Abstract
We report the results of a two‐dimensional tomographic inversion of marine seismic refraction data from an array of ocean‐bottom seismographs (OBSs), which produced an image of the crustal ...structure along the axial valley of the ultraslow spreading Mid‐Cayman Spreading Center (MCSC). The seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic‐tectonic segments along the 110 km long spreading center. Seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments, but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect. The two spreading segments both have a low‐velocity zone (LVZ) several kilometers beneath the seafloor, which may indicate the presence of shallow melt. However, the northern segment also has low seismic velocities (3 km/s) in a thick upper crustal layer (1.5–2.0 km), which we interpret as an extrusive volcanic section with high porosity and permeability. This segment hosts the Beebe vent field, the deepest known high‐temperature black smoker hydrothermal vent system. In contrast, the southern spreading segment has seismic velocities as high as 4.0 km/s near the seafloor. We suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower, thus limiting deep seawater penetration and hydrothermal recharge. This may explain why no hydrothermal vent system has been found in the southern half of the MCSC.
Key Points
Large variability in crustal structure along the axial valley of the Mid‐Cayman Spreading Center
North of Mt Dent, a seismic low‐velocity zone underlies the Beebe hydrothermal vent field
Magmatism, faulting, and high porosity in the upper crust may facilitate high‐temperature venting at an ultraslow spreading center
The responses of sedimentary systems to rifting at continental margins are three-dimensional and involve the mixing of various sediment sources through tectonic drivers and sediment response. Such ...sedimentary responses have not been well studied along magma-poor, hyperextended margins where the crust is stretched and thinned to ≤10 km. The asymmetric Mauleon Basin of the western Pyrenees is the product of such magma-poor hyperextension resulting from lateral rift propagation from the Bay of Biscay during Cretaceous time. After rifting, limited shortening during Cenozoic Pyrenean inversion uplifted the basin, resulting in preservation of outcrops of rift basin fill, upper and lower crustal sections, serpentinized lithospheric mantle, and basic rift-fault relationships. In this study ∼5800 new zircon U-Pb ages were obtained from prerift, synrift, and postrift strata; the ages constrain the proximal to distal evolution of the Mauleon Basin and define a general model for sediment routing during rifting. Zircon U-Pb analyses from lower crustal granulites indicate that granulite plutons crystallized at 279 ± 2 and 274 ± 2 Ma, and paragneissic granulites yielded zircon rim ages of ca. 295 Ma. Detrital zircon U-Pb ages from western Pyrenean prerift strata show age modes of ca. 615 and ca. 1000 Ma, suggesting continual recycling and/or well-mixed Gondwanan-sourced sediments throughout the Paleozoic and early Mesozoic; additional Paleozoic age components (ca. 300 and ca. 480 Ma) are also observed. The variation of detrital zircon U-Pb ages in synrift and postrift strata illustrates that during rifting, provenance varied spatially and temporally, and sediment routing switched from being regionally, to locally, and then back to regionally derived within individual structurally controlled subbasins.
The Black Mountain (Death Valley, CA) low angle normal, detachment faults place Pliocene-Quaternary sediment against crystalline rocks. High angle normal faults extend the sedimentary section and ...define earthquake scarps on the valley floor. The high angle faults do not offset and are coupled with the detachments. The wedge-shaped hanging wall is critically stable when the effective friction of the detachment is weakened by roughly 50% of typical earth materials. The weakening is partly because of gouge and breccia within the detachment shear zones. The fault rocks exhibit well-developed mesoscopic foliation but do not exhibit evidence for deformation from high-temperature deformation mechanisms. Measurements of the anisotropy of magnetic susceptibility (AMS) and shape-preferred orientation (SPO) of >50-micron grains define fabric with orientations consistent with the transport and extension directions of the faults. Magnetic experiments and microscopy demonstrate that the magnetic carriers within the gouge and breccia are dominantly nanometer-to-micrometer grains that grew within the shear zones prior to the most recent deformation. The kinematic development of SPO and AMS is best treated as owing to the rotation of passive markers within a passively yielding matrix. However, the similarity of fabric defined by clasts (SPO) and matrix (AMS) is more consistent with deformation via cataclastic and granular flow, each producing distinctive microstructures. Accompanying the development of the fabric was the growth of new minerals within pore spaces and the dissemination of new minerals along reaction fronts. The youngest phases are dominantly oxides and some orthoclase and dolomite, whereas earlier phases are phyllosilicates, including polytypes of illite and chlorite with interstratified swelling clays. The mineralogy can be separated into three different assemblages that grew along a retrograde reaction pathway. The reactions occurred between meteoric water and the footwall, over the last 6Ma, and from >120°C to near surface temperatures. Fault rock development does not appear to have been accompanied by elevated fluid pressure, or large water-rock ratios. Therefore, the weakening of the detachment and its propensity for seismic slip may be/have been dependent on the development of the new minerals and the change in deformation mechanism from cataclastic to granular flow.
Background As compared with open distal pancreatectomy (ODP), laparoscopic distal pancreatectomy (LDP) affords improved perioperative outcomes. The role of LDP for patients with pancreatic ductal ...adenocarcinoma (PDAC) is not defined. Study Design Records from patients undergoing distal pancreatectomy (DP) for PDAC from 2000 to 2008 from 9 academic medical centers were reviewed. Short-term (node harvest and margin status) and long-term (survival) cancer outcomes were assessed. A 3:1 matched analysis was performed for ODP and LDP cases using age, American Society of Anesthesiologists (ASA) class, and tumor size. Results There were 212 patients who underwent DP for PDAC; 23 (11%) of these were approached laparoscopically. For all 212 patients, 56 (26%) had positive margins. The mean number of nodes (± SD) examined was 12.6 ±8.4 and 114 patients (54%) had at least 1 positive node. Median overall survival was 16 months. In the matched analysis there were no significant differences in positive margin rates, number of nodes examined, number of patients with at least 1 positive node, or overall survival. Logistic regression for all 212 patients demonstrated that advanced age, larger tumors, positive margins, and node positive disease were independently associated with worse survival; however, method of resection (ODP vs. LDP) was not. Hospital stay was 2 days shorter in the matched comparison, which approached significance (LDP, 7.4 days vs. ODP, 9.4 days, p = 0.06). Conclusions LDP provides similar short- and long-term oncologic outcomes as compared with OD, with potentially shorter hospital stay. These results suggest that LDP is an acceptable approach for resection of PDAC of the left pancreas in selected patients.
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