HR: 09:15h
AN: T11G-06    [Abstracts]
TI: Counter-Rotating Microplates at the Galapagos Triple Junction, Eastern Equatorial Pacific Ocean
AU: * Schouten, H
EM: hschouten@whoi.edu
AF: WHOI, Dept. of Geology and Geophysics, MS 24, Woods Hole, MS 02543 United States
AU: Smith, D K
EM: dsmith@whoi.edu
AF: WHOI, Dept. of Geology and Geophysics, MS 24, Woods Hole, MS 02543 United States
AU: Willaims, C M
EM: clare@whoi.edu
AF: WHOI, Dept. of Geology and Geophysics, MS 24, Woods Hole, MS 02543 United States
AU: Klein, E M
EM: ek4@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Durham, NC 27708-0227 United States
AB: We recently mapped and sampled a broad region of ocean floor centered on the Incipient Rift (IR), an east-west-trending spreading center that extends eastward from the East Pacific Rise (EPR) at 2deg40minN. The IR forms a portion of the boundary of the complex Cocos-Nazca-Pacific triple junction (TJ). Based on bathymetric, side-scan (amplitude), magnetic, photographic and sampling data, we conclude that the IR is magmatically active along its length, has rifted eastward, and as it spreads, pivots about its eastern terminus (see abstract by Klein et al. this session). If the IR opens about a pivot at its eastern end, it follows that lithosphere immediately to the south of the IR rotates in a counter-clockwise direction about this pivot. This counter-clockwise rotation contrasts with the known clockwise rotation of the Galapagos microplate (GMP) to the south. It follows, then, that there must be two separate, adjacent microplates in this region: the northern microplate, herein called the northern Galapagos microplate (NGMP), and the GMP. We estimate the kinematics of the GMP and NGMP by drawing upon the concepts of edge-driven microplate mechanisms. In this approach, the rotation of a microplate is driven by a shear couple between a pair of bounding plates moving in opposite directions. The two points of coupling between the microplate and bounding plates are represented by two instantaneous relative rotation axes (IRRAs). In edge-driven microplate systems like Easter and Juan Fernandez, these axes commonly lie ahead of the tips of the microplate bounding rifts. We identify 3 such IRRAs. A flat Earth approximation yields instantaneous rotation rates of 13 deg/myr for NGMP and 21 deg/myr for the GMP relative to translating major plates. Since NGMP and GMP rotate in an opposite sense, the NGMP-GMP rate is the sum, or, 34 deg/myr. The IRRAs and their respective rotation rates predict reasonable velocities at the two ridge-ridge-ridge triple junctions, e.g., Cocos-NGMP velocity at the 2deg40minN TJ of 15 km/my (008 deg) closely matches previous estimates. The GMP-Nazca velocity at the 1deg10minN TJ of 55 km/myr (323 deg) provides a better match to the ~050deg direction of the GMP-Nazca boundary at the 1deg10minN TJ than the previous estimate of 40 km/myr (337 deg). If our new model is correct we speculate that it may be applicable to other triple junctions. In the specific case of the Cocos-Nazca-Pacific triple junction, we think that the dual microplate system acts to control the location and configuration of the Hess Deep Rift and the stability of the Cocos-Nazca-Pacific triple junction. Further work is needed to understand the evolution of this triple junction, and the nature of triple junctions and their stability in general.
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology and bottom photography
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting