Local High Relief at the Southern Margin of the Andean Plateau by 9 Ma: Evidence from Ignimbritic Valley Fills and River Incision
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doi: 10.1111/ter.12120 Local high relief at the southern margin of the Andean plateau by 9 Ma: evidence from ignimbritic valley fills and river incision Carolina Montero-Lopez,1 Manfred R. Strecker,2 Taylor F. Schildgen,2 Fernando Hongn,1 Silvina Guzman,1 Bodo Bookhagen2 and Masafumi Sudo2 1Instituto de Bio y Geociencias del NOA (IBIGEO), Universidad Nacional de Salta, CONICET, Salta 4400, Argentina; 2Institut fur€ Erd- und Umweltwissenschaften, Universitat€ Potsdam, Potsdam 14476, Germany ABSTRACT A valley-filling ignimbrite re-exposed through subsequent that the base of the ignimbrite remains unexposed in the river incision at the southern margin of the Andean (Puna) valley bottom. Our observations indicate that at least 550 m plateau preserves pristine geological evidence of pre-late of local plateau margin relief (and likely >2 km) existed by Miocene palaeotopography in the north western Argentine 9 Ma at the southern Puna margin, which likely aided the Andes. Our new 40Ar/39Ar dating of the Las Papas Ignimbrites efficiency of the orographic barrier to rainfall along the east- yields a plateau age of 9.24 0.03 Ma, indicating valley- ern and south eastern flanks of the Puna and causes aridity in Æ relief and orographic-barrier conditions comparable to the the plateau interior. present-day. A later infill of Plio–Pleistocene coarse conglo- merates has been linked to wetter conditions, but resulted in Terra Nova, 00, 1–7, 2014 no additional net incision of the Las Papas valley, considering employed stable isotopes in pedogenic Davila, 2011). These uplifted ranges Introduction carbonates and hydrated volcanic constituted orographic barriers to Unravelling the spatiotemporal pat- glass (e.g. Garzione et al., 2006; Pin- north east and east-southeast mois- terns in the topographic development gel et al., 2014; Saylor and Horton, ture-laden winds, helping to sustain of mountain belts is key to under- 2014), leaf morphology (e.g. Gregory- semiarid to arid conditions in the standing how tectonic forcing can Wodzicki et al., 1998), and geomor- plateau interior region since that influence climate and surface pro- phic/geological evidence of relief time (e.g. Strecker et al., 2007), while cesses, particularly when assessing the development (e.g. Gubbels et al., internal drainage conditions could role of deep-seated, mantle-driven 1993; Barke and Lamb, 2006; Hoke have initiated by 15 Ma (Alonso uplift mechanisms (Allmendinger et al., 2007; Schildgen et al., 2007; et al., 1991; Vandervoort et al., et al., 1997; Garzione et al., 2006). Thouret et al., 2007; Guzman and 1995). Despite this geological evi- The implications of such studies are Petrinovic, 2010; Jordan et al., 2010). dence for early topographic and even broader when orographic-barrier Most of the investigated areas lie relief development of the Puna pla- evolution is viewed in light of its along the flanks of the northern An- teau, farther north, a recent study influence on rainfall and erosion gra- dean plateau (Altiplano), and studies suggests that relief within canyon dients (Bookhagen and Strecker, suggest surface uplift of c.1–3.4 km systems did not develop along the 2012), speciation patterns (Semaw since the late Miocene (e.g. Gregory- eastern margin of the Altiplano pla- et al., 2005) or the emplacement of Wodzicki, 2000). Ambiguities remain teau in Bolivia until the onset of wet- supergene mineral deposits (Hartley owing to a lack of well-constrained ter conditions during the Pliocene and Rice, 2005). The development of chronologies, and from the potential (Lease and Ehlers, 2013), implying steep, deeply dissected flanks of for topographically induced changes potentially long delays between sur- Cenozoic orogenic plateaus and their in climate to influence the stable isoto- face uplift and river incision. impacts on climate make plateau pic (e.g. Ehlers and Poulsen, 2009) or In this study, we contribute to the margins ideal sites to investigate how incision (e.g. Lease and Ehlers, 2013) efforts to determine the timing of surface and deep-seated processes proxy data. Limited information on plateau uplift and relief development interact in creating and shaping these topographic development exists for by constraining the incision and environments. the eastern sectors of the plateau, and filling history of a deeply incised Studies attempting to elucidate the virtually nothing is known about the canyon that drains across the south- surface-uplift history of the Andean elevation history of its southern mar- ern margin of the Puna plateau in (Altiplano-Puna) plateau have gin. northwest Argentina (Fig. 1). We Shortening and surface uplift of present 40Ar/39Ar ages from two Correspondence: Carolina Montero- individual ranges in the present-day samples of an ignimbrite in the Las Lopez, Instituto de Bio y Geociencias del Puna plateau and adjacent regions Papas valley, which once covered an NOA (IBIGEO), Universidad Nacional had already occurred by the middle erosional palaeotopography and is de Salta, CONICET, Salta 4400, Argen- Eocene–Oligocene (e.g. Kraemer now being re-incised. These new tina. Tel.: +54 387 4318086; e-mail: et al., 1999; Coutand et al., 2001; dates allow us to place a minimum [email protected] Hongn et al., 2007; Nobile and age on the high relief along the © 2014 John Wiley & Sons Ltd 1 Local high relief of the southern Puna margin by 9 Ma • C. Montero-Lopez et al. Terra Nova, Vol 0, No. 0, 1–7 ............................................................................................................................................................. 69º00’ W 68º00’ W 67º00’ W 66º00’ W between 3.05 0.44 Ma and <3.77 0.10 MaÆ (Carrapa et al., 2008),Æ while ages range from c. 1.2– 25º00’ S 2.9 Ma in other basins (Bossi et al., 2001; Strecker et al., 2009). These strata are in turn unconformably ORDILLERA overlain by coarse river-terrace con- PUNA C glomerates. Terraces were sculpted 26º00’ S LC into the underlying bedrock and sedi- mentary strata, and in places are ASTERN Figure 2A E 1 km higher than the Las Papas val- C S B ley floor (Schoenbohm and Strecker, VP S.M. de 2009). 27º00’ S Tucumán A series of nearly N–S striking, FB west-dipping reverse faults associated SIERRAS with open folds has been related to AL PAMPEANAS the growth of the ranges comprising ORDILLERA BP the southern Puna margin (Rubiolo RONT C F et al., 2001), carrying basement rocks 28º00’ S S.F. del V. over the Las Papas ignimbrites and Catamarca YSTEM the Punaschotter conglomerates S (Schoenbohm and Strecker, 2009; Montero-Lopez et al., 2010a). Locally, these units are tilted approx- AMATINA meters 29º00’ S F 7000 imately 15° SW in the region of the Las Papas valley. The Las Papas river drains into the Fiambala Basin, which is 150 bounded by reverse-faulted ranges (Fig. 1). Deformation and uplift of Fig. 1 Digital Elevation Model (DEM) of northwest Argentina showing the loca- the Fiambala Basin’s northern tion of the Puna plateau. White box shows location of study area. CSB, Cordillera ranges, through which the Las Papas de San Buenaventura; FB, Fiambala Basin; BP, Bolson de Pipanaco; VP, Vicuna~ valley has incised, is inferred to have Pampa; LC, Luingo Caldera. started no later than the late Mio- cene, based on AFT exhumation ages of c. 6 Ma (Carrapa et al., 2006). southern margin of the Puna plateau. Cordillera de San Buenaventura Also, even though global cooling and (Fig. 1). The Las Papas valley has its Las Papas Ignimbrites and their possible changes in surface processes headwaters atop the Puna at eleva- topographic relationships were initiated during the Pliocene, tions around 4300 m, where it tra- we demonstrate that no additional verses Proterozoic to early Palaeozoic Along the Las Papas valley, several net incision occurred in the Las basement rocks and late Miocene– pyroclastic units with similar charac- Papas valley associated with these Pliocene volcanic rocks (Montero- teristics make for a complex volcanic changes. Lopez et al., 2010a). Towards the stratigraphy. In the section studied south, the valley exposes the Neogene here, there are at least two different Las Papas Ignimbrites (Montero- ignimbrites that we refer to as the Las Geological framework Lopez, 2009) and Quaternary ignimb- Papas Ignimbrites (Montero-Lopez, The Andean (Altiplano-Puna) plateau rites (Cerro Blanco Volcanic Complex, 2009). The ignimbrites are exposed is located between 15° and 27°S lati- Seggiaro et al., 2006; Montero-Lopez only along the central and southern tude (Fig. 1), with a mean elevation et al., 2010b). Locally, the basement sectors of the valley (Figs 2 and 3), up of 3.7 km and an areal extent of rocks and the Neogene ignimbrites to 2810 m a.s.l. in the central section c. 500,000 km2. The Puna (Turner, are covered by the Plio–Pleistocene of the valley and as low as 2260 m 1972) constitutes the southern pla- Punaschotter conglomerates (Penck, a.s.l. in the southern section, implying teau, which is characterized by inter- 1920). This diachronous unit comprises at least 550 m of topographic relief at nally drained Cenozoic sedimentary disorganized, poorly sorted boulder the Puna margin at the time of ignim- basins, widespread Cenozoic volca- conglomerates, which filled valleys brite deposition based on the outcrop nism, and N–S-oriented basement- and basins throughout the Puna mar- pattern (Fig. 3b). To the north of the cored ranges with peaks >6000 m gin (Penck, 1920; Turner, 1973; Bossi Las Papas valley, the ignimbrites are a.s.l. The Las Papas valley is one of et al., 2001; Strecker et al., 2009). U– not exposed, while to the east of the several deeply incised valleys along Pb ages of volcanic ashes intercalated valley, younger ignimbrites are dated the southern flanks of the Puna pla- within the Punaschotter of the Fiam- at 7.17 Ma (40Ar/39Ar in biotite, teau, and drains the E–W-oriented bala Basin to the south (Fig.