Post-Rift Magmatic Evolution of the Eastern North American “Passive-Aggressive” Margin

Understanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from supercontinent to rifting, to post-rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia-West Virginia region of the ENAM has experienced two postrift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long-term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new Ar/ Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bimodal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr-Nd-Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a “passive-aggressive” margin that records multiple magmatic events long after rifting ended

3-D crustal structure along the North Anatolian Fault Zone in north-central Anatolia revealed by local earthquake tomography

3-D P-wave velocity structure and Vp/Vs variations in the crust along the North Anatolian Fault Zone (NAFZ) in north-central Anatolia were investigated by the inversion of local P- and S-wave traveltimes, to gain a better understanding of the seismological characteristics of the region. The 3-D local earthquake tomography inversions included 5444 P- and 3200 S-wave readings obtained from 168 well-located earthquakes between 2006 January and 2008 May. Dense ray coverage yields good resolution, particularly in the central part of the study area. The 3-D Vp and Vp/Vs tomographic images reveal clear correlations with both the surface geology and significant tectonic units in the region. We observed the lower limit of the seismogenic zone for north-central Anatolia at 15 km depth. Final earthquake locations display a distributed pattern throughout the study area, with most of the earthquakes occurring on the major splays of the NAFZ, rather than its master strand. We identify three major high-velocity blocks in the mid-crust separated by the Izmir-Ankara-Erzincan Suture and interpret these blocks to be continental basement fragments that were accreted onto the margin following the closure of Neo-Tethyan Ocean. These basement blocks may have in part influenced the rupture propagations of the historical 1939, 1942 and 1943 earthquakes. In addition, large variations in the Vp/Vs ratio in the mid-crust were observed and have been correlated with the varying fluid contents of the existing lithologies and related tectonic structures

Structure of the crust and African slab beneath the central Anatolian plateau from receiver functions: New insights on isostatic compensation and slab dynamics

The central Anatolian plateau in Turkey is a region with a long history of subduction, continental collision, accretion of continental fragments, and slab tearing and/or breakoff and tectonic escape. Central Anatolia is currently characterized as a nascent plateau with widespread Neogene volcanism and predominantly transtensional deformation. To elucidate the present-day crustal and upper mantle structure of this region, teleseismic receiver functions were calculated from 500 seismic events recorded on 92 temporary and permanent broadband seismic stations. Overall, we see a good correlation between crustal thickness and elevation throughout central Anatolia, indicating that the crust may be well compensated throughout the region. We observe the thickest crust beneath the Taurus Mountains (>40 km); it thins rapidly to the south in the Adana Basin and Arabian plate and to the northwest across the Inner Tauride suture beneath the Tuz Gölü Basin and Kırşehir block. Within the Central Anatolian Volcanic Province, we observe several low seismic velocity layers ranging from 15 to 25 km depth that spatially correlate with the Neogene volcanism in the region, and may represent crustal magma reservoirs. Beneath the central Taurus Mountains, we observe a positive amplitude, subhorizontal receiver function arrival below the Anatolian continental Moho at ∼50–80 km that we interpret as the gently dipping Moho of the subducting African lithosphere abruptly ending near the northernmost extent of the central Taurus Mountains. We suggest that the uplift of the central Taurus Mountains (∼2 km since 8 Ma), which are capped by flat-lying carbonates of late Miocene marine units, can be explained by an isostatic uplift during the late Miocene–Pliocene followed by slab breakoff and subsequent rebound coeval with the onset of faster uplift rates during the late Pliocene–early Pleistocene. The Moho signature of the subducting African lithosphere terminates near the southernmost extent of the Central Anatolian Volcanic Province, where geochemical signatures in the Quaternary volcanics indicate that asthenospheric material is rising to shallow mantle depths

Overriding plate, mantle wedge, slab, and subslab contributions to seismic anisotropy beneath the northern Central Andean Plateau

The Central Andean Plateau, the second-highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases (SKS, SKKS, PKS, and sSKS), local S, and source-side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench-parallel to trench-oblique, contradicting predictions from the simplest two-dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two-dimensional mantle flow regime

Eskipazar havzasının evrimi ve neotektoniği,Karabük-Türkiye.

Study area, the Eskipazar Basin, is located in the western part of the North Anatolian Fault System. It is a 3-5 km wide, 10 km long and NWSE trending depression, bounded by a complex array of oblique-slip normal faults and strike-slip faults. The Eskipazar Basin is interpreted to be a superimposed basin. The basin fill is composed of two different units deposited under the control of different tectonic regimes, namely the paleotectonic and the neotectonic regimes. The latest paleotectonic fill of the basin is the fluvio-lacustrine deposits of the paleotectonic Eskipazar formation. This formation is unconformably overlain by a group of neotectonic units namely, the Budaklar, the Karkin and the Imanlar formations. The unconformity in between these paleotectonic and neotectonic units represents the time interval during which the paleotectonic period comes to end and the neotectonic period started. Thus, onset age of the strike-slip neotectonic regime in the study area is Late Pliocene (~2.6 My). Common basin margin-bounding faults of the Eskipazar Basin are the Kadilar fault set, the Beytarla Fault Zone, the Budaklar fault set, the Arslanlar fault set, the Dibek fault, the Karkin fault, the Boztepe fault and the Acisu fault. These faults display well preserved fault scarps, in places. Morphological expressions of these faults and their geometrical relationships to regional stress system indicate that these faults are mostlystrike-slip faults with normal component. However the Kadilar fault set displays a different characteristic, being the major fault controlling the basin to the west and it is indeed an oblique slip normal fault. Long term seismicity and their epicentral distribution in and very close to the study area suggest that the Eskipazar basin is located in an area of seismic quiescence, nevertheless the morphotectonic expressions of the faults exposing in theM.S. - Master of Scienc

Seismicity, focal mechanisms and active stress field around the central segment of the North Anatolian Fault in Turkey

We analysed locations and focal mechanisms of events with magnitude >= 3, which are recorded by 39 broad-band seismic stations deployed during the North Anatolian Passive Seismic Experiment (2005-2008) around central segment of the North Anatolian Fault (NAF). Using P- and S-arrival times, earthquakes are relocated and a new 1-D seismic velocity model of the region is derived. Relocated events in the area are mainly limited to a depth of 15 km and present seismicity in the southern block indicates widespread continental deformation. In the next step, focal mechanisms are derived from first motions (P, SH) and amplitude ratios (SH/P) using a grid-search algorithm in an iterative scheme. Analysis of our well-constrained focal mechanisms indicate mainly strike-slip motions apart from some normal and few thrust events that are related to complex local fault geometry. Calculated pressure/tension axes are mainly subhorizontal and maximum horizontal stress directions (SH max) are oriented predominantly in NW-SE direction which corresponds well with the slip character of NAF and its splays. In the east, E-W trending splays show right-lateral strike-slip mechanisms similar to the main strand whereas in the west, antithetic N-S trending faults show left lateral strike-slip motions. The seismic cluster that converged near Corum after relocation indicates a dominant right-lateral strike-slip mechanism along the E-W trending fault. These focal mechanisms are used to perform stress tensor inversion across the region to map out the stress field in detail. Overall, maximum (sigma(1)) and minimum (sigma(3)) principal stresses are found to be subhorizontal and the intermediate principle stress (sigma(2)) is vertically orientated, consistent with a dominant strike-slip regime. These directions point to the clockwise rotation of stress trajectories from N to S where NW-SE directed sigma(1) in the north turns towards N-S in the south away from the NAF. Moreover, the 200-km-long Ezinepazar-Sungurlu Fault which is previously mapped as an active strike-slip fault is characterized by minor seismic activity and trends perpendicular to the computed maximum stress direction in the southwest away from the main strand of NAF suggesting that the Sungurlu segment is either compressional in nature or inactive

Segmented African lithosphere beneath the Anatolian region inferred from teleseismic P-wave tomography

P>Lithospheric deformation throughout Anatolia, a part of the Alpine-Himalayan orogenic belt, is controlled mainly by collision-related tectonic escape of the Anatolian Plate and subduction roll-back along the Aegean Subduction Zone. We study the deeper lithosphere and mantle structure of Anatolia using teleseismic, finite-frequency, P-wave traveltime tomography. We use data from several temporary and permanent seismic networks deployed in the region. Approximately 34 000 P-wave relative traveltime residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms reveal segmented fast seismic anomalies beneath Anatolia that corresponds to the subducted portion of the African lithosphere along the Cyprean and the Aegean trenches. We identify these anomalies as the subducted Aegean and the Cyprus slabs that are separated from each other by a gap as wide as 300 km beneath Western Anatolia. This gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere. The eastern termination of the subducting African lithosphere is located near the transition from central Anatolia to the Eastern Anatolian Plateau or Arabian-Eurasian collision front that is underlain by large volumes of hot, underplating asthenosphere marked by slow velocity perturbations. Our tomograms also show fast velocity perturbations at shallow depths beneath northwestern Anatolia that sharply terminates towards the south at the North Anatolian Fault Zone (NAFZ). The associated velocity contrast across the NAFZ persists down to a depth of 100-150 km. Hence, our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary