Extensional transform zones and oblique spreading centers

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 99, NO. B10, PAGES 19,707-19,718, OCTOBER 10, 1994

Extensional transform zonesand oblique spreadingcenters Brian Taylor, Keith Crook, andJohnSinton Schoolof OceanandEarthScienceandTechnology, Universityof Hawaii,Honolulu

Abstract.Extensionaltransformzones(ETZs) areplateboundarysegments of order100 km long thatstrikeat anglesbetween15ø and45ø to theextensiondirection.They arecharacterized by neovolcanic/tectonic zonescomprisingoverlappingen echelonvolcanicsystemsand/orfaultsthat trend300-75ø to the extensiondirection,sometimesaccompanied by a Riedel shear.Below these surficialenechelonstructures thedeformation is aseismicandductile,andtheplateboundaryis probablycontinuous. ETZs occurin fastandslowspreadingandrifting environments andmay persistin a stableconfiguration for severalmillionyears.ETZs link obliquespreading segments to transformfaultsin the ManusandprobablytheLau backarcbasins.The ReykjanesPeninsulaand TjornesFractureZone in Iceland andthe Mak'Arrasouin Afar are ETZs that link subaerialto

submarine spreading or rifting segments. The BrawleyandCerro-Prietoseismiczonesappearto be ETZs in theImperialandMexicali valleysthatlink the SanAndreas,Imperial,andCerro-Prieto transformfaults.Experimentalandanalyticalmodelsof transtensional deformationin brittleductilesystemsmatchmanyof theobservedcharacteristics of ETZs andobliquespreadingcenters, includingvailablysigmoidalto straighten echelonfaultsthatarenotparallelto the extension direction.The contrasting faultpatternsreflectthetheologyof themodelsandlithosphere: they are more sigmoidalwhenthe strainin the lower ductilelayer is morefocused,causingthe axial faultsto curveastheypropagatetowardparallelismwith the lessductilerift margins.The angle

(0) between thefaultsandtheextension direction decreases withtheangle(ct)betweenthestrike of the zoneandthe extensiondirection.ETZs occurin therange15ø_