EXTENSIONAL TRANSFORM ZONES AND OBLIQUE SPREADING CENTERS

Extensional transform zones (ETZs) are plate boundary segments of orde r 100 km long that strike at angles between 15 degrees and 45 degrees to the extension direction. They are characterized by neovolcanic/tect onic zones comprising overlapping en echelon volcanic systems and/or f aults that trend 30 degrees-75 degrees to the extension direction, som etimes accompanied by a Riedel shear. Below these surficial en echelon structures the deformation is aseismic and ductile, and the plate bou ndary is probably continuous. ETZs occur in fast and slow spreading an d rifting environments and may persist in a stable configuration for s everal million years. ETZs link oblique spreading segments to transfor m faults in the Manus and probably the Lau backarc basins. The Reykjan es Peninsula and Tjornes Fracture Zone in Iceland and the Mak'Arrasou in Afar are ETZs that link subaerial to submarine spreading or rifting segments. The Brawley and Cerro-Prieto seismic zones appear to be ETZ s in the Imperial and Mexicali valleys that link the San Andreas, Impe rial, and Cerro-Prieto transform faults. Experimental and analytical m odels of transtensional deformation in brittle-ductile systems match m any of the observed characteristics of ETZs and oblique spreading cent ers, including variably sigmoidal to straight en echelon faults that a re not parallel to the extension direction. The contrasting fault patt erns reflect the theology of the models and lithosphere: they are more sigmoidal when the strain in the lower ductile layer is more focused, causing the axial faults to curve as they propagate toward parallelis m with the less ductile rift margins. The angle (circle divide) betwee n the faults and the extension direction decreases with the angle (a) between the strike of the zone and the extension direction. ETZs occur in the range 15 degrees less than or equal to alpha less than or equa l to 45 degrees, whereas oblique spreading centers have 45 degrees<alp ha<90 degrees and transform relay zones have 0 degrees<alpha<15 degree s. Oblique fast spreading segments exhibit ridge-parallel faults and v olcanic systems (circle divide=alpha), presumably reflecting locally r otated stress fields, whereas at oblique slow spreading centers, circl e divide is closer to orthogonal (alpha<circle divide<90 degrees).