Local High Relief at the Southern Margin of the Andean Plateau by 9 Ma: Evidence from Ignimbritic Valley Fills and River Incision

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 conglomerates 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

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 sedimentary 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. 1) are Montero-Lopez et al., 2010b).

(A) (>550 m) and river valley (>240 m) N CSB relief at the time of ignimbrite emplacement.

Geochronology

3000 The Las Papas Ignimbrites comprise 3000 pink-white to pale-purple coloured, 2500 indurated and welded pyroclastic deposits with eutaxitic texture and Pa-08 columnar jointing (Fig. 4) and a A A’ mineral association of quartz, plagio- L AS clase, K-feldspar, biotite, and zircon. LP-7 P

APAS To constrain the age of palaeotopog- 3000 raphy and relief development within

3000 V

ALLEY the Las Papas valley, we dated two samples of the Las Papas Ignimbrites using 40Ar/39Ar geochronology on biotite separates from pumice by stepwise heating of multi-grain aliqu- 2500 ots (6–14 mg of biotite separate). The results show good plateau and inverse isochron ages, with 40Ar/36Ar values close to the 40Ar atmospheric Kilometers value, implying no contamination FIAMBALÁ 27º10’ S27º10’ 0 1 2 S 27º05’ and reinforcing the robustness of our BASIN biotite ages. Additional details of 67º50’ W 67º45’ W sample analysis and summary tables are in the Supporting Information. (B) A A’ The sample collected from the 2700 WElower section of the ignimbrite pro- LP file exposed at the valley bottom 2500 river (LP-07, 2334 m a.s.l.) provided a plateau age of 9.24 0.03 Ma (Fig. 5a, Æ 2300 6 contiguous steps, 83.7% of total 39Ar released). The normal and 2100 inverse isochron ages of Elevation asl (m) Kilometers 9.33 0.09 Ma (Table S1) from the 1900 0 1 plateauÆ steps agree with the plateau age within uncertainty. The second A-A’ profile Holocene ignimbrites sample (Pa-08, 2436 m a.s.l.) was sample locations Punaschotter cgl. taken several metres up-section and 3000 elevation asl (m) Late Miocene ignimbrites yielded a plateau age of normal fault 8.47 0.04 Ma (Fig. 5b). Although reverse fault Basement rocks the plateauÆ age adopted here comprises only two contiguous steps (10 Fig. 2 (A) Geological map of the middle and southern part of the Las Papas valley and 11) and covers 43.6% of total showing the distribution of the Las Papas Ignimbrites and Punaschotter conglomer- 39Ar released, it is consistent with the ates, and sample locations. (B) AA0 cross profile illustrates the relationship between total gas age (8.47 0.02 Ma) and the Las Papas Ignimbrites and paleotopography. See the bar tilted 15° to the SW, also with normalÆ and inverse indicating the (minimum) measured thickness of the Las Papas Ignimbrites. isochron ages from the plateau steps (8.47 0.09 Ma and 8.46 0.09 Ma) (Fig.Æ 5b and Table S2).Æ Despite the incision of the Las indicating a minimum thickness of Therefore, we conclude that the Papas river, the base of the lowest 240 m after correcting for the post- plateau age is geologically meaningful. ignimbrite is not exposed anywhere depositional SW tilting (Figs 2b and along the valley bottom. In the mid- 3). Based on the overall outcrop pat- Discussion and conclusions dle part of the river-long profile, the tern (Fig. 2a), we conclude that this Las Papas Ignimbrites are exposed region records the existence of a When volcanism was active in the between elevations of 2300–2350 m southward-directed fluvial system southern Puna, pyroclastic flows at the valley bottom and up to that drained the Puna region, and followed the course of the Las Papas 2700 m on interfluves to the east, thus significant plateau margin palaeo-valley, and in some cases

(A) first deduce that the southern rim of 6868 the Puna constituted a topographic Puna 68ºW L Miocene B ignimbrites high with at least 550 m of relief and 171 PUNA meters Fig. 4 an established river network draining Puna rim the present-day plateau margin N study sample area before 9.24 0.03 Ma. Second, inci- locations sion of theÆ Las Papas river through 27ºS FB: Fiambalá Basin the ignimbrites was at least 240 m LP: Las Papas and continues to the present-day FB valley without having exposed their base, Pa-08 B-B’ profile implying that cross-valley relief has LP-7 not increased since ignimbrite deposition. Finally, because differential uplift of the plateau margin relative LP valley B’ to the Fiambala Basin would have outlet resulted in incision of the Las Papas (B) valley, the lack of exposure of the B B’ base of the ignimbrite also argues 4.5 Las Papas Valley against significant post-9 Ma differ- Holocene ignimbrites ential uplift. Hence, the total local plateau-margin relief of c. 2 km 4.0 Punaschotter cgl. Las Papas Ignimbrites (Fig. 3b) has likely changed little Basement (metamorphic since 9 Ma. and volcanic rocks) 3.5 This process of filling and renewed Long river profile incision was repeated again during the deposition of the Plio-Quaternary 3.0 Punaschotter conglomerates, which covered the erosional palaeo-topography that had developed within the 2.5 ? late Miocene ignimbrites and base- Elevation above see level (km) ment rocks. Although diachronous, Pa-08 LP-7 these coarse deposits have the unify- 2.0 ing characteristic of having partly 0 10 20 30 40 50 re-incised or filled palaeo-topography Distance downstream (km) along the southern and eastern Puna margin (Strecker et al., 2009; Pingel Fig. 3 (A) Three-dimensional perspective view of the outcrop pattern of the late et al., 2013), implying that a high- Miocene ignimbrites that filled local paleotopography across the southern Puna elevation, high-relief plateau rim plateau margin. (B) Long river profile of the Las Papas River with the sample loca- incised by river valleys already tions and the units exposed along the valley. existed during the late Miocene. Because incision failed to expose the base of the Las Papas Ignimbrites, E W we infer that the most important Las Papas Ignimbrites phase of downcutting and relief development of the Las Papas valley reflects pre-late Miocene differential uplift of the Puna margin relative to the Fiambala Basin, rather than Plio- cene climate-driven processes that may have changed the precipitation Basement and runoff regimes, a scenario that has been proposed by Lease and Las Papas River Ehlers (2013) for the more humid Bolivian Andes, but has recently Fig. 4 View of the Las Papas Ignimbrites from the Las Papas River (approximately been challenged (Gasparini and at the location of the AA0 profile of Fig. 2B). Whipple, 2014). Overall, our observations and data from the Las Papas valley indicate overtopped the interfluves. Subse- base level in the Fiambala Basin to that topographic relief structure simi- quently, this palaeo-landscape was the south. Based on the ages of the lar to that of today existed by late re-incised, and rivers draining the ignimbrites and field observations of Miocene time along the southern Puna margin adjusted to the regional their valley-filling morphology, we Puna margin. This constraint on past

zas for ﬁeld assistance. A. Hartley, T. Jor- (A) Plateau age: 9.24 ± 0.03 Ma dan and an anonymous reviewer are 10 thanked for constructive reviews. 8

)

M References ( 6

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