T43D-01 INVITED 13:40h
Using Satellite-derived Gravity Data to Infer the Existence of Localised Convective Motions in the Mantle Associated with Mid-Atlantic Ridge Transforms
Bonatti (1996) proposed that long-lived, large (greater than 200 km), transform offsets of the Mid-Atlantic Ridge in the Equatorial region are located above upper mantle thermal minima, and that they may represent semi-permanent structural and/or geochemical boundaries between adjacent ridge segments. The sites of these active transforms are regions of decreased magma supply, resulting in deep bathymetric troughs, imaged using the satellite-derived free air gravity data of Sandwell and Smith (1997) as negative anomalies, reflecting the positive correlation of free air gravity with bathymetry and the presence of low density sediments within the topographic lows. Within the South Atlantic, where the SW end of the St. Helena Seamount Chain intersects the mid-ocean ridge, we have observed distinct, southward directed, "V"-shaped, seamount trails (positive free air gravity anomalies) on the oceanic crust centred on the ridge axis. These are interpreted as an indication of excess magma production associated with a ridge-centred melting anomaly which is moving relatively, and progressively, southwards with time with respect to the underlying asthenosphere. Similar features also occur close to the ridge axis at the SW end of the Walvis Ridge and the SE end of the Rio Grande Rise. The orientation of these seamount trails is broadly concordant with the GPS-derived plate motion of Africa and South America. North of the Equatorial Fracture Zone similar, but inverted (i.e. northward directed), "V"-shaped volcanic lineaments are observed, bounded by E-W trending flowlines; these may also be associated with time-progressive migration of shallow melting anomalies, in this case moving relatively northward with respect to the asthenosphere within a particular ridge segment. The changing orientations of these near-ridge seamount trails north and south of the Equatorial Fracture Zone suggests that the source of the excess magmatism is closely associated with shallow-level seafloor spreading processes, rather than with conventional mantle plumes or hotspots. Our interpretation of the geometry of the seamount trails is consistent with localised flow directions within the asthenosphere converging in the Equatorial Zone (i.e. southward asthenosphere flow to the north of the Fracture Zone and northward flow to the south), supporting the model of Bonatti (1996). Bonatti, E. (1996) Long-lived oceanic transform boundaries formed above mantle thermal minima. Geology 24, 803-806. Sandwell,D.T. and Smith W.H.F. (1997) Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research 102:10,039-10,054.
T43D-02 INVITED 13:55h
The Strength of Fracture Zones from their Bathymetric and Gravitational Evolution
Depending on their strength, fracture zones (FZs) may play an important role in accommodating changes in plate motions and creating new plate boundaries. All previous models of FZ evolution have imposed the bathymetric offset at FZs kinematically with thin plates; here we model FZs dynamically so that they are free to slip. The models use a visco-elastoplastic rheology incorporating the influence of fault friction on FZ slip history and thus model the transition from elastic to viscous behavior more realistically than assuming a transition isotherm. By comparing synthetic gravity profiles to free-air gravity across FZs, we find that the amplitude of the gravity jump across FZs is best fit by models with weak faults that have depth-averaged yield strengths less than 10 MPa. Small-scale convective removal of the lowermost (i.e., viscous) thermal lithosphere cannot explain the older-side highs observed at many Pacific FZs. Many FZs do not fit plate subsidence models with locked or slipping faults, but are better fit by systems that are tectonically deformed by modest amounts of extension, presumably due to minor stage pole re-orientations. Longer-lived tectonic adjustments that place a FZ in compression may result in the conversion of the FZ into an incipient subduction zone. We show that $\sim$10 MPa depth-averaged strength is a critical point for plate boundary evolution: FZs with strengths less than $\sim$10 MPa can evolve into self-sustaining subduction zones with an intense phase of back-arc basin extension, as found for the Mariana's when it transitioned from a FZ to a subduction zone.
T43D-03 14:10h
3-D Melt Migration Driven by Dynamic Pressure Gradients in a Passive, Mantle Flow Field Can Explain (Almost) Everything About Melt Delivery In a Mid-Ocean Ridge System Offset by Transform Faults
Dynamic pressure gradients developed in a deforming solid medium were one of the first mechanisms suggested for driving melt from a broad region of melt production to a narrow zone of crustal accretion at the ridge axis. This idea was never fully embraced because it requires high viscosities, greater than 10*21 Pa s; otherwise melt buoyancy drives the magma vertically. Perhaps dehydration of the mantle matrix accomplished by removal of the first small melt fraction can increase the viscosity to the level required to drive porous melt flow to the axis. Assuming that the dynamic pressure gradient in a deforming mantle is the dominant driving force for melt migration, new 3-D calculations show that this mechanism quantitatively explains a remarkable number of the fundamental observations of variations in crustal structure and mid-ocean ridge basalt (MORB) composition in a ridge-transform system. More melt is predicted to be delivered to the middle of ridge segments, accounting for the thickened crust near the center, the thin crust near fracture zones, and the resultant mantle Bouguer anomaly bulls-eyes on slow spreading ridges. Deeper, low-degree melts are also preferentially delivered to the segment center. Near the end of a segment, greater cooling caused by slower upwelling rates of the mantle matrix cause melting to terminate at greater depth. As suggested by Reynolds and Langmuir (1997), these phenomena combine to produce magmas near the transform that have much lower levels of highly incompatible elements but higher levels of moderately incompatible elements than segment-center magmas. If the mantle contains "plum pudding" style heterogeneities that melt at greater depth, this model can explain the isotopic anomalies at the centers of some of the longer spreading segments on the Mid-Atlantic Ridge (Michael et al., 1994). The 230Th deficiency and highly depleted character of MORBs from East Pacific Rise intra-transform spreading centers are also predicted. The predicted transform effect in these new calculations is more pronounced than in the Phipps Morgan and Forsyth (1988) 3-D passive flow models because that study employed a different model for melt migration.
T43D-04 14:25h
The Important Roles of Oceanic Transform Faults in Hotspot-Ridge Interactions
Transform faults are an essential component of the global seafloor spreading system and plate tectonics. In this study we review evidence, especially that recently obtained from the Southwest Indian Ocean, to further highlight the important roles that transform faults play in the dynamic interaction of hotspots with mid-ocean ridges: 1) When a mid-ocean ridge system migrates sufficiently close to a hotspot, the ridge geometry often undergoes dramatic reorganization so that the ridge axis is located where the lithosphere is weakest. Changes in the geometry of transform faults, together with ridge jumps, are the basic mode of such reorganization of ridge geometry. 2) Transform faults of large age offset are often observed to compartmentalize hotspot-influenced ridges into separate corridors of different average seafloor depth, as well as to limit the along-axis extent of hotspot influence, i.e., the "waist width". These effects are observed at ridges of all spreading rates, especially at the highly segmented ultraslow Southwest Indian Ridge. 3) Representing zones of weakness in the lithosphere, transform faults and the fracture zones have been found to be associated with the apparent track of some hotspots. For example, our recent survey near the Southwest Indian Ridge indicates that the Bouvet hotspot has "pierced" through the Shaka fracture zone to form a chain of seamounts that define a pseudo hotspot track. These and other lines of evidence testify to the important roles of oceanic transform faults in hotspot-ridge interactions and the creation and evolution of the oceanic lithosphere.
T43D-05 14:40h
Transform Faults, Gravity Lineaments, and Seamounts
Age offsets along transform faults control the thermal structure and integrated strength of oceanic lithosphere. Preliminary analyses of marine gravity data from satellite altimetry suggests that seamount population density is systematically higher in younger lithospheric corridors than it is in order lithospheric corridors [Craig and Sandwell, Global Distribution of Seamounts from Seasat Profiles, J. Geophys. Res., 93, 10408-10420, 1988]. Moreover, the 150-km wavelength gravity lineaments in the central Pacific are most intense in corridors of younger lithosphere and volcanic ridges are preferentially located in troughs of gravity lineatments. These preliminary observations suggest that lithospheric thermal structure and strength have a primary control on seamount abundance. We developed a new global marine gravity anomaly model, with errors 30% lower than our previous global models, to map small seamounts and volcanic ridges in relation to the major Pacific fracture zones. The improvement in gravity precision is achieved by re-tracking the raw echoes of the Geosat and ERS-1 radar altimeters using an algorithm that is optimized for recovery of along-track sea surface slope by assuming the significant wave height varies smoothly along track [Smith and Sandwell, this meeting]. Repeat ERS-1 cycles across the South Pacific (area of high sea state) show improvement in along-track slope error from 6.45 microradian to 4.01 microradian and there is a corresponding improvement in resolution from 40 km to 33 km. We will also use the improved gravity resolution to identify plate reorganizations that are sometimes associated with higher seamount abundance.
<a href='http://topex.ucsd.edu' >http://topex.ucsd.edu
T43D-06 14:55h
Melting Anomalies, Ridge-Crossing Seamounts, and Transform Faults
Some hotspot tracks appear to cross spreading-ridges. This has been used as evidence for relatively fixed deep-mantle plumes and therefore most previous studies have focused on ridge migration over a fixed thermal anomaly. However, we have developed several conceptual models that negate the need for a fixed thermal anomaly to explain this phenomenon and instead suggest two alternative transform related mechanisms. We hypothesize that a seamount chain crossing a ridge can be caused by a) ridge migration over fertile mantle blobs and/or b) the reorganization of a ridge with seamount production at a ridge-transform-intersection (RTI). Thus, the seamount chain appears to cross the ridge because production of seamounts is initially restricted to one plate ahead of the RTI and then switches to the other plate when the ridge reorganizes or the migrates. These models are based on the premise that, as crack tips, RTIs focus extensional stress and that when transform slip is inhibited, extensional stress at RTIs becomes large enough to result in decompressional melting and the formation of a seamount and/or ridge propagation. Because changes in transform strength are strongly tied to plate stress, motion changes, and ridge-reorganizations we suggest that seamount chains crossing ridges may be indicative of both the active role that transforms play in oceanic tectonics and changes in plate motions or ridge-reorganizations. Additionally, conventional wisdom suggests that we need to find an explanation for the production of seamounts, instead these models suggest that we need to find an explanation for why we wouldn't see seamounts when the mantle is at or near its melting point. As such, these models appear to offer a possible explanation for the large number of seamounts scattered throughout the Pacific and for the non-sequential age dates found in some of the seamount chains.
T43D-07 INVITED 15:10h
Upper Crustal Deformation in Onshore Exposures of the Tjörnes Fracture Zone, Northern Iceland
The Tjörnes Fracture Zone (TFZ) links the northern rift zone (NRZ) of Iceland with the Kolbeinsey Ridge to the north. The TFZ formed 7 Ma ago as a result of an eastward jump of the spreading axis in northern Iceland. Northward propagation of the NRZ has resulted in a northward migration of the locus of transform-related deformation. Active deformation occurs in a swath 150 km long (EW) and 50 km wide (NS) involving a complex array of right-lateral (WNW) strike-slip and oblique-slip faults bounding several (NS) rifts. Previously published and new structural data from the onshore portions of the TFZ reveal pervasive fracturing and alteration, rotation of spreading-related structures, and re-activation of NS faults in a bookshelf-faulting deformational style. These structures are cut by major WNW-trending, intensely shattered, dextral, strike-slip fault zones. Enigmatic WNW-trending dikes cut NNE-trending, spreading-related features and pre-date recent TFZ deformation. In some places, late NW-trending oblique-slip faults cut this entire structural assemblage. Strike-slip faulting and reactivation of older spreading structures affects the crust over a width of at least 10 km onshore and several tens of kilometers offshore. These data support interpretations of the offshore parts of the plate boundary and may provide an analog for deformation along migrating ridge-ridge transform faults elsewhere on the mid-ocean ridge system.
T43D-08 15:25h
Ridge Transform Intersection at Troodos: Implications for 3D Time-dependent Accretion and Deformation in the Oceanic Crust
Structural features of fossil ridge transform intersection (RTI) are best exposed in the Troodos ophiolite, Cyprus. Previous works have reconstructed the two-dimensional configuration of the ophiolite. In order to understand the evolution of the crust adjacent to the RTI, we investigated deformation and magma flow patterns near and away from the key structural elements: a paleo-spreading axis and a fossil transform fault. The investigation methods included 3D paleomagnetic reconstructions and AMS surveys focused on the gabbro layer of the oceanic lithosphere. We have collected 238 oriented field drilled cores at 42 sites. The stable magnetization was measured and used to reconstruct the deformation within the lower crust. The fabric of the gabbro was determined by measuring the anisotropy of magnetic susceptibility (AMS) on all samples. Directions of magmatic flow in the axial magma chamber, as recorded by the AMS, have changed from vertical to along-axis and vice versa. Three provinces of coherent flow directions are mapped, each 5-10 km across. A province showing vertical flow is right on an orthogonal spreading axis, and the changeover to along-axis flow was accompanied by abrupt transition to a curved RTI. This study also demonstrates that the upper crust, along the spreading axis, is decoupled from the lower crust by detachment. Hence, the precise location of the ridge axis is defined where the gabbro shows no deformation. As the spreading continues and the oceanic lithosphere cools and thickens, the detachment faults at either side of the axis migrate downward into the lower parts of the lithosphere. Rotations of the paleomagnetic vectors at the outside corner reveal an extensional detachment fault, i.e., block rotation about a horizontal axis on the hanging wall of the detachment. On the other hand, the inside corner reveals a torsional detachment fault, i.e., block rotation about a vertical axis on the hanging wall of the detachment. This new type of fault formed as a result of shearing along the transform fault.