T21A-1914
Ambient Noise Tomography Around the Central North Anatolian Fault, Turkey
In eastern Turkey, the ongoing convergence of the Arabian and African plates with Eurasia has resulted in
the westward extrusion of the Anatolian plate. The North Anatolian Fault (NAF), which forms the northern
margin of the Anatolian plate, appears to be a nascent continental transform plate boundary. To study the
evolution of such a plate boundary, both at the surface and at depth, we deployed a network of 39
broadband seismometers around the central portion of the NAF from July 2006 through April 2008. We use
continuous data from this network and regional stations to image crust and uppermost mantle structure with
ambient noise tomography. We compute daily cross-correlations of noise records between all station pairs
and stack them over the entire duration of the experiment, as well as in seasonal subsets, to obtain
interstation Green's functions. After selecting station pairs with high signal-to-noise ratios and measuring
interstation phase velocities, we compute phase velocity maps at periods ranging from 8--35 s. At periods
sensitive to crustal structure (T < 20 s), the phase velocity maps highlight fault structures. The region
immediately around the NAF has high velocities and the region to the south, between the NAF and the
Ezinepazari Fault Zone, has low velocities. In contrast, at longer periods, which have maximum sensitivity in
the uppermost mantle, the phase velocity maps show a different pattern: the maps are dominated by a
northwest-to-southeast decrease in phase velocities. At all periods, the phase velocity maps are similar for
winter and summer subsets of the data, indicating that seasonal variations in noise sources do not bias our
results.
http://www.geo.arizona.edu/NAF/
T21A-1915
The Seismic Stratigraphic Record of Quaternary Deformation Across the North Anatolian Fault System in Southern Marmara Sea, Turkey
We collected high-resolution multichannel seismic reflection (MCS) and chirp seismic data across the North Anatolian Fault (NAF) system in the Marmara Sea aboard the R/V K. Piri Reis during July 2008. Three 1200+ m-deep bathymetric basins are arrayed along the North strand of the NAF. This strand passes closest to Istanbul and is considered to carry most of the current and late Holocene plate motion, but other strands to the south are active and may have been more important in the past. The transverse Central Marmara Ridge, formed by a contractional anticline, separates two of the basins. Filled sedimentary basins underlie the southern shelf, and, adjacent to that shelf, the partly-filled North Imrali basin underlies a 400 m-deep platform. Our chirp data image several strands of the southern fault system, 50 km south of the northern NAF on the inner (southern) shelf, that offset strata which postdate the ~12 ka marine transgression. Another W-striking fault that deforms post-12 ka strata cuts the mid-southern shelf. A WNW-striking segment of the Imrali fault system is associated with normal-separation, 300 m-high sea floor scarps that separate the shelf from the North Imrali basin. This basin is cut by numerous NW-striking normal-separation faults, some deforming the sea floor. At least 4 complexes of shelf edge deltas, whose tops were formed near sea level or lake level, are stacked between 500 and 900 m depth in this downthrown block of the Imrali fault. The originally sub- horizontal tops of each delta are now locally progressively tilted and folded near an ENE-striking branch of the Imrali fault (known as the Yalova fault). Lacking stratigraphic control, we infer that the deltas represent glacial intervals spaced at 100 ka during the late Pleistocene. Assuming a locally constant subsidence rate, with lowstands near -90 m, and the observed 130 m vertical spacing between the deltas, subsidence rates would be ~1.3 mm/yr, and the youngest well-preserved delta would be ~320 ka (MIS10). Alternatively, it corresponds to the pronounced 420 ka glacial (MIS12). Younger deltas did not form in this area, at least not with prograding geometries, because the water depth became too great. Possibly, outer shelf anticlinal growth may have diverted the river westward, where younger deltas are preserved on the shelf. The slope between the 400 m platform and the lower flank of the NE-trending Central Marmara Ridge is dominated by north-trending and northeast-trending 1 km-wavelength folds. These folds grew through the late Quaternary interval of deposition of the imaged deltas, and they deform the seafloor. They could be secondary shortening structures, forced folds above blind normal faults, or both. Farther east along the same slope, low-angle normal faults also grew through much of late Quaternary time. These faults root above unfaulted strata, and represent a slow collapse of the escarpment into the deep basin. NE-trending thrust- folds, NW-striking normal faults, WNW-striking transtensional faults, and ENE-striking transpressional faults are all consistent with the E-W right-lateral continental transform fault system.
T21A-1916
NAF Experiment: Seismic Anisotropy Beneath Northern Anatolia From Shear-Wave Splitting
The North Anatolian Fault (NAF) is a 1400 km long transcurrent structure located at the northern margin of the westward-extruding Anatolian plate. Despite numerous geological studies along the fault, the deeper structure of this plate margin remains relatively unknown. One important question that can be addressed is the degree of coherency between the strain field in the upper crust and the upper mantle. Another unaddressed question concerns the lateral extent of the shear zone at depth associated with the North Anatolian Fault. In order to compare the deep structure of the fault with major transcurrent or transpressive structures such as the Alpine Fault (New Zealand) and the San Andreas Fault (Western North America), we analyzed mantle anisotropy beneath NAF array, a 39 station broadband passive seismic array, using shear- wave splitting observed by SKS and SKKS phases. Our results indicated that the observed mantle strain field is uniform underneath the array with consistently NE-SW trending fast directions. This trend remains parallel or sub-parallel to the absolute plate motion vectors with a no net rotation reference frame calculated using Global Strain Rate Model (GSRM). The observations of fast polarization directions agree well with the orientations of the modeled smallest principle strain rate axis (GSRM). This indicates that the upper-mantle and upper-crust are deforming coherently, suggesting the existence of either strong coupling or similar boundary conditions. We observed that the anisotropy directions do not vary across the NAF. This might suggest that the NAF and the associated plate boundary is limited to the crust and does not extend into the upper mantle. This is in contrast to observations from the southern San Andreas Fault and Alpine Fault. The uniformity of the fast polarization directions throughout the study area also favors an asthenospheric source for anisotropy rather than a lithospheric source. Lag times between the fast and slow polarizations measured for the study area varies within the range of 0.5 seconds to 1.6 seconds. Lower lag times are obtained consistently at the eastern part of the array. The lower values of delay times might suggest either a locally thinner anisotropic source or weaker anisotropy in the upper mantle.
T21A-1917
NAF Experiment: Lithospheric Structure of the Central North Anatolia From S-wave Receiver Function Analysis
The North Anatolian Fault is a nearly 1400 km long major right-lateral strike-slip fault. It forms the northern margin of the Anatolian plate, which escapes westward as a result of the convergence between the northward moving Arabian plate and the relatively stable Eurasian plate. The North Anatolian Fault, which has caused many destructive earthquakes, is a young and active continental transform boundary located in part along an old suture zone. In order to investigate the lithospheric deformation and deep structure beneath the middle portion of the North Anatolian Fault, we deployed 39 broadband seismic stations as part of the North Anatolian Fault Passive Seismic Experiment (2005-2008), in collaboration with the Middle East Technical University, Istanbul Technical University and the Bogazici University Kandilli Observatory and Earthquake Research Institute. The deeper structure beneath the region is still poorly known. Our aim is to identify the differences in the lithospheric structure along and across the fault, and to determine how deep the surface structure and deformation along this major continental transform boundary extends into the mantle. The structure and thickness of the crust and mantle lithosphere is investigated using the high-quality teleseismic data from the NAF project through S-wave receiver function analysis. The S-wave receiver function technique (using the method of Hansen et al., 2007) isolates the S-to-P conversion generated at discontinuities beneath the station. To identify the converted phases more clearly, 30 events with epicentral distances between 60-75 degrees, magnitudes greater than 6.0 and depths shallower than 60 km have been processed. The Moho discontinuity is observed at all the stations between 4-5 seconds corresponding to a crustal thickness of 35- 40 km. The results do not indicate a distinctive change in the Moho across or along the fault zone, assuming a constant Vp/Vs ratio. A negative phase corresponding to 60-70 km depth is observed in several record sections, which we tentatively identify as the lithosphere-asthenosphere boundary (LAB). This suggests that under much of the region the lithosphere is thin and we do not see a mantle signature of the NAF.
T21A-1918
The NAF Experiment: Uppermost Mantle Structure Beneath North-Central Turkey Using Pn Tomography
The 1400km-long North Anatolian Fault (NAF), located in northern Turkey, forms a transform boundary between the Anatolian and Eurasian plates, and is capable of large destructive earthquakes. Located in part on an old suture zone, continental basement material is found north of the NAF, while accretionary complexes are found to the south. The North Anatolian Fault Passive Seismic Experiment (2005-2008), a joint project between the University of Arizona, the Middle East Technical University, Istanbul Technical University and Bogazici University Kandilli Observatory, contained 39 broadband seismometers used to investigate the lithospheric structure of this region. Using 24 regional earthquakes located up to ~1000 km from the network, we have identified over 900 Pn phases in order to investigate the structure of the uppermost mantle. The Pn phase is a compressional wave that travels through the crust, refracts into the mantle, and then propagates through the mantle lid at mantle velocities. Using a tomography code developed by Hearn (1996), Pn travel time residuals were inverted for Pn wave velocity in northern Turkey. Results show no observable contrast in Pn velocity across the North Anatolian Fault. In general, large-scale velocity variations appear to be associated with older terrain features. We have imaged a strong transition, however, from slower (~7.7-7.9 km/s) to faster (8.0-8.1 km/s) Pn velocities from east to west across the Central Anatolian Fault Zone (CAFZ), which is associated with the paleotectonic Inner Tauride Suture (ITS). The slower velocities to the east, located beneath the Eastern Anatolian Accretionary Complex, may be associated with regions of partially molten to absent mantle lid, and potentially define the edge of a slab window in this region. Higher Pn velocities are often linked to areas having a stable mantle lid. A zone of fast velocities (>8.1 km/s) is found below the Cankiri and Safranbolu Basins, regions underlain by massifs. This zone appears to cross tectonic belts, the North Anatolian Fault, and the Intra-Pontide Suture. Finally, in the western part of our network, a region of slower Pn velocity (~7.9 km/s) appears to correspond to the location of the early Miocene Galatian volcanics.
T21A-1919
Sedimentological Fingerprints of Recent Earthquakes in Lake Sediments: A Case Study on the North Anatolian Fault (NAF), Turkey
Detection of past earthquakes spatially and temporally is vital to characterize fault systems. Seismological records and GPS measurements during the instrumental period provide precise information about time, location and magnitude of earthquakes, and also slip-rates on fault zones. However, limited observation period of these techniques prevents us from long-term faulting history. For pre-instrumental period, paleoseismic data is mainly obtained from two sources; historical records and paleoseismic trenching. Both methods have constraints in terms of time and location of fault rupturing. Historical records provide relatively precise information about time of earthquakes, however they are generally not long enough and location of rupturing is not that clear. On the other hand, paleoseismic trenching precisely locates the rupture but dating of earthquakes is still problematic because of difficulties in determining the exact boundaries of event horizons in sediment sequence. Similar to fault-related sediment traps used in conventional paleoseismic trenching, lakes have potential to record earthquakes. Since depositional environment in lakes is more stagnant compared to terrestrial environments, lake sediment sequences provide more well-preserved event horizons. Detailed dating of these horizons can improve the quality of paleoseismic records. Based on this idea, lacustrine environments have frequently been investigated to reveal paleoseismic records in different places on earth such as; Chile, Dead Sea and Switzerland. In order to contribute these investigations, sedimentology of short cores from two shallow lakes located on the NAF has been studied. This part of the NAF ruptured two times in the instrumental period, in 1942 (Ms=7.1) and in 1943 (Ms=7.3). High precision fallout radionuclide (210Pb and 137Cs) dating techniques provide us with the exact location of these dates in the sediment cores. Hence, sedimentological changes around 1940s are investigated by means of physical, mineralogical and geochemical properties of the sediments. Measurements reflecting the physical properties of the sediment include magnetic susceptibility, water content, bulk density, electrical resistivity, p-wave velocity. Mineralogical and geochemical properties have been constrained using X-ray diffraction, micro-XRF (ITRAX), loss-on-ignition, atomic carbon/nitrogen and carbon isotope ratios. Ladik Lake is located in a pull- apart basin and the main strand of NAF defines the northern margin of the lake. Rupturing on the fault is expected to affect sediment source/flux to the lake and drastic changes in sedimentation are observed in the core at depths corresponding roughly to 1940s. Boraboy Lake, which is a landslide-dammed lake in origin, is not located exactly on the fault. So, drastic changes in sediment source are not as susceptible as Ladik Lake. Instead, changes in physical properties due to cyclic loading and earthquake-triggered seiche are more probable. An unusual change in bulk density trend around 1940s without any compositional change may reflect the effect of 1942 and 1943 earthquakes in Boraboy Lake. If we understand the effects of recent earthquakes on lake sedimentation, extensive earthquake chronologies far back in time can be constructed by investigating lake sediments.
T21A-1920
Stress and Structure Induced Shear-Wave Anisotropy Along the Izmit Segment of the North Anatolian Fault Zone, Northwest Turkey
The 1999 Izmit and Düzce earthquakes represent the latest of a series of westward migrating mainshocks along the North Anatolian Fault Zone (NAFZ) starting near Erzincan, Eastern Anatolia, in 1939. The Izmit Mw=7.4 event ruptured a 140 km long segment of the NAFZ reaching from the eastern Sea of Marmara towards the Karadere segment and was extended by another 50 km towards the East only 87 days later by the Mw=7.1 Düzce earthquake. Substantial segmentation along the Izmit rupture was observed both co- and post-seismically. In this study we investigate shear-wave anisotropy along the Izmit segment of the NAFZ using seismic waveforms of ~7000 aftershocks recorded by a 36-station seismic network covering the length of the entire 1999 rupture. The aim is to discriminate stress- and structure-induced shear-wave anisotropy and to relate our findings to the local seismotectonic setting. The analysis of shear wave polarization directions and differences in fast and slow shear wave arrival times is limited to events passing strict quality-based criteria and source-receiver geometry. Azimuth, angle of incidence and rectinilarity is calculated for each event in a series of moving windows. We then discard events with rectilinarity less than 0.7 or standard deviation for azimuth and incidence angle greater than 30 and 15 degrees, respectively. Waveforms of selected events are analyzed individually on a station by station basis using a moving window technique that repeatedly performs the splitting analysis in numerous windows around the S-wave arrival. In each window the fast direction and delay time are estimated by constructing a 2x2 covariance matrix of the horizontal component seismograms and performing a grid search by iteratively moving the matrix over a range of possible fast directions and delay times. The set of estimates that best minimize the smallest eigenvalue of the covariance matrix for each window are considered for further analysis. We also calculate the degree of rectinilarity of the S-wave particle motion after delay time correction and the cross correlation coefficient of the corrected waveforms to assess the quality of the splitting measurement. A total of seven stations have a sufficient number of events passing the pre-selection process to allow for a statistically significant analysis. These stations are located in different tectonic settings along the Izmit rupture including the epicentral region, the Akyazi pull-apart Basin and the Karadere fault area where the local trend of the North Anatolian Fault is 25 degrees more northerly than the regional trend.
T21A-1921
High Resolution Multichannel Imaging of Basin Growth Along a Continental Transform: The Marmara Sea Along the North Anatolian Fault in NW Turkey
The 1500-km-long North Anatolian continental transform (NAF) accommodates the westward motion of the Anatolian platelet relative to Asia. The Marmara Trough in western Turkey is a large composite Quaternary structure that includes three main extensional basins with water depths reaching ~1200m separated by shallower ridges. Syntectonic sedimentation in the basins with highly variable sea-level-related changes in accumulation rates provide valuable time-space markers for reconstructing structural growth and basin development in the Marmara Sea. The TAMAM (Turkish-American MArmara Multichannel) Project is a collaboration between several US and Turkish research institutes. During July 2008, TAMAM collected ~2700 km of multichannel profiles in the Marmara Sea using the R/V K. Piri Reis. MCS data were sampled with a 1-ms interval on the first 72 channels with 6.25m group spacing in a 600m streamer. The source was a 45/45 cu. in. GI air gun, which was fired every 12.5 or18.75m. The gun-streamer offset was 40 or 100 m depending on water depth. Both the gun and streamer were towed at a depth of 3 or 4m. This configuration yielded high-resolution images of the stratigraphy in the Marmara Sea. TAMAM follows a recent series of impressive seismotectonic studies of the NAF in the Marmara Sea area. Previous seismic cruises focused on deep penetration MCS imaging of the overall basin structure and faulting or very high-resolution imaging of the near-surface faulting. TAMAM fills a gap in resolution imaging the stratigraphy that records the history of deformation in the basins and linkages between faults. We will present preliminary high-resolution images of the stratigraphy and tectonics beneath the Marmara Sea highlighting the following exciting observations and initial results from this experiment: 1) Improved stratigraphic correlations between the major basins, a primary goal of the experiment; 2) Clearer imaging of active faults, including the NAF, the less studied southern branch of the NAF, the Imrali fault, and numerous smaller active faults; 3) Imaging of thrusts and thrust-related folds in parts of the basin; 4) Better constraints on variations in the dip and sense of motion (transpression vs. transtension) on the upper 1-2 km of the NAF; 5) Stratigraphic boundaries in the turbiditic sections in the deep basins that may be related to interactions between tectonics and changes in sedimentation rate driven by variations in sealevel and paleoclimate; 5) Extent of gravity slides at the edges of most of the subbasins; and 6) Imaging of a stack of lowstand deltas with a relatively even vertical spacing suggesting deposition tracking the ~100 ka late Quaternary glacial cycles.
T21A-1922
Crustal Structure of the Caucasus and Caspian Region Using Gravity and Receiver Functions
The intersection of the South Caspian basin and the Caucasus is a complex area with uncertain tectonic structure. Depth/VpVs stacks and modeling of receiver functions from a broadband network in Azerbaijan suggest a Moho depth of 30-35 km and high velocity lower crust at the edge of the South Caspian in the Kura Depression. Moho depths appear to increase to the west and north and reach depths of 40-45 km under the Greater and Lesser Caucasus. Unfortunately, much of the region is masked by thick sediments and the sediment/basement contact in the Kura Depression appears to be highly irregular. Multiples within the crust and lateral variations in structure create great complexity in the receiver functions and modeled receiver functions do not fit the observed data well. We attempt to develop additional constrains on the receiver function model by using gravity modeling to infer the depth of the sediment/basement contact at a regional scale. Gravity data from EGM2008 is forward modeled using a model consisting of 4 constant density layers (water, sediment, basement, and mantle). Initially, Moho depths from the receiver functions are used as a guide. We begin with a 2D profile across the South Caspian and into the low topography of the Kura Depression. Basements depths and densities from the Saatly deep (8.3 km) borehole are also used as a constraint in the Kura Depression as well as available 2D seismic reflection profiles.
T21A-1923
Tectonics in the South Caspian region constrained through earthquake source mechanism reinterpretation
Rapid subsidence of the south Caspian Sea during the last 5Ma has resulted in one of the world's deepest basins; the onset of this subsidence is still under debate. The Caspian Basin is a aseismic, rigid block overlain by a thick (up to 28km) folded sedimentary cover. It is surrounded by active orogenic belts in a highly compressional regime. Competing theories exist regarding the Caspian's past and present tectonics. Some interpretations indicate decoupling of the base sediments and the subsequent northward subduction of relatively thick oceanic crust (or thinned continental crust) beneath the Apsheron-Balkhan sill in the middle Caspian Sea. An opposing theory suggests that a gabbro-eclogite phase change, within thick continental crust, facilitated subsidence. To test these theories, earthquake mechanisms and depths have been reinterpreted from published CMT solutions. Earthquake depths are 25-60km along the sill, and source mechanisms indicate both extensional and compressive motion. It is difficult to conclusively determine the processes involved in recent subsidence. However, the presence of both extensional and compressional source mechanisms and previous interpretations of the basement structure, from Knapp (2004) and Artyushkov (2006), cannot provide definitive evidence for northward subduction. Additionally, deep basins north and south of the sill and inconclusive gravity data, further complicate interpretations. The results of this study, in combination with previous source mechanisms studies, confirm a highly compressional regime along the southern and western boundaries of the south Caspian Sea. Abundant normal faulting and few thrust faults are seen along the Apsheron-Balkhan sill to the north. These results reflect an area exposed to intense compression from the south and may indicate the presence of lithospheric flexure parallel to the sill. Initiation of subduction beneath the sill, however, cannot be confirmed with current data. Instead, a major cause of the most recent rapid subsidence is likely a result of lithospheric flexure caused by an overburden or loading from the surrounding crust as a result of regional compression along 3 of the 4 edges of the south Caspian Sea.
T21A-1924
Geochemical and Temporal Evolution of Alkaline Magmatism Along a Subduction- Transform Edge Propagator (STEP) in SW Turkey (Isparta Angle): Mantle Response to Lithospheric Tearing in a Young Orogenic Belt
The N-S-trending basaltic volcanism stretching from the Kirka and Afyon-Suhut region in the N to the Isparta- Gölcük area in the S in SW Turkey shows an age progression from 21-17 Ma to 4.6-4.0 Ma and becomes progressively more potassic toward south in time. In three distinct groups, Seyitgazi-Kırka volcanics - SKV (21-17 Ma), Afyon-Suhut-Sandıkı volcanics - ASSV (14-8 Ma), and Isparta-Gölcük-Bucak volcanics - IGBV (4.7-4 Ma), potassic (shoshonitic) and ultrapotassic rocks show close relationships in space and time. Basaltic trachyandesites-trachyandesites and coeval rhyolites-ignimbrites in the Kirka area represent the oldest phase of alkaline volcanism in the area. Younger ASSV potassic rocks are represented by shoshonite, trachyte and trachyandesite, whereas the youngest potassic rocks of Isparta are trachytic, trachyandesitic and rhyolitic in composition. Ultrapotassic rocks belonging to all three groups are transitional between lamproitic and Roman-type, although lamproitic composition is dominant in the IGBV. The overall potassic and ultrapotassic rocks of the northern groups (SKV and ASSV) and potassic rocks of the IGBV display similar trace element characteristics. They are enriched in LILE and LREE with respect to HFSE, and show negative Nb, Ta, Ti anomalies. They have no Eu anomaly, with the exception of a rhyolite sample from Kirka. First two groups are characterized by ratios of 87Sr/86Sr=0.705219-0.707450, EpsilonNd=6.3- -0.5, 206Pb/204Pb=18.90-19.07, 207Pb/204Pb=15.65-15.85, 208Pb/204Pb=39.14- 39.63, resembling the isotopic compositions of Western Anatolian High-K CA lavas and Serbian Shoshonitic and evolved lamproitic rocks. In contrast, ultra-potassic rocks of IGBV are characterized by higher contents of CaO, MgO, Cr and Ni with a narrow range of SiO2 (47-52 wt.%). These rocks are represented by nearly primitive lamproites and resemble more primary ultrapotassic rocks of Serbia. Ultrapotassic rocks of IGBV are characterized by more restricted 87Sr/86Sr=0.7035-0.7036, positive EpsilonNd=+1.7- +2.5, 206Pb/204Pb=18.7-19.1, 207Pb/204Pb=15.69-15.75, 208Pb/204Pb=39.03-39.29. These geochemical and isotopic signatures are consistent with subduction-induced sediment recycling within the upper mantle. Variations in Sr-Nd-and Pb isotopic signatures of the contemporaneous potassic and ultrapotassic rocks are explained by melting of a heterogeneous lithospheric mantle source veined through metasomatism by previous subduction events. The Within-Plate geochemical and isotopic component appears late in the evolution of IGBV ultra-potassic rocks. Isotopic differences from N to S are consistent with decreasing amounts of subduction derived crustal components in the mantle, and an increasing role of asthenospheric component. We think the sharp cusp between the Hellenic and Cyprus trenches and the significant differences in the convergence velocities of the African lithosphere at these trenches have resulted in a lithospheric tear in the downgoing African plate that allowed the sub-slab (asthenospheric) mantle to rise beneath SW Anatolia, analogous to lithospheric tearing at Subduction-Transform Edge Propagator (STEP) faults. This upwelling induced decompressional melting of shallow asthenosphere, leading to linearly distributed alkaline magmatism younging in the direction of tear propagation (southward).
T21A-1925
Crustal Structure of the Isparta Angle, Southwestern Turkey, Using P – Wave Receiver Function Analysis
The Isparta Angle (southwestern Turkey) lies at the intersection of Hellenic and Cyprus arcs related to the collision of the African and Anatolian plates and possesses a complex structure. To further understand this complexity, we deployed a temporary network consisting of nineteen broad band instruments in addition to permanent seismic stations in the region. Crustal velocity and Moho depth variations were estimated along two profiles (N-S, and E-W) formed by the fourteen temporary and permanent stations. Crustal and lithopheric velocities and Moho depths were estimated in two ways: h-k stacks and the joint inversion of receiver functions and surface wave group velocities. Receiver functions are sensitive to shear wave velocity contrast and vertical travel times while surface wave dispersion curves are sensitive to shear wave velocity averages. Combining these different properties of shear waves ensures a more robust result. We found that upper crustal shear velocities are higher beneath stations located in the west which is 2.43 km/s compared to stations in the east which is 2.11 km/s whereas mid crustal velocities do not show a significant change along the E-W profile which is 3.48 km/s. On the other hand, there is little variation in mid crustal velocities along N-S profile which is ranging between 3.47 km/s and 3.53 km/s. The upper mantle P velocities are very low beneath E-W and N-S profiles which are compatible with the regional Pn velocity tomography studies. Upper mantle velocities do not vary along E-W profile which is 7.20 km/s, however, there is an indication of velocity increase along the N-S profile towards south from 7.08 km/s to 7.30 km/s. We observed that the crust is thicker beneath western stations (38.8 km) than the eastern ones (32.1 km). Moho depth does not vary beneath the northern stations (36.9 km). Stations along the southern profile show an increased Moho depth (43.5 km) compared to the northern stations. *This research is supported by Bogazici University Research Fund under contract number 07T203.
T21A-1926
The SIMBAAD Experiment in W-Turkey and Greece: A Dense Seismic Network to Study the Crustal and Mantle Structures
SIMBAAD (Seismic Imaging of the Mantle in the Aegean-Anatolian Domain) is a temporary seismic experiment which aims at investigating the crustal and mantle structure beneath Western Turkey, the Aegean Sea, and continental Greece. This tectonically very active region has experienced a variety of geodynamic processes and its geology and kinematics have been extensively studied. It is thus a good place to test competing hypotheses on how the surface kinematics is related to mantle structure and dynamics. In the spring of 2007, we have installed a temporary network of 33 broadband stations in Turkey, Greece, and S-Bulgaria for 2- year duration. It complements the permanent broadband networks (~90 stations) to give a regular grid with an inter-station spacing of ~100 km in the area [20-34°E; 35-41°N]. The common database of broadband continuous records that we are building from permanent and temporary networks will be the first one with such a dense spacing in this area. The temporary SIMBAAD experiment also includes 2 north-south profiles of more densely-spaced stations (~15 km) crossing Western Anatolia at 18°E and 31.5°E. A preliminary receiver-function analysis of the western transect documents a rather flat Moho at ~30 km depth with negligible variations of the crustal thickness under the major grabens and core complexes (e.g. Menderes). We interpret this result as an indication for a hot and ductile lower crust which vertically decouples upper crust deformation from mantle deformation.
T21A-1927
The Deep Electrical Conductivity Structure of the Dead Sea Basin
The left-lateral Dead Sea transform (DST) is a major transform fault separating the Arabian plate in the east from the African and Sinai plates in the west. It extends from the Red Sea rift to the Taurus collision zone in eastern Turkey, with a total length of more than 1000 km. During its evolution, the DST formed several deep sedimentary (pull-apart) basins, such as the Gulf of Aqaba / Eilat, the Lake Tiberias and the Dead Sea basin (DSB). The DSB is the largest basin along the transform valley and probably the largest of these structures on earth. The basin is 135km long, 20km wide and according to an interpretation of gravity data the basin's sedimentary fill is assessed to 10km thickness. Within the framework of the multi-disciplinary DESIRE (Dead Sea Integrated Research) project, several geophysical methods were applied to investigate the southern part of the DSB, including magnetotellurics (MT), active reflection and refraction seismic, aero-gravity, and passive seismology. The MT data were acquired at 150 stations in 2006 along a 120 km long, approximately east-west oriented profile. Resistivity models obtained from two-dimensional inversion of the MT data reveal several robust features: Beneath the Dead Sea, extending to a depth of approximately 3 km, we observe extremely low electrical resistivity (0.1- 0.5 Ω m). Embedded within this structure and located beneath the Al-Lisan Peninsula we observe a localized high resistivity body (> 100 Ω m), which is interpreted as the Al Lisan salt diapir. The lateral boundaries of the DSB are clearly expressed in the resistivity model as abrupt changes from moderately low (~20 Ω m) to high resistivity (> 1000 Ω m) at depths of 3 and 4 km under the eastern and western segment of the profile, respectively. The locations of the boundaries coincide with the surface traces of the eastern and western border faults. Furthermore, the 2D model images two conductive layers beneath both, the eastern and western segments of the profile at varying depth ranges. Based on hydro-geological information the location and depth extent of these conductive layers appear to coincide with a shallow and a deeper aquifer. With our electrical conductivity model it is possible to delineate the saline/fresh water interface, particularly at the eastern shoreline of the Dead Sea. The border faults appear to prevent cross-fault fluid flow of the Dead Sea brines.
T21A-1928
A receiver function study across the Dead Sea Basin (DSB)
Beginning in September 2006, a temporary network of 30 broadband and 45 short-period seismic stations has been set up on both sides of the Dead Sea Basin (DSB). During one and a half year of successful operation, data were continuously recorded in the field at 100 Hz and 200 Hz sample frequency for the broadband and short-period seismic stations, respectively. The raw data were converted to miniseed format and archived as full seed volume in the GEOFON data center of the GFZ. In the present work, the Receiver Function Method has been applied to the three component passive source data to investigate seismic discontinuities from the crust down to the upper mantle. Unusual negative phases at about 1s delay time have been observed at several stations in the Dead Sea region on the top of the assumed salt diapir. First preliminary receiver function analysis reveals a crustal thickness of about 30 -35 km in the investigated area and possibly low-velocity layer beneath the Moho. It also shows a basin which is possibly filled with salt about 10 km thick beneath the Lisan peninsula (Dead Sea).
T21A-1929
The DESIRE Airborne gravity project in the Dead Sea Basin and 3D numerical gravity modeling
This geo-scientific research focuses on the geological setting of the Dead Sea Transform (DST) and the Dead Sea Basin (DSB) and its resulting pull-apart basins. Since the late 1970s, crustal scale geophysical experiments have been carried out in this region. However, the nature of the crust underlying the eastern and western shoulders of the DSB and underneath the DST itself is still a hotly debated topic among researchers. To address one of the central questions of plate tectonics – How do large transform systems work and what are their typical features? – An international geoscientific Dead Sea Integrated Research project (DESIRE) is being conducted by colleagues from Germany, Israel, Palestine, and Jordan. In order to provide a high resolution gravity database that support 3D numerical modeling and hence a more comprehensive understanding of the nature and segmentation of the DST, an airborne gravity survey as a part of the DESIRE project has been carried out from February to March 2007. The airborne gravity survey covered the DST from Elat/Aqaba in the South to the northern rim of the Dead Sea. The low speed and terrain-following helicopter gravity flights were performed to acquire the highest possible data quality. In total, 32 north-south profiles and 16 west-east profiles crossing the DST have been measured. Most of the profiles concentrated in areas that lacked terrestrial gravity data coverage, e. g. over the shoulders of the DSB. The airborne gravity data are merged with existing conventional (terrestrial) data sets to provide a seamless gravity map of the area of interest. Using that combined gravity dataset and DESIRE wide angle refractions seismic interpretation we modified density structures in the DSB. As results we estimated that (1) the Moho depth varies from 26 km in the Israel side to 34 km in the Jordan side. (2) The maximum thickness of the Dead Sea sediment Basin is about 15 km. (3) The salt rock with an average thickness of about 5 km is present not only in the southern part of the DSB but also in northern part underlying the entire Dead Sea.
T21A-1930
First results from a temporary seismological network in the Southern Dead Sea area
Within the framework of the international project DESIRE (Dead Sea Integrated Research Project) a local seismological network was operated in the Southern Dead Sea area as a co-operation between the GFZ Germany, GII Israel, NRA Jordan and An-Najah National Univer-sity Palestine. From October 2006 to March 2008 about 65 short period (38) and broadband (27) instruments recorded continuously the seismicity of the Dead Sea basin. This investiga-tion aims in studying the deeper structure of the Dead Sea area based on the distribution of the local seismicity. About 500 local events have been recorded and more than 300 have been processed up to now. A dominant feature in this first part of the dataset we found a cluster of 78 earthquakes, occurring in February 2007, including multiplets. We determined a 1D-reference model of P- and S-velocities using Velest (Kissling et al., 1994). The model shows a high velocity increase between 6 and 10 km depth. This could be related to a prominent reflector found in the results of the wide angle reflection experiment in the area in 2006 (Mechie et al., 2008). The station corrections suggest a 2D structure with the basin in the middle and the shoulders on the east and west. Additionally the results are compared with receiver function and magnetotelluric studies, part of the DESIRE project.
T21A-1931
Regional Crustal Velocity Models for Northern Arabian Platform and Turkish-Iranian Plateau
The geological structure of the Northern Arabian platform and surrounding mountains is dominated by the collision and suturing of the Arabian plate with the Eurasian plate and the formation of the Turkish-Iranian plateau. The structure of the Northern Arabian platform and surrounding region is poorly constrained. A recent deployment of 10 broadband seismometers in northern and central Iraq provides an opportunity to refine velocity models of the region. We have applied the Niching Genetic Algorithm waveform inversion technique to Rayleigh and Love waves traversing the Northern Arabian platform, the Zagros fold belt, the southern Turkish Plateau, the Iranian Plateau. Results show variations in crustal thickness and shear wave velocity between the Northern Arabian platform and the Turkish-Iranian plateau. In general the shear wave velocities are higher in the Northern Arabian platform than in the Plateaus. Variation of shear velocities within each of the provinces reflects the diversity in tectonic environment across the Zagros fold belt and the complex tectonic history of the region. Crustal thickness results show little crustal thickening has occurred due to collision.
T21A-1932
CRUSTAL STRUCTURE OF THE NORTHERN ZAGROS ZONE FROM SEISMIC OBSERVATIONS
The current study is concerned with estimating the local and regional crustal structure of north and northeastern Iraq, including the northern extension of the Zagros collision zone. The goal of our work is to derive local and regional seismic velocity structures using receiver function- and surface wave dispersion analyses and to use these velocity models to obtain accurate hypocenter locations and event focal mechanisms. Global seismic network coverage in this region is poor and extrapolated velocity models found in the literature lack sufficient accuracy to permit events to be located with significant precision. Ten three- component broadband stations composing the North Iraq Seismographic Network (NISN) were installed in late 2005. At present, over 650 GB of seismic waveform data have been analyzed. Our analysis of waveform data indicates clear propagation paths from the south or west across the Arabian shield as well as from the north into NISN. Phases including Pn, Pg, Sn, Lg, as well as LR are clearly observed on these seismograms. In contrast, blockage or attenuation of S-wave energy is observed for propagation paths across the Zagros- Bitlis zone from the east, as well as along the axis of the Zagros from the south-east. Supporting these findings are the results of surface wave analysis. Waves generated by events located to the south of NISN propagating across the Arabian shield produce dispersion curves with energy distributed over a broad frequency band including the development of higher modes. In contrast, waves from events to the south-east which propagate along the axis of the Zagros mountains generate dispersion curves that diverge over a broad frequency range, indicating multi-pathing caused by the complex structure of the Zagros zone. These results are corroborated by receiver function analyses which indicate a dipping Moho beneath the Zagros mountains. While the Moho below the foothills is estimated at 40-50 km depth, it dips to 45-55 km depth below the northern extension of the Zagros zone. Furthermore, lower-than-average shear wave velocities were found for north-east Iraq when compared to other crustal regions of the Earth. Common among the receiver functions is the presence of a significant velocity discontinuity at a depth of 15 km and 20 km for the stations below the foothills and Zagros mountains, respectively. The increase in velocity across this discontinuity lead to the observation of mid-crustal refracted body waves throughout NISN.
T21A-1933
Geochemistry of Quaternary Olivine Basalts From the Lut Block, Eastern Iran
Samples were collected from more than 40 small monogenetic Quaternary volcanic cones, identified using available geologic maps and satellite images, which are located along the four margins of the Lut Block in eastern Iran. The Lut Block is a ~900 km north-south by ~200 km east-west desert largely covered by Tertiary volcanic rocks, with granitoid plutonic bodies exposed in some areas. The complex regional tectonic evolution of the Lut Block microplate involves the closure of the Paleotethys ocean during collision with the Eurasian plate to the north, the closure of the Neotethys ocean during collision of the Arabian plate from the west, and ongoing subduction of the Indian oceanic plate from the south below the active Makran arc of large stratovolcanoes such as Bazman and Taftan. Several major north-south strike-slip fault systems occur along the western and eastern margins of this block, and they extend through the Makran arc to the south. The samples are mainly olivine (Fo = 63-86) basalts. Contents of MgO = 4.31 to 8.62 wt %, Ni = 35 to 180 ppm and Cr = 35 to 280 ppm indicate that some of the samples crystallized from relatively primitive mantle-derived magmas. Mantle peridotite and deep-crustal granulite xenoliths are present at one locality. In samples from small parasitic cones associated with the large Makran arc stratovolcanoes along the southern margin of the block, contents of TiO2 range from 0.9 to 1.11 wt %. In contrast, the range of TiO2 contents for samples associated with the strike-slip faults along the eastern and western margins of the Lut block to the north of the arc is significantly higher, from 1.88 to 2.65 wt %. Based on HFS elements (Nb/Y versus Zr/TiO2), all samples from the western and eastern margins of the block are alkali basalts, while the samples from the south are subalkaline basalts. Other geotectonic chemical discrimination diagram, such as those based on Th-Hf-Nb or Th-Zr-Nb, indicate that the samples from the south are calc-alkaline basalts and those from the eastern and western margins of the block are within-plate alkali basalts. The ratios of La versus La/Nb and Ce/Pb versus Nb/U also reflect the different tectonic setting for these samples. Based on these preliminary results, Quaternary calc-alkaline volcanism along the southern margin of the Lut block is related to oceanic plate subduction, while the alkali basalts along the eastern and western margins of the area form by melting of the asthenosphere unmodified by subducted components. Their parental magmas rose to the surface along the very deep strike-slip faults that border the block without significant interaction with the continental crust. Keywords: olivine basalts, Lut block, Iran, peridotite xenoliths, Makran arc
T21A-1934
Oroclinal bending, distributed thrust and strike-slip faulting, and the accomodation of Arabia-Eurasia convergence in NE Iran since the Oligocene
Regional shortening is accommodated across NE Iran in response to the collision of Arabia with Eurasia. We examine how N-S shortening is accommodated on major thrust systems bounding the eastern branch of the Alborz (east of 57°E), Sabzevar and Kuh-e-Sorkh mountain ranges, which lie south of the Kopeh Dagh mountains in NE Iran. Although these ranges have experienced relatively few large earthquakes over the last 50 years, they have been subject to a number of devastating historical events at Neyshabur, Esfarayen, and Sabzevar. A significant change in the tectonics of the eastern Alborz occurs directly south of the Central Kopeh Dagh, near 57°E. To the east, shortening occurs on major thrust faults which bound the southern margin of the range, resulting in significant crustal thickening, forming peaks up to 3,000~m high. Faulting does not continue eastward into Afghanistan, which is thought to belong to stable Eurasia. The rate of shortening across thrust faults bounding the south side of the eastern Alborz north of Neyshabur is determined using optically stimulated luminescence dating of displaced river deposits, and is likely to be 0.5-- 0.8~mm/yr (maximum upper limit of 2~mm/yr). Shortening across the Sabzevar range ~150~km west of Neyshabur has previously been determined at 0.4--0.6~mm/yr (maximum upper limit of 1~mm/yr). Basinward migration of thrust faulting is common across NE Iran, especially in the Esfarayen region near 57°E, where the northward deflection of the East Alborz range reaches a maximum of 200±20~km (from its presumed linear E-W strike at the beginning of the Oligocene). The continued northward deflection of the eastern Alborz has probably resulted in the progressive clockwise rotation of thrust faults out of their optimum orientation, promoting the formation of new thrusts within the surrounding basins which strike oblique to the range front (and remain perpendicular to the shortening direction). West of 57°E, the tectonics of the Alborz are affected by the westward motion of the South Caspian region, which results in the partitioning of shortening onto separate thrust and left-lateral strike-slip faults north and south of the range, respectively. At the longitude of ~59°E, published GPS velocities indicate that ~50% of the overall shortening across NE Iran is accommodated in the Kopeh Dagh. Of the 50% regional shortening which remains to be accommodated south of the Kopeh Dagh, thrust faulting across the eastern Alborz and Kuh-e- Sorkh ranges probably each account for ~25%. West of 59°E, additional shortening across the Sabzevar range is probably related to the westward increase in regional shortening across NE Iran as indicated by GPS data. At present day rates the total 200±20~km N-S shortening across the eastern Alborz and Kopeh Dagh mountains since the beginning of inversion of the Kopeh Dagh basin would be accommodated in 30±8~Ma. This age is consistent with geological estimates of post Early-to-Middle Oligocene (<30~Ma) for onset of Kopeh Dagh inversion.
T21A-1935
Calc-alkaline Magmatic Activities and Related Mineralization in the Northwest of the Lut Block (Eastern Iran)
The Lut Block of Eastern Iran has significant mineral potential based on its tectonic setting, geologic evolution, Tertiary igneous rock cover and old mining records. In the Ahang prospecting area (~ 40Km2), located in northwest of the Lut block, magmatic activities started in Eocene with the eruption of mafic to intermediate extrusives, mainly andesites. These rocks are intruded by Oligocene-Miocene monzodiorite and quartz monzonite stocks and cut by felsic dikes. Intrusive rocks are meta-aluminous and belong to the high-K calc-alkaline series. The results of REE analysis show that these rocks are enriched in La relative to Yb. Their ratios of Ta+Yb versus Rb suggest the VAG (volcanic arc granites) geotectonic regime for their origin. Propylitic, sericitic, argillic and silicification alteration zones are well developed in this area. Based on heavy minerals studies of stream sediments, Wulfenite, Stolzite, Galena, Sphalerite, Diaboleite, Dechenite, Mimetite, Pyrite, Celestine, Barite, Flourite, Malachite and Sapphire are present in this area. Geochemical analysis of stream sediment samples indicates considerable anomalous zones of W, Pb, Zn, Cu, Au, As, Sb and also Ba and F. Mineralization as veins, veinlet and minor stockwork are mainly controlled by structural features and brecciated zones. Mineralization occurs in three types: Quartz- Hematite- Galena (high grade of Pb, Zn, Au, As, W); Quarts- Pyrite- Chalcopyrite and Galena (high grade of Cu, Mo, Ag, Zn, Pb), and Quartz- Barite- Celestine- Galena and flurine (high grade of F, Sr, Ba, Pb). The range of value for these elements in mineralized rock samples are: Cu (5-1500ppm), Mo (0.5-109ppm), Pb (20ppm-14%), Zn (8ppm-7.8%), W (5-1350ppm), As (1-723ppm), Ag (0.5-56.5ppm), Au (7ppb-7.8ppm). According to the field observations and data presented here, this area has a high potential for base-metal, W and Au mineral exploration. Keywords: Calc-alkaline magmatism, Lut Block, mineralization, Iran
T21A-1936
Tectonic Setting and Bimodal Magmatic Evolution of Eocene Volcanic Rocks of the Bijgerd-Kuh-e Kharchin area, Uromieh-Dokhtar Zone, Iran
Geochemical composition and texture of the Middle and Late Eocene volcanic, volcaniclastic, and volcanic- sedimentary rocks in the Bijgerd-Kuh-e Kharchin area, northwest of Saveh, provide significant geochemical and geological clues for the tectonic and magmatic evolution of the Uromieh-Dokhtar volcanic-plutonic zone of Iran. The Middle Eocene volcanic rocks have an intermediate composition and include green tuff and tuffaceous sandstone with intercalated sandstone, sandy tuff, and shale. The shale has lenses of nummulite- bearing limestone with a Middle Eocene detrital age. The time between the Middle and Late Eocene volcanic activities in this area is marked by the presence of andesite and rhyolitic tuff. The Late Eocene succession is distinguished by the presence of four alternating levels (horizons) of intermediate lava and ignimbrite which we designate as Eig. The ignimbrites of the Eig sequence have a rhyolitic composition and include ignimbrite- breccia, ignimbrite-tuff, and ignimbrite-lava pairs. The volume of the felsic volcanic rocks in this sequence far exceeds that of the intermediate rocks, which makes it unlikely that they evolved through the magmatic differentiation of a basaltic magma. The presence of the nummulite-bearing limestone lenses, and sandstone and conglomerate interbeds between the ignimbrites, suggests a shallow marine environment for the pyroclastic deposition and probably the eruptions. The tuff and siltstone of the Est unit that sits above the first ignimbrite may represent deep water, Late Eocene deposit. Oligo-Miocene limestone of the Qom Formation unconformably overlies the uppermost Late Eocene ignimbrite. Washings from red marls give microfossils with Late Eocene age for the Eig sequence, which is synchronous with other paleontological evidence that puts the peak volcanic activity as Late Eocene in the Bijgerd-Kuh-e Kharchin area. Field and petrographic evidence for magma mixing/mingling is given by the presence of mafic- intermediate enclaves in the ignimbrite, hybrid breccias with felsic and mafic clasts, felsic pseudo-flames filled with intermediate lava, heterogeneity in the ignimbrite texture, and sieve texture and oscillatory zoning of plagioclase and opacitization of olivine in the intermediate lava. Geochemical analyses of the major and trace elements (including the REE) and rock texture and assemblages indicate the bimodal magmatic characteristics of the mafic-intermediate lavas and ignimbrites. The tuff and the breccia show a hybrid elemental distribution between those of rhyolite and basalt. The ignimbrites show more enriched compositions than those of the mafic and intermediate rocks on the chondrite-normalized trace element distribution diagram. The higher enrichment of the LREE in the ignimbrites may be due to a crustal contribution. The primitive mantle-normalized elemental distributions show a distinct depletion of Nb and Ti, which suggests a subduction-related volcanism during Eocene.
T21A-1937
A Backarc Basin Origin for the Eocene Volcanic Rocks North of Abbas Abad, East of Shahrud, Northeast Iran
The region in northeastern Iran, bordered by the Miami fault and the Doruneh fault, mainly exposes the Eocene volcanic and Tertiary sedimentary rocks and sporadic outcrops of pre- Jurassic metamorphic rocks such as gneiss and mica-schist. We have divided the volcanic and volcanic-sedimentary rocks into six main units: E1 through the youngest E6. North of Abbas Abad, the Lower Eocene is conglomerate, sandstone, and red shale with lenses of nummulite-bearing limestone at the base, and dacitic lava (E1) at the top. The nummulites give an Early Eocene age for the limestone lenses. The E2 unit includes vesicular basalt, intercalated, intraformational conglomerate, and lenses of nummulite-bearing limestone. E3 is volcanic- sedimentary, and is made of green tuff, tuffite, shale, and nummulite bearing limestone. E4 includes basalt and vesicular trachy-basalt, and E5 is mostly sedimentary, made of tan marl, sandstone, shale, and lenses of Middle Eocene nummulite-bearing limestone. The E6 unit is the most extensive, with at least three levels of nummulite-bearing limestone lenses which give a Middle to Early Eocene age. The volcanic rocks of the E6 unit include few hundred meters of epiclastic to hyaloclastic breccia, with intercalations of lava at the base. These are overlain by four horizons of aphyric olivine basalt and basalt, and phyric trachy-andesite and trachy-basalt. The volume of the aphyric lavas decreases, and that of the phyric lavas increases upsection. The Eocene volcanic sequence is covered by turbidite; the marl washings give an Eocene-Oligocene age range. Chondrite-normalized multi-element plots indicate enrichment of the Eocene Abbas Abad volcanic rocks in the LILE elements, with variable ratios of La/Yb (4.36-19.33) and La/Sm (3.10-7.91). These plots show a gentle slope, and the volcanic rocks in the E1 to E4 units are less enriched than those in the E6 unit, probably reflecting the difference in the original source for the melt. The multi-element plots of the volcanic rocks in E6 also indicate an enrichment of the phyric lavas in the upper parts of this unit than those in the aphyric lavas. The primitive mantle-normalized multi-element plots, in addition to indicating enrichment in the LILE, show clear anomalies for Nb and Ti, indicating a backarc basin-related volcanism. The Eocene Abbas Abad basaltic lavas show a variable La/Nb=2.15-4.63 ratio, which is comparable with that in the in-arc basalt. The E1-E4 basic volcanic rocks have a Th/La ratio of 0.14-0.2, which is comparable with that of oceanic basalt or bulk continent. However, the Th/La ratio in the E6 lavas ranges from a 0.18 minimum, in the lower olivine basalt, to 0.4 in the upper, phyric trachy-andesite, which may indicate the elemental contributions of sediments to these lavas and enrichment of the LILE. Our data suggest that the Eocene volcanism in the Abbas Abad area occurred in a backarc basin, a conclusion which is consistent with the model that assumes continued subduction of the Arabian plate under the Iranian plate, and partial melting of the Paleotethys ocean and supra-ophiolite sedimentary rocks in northeastern Iran. It seems that the partial melting of the ultramafic parts of the Paleotethys has led to the formation of the E1 to E4 volcanic rocks, and the partial melting of these rocks and the supra-ophiolitic sedimentary rocks formed the volcanic rocks in the E6 unit.
T21A-1938
Variation of Moho depth in the Central Alborz Mountains, Northern Iran
The Alborz Mountains of northern Iran form a belt of active crustal deformation along the southern side of the Caspian Sea within the broad Arabian-Eurasia continental collision zone. Although the range has an average elevation of about 3000 m with peaks reaching elevations in excess of 5000 m, previous geophysical studies using meagre data, have suggested that the range is not supported by a crustal root. This study presents the first measurements of Moho depth below the Alborz Mountains region based on substantial seismic data recorded in the Alborz. We determine a model for the crust of the central Alborz Mountains using teleseismic receiver functions from data recorded on a net work of broadband seismographs temporarily deployed across the region. The receiver functions from these recordings have been inverted simultaneously with fundamental-mode Rayleigh-wave group velocities measurements in the 10-100 s period range. Our analysis shows a thickening of the crust from ~46 km beneath the northern part of the Central Iranian Plateau to ~55 km below the central part of the Alborz Mountains, then a thinning of the crust to ~44 km north of the Alborz Mountains beneath the coastal region of the South Caspian Sea Basin. Our seismological results, together with an analysis of gravity, show that the central Alborz Mountains have a moderate crustal root but are not fully compensated.
T21A-1939
Geochemistry of High-Ca Boninite Dike Swarms and the Related Plutonic Rocks in the Oman Ophiolite
It has been debated whether the tectonic setting of the Oman ophiolite is mid-ocean ridge (MOR) or supra- subduction zone (SSZ). The study of the timing and geochemical modeling of boninite magmatism in the Oman ophiolite suggested that the tectonic setting changed from MOR to SSZ (island arc) setting due to an intra oceanic thrusting (Ishikawa et al., 2004). If this model is acceptable, it is expected that the crustal section of the Oman ophiolite contains the early stage products of island arc magmatism. In the Fizh block of the northern part of the Oman ophiolite, ultramafic cumulate, gabbronorite, plagiogranite, and boninitic dike swarms are recognized as late intrusive rocks, which are largely intruding into MOR type gabbroic crust. The boninitic dikes and the olivine-clinopyroxene ultramafic cumulates have the geochemical similarity of their mineral and parental melt compositions. Their Cr-spinels have an island arc character with high Cr# (> 65) and low TiO2 (< 0.5 wt%). Chondrite normalized multi-elements patterns of the parental magma calculated from clinopyroxene composition of the ultramafic cumulates have a closely resemblance to boninitic dikes with depletion of HFS elements and enrichment of LIL elements than MORB volcanic rocks of the Oman ophiolite. Boninite melt generation requires hydration melting of refractory mantle peridotite under an extremely high temperature and low pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic crust, which previously experienced MORB melt extraction. In this area, boninitic dikes form 500 m to 2 km width dike swarms, which are rooted in the ultramafic cumulates, and strike WNW direction oblique to N-S direction of MORB sheeted dike complex. This indicates that the principal stress direction of Oman ophiolite changed from E-W extension to E-W compression (Yanai et al., 1989). These evidence support a model in which the tectonic setting changes from MOR to SSZ (island arc). And the ultramafic cumulates with the boninitic affinity are interpreted as the early stage products of island arc magmatism. Refference: Ishikawa et al. (2005), EPSL, 240, 355-377; Yanai et al. (1989), jornal of Geography (Japanese), 98, 278-289.