T52B-01 INVITED
Unexpected Widespread Detachment Faulting During Formation of Lithosphere at the Northern Mid-Atlantic Ridge
The region of the Mid-Atlantic Ridge (MAR) near 13N displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults show large amounts of rotation, dome-shaped corrugated detachment surfaces (core complexes) intersect the seafloor within the inner valley floor, and dead core complexes cover the seafloor off-axis. Steep outward facing slopes indicate that the footwalls of many of the normal faults have rotated by more than 30 degrees suggesting at least 3 km of extension. The rotation occurs very close to the ridge axis and produces distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at magmatic sections of the ridge which display a smaller amount of backtilt (typically less than 15 degrees implying 1-1.5 km of extension). We suggest that the severe backtilt of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long-lived (more than 5 km extension) a dome-shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed. We have applied our understanding of the 13N region to the northern MAR between 15 and 35N and find up to15 ridge segments dominated by extreme fault rotation and core complex exhumation. Core complexes are not limited to inside corners of ridge-transform intersections but may occur anywhere along the length of a segment. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the morphology that we observe. If there is a continuous detachment fault along axis then that suggests differential uplift of the footwall to explain the highs and lows of the domes and swales, perhaps indicating that relatively more gabbro is emplaced under the domes and less under the swales.
T52B-02
Visualizing Core Complex Formation
We visualize the flexural rotation of high-angle normal faults at the axis and their development into low-angle detachment surfaces of oceanic core complexes. In regions of extreme tectonic extension normal faults display large rotations and we use the visualizations to obtain a better understanding of how the rotations occur and thus, the kinematics of detachment faulting and core complex formation. The visualizations emphasize how flexural rotation of the footwall occurs immediately after the formation of a fault and is mostly complete by about 10 km from the axis, even on faults that are long-lived and have developed into core complexes. We use our visualizations to synthesize the seafloor topography of several well-known North Atlantic core complexes including those at 13N at the Mid-Atlantic Ridge, the Logachev massif at 15.75N, the TAG core complex at 26N, and the Kane megamullion at 24N. One of the results of these visualizations is that on the basis of the morphology alone it is difficult to distinguish whether the seafloor topography is generated by long-lived detachment faults or by multiple short-lived faults since each can produce the backtilts that are observed on the faults. Our visualizations also show that gabbro bodies drilled at the low-angle detachment surface of core complexes were probably emplaced near the base of the active detachment fault and rode passively with the upward flexing footwall to the seafloor.
T52B-03
Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from SHRIMP Pb/U zircon ages in IODP hole 1309D, Atlantis Massif, MAR (30°N)
We present U-Pb zircon ages for 18 samples from lower oceanic crust recovered by IODP Hole 1309D in the footwall to an oceanic detachment fault. These samples are evolved oxide gabbros and felsic dikes from between 40-1415 meters below sea floor (mbsf). Ages range from 1.08±0.07 Ma to 1.28±0.05 Ma with errors as low as 1.6%, and reveal a protracted history of accretion. U-Pb zircon dating was performed using the U.S.G.S.-Stanford SHRIMP-RG. Seven ages from both oxide gabbros and felsic dikes above 570 mbsf give a weighted mean of 1.17±0.02 Ma (MSWD=1.03). Oxide gabbros between 620-1040 mbsf are consistently older and give a weighted mean age of 1.24±0.03 (MSWD=1.4). Two felsic dikes within this interval (867 and 1040 mbsf) give younger ages of 1.14±0.05 Ma. In the deepest section (below ~1040 mbsf) oxide gabbros give varied ages of 1.27±0.05 Ma (1175 mbsf) and 1.14±0.04 (1240 and 1327 mbsf). The deepest sample is a felsic dike intruding gabbro at 1415 mbsf, and has an age of 1.28±0.05 Ma. Abrupt changes in the age of the oxide gabbros (at ~600 and below 1040 mbsf) coincide with petrologic and geochemical variations, and indicate the presence of distinct intrusive bodies, which we interpret as sills. The overall weighted mean age of the crust penetrated by 1309D is 1.20±0.02 Ma (MSWD=7.1). However, the range of zircon crystallization ages indicates that this section of crust was constructed over at least ~100-200kyr. This is a minimum estimate because dated samples intrude more primitive olivine-rich rocks and are cut by later diabase. Shallow paleomagnetic remanence inclinations of -38° to -31.5° from below 180 mbsf in Hole 1309D, along with generally steep magmatic fabrics (~40-60°), imply up to 55° of counter- clockwise rotation associated with detachment faulting. The mean age of the hole, together with additional Pb/U zircon ages determined from dive and dredge samples from the southern wall of Atlantis Massif constrain the slip rate of the detachment fault to be between 12 and 15.8 mm/yr. The higher rate implies asymmetric spreading and is consistent with the gabbros being emplaced at depths of 5-7 km below the ridge axis and transported to the seafloor along a curved detachment fault. A significant result is the observation of increasingly older ages with depth in the hole. This result is not consistent with a model whereby melt intrudes at a constant depth beneath the detachment fault, because such a model predicts younging downwards. The data are however consistent with a multiple sill model whereby sills intrude at random depths.
T52B-04
Deformation and Melt-Rock Interaction in Peridotites From the Atlantis II Transform, SWIR: Evidence for Diffuse Melt Percolation in Deep Lithospheric Mantle
Abyssal peridotites that record both melt-rock interaction and crystal-plastic deformation provide the most direct means of assessing the interaction of tectonic processes and melt generation and transport processes in the mantle beneath mid-ocean ridges. We present a preliminary study of microstructural observations, crystallographic fabric analysis, modal compositions, and mineral chemistry for 10 spinel lherzolites from the Atlantis II Transform Fault on the ultraslow-spreading (~1.4 cm/year) Southwest Indian Ridge (SWIR). The samples were collected via submersible along a 1km traverse up the side of the valley. All preserve protogranular to weakly porphyroclastic textures characteristic of high-temperature (~1100-1250°C) deformation. Deformation microstructures include: a) patchy zones of recrystallized olivine, ortho-and clino- pyroxene, and b) a weakly developed pyroxene shape-preferred orientation. Electron backscatter diffraction (EBSD) analysis indicates that olivine, ortho- and clino-pyroxene exhibit a weak but consistent lattice-preferred orientation (LPO). The average olivine and orthopyroxene M-indices are ~0.1, comparable to peridotites from the upwelling zones of ophiolites. Clinopyroxene M-indices are very weak (~0.05). The samples also preserve microstructural evidence for melt-rock interaction, including: a) olivine "embayments" in orthopyroxene resulting in extremely complex grain shapes; b) thin (<0.1mm) interstitial clinopyroxene and spinel, oriented roughly parallel to the foliation and LPO slip direction; c) vermicular spinel replacing the edges of orthopyroxene porphyroclasts; and d) large (3-5mm) complex spinels interstitial to aggregates of ortho- and clino-pyroxene. These delicate microstructures are not deformed; thus at least some melt percolation and crystallization must postdate the high temperature deformation. The degree to which melt-rock interaction has overprinted the deformation varies from sample to sample, showing that the last stage of melt percolation was heterogeneously distributed on the scale of <1km. Two samples show plagioclase-rich melt impregnations and one contains a 5mm wide micrograbbro vein. Our study reveals the upwelling and emplacement history of these peridotites: 1) partial melting, along with deformation and melt-rock reaction from ~60-30km in the upwelling mantle, 2) accretion to the lithosphere at a depth of >25km beneath the SWIR while still upwelling (before "corner flow"); this depth is consistent with the expected mantle temperatures adjacent to a large-offset transform fault, 3) cessation of penetrative deformation, and onset of localized deformation in shear zones not sampled by this study, 4) heterogeneously distributed melt percolation while still in the spinel stability field at depths >25km, 5) minor late-stage intrusion of gabbro veins and interstitial melt in the plagioclase stability field (<25km depth), and 6) emplacement of peridotites to the wall of the Atlantis II Transform Fault. These samples record the cessation of both melting and penetrative high temperature deformation at a depth >25 km. Therefore, we suggest that the majority of melt delivery and heat supply may have been focused toward the ridge segment center and away from the segment end near the Atlantis II transform. The last recorded stage of significant melt percolation in these samples was diffuse and spatially heterogeneous.
T52B-05
Thermal History of an Oceanic Core Complex, Atlantis Bank, Southwest Indian Ridge: Evidence for Hydrothermal Activity 2.6 Myr Off-Axis
We report 26 new (U-Th)/He zircon dates from the Atlantis Bank Oceanic Core Complex (OCC), Southwest Indian Ridge. The low (~200 °C) closure temperature of the (U-Th)/He isotopic system, together with higher temperature (850 °C) crystallization ages from U-Pb zircon dating, allow us to constrain the timescales and rates of lower crustal cooling in oceanic crust. Samples from the detachment fault surface exposed at the sea floor, indicate that the denuded crust cooled rapidly through 200 °C in <1 Myr, yielding mean cooling rates >1200 °C/Myr, consistent with existing models for cooling of oceanic crust. However, samples collected along post-detachment, N-S- and E-W-trending fault scarps record (U-Th)/He ages averaging 2.6 Myr younger than their corresponding igneous crystallization ages. These ages are inconsistent with steady-state conductive cooling models for lower oceanic crust and cannot be explained by simple monotonic cooling. Instead, they record cooling through 200 °C when the crust was well outside the rift valley, ~36 km off-axis assuming a half spreading rate of 14km/Myr. These samples display extensive post-crystallization greenschist-facies alteration and contain metamorphic mineral assemblages of chlorite + actinolite ± hornblende ± epidote ± serpentine ± clay, consistent with hydrothermal alteration. Therefore, we suggest that these anomalously young (U-Th)/He zircon ages record localized thermal heating events associated with high- temperature (>300 °C) hydrothermal fluid flow along transform-parallel and transform-normal faults that were active outside the rift valley during transtension along the bounding Atlantis II transform fault. A significant component of the heat driving hydrothermal fluid flow may have been derived from underplated mafic magmas emplaced during transtension. The young (U-Th)/He ages therefore delimit zones of hydrothermal upflow, and record evidence of protracted hydrothermal circulation up to ~3 Myr off-axis at the Atlantis Bank OCC.
T52B-06
Crust-Poor Lithosphere at Cold Spots in the Mid Atlantic and SW Indian Ridges
The Equatorial portion of the Mid Atlantic Ridge is thought to reflect a thermal minimum in the subridge structure, with deeper than normal axial topography underlain by high upper mantle seismic velocities revealed by tomography. This stretch of Ridge is intersected by a number of long offset transforms, the longest being the Romanche (offset ~950 km corresponding to ~50 Myr). As the Mid Atlantic Ridge axis approaches the Romanche transform from the south, it gradually deepens; its rift valley disappears, and, starting roughly 50 km from the transform, the basaltic crust becomes patchy and then disappears, leaving mantle ultramafics outcropping on the sea floor. Modelling the "cold edge" effect of the transform on the axial Ridge segment shows that partial melting of the subridge mantle decreases as the transform is approached. Crust-free lithosphere outcrops continuously for several hundred kilometers in a ~30 km wide belt south of the Romanche, indicating that the present-day lack of crustal production has been prevailing for at least 30 million years. The mantle derived serpentinized peridotites are of two types. The majority of the samples show evidence of strong impregnation by basaltic melts. The mineral chemistry of the samples that are free of impregnation suggests that they have undergone a very low degree of melting. These results suggest a quasi-crust-free lithosphere, produce by a mantle that has undergone little or no partial melting, unable to expel the small quantities of melt it generates. The small quantities of basalt produced in this area tend to have alkali affinity and are strongly enriched in H2O. Their REE content show a strong garnet signature, suggesting that they were generated mostly in the garnet peridotite mantle zone (> 20 kbar). This quasi-crust-free impregnated lithosphere probably prevails at cold spots along mid ocean ridges. Peridotites obtained recently from the SW end of the Andrew Bain transform, that offsets the SW Indian Ridge by ~750 km (~50 Myr), are strongly impregnated by basaltic melt, in a situation very similar to that observed near the Romanche. In contrast, peridotites from the NE end of the Andrew Bain transform are not impregnated, and are residual of a significant degree of melting, probably due to the influence of the Marion plume located a few hundred km away.
T52B-07
First Discovery and Investigation of a High-Temperature Hydrothermal Vent Field on the Ultra- Slow Spreading Southwest Indian Ridge
Two recent cruises on board the Chinese research vessel Dayang Yihao have successfully investigated the first active hydrothermal vent field to be located along the ultraslow spreading Southwest Indian Ridge (SWIR) and collected hydrothermal sulfide deposit samples. The newly discovered hydrothermal vent field is located on the western end of a magmatically robust spreading segment immediately west of the Gallieni transform fault. Preliminary evidence of strong turbidity anomalies was first measured during a Nov. 2005 cruise on board Dayang Yihao (InterRidge News, vol. 15, pp. 33-34, 2006). Color video footages of the seafloor in the vent-field area were first obtained by a deep-towed video camera in February 2007 during DY115-19 Leg 1, when significant water column turbidity anomalies, noticeable temperature anomalies and methane anomalies were also measured. The vent field was then precisely located, mapped, and photographed in great detail in February- March 2007 during the DY115-19 Leg 2, using the autonomous underwater vehicle ABE of the Woods Hole Oceanographic Institution. A high-resolution bathymetric map, more than 5,000 near-bottom color photos, and several types of water column data were all obtained during three phases of ABE dives. Within the approximately 120-m-long by 100-m-wide hydrothermal field, three groups of active high-temperature vents were identified and color images of black smokers and associated biological communities were obtained from ABE, flying 5 m above the seafloor. Hydrothermal sulfide deposits were then successfully obtained using a TV-guided grab.
T52B-08
Shearing within in situ lower crust during progressive retrogression: a structural study of the Godzilla Mullion, Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea
Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a detachment fault at depth as part of an oceanic core complex. Microstructures indicative of intense deformation are observed within samples of gabbro dredged from the breakaway of the mullion (dredge site D6), at the site of its initiation. The sizes of recrystallized grains, nature of crystal-preferred orientation, and chemical composition all vary systematically with respect to the temperature of deformation. The sizes of dynamically recrystallized grains of plagioclase can be divided into three types: coarse (80–130 µm), medium (25 µm), and fine (8 µm). Furthermore, although the chemical composition of plagioclase porphyroclasts is consistently An 40–50 among all grain sizes, the compositions of dynamically recrystallized grains vary with size, being An 40–50 for the coarse type, An 30–40 for the medium type, and An 20–30 for the fine type. Given that the chemical composition of plagioclase potentially results from a change in temperature during dynamic recrystallization, this finding suggests that the microstructural development of plagioclase occurred under increasing stress during uplift-related cooling of the gabbro body. Plagioclase crystal-preferred orientations (CPOs) shows a (010)[100] pattern for coarse and medium grains, but a largely random pattern for fine grains. This indicates that grain-size-sensitive creep became progressively dominant in the fine type, thereby leading to strain softening and localization during cooling. Although the chemical composition of hornblende varies from pargastic-hornblende to actinolite and tremolite within each of the gabbro samples, the pattern of variation is similar among the three sample types. However, hornblende in the coarse type shows no evidence of deformation, whereas hornblende in the medium and fine types is plastically deformed. Consequently, we propose that a primary shear zone developed at the breakaway under high temperatures (>700E#8249;C) and at depth under anhydrous conditions. The shear zone subsequently evolved during progressive retrogression in association with hydration of the shear zone, eventually resulting in the development of the detachment fault that gave rise to the Godzilla Mullion.