COMPARING MODERN AND ANCIENT OROGENS: HIMALAYAS AND APPALACHIANS SIMILARITIES AND DIFFERENCES

Ancient and modern orogens present opportunities to compare components preserved in one but not the other. Both the Appalachians (APs) and Himalayas (HMs) are products of continent-continent collision, with both being about the same length, and both record diachronous events along strike. The APs, however, are the product of arc-continent collision (Ordovician Taconic orogeny), collision of a composite terrane (mid-Paleozoic Acadian-Neoacadian orogeny), and transpressive continent-continent collision (late Pennsylvanian-Permian Alleghanian orogeny). The HMs are the product of Eurasian-Indian plates collision beginning ~50 Ma, continuing today. Suturing along the Indus-Tsangpo is defined by dismembered ophiolites. Crustal thickening of passive Indian margin sedimentary rocks produced Barrovian metamorphism and a fold-thrust belt in the late Eocene. The basal detachment that initially accommodated south-directed thrusting was then reactivated as the South Tibetan detachment (STD) when the locus of exhumation propagated toward the foreland following onset of coeval movement on STD and Main Central thrust. Together these produced south-directed extrusion of the anatectic core of the Himalayas from beneath the southern margin of the Tibetan Plateau (TP) as India is A subducted beneath southern Asia. Motion vectors from GPS measurements record complex kinematics in the TP and toward SE Asia. The processes that formed the High Himalaya have been interpreted as a combination of south-directed extrusion and orogen-parallel extension that exhumed mid-crustal rocks from different structural positions. Flow patterns in the AP Inner Piedmont (IP) are complex and broadly similar to those in the TP, so flow in the eroded mid-crustal IP could reveal the flow pattern beneath the TP. These patterns, however, are products of oblique, transpressive collision of Carolina (Avalon) superterrane, buttressing along the dextral Brevard fault and A-subduction of Laurentia, not head-on collision as in the HM.