HR: 14:10h
AN: T23F-03 INVITED     [Abstracts]
TI: Internal Structure of the Upper Oceanic Crust Generated at Fast to Intermediate Rates: The View From Tectonic Windows in the Pacific
AU: * Karson, J
EM: jkarson@duke.edu
AF: Division of Earth & Ocean Sciences, Duke University, Durham, NC 27708 United States
AB: Faulted escarpments provide tectonic windows into the upper oceanic crust generated at the East Pacific Rise (EPR) at spreading rates ranging from 60 mm/yr to 140 mm/yr (full rates). These natural cross sections reveal the architecture of upper crustal rock units and have implications for sub-axial processes at spreading centers like the EPR. Exposures that are a few tens of kilometers across and 2000 to 3000 m high reveal lateral variations in the thickness and internal structure of basaltic lava and sheeted dike units that can be compared to the essentially 1-dimensional samples from deep drill holes and inferences based on seismic studies of the upper crust. Near-bottom investigations at the Blanco Transform Scarp (60 mm/yr; 1 Ma), Hess Deep Rift (110 mm/yr; 1 Ma), Pito Deep Rift (140 mm/yr; 3 Ma) and Endeavor Deep (160 mm/yr; 3 Ma) reveal a spectrum of structures that may be related to spreading-rate-sensitive parameters. All of these cross sections are approximately parallel to spreading flow-lines, but their positions relative to spreading segment boundaries is not well constrained. Based on these areas, the following variations occur with increasing spreading rate: 1) decreasing variability of the total thickness of lavas and sheeted dikes, though the average thickness remains constant; 2) decreasing thickness of basaltic lavas with highly variable proportions of sheet flows relative to pillow and lobate forms; 3) increasing thickness of the sheeted dike unit; 4) decreasing fracturing and tilting of lavas and dikes during axial magmatic construction and hydrothermal metamorphism; 5) increasingly localized faulting and fluid flow. These relationships indicate that sub-axial subsidence that accommodates thickening of the lava unit and results in brittle damage to the underlying crustal units diminishes with spreading rate. Although many different processes interact during crustal accretion, magma pressure and frequency of dike intrusion relative to plate separation rate can account for many of these features. These results must be reconciled with seafloor investigations of volcanic, tectonic, and hydrothermal features along active spreading center segments in developing a comprehensive understanding of processes that build and modify the upper oceanic crust at relatively high spreading rates.
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3641 Extrusive structures and rocks
DE: 3642 Intrusive structures and rocks
DE: 8010 Fractures and faults
DE: 8011 Kinematics of crustal and mantle deformation
SC: Tectonophysics [T]
MN: Fall Meeting 2005