Pangaean divergent margins: historical perspective

Pangaean divergent margins caused by the breakup by rifting of that supercontinent display three distinct evolutionary stages. In their youth or rift stage, divergent margins display a synsediment fault style that has considerable tectonic instability. During the mature or drift stage (seafloor spreading) continental margins are relatively stable and are dominated by thermal subsidence. These two stages (rifting and drifting) are separated in many areas by an unconformity coeval with the oldest associated oceanic crust — the breakup unconformity; in some areas the onset of seafloor spreading is marked by intense magmatic activity. Sediment facies the rifting stage range from continental clastics landward of the continental basement hinge and along the flanks of the basin to saline deposits and more open-water carbonates in the axial zone of the rift. Lithologies emplaced during the rift or mature stage of margin development range from shallow clastics/carbonates to deep-water carbonate/siliceous oozes, turbidites and contourites, some of which were deposited in an anoxic environment. Deposition then is controlled by changes in configuration of basins, changes in sea level, and plate migration through several climatic belts. During the final, or old-age, of margin development, divergence gives way to convergence. This stage, which generally terminates with continental collision, is characterized by tectonism, magmatic activity, and crustal shortening. Mesozoic divergent margins along the east-west trending former Tethys Ocean have reached this stage in their evolution.