HR: 1330h
AN: T52A-0238    [PDF]
TI: Interplate and Intraplate Decoupling: A 3D View from Surface Geology and Seismicity, Eastern Hellenic Forearc, Greece
AU: Kleinspehn, K L
EM: klein004@umn.edu
AF: Dept. of Geology and Geophysics, University of Minnesota, Pillsbury Hall, Minneapolis, MN 55455 United States
AU: * Russo, R M
EM: ray@earth.northwestern.edu
AF: Dept. of Geological Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208 United States
AB: Shallow active seismicity and neotectonic structures reveal important changes in the degree of interplate and intraplate coupling along the convergent Hellenic plate boundary from Crete to Rhodes. The onshore/offshore Pliocene-Holocene surface geology of the Hellenic forearc records three different deformation states: 1) A western segment (western Crete) where incipient continent-continent collision produces shortening under strong interplate coupling; 2) a central segment (central-eastern Crete) partly coupled to Africa where oblique convergence is partitioned into sinistral strike slip and orthogonal shortening which is confined to the accretionary wedge; and 3) an eastern trantensional segment (Rhodes), mechanically decoupled from African oblique convergence, instead reflecting slab rollback and Aegea's southward motion relative to Anatolia. Such along-strike heterogeneity of neotectonic structures suggests each segment should also display distinct crustal-scale stress patterns. Abundant earthquake focal mechanisms provide a means to gauge stress regimes. Shallowly plunging P (compression) and T (tension) axes of crustal events differ systematically along the three forearc segments. Above the brittle-ductile transition ($<13 km$), the western segment records N-S P axes and E-W T axes. In the central Crete transition zone, P and T axes vary, whereas sparse P axes in the decoupled eastern forearc (Rhodes) parallel the NNE plate margin. Below the brittle-ductile transition ($13 < h < 40 km$), P axes beneath western Crete trend N-S normal to the subduction trace, signifying interplate coupling given their similarity to plate-convergence vectors. T axes trend WNW consistent with margin-parallel extension at depth due to Africa's northward convergence. Stress patterns reverse for the wrench-dominated transition zone: P axes trend WNW-ESE and T axes trend N-S, indicating that northward convergence is less important than slab roll back. In the transtensional forearc east of Crete, P axes vary, but T axes trend normal (WNW) to the plate margin (NNE), consistent with forces due to rollback. Thus, the maximum compressive horizontal stress, delineated by shallowly plunging P axes, varies both laterally and vertically within the Hellenic forearc. The western coupled forearc segment displays similar P-axis trends at all depths 0-40 km where the base of forearc crust directly overlies subducting African lithosphere. Eastward, variable P axes become systematic below the brittle-ductile transition where Aegean upper mantle lies between the eastern Cretan Moho and the top of the African slab, suggesting vertical intraplate decoupling where upper mantle underlies forearc crust.
DE: 7218 Lithosphere and upper mantle
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 8158 Plate motions--present and recent (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting