NEOPROTEROZOIC-PALEOZOIC TECTONICS AND PALEOGEOGRAPHY OF THE WEST-CENTRAL SOUTH AMERICA CONVERGENT MARGIN FROM DETRITAL ZIRCON GEOCHRONOLOGY

Detrital zircon U-Pb results for 17 sandstones from a >15 km-thick Neoproterozoic-Carboniferous succession in the Andes of southern Bolivia links sediment source regions and basin subsidence patterns to the evolution of the proto-Pacific convergent plate boundary of western South America. Neoproterozoic-Cambrian (750-510 Ma) U-Pb ages for Neoproterozoic-Middle Ordovician subduction complexes and marginal basins indicate east-derived, cratonic Braziliano and Pampean orogen sources. Famatina and Ocloyic age zircons (500-420 Ma) in Early to Middle Ordovician-Devonian backarc extensional and retroarc foreland basins mark the growth of a western arc source. Renewed Pampean-Braziliano (750-510 Ma) signatures for Silurian and Carboniferous retroarc foreland settings suggest both western orogenic and southeastern cratonic sources.

A synthesis of central Andean U-Pb ages and geologic records between the Arequipa-Antofalla terrane (AAT) and Pampia craton (PC) helps define the paleogeography of eroding highlands and sediment routing systems across the pre-Andean convergent plate margin. Early Cambrian orogenic growth and collision of the AAT and PC shed detritus to shallow marine systems during extensional collapse. Ordovician uplift of rift shoulders induced by westward-retreating subduction beneath the AAT provided both, east-derived Late Cambrian and cratonic sources and west-derived magmatic sources to deltaic and deep marine systems. The middle Ordovician expression of the Transpampean Arch coincided with reaccretion of the AAT along the southern margin of the PC. Silurian-Devonian oblique subduction modulated the Transpampean Arch growth, and expansion of the fringing clastic ramp, where coastal to shallow-marine systems were fed by west- and south-derived arc and orogen sources. Carboniferous steeping of the subducting slab may have culminated in eastward orogenic growth with uplifted source regions providing lower Paleozoic detritus coeval with NW advance of ice sheets. Unraveling the complexities of temporally and spatially diverse source regions in the sedimentary record of the pre-Andean convergent margin not only refines the complex interactions at an active accretionary margin but also provides key predictions of tectonic inheritance for later Andean orogenesis.