ANDINOTYPE VS. INTRAPLATE OROGENESIS – HIGH PLATEAUS AND LOWER CRUSTAL STRUCTURES

Mesozoic to recent andinotype tectonism spanning the entire western margins of both North and South America has been superimposed upon older intraplate orogens resulting from Neoproterozoic-earliest Paleozoic assembly of Gondwana and from late Paleozoic collision of Gondwana with Laurentia. The later episodes have exhumed deeper portions of the older orogens, exemplified by structures of the late Paleozoic Ancestral Rocky Mts., which are exposed in Laramide (Mesozoic-Cenozoic) Rocky Mt. uplifts from NW Canada to NE Mexico. Similarly, lower crustal structures of the Sierras Pampeanas, an intraplate consequence of South America-Africa collision during Gondwana assembly, have been exposed through ongoing Andean orogenesis (e.g., Escayola et al., 2005, Gondwana 12 abstracts, p. 147).

Both intraplate and andinotype systems give rise to high plateaus – volcanic plateaus during andinotype orogenesis (Andean Altiplano) and vast nonvolcanic plateaus during intraplate tectonism (Tibetan Plateau). Andesitic volcanism characterizes andinotype systems, whereas peraluminous intrusive activity, crustal thickening and anatexis, metamorphism and ductile deformation at depth are attributes of intraplate systems. The crust of the Tibetan Plateau, for example, approaches twice the thickness (>60 km) of normal continental crust and ten times that of subducting oceanic slabs (e.g., Jordan and Watts, 2005, EPSL, p. 732-750). Thickening of Himalayan upper crustal rocks at the expense of overridden Indian continental crust during continental collision is interpreted from broadband seismic data (Shulte-Pelkum et al., Nature, 30 June 2005).

Unambiguous discriminants for the two orogenic styles include crustal thickness, fault-plane dips in the lower vs. upper crust, and structural relief on the lithospheric mantle. Deep seismic investigations of intraplate complexes unsullied by later subduction-related deformation, as in central Australia (Korsch et al., 1998, Tectonophysics, p. 57-69), reveal structures that are specific to intraplate mountain building. Mantle tomography, broadband seismometry, and other deep imaging methods aid in discrimination of andinotype and intraplate orogenic elements.