T41G-01
Active deformation at the junction between the East Anatolian Fault, Dead Sea Fault and Cyprus Arc (Hatay Province, South Turkey): Kinematic modeling from Tectonic and GPS data
Field investigations at the junction between the East Anatolian fault (EAF) and Dead Sea Fault (DSF) using quantitative geopmorphology and paleoseismology are compared with GPS measurements. These tectonic structures meet at a triple junction between the Arabian plate, the African plate and the Anatolian block in south Turkey. The ENE-WSW trending EAF and segment consists on a quasi-linear rupture of gouge zone with ridges and young scarps showing left-lateral offset streams and young alluvial terraces. Detailed measurements of offset stream channels and terraces along the Maras segment of the EAF ranges from 3.8 m to 1500 m and trench studies attest for late Pleistocene and Holocene tectonic activity. These recent surface ruptures along the EAF segment may be correlated with the AD 1114 and AD 1513 large earthquakes. To the south, the ENE-WSW trending and left-lateral Amanos Fault with normal component and related Karasu Valley to the east, was the site of the 1822 large earthquake and form the connection with the DSF. The north-south trending DSF also shows left-lateral offset streams and alluvial terraces ranging from 14 m to 650 m. The northern DSF segment which is correlated with the AD 1408 large earthquake, meets the Karasu Valley through the Amik Basin and displays 43 +-1.5 m left-lateral offset (measured from the detailed morphology and magnetic survey) of a 6 Ka to 8 Ka old archeological site (the Sicantarla Tell). These new results in active tectonics combined with an analysis of GPS velocities (from a dense network of 22 stations, Reilinger et al., 2006) provide some constraints on the kinematic modeling at the junction. The modeling is consistent with the seismotectonic framework, the newly identified Iskanderun Block and westward escape of Anatolia. The junction can be interpreted as a triple point where the DSF transforms the Cyprus arc subduction into the East Anatolian Fault system.
T41G-02 INVITED
Exploring the Pacific-Australian transform plate-boundary in central South Island, New Zealand
Lithospheric structure beneath and adjacent to the continental transform of central South Island, New Zealand (the Alpine Fault) has been investigated with both geophysical and geological methods. Principal features that have been seismically imaged include a spectacular example of a lower crustal detachment that separates the obducting greywacke-schist rocks from lower crust and mantle below. This detachment can be seen as a zone of strong reflectivity that is sub-horizontal at 35 km depth beneath the Southern Alps. The detachment then forms a SE-dipping ramp that gets progressively steeper until at a depth of ~15 km it dips at 60 degrees. If the reflectivity that defines the detachment is projected upwards it would intersect with the surface trace of the Alpine Fault. In the hanging wall of the Alpine Fault low P-wave seismic wave speeds and high electrical conductivity are mapped. These properties are interpreted to mean inter-connected fluid, high fluid pressures and reduced effective normal stresses. Consistent with such high fluid pressures are extensive quartz veining and geological evidence for deep crustal embrittlement along vertical shear planes. Mantle deformation adjacent to the Alpine Fault is detected with SKS splitting results and Pn wave speeds from mutually perpendicular, offshore, seismic lines. P-wave anisotropy of up to 13% is seen in the mantle lid within 20 km of the fault. Moreover, combining SKS and Pn observations suggest that the lateral extent of mantle deformation may be as much as 200 km from the Alpine Fault, and that all the anisotropy can be assigned to finite deformation of the lithospheric mantle. Flexural modeling shows the effective elastic thickness (Te) to be vanishingly small beneath the Southern Alps. Beyond the coastlines values of Te are greater than 20 km. We propose that the weakness and the wide zone of deformation are phenomena of plate boundaries where both strike-slip and convergence have persisted for several millions of years.
T41G-03
Structure, slip rate and seismic potential of the Alpine fault
The Alpine fault is steeply-dipping and strike-slip in character over most of its 800 km length. The southern onshore section has a surface strike-slip rate of 23 +/- 2 mm/yr and partitioning of crustal convergence occurs across a zone 300 km wide; partitioning and localisation is also implied in the mantle by a steeply-dipping Benioff zone. In central South Island, the fault dips moderately southeast and oblique-slip occurs over an anomalous 200 km length; the width of the zone is reduced to c. 100 km; the fault in the mid and lower crust is characterised by high conductivity, low Vp and low Q; and it is unclear how active deformation is localised or partitioned within the mantle. Does this anomalous zone of potentially low strength inhibit great earthquake ruptures? Paleoseismic evidence suggests Alpine fault ruptures of 300-500, 200-300, and 350-600 km at c. 1717, 1620, and 1430 AD, respectively. The rupture lengths and local discreet displacements of c. 8 m suggest moment magnitudes in the range 7.3-8.3, if interpreted as single events. The lack of significant step-overs and the results of a simplified dynamic rupture model are consistent with paleoseismic evidence and suggest there are no theoretical grounds for ruling out continuous ruptures over most of the fault length. However, a triggered sequence of smaller events or significant afterslip can not be ruled out by the data. The spatial and genetic relationship between the Alpine fault and Fiordland subduction zone is significant for understanding both the large-scale structure and seismic potential.
T41G-04
How does a lithospheric plate boundary adapt to changes in plate motion? An example from South Island of New Zealand
Since ~ 20 Ma the plate boundary along the Alpine Fault in South Island of New Zealand has undergone substantial changes. The combination of movement of the Eulerian pole describing the Pacific-Australian relative motion with the southern migration of the Hikurangi subduction zone has changed the conditions along the Alpine Fault to progressively more transpressive. On a crustal scale this change has been accommodated by crustal thickening and the creation of the Southern Alps, on the full lithospheric scale it is still not clear how these changes in plate motion are accommodated. At shallow to middle level where the lithosphere is predominantly brittle these changes of motion are normally accommodated through strain partitioning along preexisting weakness (faults) and/or through crustal thickening. Within the ductile regime, the plate boundary shear may reorient along a more favorable direction (a typical behavior for a linear viscous flow) or it may localize along a preexisting weak shear zone with lithospheric thickening (typical for power law rheology). Here we use a FEM to simulate the evolution of the Southern Alps and to analyze the effects of the coupling between ductile and brittle lithosphere (in particular the presence of a fault), the effects of boundary conditions (southern migration of the subduction) and of the rheology (linear vs. power law) on the localization of the flow in the ductile part of the lithosphere and how the lithospheric scale plate boundary adapts to the new tectonic regime.
T41G-05
Evolution of the Plate Boundary Through New Zealand since 25 Ma � Implications for the Development of the Alpine Fault
The current plate boundary configuration through New Zealand of a (transpressional) continental transform linking two convergent/subduction zones initiated approximately 25 Ma. Its present configuration, kinematics, and patterns of deformation are reasonably well constrained, although the tectonic history of boundary is less clear. In particular, the extent, geometry, location, and deformational behavior of the plate boundary linking the Hikurangi and Puysegur subduction regimes � the proto-Alpine Fault (AF) plate boundary structure prior to approximately 10 Ma � is not well understood. Combining plate reconstructions, patterns of basin evolution and volcanism recording the southward migration of Hikurangi subduction, and plate kinematics through time allow us to place important constraints on the development of the proto-Alpine fault plate boundary through the Miocene. Results include (1) a substantial volume of crustal material must have been removed along the proto-AF adjacent/within the present North Island, implying transpression along that segment of the plate boundary in the Middle Miocene and the development of a Southern Alps-like orogen bounding the eastern margin of the present-day North Island; (2) Hikurangi margin forearc terranes were likely originally part of the Pacific plate (i.e. east of the proto-AF) and have been sequentially captured by the Australian plate in a manner similar to present capture by Australia of the Marlborough terranes in the northern South Island; (3) remnants of the proto-AF structures should lie inboard of these forearc terranes and may be associated with faults of the North Island Axial Ranges; and (4) the proto-AF plate boundary may have had different geometry, orientation, and attitude from the current Alpine Fault through the South Island � characteristics that are in part controlled by the structure and mechanical properties of the Australia plate margin. This analysis implies that the Alpine Fault system was transpressional throughout much of its history and not only post-Miocene as is normally assumed.
T41G-06
Kinematics and interseismic coupling on the Alpine Fault, New Zealand from GPS, geological, and seismological data
The central Alpine Fault is one of the world's classic oblique-slip faults. To better understand the role that the Alpine Fault plays in overall Pacific/Australia plate boundary deformation, we integrate GPS, geological and seismological data to describe the active deformation in the South Island, New Zealand by using an elastic, rotating block approach. This approach also allows us to estimate the degree of interseismic coupling on the Alpine Fault, which has implications for future seismic hazard estimates. The data are fit to within uncertainty when inverted simultaneously for angular velocities of rotating tectonic blocks and the degree of interseismic coupling on faults bounding the blocks. Consistent with previous geological studies, we find that most (70-82 %) of the relative plate motion budget in the central South Island is accommodated on the Alpine Fault (27-31 mm/yr strike slip, 5 mm/yr reverse slip). The GPS velocities also suggest that up to 5 mm/yr of active dextral deformation on faults distributed within the Southern Alps < 100 km to the east of the Alpine Fault is possible. The degree of interseismic coupling on the Alpine fault changes markedly along-strike, with shallower maximum coupling depths on the central portion of the Alpine Fault relative to northern and southern portions of the fault. This is consistent with previous suggestions based on thermal observations in the region of the central Alpine Fault, as well as seismological interpretations of high fluid pressures at depth. We also find that vertical axis rotation rates of tectonic blocks in the central South Island are similar to that of the Pacific Plate, suggesting that edge forces dominate the tectonic block kinematics in the central South Island.
T41G-07
Linking Seismic and Tectonic Timescales in the Central Southern Alps, New Zealand
The Alpine Fault marks the transpressional plate boundary between the Pacific and Australian continental plates in South Island, New Zealand, and is known to rupture in large earthquakes with a recurrence interval of several hundred years. Slip rates derived from paleoseismology and inferred from present surface deformation (GPS) indicate that up to 70% of the relative plate motion is taken up along the Alpine Fault, and geological and geophysical evidence suggest that a mylonitic shear zone extends into the lower crust beneath it. We investigate the linkages between deformation on seismic timescales and the slowly accumulating aseismic deformation in the lower crust using numerical models that incorporate the earthquake cycle and elastic, brittle, and viscous deformation. Although simple, the models demonstrate how the accumulation and release of elastic stress near the brittle-ductile transition zone can cause deformation in the overlying crust near the Main Divide fault zone, and how a weak frictional fault contributes to localization of ductile shearing beneath it. The uplift rates and slip rates derived from the models compare well to geological and geodetic observations, and help to illustrate the interplay between elastic and inelastic mechanisms. Transient deformation and changes in fluid pressure are evident in rocks exhumed in a fossil brittle-ductile array structurally above the Alpine Fault in the central Southern Alps. We use the models to test the influence of fault geometry, ductile flow-laws, and high fluid pressures on the development of such transient deformation in the mid-lower crust.