The intersection of the Nazca, Antarctic, and South American tectonic plates is a trench-ridge-trench triple junction (Chile Triple Junction, or CTJ) at latitude 46°S along the Chilean margin of South America. Pliocene-Pleistocene sand and sandstone samples were collected during Leg 141 from Sites 859 and 860 (located 10 km north of the CTJ), Site 863 (located a few km south of the CTJ), and Site 862 (located farther south on the Taitao Ridge). Sand detrital modes of 107 samples were determined using the Gazzi-Dickinson method of point counting. Sand compositions are fairly uniform, with the following mean values: QFL%Q 31, QFL%F 35, and QFL%L 34. These sediments have a dissected arc provenance, reflecting extensive onshore erosion of quartzose Paleozoic metasedimentary basement rocks and the Mesozoic Patagonian batholith in conjunction with late Cenozoic glaciation and tectonism. The lithic component is dominantly intermediate to mafic volcaniclastic debris derived from arc volcanoes and perhaps near-trench volcanic centers on the Taitao Peninsula. Sites south of the CTJ show higher metamorphic and sedimentary lithic proportions, probably due to uplift and erosion in the Taitao Peninsula region during northward migration of the CTJ.
Sand layers recovered at Ocean Drilling Program Site 893 in the Santa Barbara Basin were sampled and petrographically analyzed to determine their provenance. Sand deposited at this site is quartzo-feldspathic with a minor lithic component. Plagioclase feldspar dominates over potassium feldspar, and sedimentary and metamorphic lithic fragments are more common than volcanic lithic fragments. Sand detrital modes are consistent with a source along the eastern flank of the Santa Barbara Basin, most probably a combination of the Santa Clara and Ventura rivers. Compositional trends observed within the thickest subunit (1C) can be explained in part by the lengthening of the Santa Clara River channel and the merging of the Santa Clara and Ventura river drainages with falling sea level.
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