T23B-1407
Hydrothermal Circulation across the Axis of (Slow-Spreading) Mid-Ocean Ridges: the Role of Faults and Variations in Lithospheric thickness.
The permeable (and thermal) structure(s) of mid-ocean ridges control the dynamics, distribution and geometry of sub-seafloor hydrothermal circulations. Ridge perpendicular extension likely leads to faulting and a permeability that is strongly anisotropic favoring along-axis flow. Across-axis flow can not be ruled out, especially at slow- spreading mid-ocean ridges where the lithosphere is crossed cut by ridge-parallel faults that can root near the magma chamber as recently evidenced by Singh et al. (2006) on the mid-Atlantic ridge at the Lucky Strike site. It may thus be that hydrothermal flow at slow spreading ridges has a significant across-axis component. The dynamical pattern of this across-axis flow critically depends on (i) the across-axis permeability and temperature structures and (ii) the characteristics of the fault system (e.g., fault location, dipping angle, interconnection and hydraulic properties). In this communication we study the characteristics of this across-axis flow by means of numerical models of hydrothermal circulation. The models incorporate realistic fluid properties, permeability (i.e., layered versus homogeneous systems) and thermal (i.e., variations in across-axis thickness of the brittle lithosphere) structures. As it has been proposed that faults can be both permeable conduits and impermeable barriers, and that the hydraulic properties may be transient, we model the effects of both enhanced and reduced fault zone permeability. The different fault models provide insight on possible temperature and geometry (e.g., cell aspect ratio, coupling of fluid flow between faults) of the hydrothermal circulation, and help constrain the role of fault systems in focusing hot or cold fluid flows and their possible contribution to the general sub-seafloor hydrothermal circulation.
T23B-1408
Three-dimensional Geometry of Magma Chamber Roof and Faults from 3D Seismic Reflection Data at the Lucky Strike Volcano, Mid-Atlantic Ridge
A three-dimensional (3D) seismic reflection survey was carried out during the SISMOMAR 2005 experiment covering an area of 18x3.8 km2, which includes the Lucky Strike volcano and associated hydrothermal vent sites, part of the graben on top of the volcano, and extends out to the median valley bounding faults. The survey consisted of 39 lines shot at 100 m spacing using a 4.5 km-long streamer resulting in a sixty-fold coverage and 6.25 m CDP spacing. We present here the resulting 3D seismic volume migrated and converted to depth with the velocities from the 3D refraction survey (Seher et al). 3D processing allowed us to determine the geometry and depth of a bright reflector identified at depth beneath the volcano, which is interpreted as the roof of a magma chamber. The magma chamber reflector is 1 to 2 km wide and located 3.4±0.3 km deep beneath the seafloor. Several faults, among which the axial valley bounding faults and faults on the volcano, are imaged at depth and some reach the vicinity of the magma chamber reflector (< 1 km). The dip of faults is constrained; it is comprised between 40 and 55°, except the western bounding fault which dips 33±5°. A map of layer 2A thickness was derived from picks of layer 2A arrivals on the reflection volume, which shows a characteristic thinning of layer 2A associated with major faults. We also derived a high-resolution seafloor bathymetric map from picks of the seafloor arrival on the reflection volume. The 3D geometry of the magma chamber and faults, along with the overlying tectonic and accretion expressions at the seafloor provide new insights into the interplay between magmatic and tectonic processes at slow spreading ridges. By constraining the heat source and probably the main fluid paths locations, these results also provide a geometric pattern for the hydrothermal circulation feeding the vents.
T23B-1409
Ocean deformation processes at the Caribbean-North America-South America triple junction: Initial results of the 2007 ANTIPLAC marine survey
Marine geophysical data (multibeam and seismic lines) acquired in 2007 (ANTIPLAC survey) in the North-South Americas-Caribbean triple point (Central Atlantic, Barracuda and Tiburon ridges area), provide information about the structure, the tectonic processes and the timing of the deformation in this large diffuse zone of polyphase deformation. The deformation of the plate boundary between the north and south Americas is distributed on several structures located in the Atlantic plain, at the front of the Barbados accretionary prism. In this area of deformation of the Atlantic oceanic lithosphere, the main depressions and transform troughs are filled by Late Pliocene-Pleistocene turbidite sediments, especially in the Barracuda trough, north of Barracuda ridge. These sediments are not issued from the Lesser Antilles volcanic arc but they are sourced from the East, probably by the Orinoco turbidite distal system, through channels transiting in the Atlantic abyssal plain. These Late Pliocene- Quaternary sediments show locally spectacular evidences of syntectonic deformation. It can be shown notably that Barracuda ridge includes a pre-existing transform fault system which has been folded and uplifted very recently during Pleistocene times. This recent deformation has generate relieves up to 2 km high with associated erosion processes notably along the northern flank the Barracuda ridge. The subduction of these recently deformed ridges induces deformation of earlier structures within the Barbados accretionary prism. These asperities within the Atlantic oceanic lithosphere which is subducted in the Lesser Antilles active margin are correlated with the zone of intense seismic activity below the volcanic arc.
T23B-1410 INVITED
Quantifying the Role of Active Detachment Faulting in Lithospheric Accretion Along Slow–Spreading Ridges (MAR 12-35°N)
We have systematically examined the seafloor morphology along both flanks of the Mid-Atlantic Ridge (MAR) axis between the Marathon and the Oceanographer Fracture zones (FZs), using available multibeam data. We find that detachment faulting may be active along more than 35% of this section of the northern MAR. Our interpretation is based on the findings that showed that the seafloor along the 12- 14°N section of the the MAR [Smith et al. 2006] is characterized by prominent linear ridges with significant outward facing slopes, and associated striated fault surfaces exposed at the seafloor. This terrain, which is interpreted to be the result of detachment faulting and core complex formation, is common throughout the northern MAR, and is systematically associated with zones of enhanced seismicity. The proportion of the axis dominated by detachment faulting varies from ~15% between the Hayes and Oceanographer FZs to >60% between the Marathon and Fifteen-Twenty FZs. Magmatic terrain is characterized by volcanic morphology and fault-bound abyssal hills subparallel to the axis. This terrain, which is associated with limited tectonic extension and aseismic zones, dominates accretion along 30% of the ridge axis. The remaining seafloor flanking the axis (25%) is difficult to classify, and corresponds to either oblique portions of the MAR with complex tectonic and magmatic features, or to areas with limited data coverage. Detachment faulting thus appears to play a larger role in lithospheric accretion along slow spreading ridges than that suggested by the striated fault surfaces alone. The two distinct seafloor morphologies correspond to sections of the ridge with different thermal states, as indicated by the seismic patterns, and probably linked to differences in magmatic supply to the axis.
T23B-1411
Models of Deformation of Uppermost Oceanic Lithosphere: Comparison of Crustal Flexure in the Blönduós Area, Northern Iceland, and Structure of East Pacific Rise Crust at Hess Deep
Models of the internal structure of oceanic crust have been constructed from studies of ophiolites and from more recent observations of tectonic windows into the upper crust. Spreading rate and/or magma supply are the central variables that control ridge processes and the ultimate architecture of ocean crust. In addition to ophiolites, Iceland also provides an important analog to study mid-ocean ridge processes and structure. Flexure zones in Iceland characterize the structure of Tertiary-Recent lava flows, and are areas wherein lavas dip regionally inward toward the axis of one of several ~N/S-trending rift zones. These rift zones are interpreted to represent fossil spreading centers which were abandoned during a series of eastward-directed ridge jumps. In the Hildará area, north-central Iceland, the eastern side of a regional flexure is characterized by westward-dipping lavas, approximately 6-8 Ma, which are cut by east-dipping normal faults and dikes. The upper-crustal structure within this flexure zone from slow spread (~20 mm/yr) crust exhibits remarkable similarities to the structure of the upper crust created at a fast-spreading (110 mm/yr) segment of the East Pacific Rise (EPR) observed at Hess Deep. In this modern ocean setting, ~1 Ma crust is characterized by west-dipping lavas above consistently east-dipping (away from the EPR) dikes and dike-subparallel fault zones. In both locations, paleomagnetic and structural data indicate that west-dipping lavas and east-dipping dikes result from tectonic rotations. In addition, cross-cutting dike relationships demonstrate that dike intrusion occurred both during and after normal fault- related tilting. These data indicate that fault-controlled tilting was initiated within the narrow neovolcanic zone of the ridge and is not associated with off-axis processes. Lavas at magmatically robust ridges commonly flow away from elevated ridge-crests. Measurement of anisotropy of magnetic susceptibility (AMS) of the lavas from the flexure in Iceland suggests a mean flow direction to the northeast, that is, away from the fossil-ridge axis, demonstrating that the fossil spreading center from which the lavas were extruded was located to the west. Despite the distinct differences in spreading rates, the high magma supply in both environments resulted in a very similar upper crustal architecture.
T23B-1412
Cemented Mounds Perched on the Kane Megamullion Detachment Surface: A New Manifestation of Hydrothermal Venting?
Hydrothermal venting has been widely investigated along the axes of mid-ocean ridges, but there has been little evidence of hydrothermal flow directly associated with major normal faults. A notable exception is the TAG hydrothermal field on the Mid-Atlantic Ridge (MAR), which is nucleated in the hanging wall above such a fault and which suggests that these fault/flow relations may be more common than heretofore recognized. We have encountered a probable example of such an association at Kane megamullion, which was formed by a long-lived (ca. 1.2 m.y.) detachment fault just south of Kane Fracture Zone on the MAR. The detachment fault exhumed sheeted dikes, gabbros, and mantle peridotites. Interpreted hydrothermal cementation and sediments occur on the detachment surface and take two forms: 1) consolidated mounds of mixed rock debris and cemented sediment that appears to be enriched in Mn- and Fe-rich hydrothermal precipitates, and 2) cemented, slabby sedimentary layers that also are Mn- and Fe-rich. The mounds have variable shapes and sizes. They range from conical to elongate or ridge-like and have heights from ca. 1 to 8 meters, with flank slopes of ca. 30 degrees to near-vertical. Compositions of the mound surfaces range from nearly pure rock debris (primarily basalt, but also occasional gabbro and serpentinite), to polymict breccia, to nearly pure sediment. Some of the sedimentary mounds show flow structures that resemble basalt pillows. The slabby sedimentary layers in some places occur on the flanks or at the bases of mounds and in other places occur on relatively smooth seafloor. The latter form is often cracked in linear to polygonal patterns, with upturned ridges produced at the cracks. We hypothesize that the debris mounds were produced where point-source fluid venting occurred through the hanging wall close to the active trace of the detachment fault. The fluids cemented the hanging-wall debris and affixed it to the emerging footwall while the surrounding unconsolidated debris wasted away down the sloping fault surface. The predominantly sedimentary mounds, as well as the cemented slabby sediments, may have formed in association with diffuse venting of low-temperature fluids through the footwall as it was exhumed and subjected to extensional bending stresses. If our hypothesis is correct, it indicates that primary pathways of fluid flow are intimately associated with major normal faults and probably also with fracture patterns in the emerging, bending footwalls of the faults.
T23B-1413
High-resolution 3D models of mid-ocean ridge hydrothermal systems: The relative importance of axial versus cross-axial flow.
We investigated fluid flow patterns at mid-ocean ridge hydrothermal systems in 3D, using a range of permeability patterns. We use high-resolution meshes, containing several million elements, and account for the non- linearities in the properties of pure water at supercritical conditions. These naturally make the simulations extremely cpu intensive. Therefore, we apply recently implemented parallel techniques in CSMP++ and solve the simulations on a large Linux cluster using up to 32 processors. The results show that only for specific permeability distributions, where the permeability is increased towards the ridge axis, spatially separated black smoker fields emerge, as observed in nature. We studied this effect in more detail by solving for a range of permeability models and determining the relative importance of heat and mass transport in the axial plane versus in the cross-axial plane. Our results show that an increased permeability due to faults and fractures in the near axial region play an important role in the formation of hydrothermal convection cells. Further the results show that phenomena like viscous fingering and oscillating vent temperatures, which have been observed previously in 2D models, also occur in 3D.
T23B-1414
Fault-controlled hydrothermal fluid flow at the EPR.
Investigation of an axial mid-ocean ridge fault to determine the character of focused hydrothermal fluid flow. Tectonic escarpments at Pito Deep expose ocean crust formed at the super-fast spreading portion of the southern EPR (ca. 3 Ma). Ocean crust from Pito Deep is cut by high angle axial faults striking NE-SW parallel to the sheeted dykes. We focus on trace element and Sr isotopic compositional variations across an approximately 35 m fault zone within the sheeted dyke complex, ca. 100m below the lava-dyke transition, to explore the relationship between fluid flow, alteration and deformation. Samples from across this axial fault can be divided into 1) wall-rock basalts, 2) fault zone basalts and 3) fault zone breccias, ranging from simple cataclasites, veined cataclasites to quartz cataclasites. Breccias and basalts occur closely juxtaposed within the fault zone suggesting heterogeneous, highly localized deformation and fluid flow. Wall-rock basalts have been sampled a few to 30 m away from the fault zone and show extents of alteration typical of dykes away from fault zones with dominant replacement of primary phases by amphibole. The alteration in the fault zone basalts is also amphibole dominated with wider veins than the wall-rocks of quartz, chlorite ± amphibole. The fault zone breccias range in alteration mineralogy from chlorite to quartz dominated assemblages. Wall rock and fault zone basalts have similar trace element compositions to fresh crust. Breccias show large deviations in trace element compositions in comparison to adjacent altered basalts, with depletion of highly mobile elements such as K, Sr and both enrichment and depletion of Cu and Zn. 87Sr/86Sr of the dykes away from the fault zones are slightly elevated over fresh oceanic crust (0.7025- 0.7029) and fault zone samples have similar to higher 87Sr/86Sr. Quartz-epidote veins precipitated from fluids at Pito Deep have 87Sr/86Sr of approximately 0.7040. These new data provide insight into the chemical evolution of axial hydrothermal fluids during focused fluid flow.
T23B-1415
Faults of subaxial origin exposed in the Hess and Pito Deep Rift walls: implications for hydrothermal systems and seismicity of axial regions.
The rift walls adjacent to Hess and Pito Deep Rifts (HDR and PDR) provide tectonic windows into ocean crust that was accreted and spread from the EPR ~1 mya at >110 mm/yr (HDR), and ~3 mya at >140 Ma/yr (PDR). Faults are exposed in each area that: strike parallel to the ridge axis and abyssal hill lineaments, bound tilted dikes and lavas, are cut by dikes, have growth-relationships with lavas, and are rich in hydrothermal minerals. Thus, the faults developed within the intrusive and eruptive center of the EPR in the presence of high- temperature fluids. The faults contain cataclastic units that have bulk-rock variations above igneous compositions of up to 6 wt% MgO and 20 wt% SiO2. Mechanically comminuted gouge preserves relatively unaltered compositions, likely having sealed the faults to hydrothermal fluid flow. Differences between the two localities abound. In the HDR exposures, >10 m wide damage zones rich in <1 cm-wide chlorite and actinolite veins surround rare, <1 m-wide faults. In contrast, the PDR exposures contain narrow damage zones localized around multiple splays of well-defined fault planes and multiple bands of cataclastic and gouge material, all with mutually crosscutting relationships with networks of >1 cm-wide quartz- (and locally epidote-) rich veins. The multiple increments of deformation, fault slip, and hydrothermal fluid flow occurred within the subaxial center, before possible reactivation by abyssal hill faults. This environment is largely aseismic in the modern EPR. The patterns of deformation and alteration of oceanic crust therefore require a model of faulting that depends less on the strain-weakening evolution intrinsic to seismogenic faults, and more on the coupling of hydrothermal processes and distributed cracking.
T23B-1416
Spatial and temporal patterns of microearthquake frequency-magnitude distributions (b- values) within the axial region of the East Pacific Rise near 9-50 N
Relocated hypocentral data from a seven-month deployment of ocean bottom seismometers provides an opportunity to map microearthquake frequency-magnitude distributions (FMDs) along the 9 49-52 N region on the East Pacific Rise. These analyses, which incorporate more 9000 earthquakes, represent the first investigation of the 3-D spatial and temporal patterns of FMD's along any mid-ocean ridge spreading center. The data are well described by a Gutenberg-Richter model, indicating a power-law or fractal relationship between earthquake size and frequency. The scaling exponent, or b-value, shows significant spatial variability, exceeding a value of 2.0 at the shallowest depths (less than 0.5 km) on axis and dropping below 1.0 away from the axial trough. This spatial pattern is consistent with an inverse relationship between b-value and effective stress, with the lowest stress levels at shallow depths and relatively high stress levels (or low pore pressures) observed away from the hydrothermal cell. Intermediate b-values are observed in the 0.5-1.5 km depth range above the axial magma chamber; however, within this region significant variability also exists along axis. This heterogeneity suggests that the effective stress may vary at sub-kilometer scales within the hydrothermal reservoir. Although the rate of seismic activity increases by roughly a factor of three throughout the deployment (Oct. 2003-Apr. 2004), the frequency-magnitude distribution shows no temporal trend. As a by-product of this b- value analysis the detection capabilities of the array are assessed empirically in both space and time. http://web.mac.com/drbohnen/iWeb/MG_NCSU/Welcome.html
T23B-1417
Long-term Seismicity Comparisons from Oceanic Transforms Bounded by Slow, Intermediate, and Fast Mid-ocean Ridge Spreading Segments
Long-term observations of seismicity along oceanic transform faults have traditionally been difficult due to limited coverage provided by land based seismic networks. More recently, hydroacoustically recorded earthquakes have been catalogued along the East Pacific Rise (EPR), Mid Atlantic Ridge (MAR), and in the northeast Pacific by the NOAA/PMEL and Oregon State University Acoustic Monitoring Program. These catalogs reduce earthquake detection thresholds by nearly 2 orders of magnitude for the slow spreading MAR, the intermediate spreading Juan de Fuca system, and the fast spreading EPR allowing for a more complete long-term time series of seismic activity along the associated transforms in each spreading regime. Using these hydroacoustically derived earthquake catalogs from 1996-2005, this study examines the long-term temporal and spatial seismicity rate patterns of oceanic transform faults bounded by slow, intermediate, and fast mid-ocean ridge spreading. Our analysis includes 5 MAR transforms, 1 northeast Pacific, and 7 EPR tranform faults. Using standard time series analysis techniques in addition to empirical orthogonal functions (EOF), we describe time space patterns along each transform, characterize seismic behavior between transforms within each spreading regime, and finally compare seismicity time series between transforms bounded by different spreading rates. Through our analysis we anticipate the development of an oceanic tranform fault index parameterized by background seismicity rate, seismicity rate variability during seismic events, fault length, degree of tranform segmentation, and rate of spreading along bounding ridge segments. Utilizing a more complete hydroacoustically derived earthquake catalog provides an unprecedented and comprehensive approach for examining long-term seismicity patterns in transform faulting within these 3 mid-ocean ridge settings.
T23B-1418
Microseismicity in the Lau Basin from T-phases recorded by the LABATTS ocean bottom seismograph experiment
We use the 1994 LABATTS Ocean Bottom Seismograph (OBS) array to locate microseismicity within the Lau Basin, delineating intrabasin tectonic, volcanic, and hydrothermal sources. The main line of OBSs was deployed across the Central Lau Spreading Center (CLSC), stretching from the Lau ridge to past the Tonga trench, and several OBSs were deployed off -line. Although the array was not optimally designed for microseismicity studies, a large number of T-phases, were recorded at multiple stations, allowing for hypocenter determination. The OBSs were 4-component broadband stations, and recorded at 32sps for three months in late 1994. Body waves were often recorded for these smaller events at a couple of stations, which are included in the location algorithm, but the larger number of T-phases recorded on the hydrophones typically govern the location hypocenter determinations. Given the large number of T-wave arrivals and the difficulty in precisely picking these largely non-impulsive seismic signals, we automate the picking by performing a gridsearch over the basin and stacking the envelopes of the time-shifted data, for predicted relative arrival times at each station in the experiment layout. After initial hypocenter determination, we re-locate each of the hypocenters in a weighted least-squares fashion. A few patterns are apparent in the data. A significant percentage of located events are to the north of the CLSC and associated with the Peggy Ridge extensional relay zone, possibly the most seismically active feature within the Lau basin. Another significant concentration of seismicity occurs near the northern extension of the ELSC, with some scattered off-axis events as well. Many of these events occur in swarms with dozens of events occurring over the space of a few hours.
T23B-1419
Ocean bottom seismographic study on the 2005 off-Sanriku intralate earthquake (M7.1) occurred in the outer rise of the Japan Trench
An earthquake with magnitude of 7.1 occurred on November 15, 2005 in the outer rise region of the middle part of the Japan Trench subudction system. This earthquake was the largest earthquake in this area since the occurrence of the 1933 Sanriku earthquake (M 8.1), which was accompanied with devastating tsunami along the Pacific coast of northeastern Japan. The normal fault type focal mechanism solutions estimated from global seismic network data (Harvard CMT, F-net) of the 2005 earthquake indicate that this earthquake is related to the bending of the pre-subducted Pacific plate as well as the 1933 earthquake (Kanamori, 1971). We deployed six ocean bottom seismographs (OBS) to observe the aftershocks of the 2005 earthquake for two months, from April to June, 2007. Although it has passed one and a half year after the mainshock occurrence, our OBS observed intense micro-seismicity in the aftershock area presumed from the land-based seismic data. Source regions of the outer rise earthquake are usually quite far from land area and analyses based on teleseismic data have been only the way to understand the faulting process of earthquakes of this kind. The present OBS observation provides us an invaluable opportunity to study outer rise intraplate earthquakes in anatomical way. By using P- and S-waves arrival time data, we estimated the hypocenter locations of more than 300 earthquakes. We also sought optimum Vp and Vp/Vs in the aftershock area to minimize travel time residuals in the hypocenter determination. Obtained Vp is about 6 km/s and 7 km/s at the top and the bottom of the crust, respectively, well within the range of the standard Vp value of the oceanic crust (e.g. White et al., 1992). The Vp/Vs estimated from slope of the Wadachi diagram is 1.73 or less, smaller than that estimated in the NE Pacific Basin (Fukano et al., 2006). The relocated epicenters are constrained within a rectangular area with size of 40 x 20 km, in along-strike and across-strike directions of the trench strike. This size of the aftershock area is consistent with the fault dimension approximated from the magnitude of the mainshock. Estimated focal depths of the aftershocks are ranging from about 1 to 15 km measured from the sea-bottom, indicating that the aftershocks occurred both within the oceanic crust and the uppermost part of the mantle. The hypocenters form two planes perpendicular to each other; one of them dips towards the trench by about 60 degree and the other dips to opposite direction by about 30 degree. It is notable that the strike and dip of these planes almost agree with those of the nodal planes of the CMT solutions estimated by the F-net data. This suggests that the aftershocks distribute not only along the fault plane of the mainshock rupture but also its conjugate plane. The two planes are connected at the bottom to form 'V' shape geometry. The steeper plane seems to cut through the oceanic crust but does not reach the sea bottom. On the other hand, the shallow dipping plane remains in the deeper part of the crust and in the mantle. The combination of a steep trenchward dipping fault and an oceanward dipping shallow fault system is concordant with the asymmetric fault system of outer rise earthquakes found by Jiao et al. (2000). Present result may help to understand how the asymmetric fault system in the trench outer rise.
T23B-1420
The January 13, 2007, Kuril Islands Outer Rise Earthquake
We will present the results of an investigation of the January 13, 2007, Kuril Islands Mw=8.1 normal faulting outer rise earthquake that occurred in close proximity of an Mw=8.3 interplate event that took place two months earlier. Our primary interest is in determining the depth extent of its rupture process through the modeling of seismic waveforms and tsunami data. This earthquake was one of the greatest outer rise earthquakes of the past century and is certainly the best recorded. We will put this event in the context of a larger framework of worldwide outer rise earthquake occurrence in the past 35 years, using a catalog that we created by applying a search algorithm, based on earthquake mechanism in combination with local subduction zone geometry, to the Global CMT catalog. The January 13 earthquake was followed by a unproductive aftershock sequence and preceded by a significant number of events in the outer rise area following the Mw=8.3 interplate event of December 2006. We will also show the results of a basic analysis of the temporal and spatial characteristics of both of these sequences.
T23B-1421
Hypocenter depths and focal mechanisms of trench-outer rise earthquakes from a local seismic network
At subduction zones, the bending of the incoming oceanic plate leads to a reactivation or creation of normal faults that have been inferred to cut deep enough into the mantle to provide a pathway for seawater to penetrate into the lithosphere, changing "dry" peridotites into "wet" serpentinites. Such a mechanism would present an efficient system to transport fluids into the slab and therefore would influence a wealth of subduction zone processes as dehydration at depths of 50-300 km can trigger intermediate-depth earthquakes and promote the melt generation under volcanic arcs. Supported is the idea perhaps by seismic wide-angle and refraction lines that show unusually low P-wave velocities in the crust and upper mantle seawards of the trench axis, which are best explained by a fractured and serpentinized lithosphere. Yet the extent and the importance of this process remain under debate, since bending-related faults might cut much deeper into the mantle than tomographic inversions could resolve so far. We present implications for a unique data set obtained from a dense, local seismic monitoring network. A combination of local earthquakes within or nearby the network and well-located earthquakes in distances of 200- 700km allow us to constrain a velocity model that reflects almost the entire lithosphere. We document a relation between bending-related faults and trench-outer rise seismicity, but state that micro-seismicity reaches much deeper than velocity anomalies and therewith possible serpentinization. We explain the latter with a change of the stress regime from tensional at the top of the incoming lithosphere to compressional beneath, as we infer from the determination of focal mechanism for a number of events. Seawater might not be able to penetrate into an area governed by compressional stresses.
T23B-1422
Intraplate Deformation Adjacent to the Macquarie Ridge South of New Zealand - The Tectonic Evolution of a Complex Plate Boundary
The response of lithospheric plate boundaries to rapid changes in plate motions provide constraints used to determine the manner in which transitions in plate motions and plate boundary configurations can occur. In the case of the Australia – Pacific plate boundary in the Macquarie Ridge region south of New Zealand a substantial change in plate motions has occurred since the Oligocene. Over a period of less than 15Ma, this boundary changed from mid-ocean ridge spreading to simple translation, the record of which is recorded in the fabric and fracture zones of the oceanic lithosphere. Application of available well-constrained plate motions imply that substantial deformation of the oceanic lithosphere must have occurred after fracture zone formation to create the arcuate structure of these fracture zones today. Plate reconstructions of this plate boundary system from the Oligocene through the Early-Mid Miocene are used here to isolate the timing of transitions in plate motion from divergence to translational motion. These reconstructions identify rapid rotations in plate motions after approximately 25Ma. By 20Ma, the majority of crust created along this plate boundary was already in place, and the Australian Plate was translating northwards relative to the Pacific towards New Zealand, where a corner of Australian Plate is ultimately subducted. The timing of this transition in plate motions implies that the onset of subduction at the Puysegur Trench may have been as early as approximately 20Ma. These reconstructions also identify the shape of fracture zones either side of the relic mid-ocean ridge through the time of their formation. Comparison of these restored fracture zones with their present-day appearance delineates a broad zone of deformation extending ~150km into the plate interior from the Macquarie Ridge Complex, the modern plate boundary structure. This area of deformation coincides with a broad distribution of seismicity in the Australian Plate on both inter- and intra-plate structures, including two great (M8+) earthquakes over the past twenty years, one of which occurred over 130km from the plate boundary. The persistence of this deformation through time indicates a link with the evolution of the plate boundary from divergence to translation and subduction, and may be a result of stress build-up within the Australian Plate as a consequence of the impingement of the subducting plate on the thickened lithosphere of southern New Zealand. Such a collision may act as a resisting force to subduction, and if it continues, further deformation internal to the Macquarie Block may lead to a southward migration of the Australia:Pacific subduction interface and the capturing of this section of lithosphere onto the Pacific Plate.
T23B-1423
Sediment Gravity Flows in a Mid-Ocean Ridge Environment and Their Relationship to Spreading Related Faults, Macquarie Island
Despite the vast body of work devoted to deep water sedimentary processes, relatively little work has been done on sediment gravity flow deposits (gravites) produced along mid-ocean ridges. This is due to difficult access in modern ocean basins and structural overprinting in obducted ophiolites. Nevertheless, a rugged bathymetry (typically associated with slow-spreading ridges) coupled with ongoing tectonic activity makes the mid-ocean ridge depositional environment a prime location to study deep marine sediment gravity flow processes and their relationship to spreading related faults. Here we present data from a range of syntectonic gravites (turbidites, densites and debrites) that were deposited within the Late Miocene proto Macquarie spreading center, a slow to intermediate rate spreading center. Gravites are interbedded with MORB basalt and now uplifted and well-exposed on Macquarie Island. Geophysical data offshore and structural data onshore has constrained the paleo-tectonic setting of the island and show that it formed close to a ridge-transform intersection. Petrographic data indicate that gravites, which are most prevalent adjacent to major spreading-related faults, were solely derived from oceanic crust. Paleoflow indicators and stratigraphic architecture show a strong influence by major spreading related faults and hence paleobathymetry. A unique disparity exists between: 1) sandy polymict gravites produced by unconfined flow with paleocurrent data transverse to major structures and interbedded with thick tabular basalt flows; 2) sandy polymict gravites produced by confined flow with paleocurrents parallel to major structures and interbedded with pillow lavas; and 3) gravelly oligomict gravites lacking sedimentary structures and juxtaposed against local structures. We propose that these relationships correspond to differences in fault proximity, oceanic faulting style and volcanic center effusivity.
T23B-1424
Internal Structure of the Extinct Skagi-Hunafloi Rift Zone and Implications for Magmatic Construction
Structural relief ~1.5 km of basaltic crust on the eastern flank of the now-extinct Skagi-Hunafloi rift zone (active between 8 and 4 Ma) is exposed in two mountain ranges bounding the glacially excavated Vatnsdalur in northwestern Iceland. This area reveals both the along- and across-strike variations within an abandoned Tertiary rift zone. Regionally the rift zone is characterized by a "flexure zone" with basaltic lavas that dip 5-10° westward, toward the rift axis. Superimposed on this structure is a bowl-shaped depression at least 800 m deep and ~5 km in diameter in which lava flows dip as much as 50° inward. The lowest package of lavas associated with the depression are basaltic and minor rhyolitic flows (7.62±0.32 Ma), which are at least 400 m thick and have been tectonically rotated during subsidence, as indicated by field relationships and preliminary paleomagnetic data. Overlying these tilted, pre-subsidence flows are syn-subsidence basaltic lavas, which are at least 150 m thick and breccias all of which thicken toward the center of the depression. Overlying the syn- subsidence flows is a ~250 m thick basaltic lens (6.98±0.32 Ma). Ages and thicknesses of units associated with the depression roughly constrain the local subsidence rates at ~1 km/my. Just to the west, gabbroic to granophyric intrusions and hydrothermally altered igneous rocks and breccias mark a dissected volcanic center. Dense (cone?) sheet swarms dip radially inward. This entire assemblage is overlain by gently dipping basaltic to rhyolitic lavas showing that magmatic construction did not result in generation of high relief. Crustal thickening was accommodated by subsidence and backfilling near the depression and by basaltic to minor rhyolitic sheet intrusions over a broader surrounding area. Folding of the pre-subsidence lavas was likely accommodated by slip on steeply dipping fractures and within flow top breccia units. Focused subaxial subsidence in rift zones may result from subsidence into accommodation space created by evacuation of shallow crustal magma chambers beneath central volcanoes. The along- and across-strike variations in Tertiary rift zones in Iceland may provide an analog for the internal structure of mid-ocean ridge spreading segments.
T23B-1425
Ocean crust deformation at the North America-South America plate boundary: Results of the 2007 ANTIPLAC marine survey
East of the Lesser Antilles active margin, the area of the Barracuda and Tiburon ridges is resulting from of a multidirectional and polyphase tectonic history at the diffuse plate boundary between the North and South American plates. These WNW-ESE trending ridges control the sediment distribution and they are bounded by sedimentary trenches, both ridges and trenches trending parallel to the Mid-Atlantic oceanic fracture zones. A marine survey (called ANTIPLAC) conducted in the beginning of the year (January 2007) has provide new evidences (multibeam and seismic acquisition) of the deformation processes which occurred at this plate boundary. On the seismic lines, a major angular unconformity can be recognized in the whole area of the survey. Interpreting the acquired seismic grid, the lower part of the stratigraphic series can be easily tied to the DSDP/ODP holes of legs 78A, 110, 156, 171A, especially with wells 543 and 672. Thus a Maastrichtian-Pliocene age can be attributed to the geological formations located below the regional unconformity. The very recent geological formations located above the unconformity (attributed to the Late Pliocene-Pleistocene) tend to fill the main depressions of the area and show very heterogeneous thickness. These recent deposits can be more than 3 s(TWT) thick in the Barracuda trough (north of Barracuda ridge). Globally they show clear onlapping characters above the older levels, but in some places these levels show spectacularly evidences of syntectonic deposition. This is notably the case of a narrow WNW-ESE trending fold and fault system trending along the axis of the Barracuda trough. South of Barracuda ridge the recent deposits show also locally spectacular fan geometries characterizing deposition during significant tilting. Also, between Barracuda and Tiburon ridges several fracture zones show evidences of very recent (and probably active) reactivation. This recent deformation is also characterized by recent basin inversion structures. Finally and more generally, the data acquired during the ANTIPLAC survey demonstrate that high deformation occurred at the boundary between the North and South American plates during much more recent times than previously thought, and that notably spectacular compressional structures resulting from the convergence between the two american plates developed recently during Late Pliocene-Quaternary times. The subduction of this structural pattern and its partial incorporation within the Barbados tectonic wedge has widely influenced the deformation processes within the accretionary prism and has also induced segmentation within the overriding Caribbean plate.