tectonics_revisions.pdfTable1.pdffigure1.aifigure2.aifigure3.aifigure4.aifigure5a.aifigure5b.aifigure7.aifigure8.aifigure9.aifigure10.ai1 Geomorphic evidence for post-10 Ma uplift of the western flank of the Central 1 Andes 18°30-22°S 2 3 Gregory D. Hoke1*, Bryan L. Isacks1, Teresa E. Jordan1, Nicolás Blanco2, Andrew J. Tomlinson2, 4 and Jahandar Ramezani3 5 6 1 Department of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca NY 7 14853, U.S.A. 8 2 Servicio Nacional de Geologia y Minera, Av. Santa Maria 0104, Santiago, Chile 9 3 Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of 10 Technology, 54-1117, Cambridge, MA 02139 U.S.A. 11 * now at: Department of Earth and Environmental Sciences, University of Rochester, 227 12 Hutchinson Hall, Rochester, NY 14627 U.S.A. 13 Abstract 14 The western Andean mountain front forms the western edge of the Central Andean 15 Plateau. Between 18.5° and 22°S latitude, the mountain front has ~3000 m of relief over ~ 50 km 16 horizontal distance that has developed in the absence of major local Neogene deformation. 17 Models of the evolution of the plateau, as well as paleoaltimetry estimates, all call for continued 18 large magnitude uplift of the plateau surface into the late Miocene (i.e. younger than 10 Ma). 19 Longitudinal river profiles from 20 catchments that drain the western Andean mountain front 20 contain several streams with knickpoint-bounded segments that we use to reconstruct the history 21 of post 10 Ma surface uplift of the western flank of the Central Andean Plateau. The generation 22 of knickpoints is attributed to tectonic processes and are not a consequence of base level change 23 related to Pacific Ocean capture, eustatic change, or climate change as causes for creating the 24 knickpoint-bounded stream segments observed. Minor valley filling alluvial gravels intercalated 25 with the 5.4 Ma Carcote ignimbrite suggest uplift related river incision was well under way by 26 5.4 Ma. The maximum age of river incision is provided by the regionally extensive, 27 approximately 10 Ma El Diablo-Altos de Pica paleosurface. The river profiles reveal that relative 28 surface uplift of at least1 km occurred after 10 Ma. 29 Introduction 30 The processes and rates related to the formation of large continental plateaus remain 31 controversial. Many different mechanisms and ideas have been invoked to explain their 32 evolution, spanning climatic controls [e.g. Montgomery, et al., 2001], pure structural thickening 33 via horizontal shortening [e.g. McQuarrie, 2002], to viscous flow of lower crustal material 34 [Clark and Royden, 2000]. The outcomes of some models of plateau forming processes are much 352 easier to test with observation than others. The simplest metric of tectonic activity is upper 36 crustal deformation combined with chronologic constraints. In contrast, proposed mechanisms of 37 plateau formation not resulting in surface-breaking faults, are much more difficult to constrain. 38 The Andes of South America are home to the Central Andean Plateau, the second highest 39 continental plateau on Earth at ~4 km average elevation. The plateau is bounded on either side by 40 mountainous topography of the Eastern and Western Cordilleras (Figure 1). The eastern side of 41 the Andean orogen is characterized by a wide belt of Neogene supracrustal deformation 42 expressed by numerous surface-breaking faults and folds. The belt has varied tectonic style, 43 typified by basement uplifts in the Argentine and Peruvian forelands, as well as the spectacular 44 thin-skinned Sub-Andean fold-and-thrust belt in Bolivia. Structural studies indicate that as much 45 as 300 km of Neogene horizontal shortening [McQuarrie, et al., 2005] has occurred across the 46 entire plateau and postulate that it generated 3000-4000 m of relief between the plateau and the 47 undeformed foreland [Elger, et al., 2005]. By contrast, the western mountain front of the Central 48 Andes contains a similar amount of relief but only minor Neogene supracrustal deformation 49 (Figure 2). Several studies related to the development of the plateau or the Central Andes as a 50 whole [Garzione, et al., 2006; Gregory-Wodzicki, 2000; Isacks, 1988; Kay and Kay, 1993; Kay, 51 et al., 1994; Lamb and Hoke, 1997] postulate that uplift of the plateau surface occurred 52 throughout the Miocene. However, the western mountain front has provided little evidence in 53 support of the uplift history interpretations that were based on observations from other parts of 54 the plateau, due to the lack of well-defined geologic structures of appropriate age. Where 55 structural geology approaches cannot be applied, we must explore other methods to test the 56 various hypotheses for plateau formation. Here we examine the longitudinal stream profiles from 57 20 drainage networks (Figure 2) descending the western flank of the Central Andean Plateau in 58 an effort to detect and quantify post-10 Ma relief formation in the region. To do so we perform 59 three tasks. The first is the extraction of stream profiles from digital elevation data, the second is 60 demonstration of the ages of stream segments, and third is reconstruction of the surface relief at 61 the times when the dated stream segments were created. 62 63 Geologic setting 64 The Andes are the type example of a non-collisional orogen in a convergent margin 65 setting. Here the Nazca plate subducts below the overriding South American plate. Subduction 663 along this margin has been active since the Mesozoic, with the modern phase of mountain 67 building beginning in the latest Oligocene [Jordan, et al., 1997; Mpodozis and Ramos, 1989]. 68 The Central Andean segment of the orogen is dominated by the Central Andean Plateau which 69 spans some ~1,500 km N-S and 300 km E-W at its widest point. It is bounded to the west by the 70 volcanic peaks of the Western Cordillera (the volcanic arc) and to the east by the Eastern 71 Cordillera (the hinterland of the modern Sub-Andean fold-thrust belt). 72 Our study area lies within the modern Andean forearc and forms the western flank of the 73 Central Andean Plateau. Several physiographic provinces exist in the study area (Figure 2). From 74 west to east, they are the: 1) Coastal Cordillera, 2) Central Depression (Pampa del Tamarugal), 75 3) western mountain front, 4) Western Cordillera and 5) Altiplano (Figure 2). In the study area 76 the Coastal Cordillera