396 TECTONICS/Propagating Rifts and Microplates At Mid-Ocean Ridges Propagating Rifts and Microplates At Mid-Ocean Ridges R N Hey, University of Hawaii at Manoa, Honolulu, HI, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Propagating rifts are extensional plate boundaries that gradually break through lithospheric plates, forming new plate boundaries and rearranging the geometries of old ones If the rifting advances to the seafloorspreading stage, propagating seafloor-spreading centres gradually extend through the rifted lithosphere This evolution occurs rapidly ($105 years) for oceanic propagators and much more slowly ($107 years) for continental ones The orthogonal combination of seafloor spreading and propagation produces a characteristic V-shaped wedge of lithosphere, with progressively younger and longer isochrons abutting the ‘pseudofaults’ that bound this wedge Oceanic propagators generally replace preexisting spreading centres, causing lithospheric transfer from one plate to another, and sequences of spreadingcentre jumps, leaving failed rift systems in their wakes This changes the classic plate tectonic geometry and results in asymmetric accretion of lithosphere to the plates There is pervasive shear deformation in the overlap zone between the propagating and failing rifts, much of it accommodated by bookshelf faulting, in which, e.g., right-lateral plate motion shear produces high-angle left-lateral fault slip When the scale or strength of the overlap zone becomes large enough, it stops deforming, and instead begins to rotate as a separate microplate between dual active spreading centres Continental propagators break apart continents and can leave failed rifts (aulocogenes) along unsuccessful propagation paths, as well as predictable patterns of deformation ahead of the propagator tips For continental rifting, the pseudofaults are the continental margins Rift propagation appears to be the primary mechanism by which Earth’s accretional plate boundary geometry is reorganized Oceanic Propagators Figure shows several variations of typical oceanic ridge propagation geometry, in which a pre-existing ‘doomed rift’ is replaced by the propagator This always seems to result in at least slight spreading-centre reorientation Whether this is because rifts propagate in response to changes in direction of seafloor spreading, or because the spreading direction changes while rifts propagate, rift propagation is the primary mechanism by which many seafloor-spreading systems have adjusted to changes in spreading direction Propagation rates and local spreading rates are often similar in magnitude, although propagation rates as high as 1000 km My have been discovered Figure 1A shows the discontinuous propagation model, in which periods of seafloor spreading alternate with periods of instantaneous propagation, Figure (A) Discontinuous, (B) continuous, and (C) non trans form zone oceanic propagating/failing rift models Propagating rift lithosphere is marked by dark stipple, normal lithosphere created at the doomed rift is indicated by light stipple, and trans ferred lithosphere is cross hatched Heavy lines show active plate boundaries In the non transform zone model(C), active axes with full spreading rate are shown as heavy lines; active axes with transitional rates are shown as dashed lines The overlap zone joins these transitional spreading axes Reproduced with permission from Hey RN, Sinton JM, and Duennebier FK (1989) Propagating rifts and spreading centers In: Winters EL, Hussong DM, and Decker RW (eds.) Decade of North American Geology: The Eastern Pacific Ocean and Hawaii, pp 161 176 Boulder, CO: Geo logical Society of America