Investigating the Seismic Gap in North Carolina: A Seismic Survey of the Deep River Basin in Central North Carolina

The Deep River Basin is one of a series of Triassic rift basins along the east coast of the United States that make up the Newark Supergroup (Olsen et al. 1989). The supergroup of basins formed from preliminary extension during the breakup of Pangaea before the Atlantic Ocean began to spread (Olsen et al. 1989). The Deep River Basin consists of three sub-basins, the Wadesboro sub-basin, the Sanford sub-basin, and the Durham sub-basin (Olsen et al. 1989). A bed of shale and coal known as the Cumnock formation surfaces along the northern margin of the Sanford sub-basin (Olsen et al. 1991). In the past, this has been exploited by coal mining operations. More recently, however, it has been the focus of local gas companies. Recent legislation suggests that hydraulic fracturing operations may occur in the Sanford sub-basin in the near future. While hydraulic fracturing itself does not cause seismicity large enough to be recorded by the type of data analyzed here, the reinjection of wastewater into basement rocks has been linked to more significant seismicity in certain areas. Although it is unknown whether or not hydraulic fracturing or the associated reinjection of wastewater will occur in the Sanford sub-basin, a baseline of natural seismicity can be recorded before operations to be used for any comparative studies of seismicity.Bachelor of Scienc

Mantle dynamics of the Andean Subduction Zone from continent-scale teleseismic S-wave tomography

The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS traveltime residuals at >1000 seismic stations that have been deployed across South America over the last 30 yr to produce a finite-frequency teleseismic S-wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ~5°N to 45°S and from depths of ~130 to 1200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ~1000 km depth and extend eastward subhorizontally for >2000 km. South of 25°S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ~400 to ~1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca-South America subduction zone remains enigmatic.Fil: Rodríguez, Emily E.. University of Arizona; Estados UnidosFil: Portner, Daniel Evan. No especifíca;Fil: Beck, Susan L.. University of Arizona; Estados UnidosFil: Rocha, Marcelo P.. Universidade do Brasília; BrasilFil: Bianchi, Marcelo B.. Universidade de Sao Paulo; BrasilFil: Assumpção, Marcelo. Universidade de Sao Paulo; BrasilFil: Ruiz, Mario. Escuela Politécnica Nacional; EcuadorFil: Alvarado, Patricia Monica. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Condori, Cristobal. Universidade do Brasília; BrasilFil: Lynner, Colton. University Of Delaware; Estados Unido

Variability in Slab Structure and Behavior Within and Among the South American and Eastern Mediterranean Subduction Systems

As subduction science advances, it is becoming increasingly clear that the conventional model of subduction driven by cold, strong, dense slabs sinking cohesively in the mantle is insufficient to explain observed heterogeneities across a range of scales. The breadth of observed features of subduction zones is expanding at the same time as along strike variability is revealed within individual subduction systems. Not surprisingly, the driving components of subduction zones, and by extension this variability, are the subducting slabs. Thus, in order to revisit the traditional model of subduction, this dissertation uses seismic imaging techniques in four distinct studies to characterize the structure and behavior of the subducting slabs across two extensive subduction systems: South America and the Eastern Mediterranean. These two systems serve as the ideal test cases for characterizing variability within and across subduction zones because they represent end-member styles of subduction: Andean-type subduction and Mediterranean-type subduction, respectively. The first study presents the results of a regional-scale teleseismic tomography inversion surrounding the Sierras Pampeanas of Argentina. This study targets the observable response of the slab-mantle interaction to subduction of the hot spot-generated Juan Fernández Ridge, identifying entrainment of hot subslab asthenosphere and induced tearing of the subducting Nazca slab. Through this study I also present a model for explaining the infrequent distribution of subslab slow seismic velocity anomalies in subduction systems globally. The second study presents the results of a continental-scale teleseismic tomography inversion across the Anatolian sub-continent in the Eastern Mediterranean. This study targets the response of the slab to the major tectonic transition seen in central Anatolia from ocean subduction in the west to continental collision in the east. The resulting seismic images show an increasingly deforming slab from west to east that illustrates the evolutionary process of subduction termination. The third study presents new software developed to automatically extract slab geometry from teleseismic tomography models. The efficacy of the software is presented through applications to the Juan de Fuca/Gorda and Nazca slabs, signifying its broad utility in interpreting tomographic images. The fourth study utilizes this new software along with a continental-scale teleseismic tomography model across South America to present a detailed model of Nazca slab geometry from the surface into the lower mantle. The results of this study reveal that the slab penetrates the 660 km discontinuity uninhibited while instead interacting with a lower mantle discontinuity. This result has far reaching implications for geodynamic models of South American evolution. Together, these studies reveal a broad consistency across subduction zones – that slabs do not remain cohesive during their descent in the mantle. Instead, the presence of heterogeneities in the downgoing plate induces slab tearing and possibly fragmenting within the upper mantle. Despite the relative complexity of the multi-plate Eastern Mediterranean system compared to the two-plate South American system, both systems display significant variability along the margins, including regions with varying degrees of slab tearing. This observation directly contradicts the traditional view of cohesive, strong slabs sinking in the mantle, which has related implications for the role of subducting slabs in mantle convection and material recycling over geological timescales

Incorporating teleseismic tomography data into models of upper mantle slab geometry

Earthquake-based models of slab geometry are limited by the distribution of earthquakes within a subducting slab, which is often heterogeneous. The fast seismic velocity signature of slabs in tomography studies is independent of the distribution of earthquakes within the slab, providing a critical constraint on slab geometry when earthquakes are absent. In order to utilize this constraint, researchers typically hand-contour images of subducting slabs in tomography models, leading to a subjective final slab model. With this paper, we present an automated procedure for extracting slab geometry from teleseismic tomography volumes that limits this subjectivity and provides constraints on the structure of aseismic segments of slabs. This procedure is designed as a complement to earthquake-based slab models rather than as a replacement, which can help to broaden the extent of existing subduction zone geometry databases.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

The nature of subslab slow velocity anomalies beneath South America

Slow seismic velocity anomalies are commonly imaged beneath subducting slabs in tomographic studies, yet a unifying explanation for their distribution has not been agreed upon. In South America two such anomalies have been imaged associated with subduction of the Nazca Ridge in Peru and the Juan Fernandez Ridge in Chile. Here we present new seismic images of the subslab slow velocity anomaly beneath Chile, which give a unique view of the nature of such anomalies. Slow seismic velocities within a large hole in the subducted Nazca slab connect with a subslab slow anomaly that appears correlated with the extent of the subducted Juan Fernandez Ridge. The hole in the slab may allow the subslab material to rise into the mantle wedge, revealing the positive buoyancy of the slow material. We propose a new model for subslab slow velocity anomalies beneath the Nazca slab related to the entrainment of hot spot material.NSF [EAR-1415914, EAR-1565475]6 month embargo; First published: 21 May 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

The nature of subslab slow velocity anomalies beneath South America

Slow seismic velocity anomalies are commonly imaged beneath subducting slabs in tomographic studies, yet a unifying explanation for their distribution has not been agreed upon. In South America two such anomalies have been imaged associated with subduction of the Nazca Ridge in Peru and the Juan Fernandez Ridge in Chile. Here we present new seismic images of the subslab slow velocity anomaly beneath Chile, which give a unique view of the nature of such anomalies. Slow seismic velocities within a large hole in the subducted Nazca slab connect with a subslab slow anomaly that appears correlated with the extent of the subducted Juan Fernandez Ridge. The hole in the slab may allow the subslab material to rise into the mantle wedge, revealing the positive buoyancy of the slow material. We propose a new model for subslab slow velocity anomalies beneath the Nazca slab related to the entrainment of hot spot material.NSF [EAR-1415914, EAR-1565475]6 month embargo; First published: 21 May 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

A tear in the subducting Nazca slab is located between the end of the Pampean flat slab and normally subducting oceanic lithosphere. Tomographic studies suggest mantle material flows through this opening. The best way to probe this hypothesis is through observations of seismic anisotropy, such as shear wave splitting. We examine patterns of shear wave splitting using data from two seismic deployments in Argentina that lay updip of the slab tear. We observe a simple pattern of plate-motion-parallel fast splitting directions, indicative of plate-motion-parallel mantle flow, beneath the majority of the stations. Our observed splitting contrasts previous observations to the north and south of the flat slab region. Since plate-motion-parallel splitting occurs only coincidentally with the slab tear, we propose mantle material flows through the opening resulting in Nazca plate-motion-parallel flow in both the subslab mantle and mantle wedge.NSF [EAR-0738935, EAR-0739001, EAR-1565475]; Colorado College Patricia Buster Scholarship Fund; National Science Foundation through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation [EAR-1261681]6 month embargo; published online: 13 July 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Subduction termination through progressive slab deformation across Eastern Mediterranean subduction zones from updated P-wave tomography beneath Anatolia

Using finite-frequency teleseismic P-wave tomography, we developed a new three-dimensional (3-D) velocity model of the mantle beneath Anatolia down to 900 km depth that reveals the structure and behavior of the sub-ducting African lithosphere beneath three convergent domains of Anatolia: the Aegean, Cyprean, and Bitlis-Zagros domains. The Aegean slab has a relatively simple structure and extends into the lower mantle; the Cyprean slab has a more complex structure, with a western section that extends to the lower mantle with a consistent dip and an eastern section that is broken up into several pieces; and the Bitlis slab appears severely deformed, with only fragments visible in the mantle transition zone and uppermost lower mantle. In addition to the subducting slabs, high-amplitude slow velocity anomalies are imaged in the shallow mantle beneath recently active volcanic centers, and a prominent fast velocity anomaly dominates the shallow mantle beneath northern Anatolia and the southern Black Sea. As a whole, our model confirms the presence of well-established slow and fast velocity anomalies in the upper mantle beneath Anatolia and motivates two major findings about Eastern Mediterranean subduction: (1) Each of the slabs penetrates into the lower mantle, making the Eastern Mediterranean unique within the Mediterranean system, and (2) the distinct character of each slab segment represents different stages of subduction termination through progressive slab deformation. Our findings on the destructive processes of subduction termination and slab detachment have significant implications for understanding of the post-detachment-behavior of subducted lithosphere.National Science Foundation [EAR-1109336]; Wiess Postdoctoral Research Fellowship at Rice University12 month embargo; published online: 09 May 2018This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

New Finite Frequency Teleseismic P wave Tomography of the Anatolian Sub continent and the Fate of the SubductedCyprean Slab

The eastern Mediterranean region is characterized by active subduction of Tethyan lithosphere beneath the Anatolian sub-continent at the Aegean and Cyprean trenches. The subduction system is historically characterized by slab roll-back, detachment, and slab settling in the mantle transition zone. Prior mantle tomography studies reveal segmentation of the subducted Tethyan lithosphere, which is thought to have a strong control on surface volcanism and uplift across Anatolia. However, tomographic resolution, particularly in central Anatolia, has been limited, thus making detailed delineations of the subducted slab segments difficult. To improve resolution, we combine two years of seismic data from the recent Continental Dynamics - Central Anatolia Tectonics (CD-CAT) seismic deployment and Turkey's national seismic network ( 33,000 residuals) to 33,000 travel time residuals from Biryol et al. (2011, GJI) in a new finite-frequency teleseismic P-wave tomographic inversion. Our new images reveal with detail a complicated geometry of fast velocity anomalies associated with subducted Tethyan lithosphere. At shallow depths, slow velocities separate the fast anomalies connected to the Aegean and Cyprean trenches. The fast anomaly connected to the Cyprean trench has an arcuate shape in map view, following the trace of the Central Taurus Mountains. This anomaly is separated from a high-amplitude block to the north that appears to dip sub-vertically throughout the upper mantle (200-660 km depth). Other blocks of fast material that may represent subducted Tethyan lithosphere appear down-dip of the vertical block. Additionally, our images indicate that some of the fast velocity anomalies previously seen to flatten in the mantle transition zone may continue into the lower mantle. Thus, our new images provide a more detailed picture of the fate of the Cyprean slab and suggest that some of the fast anomalies associated with the slab continue into the lower mantle, bringing to question the traditional view of a slab graveyard in the mantle transition zone in this region