformation in the west and youngest in the east [Hatcher and Odom, 1980]. The southern and central segment of the Appalachi- ans includes youngest deformation on the eastern and western flanks of the mountain 2 belt and oldest deformation in the center [Hatcher and Odom, 1980]. The boundary between the Northern and Southern Appalachians lies directly west of New York City and is also marked by the thinnest area of the orogen widening both to the north and to the south (Figure 1.2). The aim of this study is to investigate the nature of seismic anisotropy in the upper mantle beneath the central Appalachian Mountains in order to identify any first-order boundary between the northern and southern segments. Any remnant anisotropy in the lithosphere will also provide information as to how anisotropy is created and retained through such a complex deformational system. Using shear-wave splitting analyses, I characterize anisotropic fabric in the upper mantle beneath 73 broadband seismic stations spanning 10◦ longitude and 5◦ latitude encompassing Pennsylvania and portions of the surrounding states. The results of this analysis are used to further constrain deformational history of the Appalachian Mountain range. This thesis is composed of three main chapters. The remainder of this chapter includes background information on the geology and

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