T51C-1349
The Geologic Structure of the Tebesti and Ennedi; Not Just Plateaus, Mountains, and Inland Seas
Research into the origins of the Tebesti and Ennedi structures where sources of dust streaming into the Bodele are of major importance to the study of global climate forcing, have resulted in a geologic story quite different from previous research results. The volcanic nature of both structures are not only similar in geologic time; when comparing the mineralogy and structure throughout the Saharan, based on extensive geological studies conducted in the region early to mid 1900's, the two areas appear to be connected in time and composition. It is quite possible the Tebesti Mountains and Ennedi Plateau region are one structurally cohesive unit with common mineralogy and geologic history. Two scenarios appear to predominate at this time. One, the entire geomorphology of the Sahara is a huge syncline/anti-syncline system ranging from the East to West coast with marked synclinal depressions affording opportunity for inland seas to develop during crustal deformation in the past. Second, geologic indicators suggest that the entire Tebesti and Ennedi region form an ancient caldera extending northeast into Egypt and southwest into Chad, just north of the Bodele. Mineralogy and structure of the region could be interpreted as a caldera of massive proportion existing early in the African continent's history prior to possible crustal deformation and the formation of the syncline/anti-syncline basin and range system.
T51C-1350
Electrical Resistivity Structure of the Arabia-Eurasia Collision Zone in Eastern Anatolia
The tectonics of eastern Anatolia is dominated by the collision of the Arabian and Eurasian plates. Recent passive seismic exploration has provided new constraints on the seismic velocity structure of this region and suggest that asthenosphere may be present at shallow depth within the collision zone. Magnetotelluric data can provide independent constraints on processes in regions of active tectonics by remotely sensing the electrical resistivity of the crust and upper mantle. From May to November 2005 magnetotelluric (MT) data were collected in Eastern Anatolia in a joint Canadian-Turkish project by the University of Alberta and Istanbul Technical University. Long period MT data give penetration to upper mantle depths and were collected on profiles extending from the Arabian plate to the Black Sea with a spacing of 10-15 km. More detailed broadband MT data were collected on shorter profiles crossing the North and East Anatolian Fault Zones with a spacing of 1-2 km. Preliminary resistivity models will be presented for each of the profiles and indicate that: (1) A mid-crustal conductor is present beneath the Anatolian Block in the vicinity of Elazig and terminates directly beneath the surface trace of the East Anatolian Fault. This geometry is very similar to that observed on major strike-slip faults in Northern and Eastern Tibet. (2)East of the Karliova triple junction, the Anatolian crust has a high resistivity. A low resistivity zone is present at depths of 40-50 km and may represent a shallow asthenosphere. (3)Low resistivity is observed in the upper mantle beneath Karacadag, a center of recent basaltic eruptions on the Arabian Plate, and in the crust beneath the Nemrut Golu - Suphan Dagi volcanic belt. The tectonic significance of these new MT data will be discussed in the context of other geological and geophysical data.
T51C-1351
GPS measurements of present-day crustal deformation within the restraining bend of the Dead Sea fault system in Lebanon
Lebanon and southwestern Syria comprise a prominent, 200-km long restraining bend along the Dead Sea fault system (DSFS) - the continental transform boundary between the Arabian and Sinai plates. Within this "Lebanese Restraining Bend", the DSFS splays into several prominent, left-lateral strike-slip faults, in addition to uplift owing to regional transpression. Hence, upper crustal deformation resulting from the relative motion between the Arabian and Sinai plates is likely distributed among multiple geological structures. We present new Global Positioning System (GPS) measurements that help constrain relative plate motion along this portion of the plate boundary and explore strain partitioning of the plate motion within the restraining bend. Our GPS observations were collected during 5 survey campaigns in Lebanon spanning more than 3 1/2 years. The regional survey network consists of 14 survey sites, along with one continuous GPS station (LAUG in Jbail, Lebanon). Our measurements are processed along with other continuous stations in the region and stabilized using the global ITRF2000 reference frame core sites. For tectonic interpretation we rotate this solution into the Arabian, Nubian and Sinai fixed plate reference frames. Across the restraining bend, we constrain the 4 - 6 mm/yr of plate motion, with 1-sigma uncertainties typically less than 1 mm/yr. The plate motion is particularly well constrained between the continuous GPS stations in Jbail and Damascus (Syria). Within the uncertainties, GPS-based velocities compare well with Holocene-averaged estimates of slip along the Yammouneh and Serghaya faults. Furthermore, displacement gradients also suggest small, but measurable, horizontal shortening across the Mt. Lebanon range. We develop our kinematic model further by applying an elastic block model to assess the kinematics of the restraining bend and constrain geodetic locking depths. These results provide critical constraints for documenting and assessing the earthquake hazard in Lebanon and nearby regions of Syria.
T51C-1352
Relocation and Assessment of Seismicity in the Iran Region
We have relocated Iranian earthquakes occurring between 1909 and 2005. The image of seismic activity occurring at the boundaries between distinct tectonic blocks is sharpened, and - most significantly - event depths are refined. ISC locations throughout Iran (especially in the Zagros) tend to be in the lower crust or upper mantle. Our results suggest that the vast majority of Iranian events occur in the upper crust, consistent with focal depths of available local seismic network hypocenters. Most confirmed lower crustal events are located in the Oman line region. Mantle events are associated with the Makran subduction zone in southern Iran and incipient subduction across the central Caspian north of the south Caspian Basin. Iranian seismicity is the result of the early stages of continent/continent collision (25-35mm/yr of northwards overall shortening) between the Arabian Peninsula and Eurasia. This shortening across Iran results in thrust and strike-slip faulting. Distinct tectonic blocks respond to the nascent collision through relative motion, resulting in seismicity at the boundaries. Areas of heightened strain (collision with the Oman Peninsula and drastic variations in crustal structure around the southern Caspian) result in seismicity in the lower crust.
T51C-1353
Geophysical expression of the Gadamota caldera within the central Ethiopian Rift
One of the main characteristics of the Ethiopian rift is the distribution of several quaternary calderas on the rift floor. The majority of these calderas have been analyzed geologically in some detail. However, the Gadamota caldera has not been studied in detail. In this study, we present the results of a gravity investigation of the Gadamota caldera, which lies within the central section of the main Ethiopian Rift. This caldera has a physiographic expression on its western side; however its eastern side is not clearly visible. In order to define the lateral extent and the structural setting of this caldera, a gravity survey was conducted in the summer of 2005. The data were acquired using a Lacoste and Romberg Model G-780 gravity meter and Trimble 4000 SSE GPS instruments for high precision measurements. From our preliminary analyzes, we are able to delineate the extent of the caldera to be about 18 km wide. DEM data together with ETM Landsat data also assisted in the control of surface features such as the extent of the caldera rim and the shape of the caldera. The western and northwestern caldera rim can be seen clearly in the field and on SRTM 90-meter DEM data. In addition, numerous northeast-trending faults are depicted outside the caldera on the Landsat image, which can be associated with the Wonji Fault Belt system but do not cut the caldera. The caldera is evidenced by a gravity maximum whose apex occupies the southeastern portion of the caldera. This anomaly has been previously detected in part both on the gravity and refraction seismic profiles that were part of the first phase of the EAGLE project. The gravity survey has proved that a high density causative body is situated in the southeastern portion of the caldera and that the volcanic fill is shallow. The distinct gravity anomaly we detected shows that the Gadamota caldera is a separate feature from the Aluto volcano.
T51C-1354
Extension in NW Iran inferred from GPS enlightens the behavior of the south Caspian Basin
Large scale velocity field of Iran has been obtained from recent repeated GPS surveys (Vernant et al. 2004) and suggests about 7 mm/yr of right-lateral displacement for the NW-SE faults in the Tabriz region (NW Iran). These faults are well-known for their large historical seismicity (Berberian & Yeats, 1999). East of NW Iran, the south Caspian basin (SCB) is a relatively aseismic block involved in the collision zone between Eurasia and Arabia. The SCB is expected to be relatively rigid. In order to 1- localize more precisely the right-lateral movement observed in NW Iran and to 2- better constrain the relative motion of the SCB with respect to NW Iran, we have installed and measured a dense GPS network in NW Iran, from Central Iran to the Turkish, Armenian and Azerbaijan borders. 19 points has been installed and measured in the framework of French Iranian cooperation. All sites have been surveyed at least three times in September 2002, 2003 and 2004 during 48 hours. Some sites have been measured four, five or six times since 1999. Surprisingly, an unexpected result has been obtained: the NW Iran right-lateral movement is combined with SW-NE extension north of the Tabriz fault (TF). On the base of our GPS results in NW Iran, we address three aspects of the Iranian tectonics. (1) The right-lateral movement observed in NW Iran is concentrated on the TF which suffers about 7 mm/year of right-lateral movement. This rate is in agreement with a recurrence interval time of 250-300 years proposed from historical seismicity studies. (2) About 8 mm/year of extension is observed north of the TF, between the TF and the Azerbaijan border, within the Talesh. This extension is in agreement with the existence of a southwestward subduction of the SCB. (3) The Talesh and Alborz mountain ranges are not a single continuous structure wrapping the Caspian shoreline because of the northward extension of Central Iran up to the southern Caspian shoreline documented by the velocity of ATTA.
T51C-1355
Active Fault Pattern and Kinematics in the Northwest Anatolia: From Strike Slip Deformation to Extensional Tectonic Regime
NW Anatolia is a transition region between North Anatolian Transform Fault System (NAFS) and Aegean Extensional Tectonic Regime. However, characteristics and regional pattern of the active faults provide data to understand regional present kinematics, which are poorly known. The NW Anatolia is deformed under the complex tectonic regime that bounded by NAFS in the north and Sindirgi-Sincanli Fault zone in the south. Complex active fault systems form large bends or arcs concave to south roughly parallel to Gediz and Dinar graben systems that take place between those major tectonic structures. In the Marmara region, the NAF turns into a large deformation zone and bifurcates into two main strands as the northern and southern. Recent GPS data reveal that the northern strand is the plate boundary and this strand essentially accommodates lateral motion along the NAF. However, southern strand is included in the complex fault systems in the Southern Marmara and Biga peninsula. Active faults in NW Anatolia form major bend systems generally trend E-W. The faults on the both flanks of the bends are right lateral strike slip faults and at the apex of the bends in the west NE-SW trending faults have reverse or thrust component, whereas in the east NW-SE trending faults have normal component. The region was divided into sub-blocks under the neotectonic regime. The cores of those are formed from paleotectonic massifs and active fault systems mostly have been were developed as a result of the reactivitation of paleo-tectonic structures around the massifs like Uludag, Kazdag, Muratdag, etc. Bursa bend is connected with right lateral Eskihehir-Tuz Golu fault system trending NW which is easternmost boundary of Aegean extensional tectonic regime within the Central Anatolia. Right lateral Sindirgi-Sincanli zone extends Sultandagy and Soma normal fault systems that separate the Aegean grabens from the NW Anatolia. All of cross faults on the hanging wall block of the Gediz detachment fault terminate at this zone. We concluded that; 1) The NW Anatolia is deformed under the complex tectonic regime between NAF and Aegean graben systems as a transition region. 2) Right lateral active faults are dominant structures within the regional kinematics and fault systems form arc patterns concave to south 3) Active faults in the southern Marmara and Biga Peninsula can not be directly evaluated within the NAFS as the identified in previous studies. Contrary, those fault systems reach within the Anatolian block towards to east via bends. 4) Active fault pattern in the region was mostly developed on the paleo-tectonic structures. 5) Anti-clockwise rotation on the Western Anatolia that consequence of escape tectonic regime of Turkey, is accommodated by regional active fault pattern.
T51C-1356
An active normal fault NW of the Mont Blanc massif, France: evidence of extensive tectonics near the main thrust zone of the Chamonix valley ?
Examination of historical seismograms and macroseismic data of the two 1905 Chamonix moderate sized earthquakes, together with a close inspection of a fault that follows the southeastern flank of the Aiguilles Rouges massif and continues northward through the Vallorcine valley for 10 km, suggest that the southern part of this fault - a normal left-lateral fault - may be active. These results question recent geological surveys and geochronological data from the Mont-Blanc and Aiguilles Rouges massifs suggesting that the relative uplift between both massifs separated by the main thrust zone of the Chamonix valley became inactive around 4 millions years ago (Leloup et al., 2005). Among the two earthquakes occurring in the immediate vicinity of the Chamonix and Vallorcine valleys in 1905, only the first one (magnitude Mw=5.5 on April 29) is large enough to have possibly caused surficial fault breaks. Direct evidence of activity on the fault are: 1) a fresh, steep and linear topographic scarp, 2) fine striations over at least 3 m height at the bottom of the fault scarp, and 3) compatibility between the focal mechanism inferred from the striations and a record made in Goettingen, Germany, of the 1905 earthquake. The observed striations most probably postdate the retreat of the northern branch of the Mont Blanc glacier which disappeared there around 8 000 - 11 000 years ago according to cosmogenic Be10 data of glacially polished rocks. A georadar survey of a pond within a small grabben located at 2060 m elevation along the fault suggests that deepening of about 2 m occurred since the glacier stopped eroding the crystalline outcrop of this area. Our observations support the idea that the fault may be active with a normal slip rate on the order of 0,3 mm/year, and a recurrence time for Mw=5.5 earthquakes around 300 years. Extrapolating to 4 million years, a differential height of the Aiguilles Rouges massif with respect to the eastern side of the fault, could reach about 800 m, explaining the relative elevation difference of the top of the crystalline basement observed in the area. Leloup et al., Tectonics, 24, TC4002, doi;10.1029/2004TC001676, 2005.
T51C-1357
Stress Regime in Italy: State of the art
Stress data collection and analysis in Italy increased a lot in the last decade but there is still a lot of work to be done. First of all concerning stress magnitudes: deep data are very few and an organized collection of shallow depth data is not yet available for scientific purposes, despite the large amount of shallow rock stress measurements. Stress magnitude data could contribute to deal with a wide spectrum of engineering problems but also to get reliable stress profiles from surface to earthquake depths useful for seismotectonic models. Here we present our latest work on stress magnitude determination inferred from more than two hundreds new leak-off test data from oil wells, kindly provided by ENI S.p.A (Italian Oil Company). We analyzed them notwithstanding the problems of the datum itself and did our best to obtain as much as possible reliable information on the state of stress of Italian peninsula. We calculated the values of the principal stress axis and stress regime at different depths, ranging from about 200m to 5000m, also considering the possible scattering of data and their uncertainties. We compared the results with horizontal stress orientations from borehole breakout analysis and other stress indicators. We analyzed the pictures of stress regime at different depths and along different transects through the Apennines, speculating about the reasons of regime changes that are observed in some areas. The state of stress depicted by the new data confirms the results of previous studies making them and the new ones more confident.
T51C-1358
Neotectonic Southeast Carpathian Foreland Deformation and Genetic Association with the Vrancea Seismogenic Zone
Three deep (18-20 s TWTT), industry type seismic reflection profiles and the DRACULA II (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) multichannel transect (30 s), collected in the southeast Carpathian foreland basin were processed targeting crustal and upper mantle structures that could substantiate a genetic and geometric association between the ongoing foreland deformation and the seismically active Vrancea zone. Of specific interest was the imaging of (1) active crustal faults, possibly offsetting the Moho, (2) geometry of crustal reflectors, and (3) position and geometry of the Moho. These features are viewed as essential in differentiating between existing models (subduction of oceanic lithosphere versus delamination of continental lithosphere) explaining intermediate-depth (50-210 km) seismicity in the SE Carpathians. In the central-southern part of the Focsani basin, two combined profiles oriented W-E document (1) a gently eastward dipping Moho at a depth of 42-45 km, (2) a dramatic increase in the sedimentary cover from 14 km (W) to less than 1 km (E), (3) interpreted normal faults with surface offsets, some cutting the crystalline basement, (4) horizontal mid-crustal reflectivity, and (5) the absence of prominent west dipping reflectors expected from the subduction model. A NNW-SSE oriented profile located in the southern extreme of the basin shows (1) 7-10 km sediment fill, (2) a crust-mantle transition zone between 42-38 km, (3) similar mid-crustal horizontal packages of reflectors, and (4) intense basement fracturing with some faults possibly active and propagating to the surface. DRACULA II profile was acquired in the summer of 2004 with the purpose of imaging structures over the seismically active crustal scale Peceneaga-Camena fault, in the northern extremity of the basin. It displays unusual reflective sequences down to 15 s (45 km) where we interpret the Moho on the eastern side but a seemingly transparent crust on the western side, possibly a result of the depth extent of the Peceneaga-Camena fault. No clear offsets are imaged in the shallow section but seismic activity suggests a recent reactivation of this fault. To a first order, the recent deformation, localized subsidence, and crustal seismic activity of the SE Carpathian foreland may favor delamination of the continental mantle lithosphere as a viable mechanism to produce the intermediate-depth Vrancea seismicity.
T51C-1359
Discriminating Between Subhorizontal Lower Crustal and Upper Mantle Reflectivity to Delineate Moho Geometry Under the Transylvanian Basin, Romania
A thick interval (~16 km) of reflective layered material at the crust/mantle boundary beneath the Transylvanian Basin, Romania, complicates delineation of the Mohorovicic discontinuity based on the deep seismic reflection method. Seismic reflection data from DRACULA I, collected during Project DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity), correlated with existing deep (17 s) industry reflection data constrains this regional subhorizontal fabric and may show a genetic relationship to the Vrancea Seismogenic zone. Portions of the DRACULA I line exhibit subhorizontal reflectivity in excess of 25 seconds (~75 km) with no clear reflective break indicating probable Moho location. Other bright reflectors in the southeastern end of DRACULA I at 7-10 seconds TWTT may represent the westward continuation of structural detachments from the Eastern Carpathians. The subhorizontal laterally continuous 10-17 second TWTT crust/mantle reflective interval is the most pervasive feature observed on all the profiles used in this study, and shows continuity of the Transylvanian crust into the Eastern Carpathians. The subhorizontal nature of this crust/mantle reflective interval seems to preclude the existence of a former Miocene-age suture zone to account for Miocene subduction as currently proposed for the geodynamic setting of the Vrancea seismogenic zone. A continuous lower crust without a plate boundary beneath the Transylvanian Basin advocates the less invasive process of active continental lithospheric delamination as a means to explain the Vrancea seismicity. Although still preliminary, results from the DRACULA I and supporting industry profiles, speak strongly against Miocene-age northwestward subduction in the Southeast Carpathians to be responsible for the intermediate-depth seismicity.
T51C-1360
The Mesozoic and cenozoivc evolution of the Bays of Kiel and Mecklenburg - A part of the NeoBaltic project
In the frame of the Priority Program 1135 of the German Research Foundation (DFG) "Dynamics of sedimentary systems under varying stress conditions by example of the Central European Basin System", the scientific goal of the NeoBaltic project is to describe the post-Permian to recent structural and sedimentary evolution of the entire western Baltic Sea region, with a special emphasis on neotectonic activity and its relation to salt dynamics and the possible reactivation of deeper inherited structures. In order to investigate these scientific goals the Universities of Aarhus (Denmark) and Hamburg (Germany) has since 1998 completed eight marine campaigns in the western Baltic Sea, collecting 2D high resolution seismic (HRS), gravity and magnetic data in the entire region during different projects. Since 2003 all these data has been available for the NeoBaltic project. All together the data pool have more than 8000 km HRS, 6000 km gravity and 5000 km magnetic data. Until now the project work has been focused on the completion of the data processing and the digital interpretation of important Mesozoic and Cenozoic markers on the seismic sections from the Bays of Kiel and Mecklenburg. Furthermore several maps have been completed from the potential field data (gravity and magnetic). As a result of the digital interpretation of the HRS data, the overall geological evolution of the northern part of the NGB can be subdivided into four distinct periods. During the Triassic and the Early Jurassic, E-W directed extension and the deposition of clastic sediments initiated the movement of the underlying Zechstein evaporites. This is seen by the presence of several salt pillows in the region. The deposition ceased during the Middle Jurassic, when the entire area was uplifted, due to the Mid North Sea Doming. The uplift resulted in a pronounced erosion of Upper Triassic and Lower Jurassic strata. This event is marked by a clear unconformity on the seismic sections. The region remained an area of non-deposition until the end of the Early Cretaceous. The sedimentation resumed in the late part of the Early Cretaceous and continued throughout the Late Cretaceous. No pronounced halokinesis has been detectedduring this period. Towards the end of the Late Cretaceous, the Alpine Collision caused the reactivation of salt structures seen on the thinning of the Cenozoic sequence across the different structures. As a result of the different Pleistocene glaciasations, several buried valleys have been detected on the seismic sections, especially in the Bay of Kiel and the Danish Lillebelt region. Some of these buried valleys contain biogenic gas, which results in a sufficient pull-down of the underlying markers on the seismic sections.
T51C-1361
Surface Monocline Development Along Normal Faults in Basalt, Southwest Iceland
The subaerial exposure of the Mid-Atlantic Ridge in southwest Iceland is characterized by a high angle of spreading obliquity ($30°), resulting in a complex structural fabric along the ridge segment at the Reykjanes Peninsula (RP). Tension fractures, normal, oblique-slip, and strike-slip faults, eruptive fissures, and prominent hyaloclastite ridges demarcate a series of four closely-spaced fissure swarms across the length of the RP. Monoclines flank the hanging wall sides of numerous normal and oblique-slip faults in the fissure swarms. The formation of these surface flexures is likely the result of upward fault propagation from depth, facilitated by the growth of vertical fractures that nucleate at the upper fault tip and then rip through the highly fractured basalt lava pile. Subsurface slip along the buried fault causes a monocline to develop at the surface above the fault tip. As slip continues, the monocline grows in both height and width, accommodating throw at the surface purely by flexure. Bending stresses within the flexure induce tensile fracturing that breaches the upper hinge at the surface. Further movement along the underlying fault increases the dilation of this fracture resulting in a gaping chasm that may widen by collapse along the fracture walls. No throw occurs along this hinge fracture until the fault ultimately breaches the surface via the fracture. At this time, the monocline becomes a passive and detached hanging wall structure along a vertical fault scarp, thus preventing further growth of the flexure. Detailed field and aerial photo mapping from scanned then digitally orthorectified photos (0.125m/pixel) were used to analyze fracture style, surface monocline geometries, and to assess the spatial variability of monoclinal flexures across the RP and northwards into the Western Volcanic Zone (WVZ). To capture along-strike changes in monocline shape, elevation profiles were constructed at numerous locations along, and orthogonal to, fault traces using a Trimble real-time differential GPS Pathfinder Pro XR data collection system. The observed monoclines are variable in scale, the largest of which exceeds 8 km in length, located on the SE side of the western graben-bounding fault at Thingvellir in the WVZ. Near the center of this fault, a throw of nearly 20 m across the upper hinge fracture combines with an additional 15 m of throw that was accommodated by the monocline itself prior to the fault breaching the surface, for a total of 35 m of throw. As the throw approaches zero towards the tips of the fault, there is an associated decrease in monocline-accommodated throw and monocline width until the monocline eventually disappears at the fault tips. Monocline geometries display the same relationship along individual, 10s of meters long fault segments near Burfell on the RP. An oblique-slip fault here is characterized by left-stepping en echelon segments that each exhibit a discrete monocline. Elsewhere on the RP, examples of monoclines lacking surface rupture (i.e., blind faults) have been mapped in regions of active faulting in the rift zone, suggesting that monocline formation is an active and ongoing process. Some examples of unbreached surface flexures contain underlying cavities several meters across that appear to occur above a vertical fracture linked to the upwardly propagating, underlying fault. The occurrence of these cavities suggests that the formation of monoclines may not necessarily be related to broad-scale warping of the surface (as appears to be true of Thingvellir), but rather a delamination phenomenon involving only the most recent lava flows that covered an active fault.