IMPLICATIONS OF INTRACRATONIC SYNRIFT FAULTS FOR LITHOSPHERIC ROLE OF TRANSFORM FAULTS ALONG THE IAPETAN LAURENTIAN MARGIN

During Late Proterozoic-Early Cambrian continental rifting, breakup of Rodinia, and opening of the Iapetus Ocean, northwest-trending transform faults offset northeast-trending rift segments to outline promontories and embayments of the rifted margin of eastern Laurentia. Intracratonic fault systems, primarily oriented northeast and northwest, parallel rift and transform segments, respectively, of the Laurentian margin.

The Mississippi Valley—Rough Creek—Rome fault system includes dextrally offset northeast-striking graben systems on the south (Mississippi Valley) and north (northern Rome) linked by easterly trending fault systems (Rough Creek and western Rome). East-striking faults of the Rough Creek and western Rome systems apparently are not continuous along strike; however, the dextral transform offset of the northeast-striking Mississippi Valley and northern Rome extensional fault systems is aligned with transform faults of the rifted continental margin. The northeast-striking Birmingham graben ends northeastward against northwest-trending transverse faults, which are aligned with a transform fault of the rifted margin. Clastic sedimentary fill of the grabens ranges from Early to early Late Cambrian in age, contemporaneous with the latest stages of continental rifting.

The Southern Oklahoma fault system comprises laterally variable and discontinuous faults along an alignment parallel with transform faults of the continental margin. Associated bimodal igneous rocks have ages of 539-530 Ma, similar to the ages of synrift sediment in intracratonic grabens. Gravity and magnetic highs and steep gradients are consistent with dense mafic rocks along a near-vertical fracture system with minor extension.

The discontinuity and obliquity of transform-parallel faults suggest a brittle, shallow crustal response to ductile transform motion in the deeper lithosphere. The more continuous rift-parallel fault systems reflect brittle extension of the shallow crust associated with ductile extension of the deeper lithosphere. Lithosphere-penetrating fabric documented by seismic anisotropy defines zones of distributed shear in the deep lithosphere beneath shallow crustal transforms, further linking upper crustal transform faults to transform motion in the ductile lithosphere.