HR: 10:20h
AN: T52B-01 INVITED     [Abstracts]
TI: Rifting and Breakup Between Newfoundland and Iberia
AU: * Tucholke, B E
EM: btucholke@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Sawyer, D S
EM: dale@rice.edu
AF: Department of Earth Science, Rice University, Houston, TX 77005 United States
AU: Sibuet, J
EM: jcsibuet@ifremer.fr
AF: Ifremer Centre de Brest, B.P. 70, Plouzane, 29280 France
AB: The Newfoundland-Iberia rift is considered to be a type example of non-volcanic rifted margins. A key feature of the conjugate margins is a transition zone (TZ) that lies between clearly continental crust and assumed 'normal oceanic crust' that appears up to 200 km farther seaward near magnetic anomaly M5-M3 (Hauterivian-Barremian). Basement ridges drilled in the Iberia TZ appear to consist of serpentinized peridotite with continental affinity, consistent with seismic refraction studies. The boundaries between continental crust and the TZ can be defined with relative confidence from basement morphology and velocity structure, but major questions remain about the nature and position of the seaward limit of the TZ. Notably, drilling at Site 1070 near anomaly M2r on the Iberia margin and at Site 1277 near anomaly M1 in the Newfoundland Basin recovered serpentinized peridotite from supposedly oceanic crust. In addition, existing interpretations place the 'breakup unconformity', classically associated with both the end of continental rifting and the first formation of oceanic crust, near the Aptian-Albian boundary (ca. 112 Ma). This age is some 15 m.y. younger than the oldest magnetic anomaly (M5) that has been proposed to have oceanic crustal affinity off Iberia, and it is 9 m.y. younger than anomaly M0, which many investigators argue is certain oceanic crust. There are three possible ways to explain this paradox: 1) the breakup unconformity has been misidentified, 2) the breakup-unconformity paradigm is invalid, or 3) actual `breakup' occurred within the domain currently considered to be oceanic crust. We analyzed the tectonic structure and basal (pre-Cenomanian) stratigraphy of the rift using seismic reflection profiles and drilling results. Rifting occurred in three phases (Norian-Hettangian, Oxfordian-Kimmeridgian, and Tithonian-Aptian). The first two formed continental rift basins without significant thinning of continental crust, but the third phase led to continental breakup, with extension concentrated in three episodes that culminated near the end of Berriasian, Hauterivian, and Aptian time. Our analysis suggests that consistent accretion of 'normal' oceanic crust did not occur until near the Aptian/Albian boundary, coincident in time with the formation of the historically proposed breakup unconformity. Up to that time, the entire rift probably was under elevated in-plane tensile stress, manifested by continued mantle exhumation (with minor amounts of introduced melt) as well as by localized extensional deformation throughout both plates. At the Aptian/Albian boundary, relaxation of tensile stress caused relative compression of the plates and associated deformation in weak zones (e.g., serpentinite ridges), enhanced mass wasting from such ridges in the deep basins, shallow-water shelf erosion, and basin flooding by turbidites. The coarse debris that accumulated in the deep basins created a strong reflection that characterizes the event. Thus the 'breakup unconformity' here is interpreted to mark the onset of seafloor spreading at the end of mantle exhumation that formed the TZ, and it postdates the final separation of continental crust by 14-15 m.y. The event may mark the ultimate separation of subcontinental mantle by upwelling asthenosphere in the rift.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3075 Submarine tectonics and volcanism
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005