ACCRETIONARY COMPLEXES: RECORDERS ON EARTH AND POSSIBLY MARS

Accretionary complexes occur in parts of the world, highlighted in Japan, North America, Europe, and Greenland. They comprehensively record information compiled while the oceanic crust is en route from the mid-oceanic ridge to the subduction zone, spanning hundreds of millions of years. At the zone, oceanic crustal materials are stacked along thrust faults and/or subducted to be eventually recycled into the mantle. The surviving accretionary-complex materials include Ocean Plate Stratigraphy (OPS). The ideal succession of the OPS (from oldest to youngest) is mid-ocean ridge basalt, pelagic sediment including radiolarian chert, hemipelagic sediment including siliceous shale, and trench turbidite deposits. Therefore, accretionary complexes often record diverse environmental conditions from deep- to shallow-marine environments, including those perturbed by endogenic (e.g., magmatic) and exogenic (e.g., impacts and Snowball-Earth) events. During an ancient, dynamic phase of its evolution (~> 4.0 Ga), Mars had Earth-like conditions, including an interacting ocean, landmass, and atmosphere, as well as possible plate tectonism. A candidate accretionary complex and nearby outcrop of steeply dipping beds comprising olistostrome-like blocks, nearby and in the Claritas rise, respectively, southwest margin of the Tharsis superplume, may be key evidence of major crustal shortening related to plate tectonism. Claritas rise is a rugged quasi-circular promontory about 250 km across, which forms the northwest part of an extremely ancient and large mountain range, Thaumasia highlands, extending west to east for nearly 2,400 km, or approximating the length of the Himalayas on Earth. Future investigation of the ancient Martian basement, which includes geochemical analyses for possible OPS sequences (an important candidate test), as well as reconnaissance for other parts of Pacific-type orogenic complexes, which includes metamorphic belts, ophiolite sequences, and belts of felsic materials, will be an important next phase in the geologic investigation of Mars. Such features could contain far-reaching records dating back more than 4.0 Ga based on stratigraphy, crater statistics, and magnetic-anomaly map information, equivalent to Hadean records on Earth which have been all but destroyed.