INITIAL BREAKUP OF PANGEA AND BIRTH OF THE ATLANTIC: A DIFFICULT AND COMPLEX PROCESS COMPARBLE TO THE BIRTH OF LARGE ANIMALS

The breakup of Pangea had a long and far reaching precursor history. Some events are well known. Late Triassic-Early Jurassic extensional-basins and 200 Ma CAMP-province diabase-dikes and volcanic rocks, for example, have been studied on both sides of the Central Atlantic for over a century. Less well-studied, however, are widely distributed small-displacement fault-systems that also formed in eastern North America at this time. Locally these faults offset diabase dikes; elsewhere, however, dikes cut the faults. There are three dominant fault orientations: ~N-S: ~E-W; and 035-050. Displacement sense depends upon orientation and varies from normal to dextral to sinistral. Some faults reactivate old faults, including some basin border-faults; other faults, however, were not controlled by the dominant orogenic fabric of the Appalachians (035-070). Both brittle and ductile fault rocks occur. Where dilational, many brittle faults record a complex history of vein-filling and fault-slip. Faults, initially filled with quartz, can become multiply reactivated, brecciating the quartz-fill and re-filling the fault zone with more, sometimes vuggy, vein-quartz, only to be broken again. Some larger fault-systems are filled with large siliceous cataclasite bodies that are rhomb-shaped in dextral stepovers, and more linear when parallel to faults. Time-equivalent very low temperature quartz-mylonite layers formed in similar orientations over a wide area. These mylonites have a very distinctive fabric comprising long, low-angle boundary-dominated quartz ribbons that flow around rare, rigid feldspar grains. These mylonites formed at temperatures close to the brittle-plastic transition, but with insufficient thermal energy to anneal the strain fabric, with the exception of very local grain boundary recrystallization. Metamorphic assemblages in veins include zeolites (e.g., heulandite) and prehnite, indicating burial depths of <10 km; VR-data from coal seams in several basins suggest burial depths could be <3 km. The eventual onset of more focussed separation was accompanied by the formation of the East Coast magnetic anomaly, which is possibly related to a linear mafic body produced by break-up related decompression melting of lower crust and upper mantle. The initial breakup of Pangea was now well on its way. . .