Extensional Tectonics in Convergent Margin Basins: an Example from the Salar De Atacama, Chilean Andes

Extensional tectonics in convergent margin basins: An example from the Salar de Atacama, Chilean Andes

S. FLINT Department of Earth Sciences, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, United Kingdom P TURNER 1 " r_T T _ „ > School of Earth Sciences, University of Birmingham, P.O. Bcoc 363, Birmingham B15 2TT, United Kingdom b. J. JUJLLifc/Y J A. J. HARTLEY Department of Geology & Petroleum Geology, University of Aberdeen, Aberdeen AB9 2UE, United Kingdom

ABSTRACT enance area for a 2-km-thick Oligocene conti- The modern Salar de Atacama basin nental basin-fill component (Paciencia Group). (Figs. 1 and 2) is situated in the Pre-Andean The Salar de Atacama basin of northern The Oligocene basin was an extensional to trans- Depression, bounded to the east by the Mio- Chile preserves stratigraphic evidence for the tensional basin. cene-Holocene Andean volcanic arc (High evolution of the Andean cycle. It has evolved The Miocene-Holocene Salar basin is a con- Andes) and to the west by thrusted Paleozo- from a non-arc-related rift, through back-arc tinental fore-arc basin. This latest segment of ic-Mesozoic sedimentary strata and Late Cre- and inter-are stages, to a Neogene fore-arc ba- the basin fill comprises pyroclastic and conti- taceous igneous intrusions of the Cordillera sin. Accumulation of the sedimentary succes- nental sedimentary rocks thrust over Quater- de Domeyko. The modern basin is —100 km sion was mainly due to extensional faulting. nary gravels in many places. The Cordillera de long (north-south) and 40 km wide; most of Important but short-duration contractional ep- la Sal is an intrabasinal uplift, initiated as a the surface is occupied by a saline playa com- isodes do link to known first-order plate-mar- thin-skinned contractional feature. Thus, the plex with a cultivated northern area around gin changes, but their stratigraphic effect ap- superposed basin-fill components represent the Rio San Pedro delta (Figs. 2 and 3). The pears to be restricted to uplift/erosion rather responses to distinctly different geodynamic western side of the basin is occupied by a than creation of significant flexural subsidence. settings. gravel plain (the Llano de la Paciencia), which The Salar de Atacama basin originated as is now separated from the main Salar by the part of a regional rift system during Permian INTRODUCTION Cordillera de la Sal, an intrabasinal range of time and contains 2 km of Permo-Triassic hills. continental detritus and volcanic rocks. The Current models for sedimentary basins in- area remained above depositional base level clude rift, thermal-sag, foreland, and pull- TECTONIC SETTING OF THE SALAR throughout Late Triassic to Late Cretaceous apart types, which have generally accepted DE ATACAMA BASIN time. Syn-rift continental red beds were depos- spatial and temporal geodynamic positions in ited to a thickness of at least 2 km on a parallel orogenic cycles (Ingersoll, 1988). In this pa- The Central Andes of northern Chile are rift segment in the Domeyko area to the west. per, we describe the history of the Salar de constructed of five longitudinal morphotec- Continued Triassic-Jurassic extension in the Atacama, a long-lived (Permo-Triassic-Hol- tonic elements. The Coastal Cordillera Domeyko basin resulted in a classic continental ocene) nonmarine basin that, owing to the (Fig. 1) is built of Jurassic andesitic lavas and to marine transition, with deposition of a 2 geodynamic development of the Central intercalated marine sedimentary rocks of the km+ Jurassic mixed carbonate/clastic sequence. Andes, has evolved from a non-arc-related La Negra Formation, interpreted as a volcan- In latest Cretaceous-Eocene time, the Salar rift basin, through complex "arc-related" ic arc complex (Coira and others, 1982). To basin was an arc-related basin and accommo- stages, to a Miocene-Holocene fore-arc ba- the east, the Central Depression is a late Ter- dated some 4 km+ of continental detritus (Pu- sin. Mechanisms of subsidence, accommo- tiary-Quaternary basin that is continuous rilactis Group) due to back-arc extension, sed- dation space generation, and base-level con- along much of Chile and contiguous with the iment being derived from the Domeyko trol are assessed in relation to both tectonic Atacama Fault Zone; however, in the latitude Cordillera and arc rocks to the west. Late and thermal evolutionary processes. We also of the study area, the continuous nature of the Eocene right-lateral strike-slip faulting and as- attempt to demonstrate that sedimentary ba- depression is broken by Mesozoic intrusions sociated restraining bend uplift were driven by sins at convergent plate margins preserve the aligned on west-east-striking faults. East of a high rate of oblique convergence between the most complete record of the local geody- the Central Depression is the Cordillera de Farallon and South American plates. This de- namic history of the orogen, and that their Domeyko. This mountain range comprises a formation complicated the stratigraphy of the analysis is an essential component in studies basement-cored, thrusted series of Paleozoic Purilactis Group and inverted the western ba- of crustal evolution. Our data base includes and Mesozoic clastic and carbonate sedimen- sin margin, which formed the dominant prov- over 100 logged sections (some 20 km of tary rocks and middle Cretaceous to early stratigraphy), field maps, and the interpreta- Tertiary intrusive rocks (Fig. 2). This base- tions of aerial photographs/satellite images ment block consists of early Paleozoic sedi- *Present address: B.P. Petroleum Development Ltd., 301 St. Vincent Street, Glasgow G2 5DD, and several regional seismic lines across the mentary rocks and Cretaceous plutons, United Kingdom. basin. bounded by normal faults. The eastern mar-

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ity-bounded depositional basin-fill units are discussed below, starting with the strati- graphically oldest unit.

Fill Unit 1: Peine Formation (Permo-Triassic)

Description. The Peine Formation (Moraga and others, 1974) crops out as isolated, west- dipping inliers on the eastern margin of the Salar basin (Figs. 3 and 5). The exposed sequence consists of more than 600 m of continental clastic and acid/intermediate volcanic rocks. The sequence was divided into three members by Ramirez and Gardeweg (1982) Figure 1. Location of the Salar and is assigned an Early Triassic age. A lower de Atacama basin within the unit, dominated by volcanic breccia and con- present-day morphotectonic glomerate is separated by an unconformity framework of the north Chilean from a sequence of clastic sedimentary rocks fore arc. Boxed area shows posi- (middle member) and an upper pyroclastic- tion of Figure 3. dominated unit. In the main Salar area, seismic lines reveal a deep half graben with 2 s (~3 km) of fill. The Cordon de Lila (Fig. 3) was a basement high during the deposition of the unit because the reflector geometries imply onlap onto the high, which was bounded by a major extensional fault (Fig. 6). The seismic facies include both laterally continuous high-amplitude reflections and discontinuous reflections close to the major western boundary fault. The western basin under the Llano de la Paciencia shows only poor distinction of the early fill. A log through part of the middle member of the Peine Formation (Fig. 1), 10 km north of Peine (Fig. 3), shows the sequence to consist of red siltstones that exhibit rare desiccation gin of this uplift is a structurally complex zone saliferous deposits. The major types and pe- cracks and interbedded coarse sandstone in 1- that exposes much of the Mesozoic-Holocene riods of faulting and folding within the com- to 100-cm-thick beds with erosive bases. The stratigraphy of the Salar basin fill (Figs. 3 and posite stratigraphy of the Salar de Atacama sandstone is dominated by parallel lamination 4). The eastern margin of the Salar basin is a include (1) Permian listric normal faults, strik- interpreted as upper phase plane bedding, monoclinal upwarp created by the middle ing north-south, downthrowing to the east; with abundant siltstone rip-up clasts and con- Tertiaiy-Holocene Andean volcanic arc and (2) Late Cretaceous reactivation of the Perm- volute lamination. Palynological studies indi- Quaternary stratovolcanoes of the High ian fault system, resulting in thickening of the cate a Late Permian to Scythian age (Em- Andes, marking the international frontier Purilactis Group in the hanging walls of these presa Nacional del Petroleo, 1991, personal with Bolivia and Argentina (Figs. 2 and 3). faults; (3) local intense folding of the Purilactis commun.). Group in the northwest of the basin, linked to The nonmarine character of the sequence STRATIGRAPHY AND STRUCTURAL dextral strike-slip faulting (late Eocene); (4) reduces the number of age determinations DEVELOPMENT OF THE BASIN FILL east to southeast thin-skinned thrusting and possible, which are based exclusively on iso- related folding in early Miocene time; and (5) topic dating of igneous rocks by other authors The Salar de Atacama basin has a long ge- neotectonic thrusting of Tertiary strata over and us. A burial history analysis within the ologic history, from Permo-Triassic to Holo- Holocene gravels. available data base, however, indicates an av- cene, all of it nonmarine. The composite —10 The seismic stratigraphy presented herein eraged subsidence rate of200 m/m.y. through km of stratigraphy can be divided into five is proven by an oil exploration well, drilled by the Permo-Triassic (Fig. 8a). Given the large unconformity-bounded megasequences: the Hunt Oil Company in 1991, but the well data volume of igneous rocks present and the clear Permo-Triassic Peine Formation and equiv- are not public at this time. The pre-Andean presence of alluvial-fan deposits in outcrop alents, the latest Cretaceous-Eocene Purilac- basement to the Salar basin is Paleozoic sed- and inferred from the seismic data, actual tis Group, the Oligocene-earliest Miocene imentary rocks of Ordovician to Carbonifer- rates of subsidence over shorter time periods Paciencia Group, the early Miocene-Plio- ous age, exposed as inliers along the eastern are likely to have been much greater. Pleistocene San Bartolo Group/Vilama For- and southern basin margins (Fig. 3; Ramirez Interpretation. We interpret our basin-fill mation, and the Holocene alluvial fans and and Gardeweg, 1982). The main, unconform- unit 1 as a continental rift sequence. These

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limits the upper age of the Purilactis Group to late Eocene. Our isotopic age determination on an intra- Purilactis lava flow (63 ± 10 Ma, Ar/Ar, Flint and others, 1989) allows a two-stage subsidence history to be determined for the unit (Figs. 8a and 8b). An early, averaged subsidence rate of —300 m/m.y. reduces to —100 m/m.y. in the upper part of the stratigraphy. Again, we stress that actual short-term rates may have been much higher. The stratigraphy of the Purilactis Group (Fig. 5) is well documented (Hartley and others, 1988; Fig. 9). The lowermost unit consists of 600 m of red siltstone and evaporite (Fig. 10a), sharply overlain by a 300-m-thick sequence of coarse-grained, cross-bedded sandstone. The upper 3 km of the Group may be divided into a series of large, upward- coarsening and upward-fining depositional cycles, some 200-500 m thick, composed of fining-upward sequences 5-15 m thick (Fig. 9). Seismic data indicate clear evidence of important extensional normal faulting active during deposition of the Purilactis Group. Two sub-basins are visible (Fig. 6) to the west and east of the Cordon de Lila. The Purilactis Group onlaps onto the Permo-Triassic Unit 1 and has a thickness of as much as 2.5 km (1.5 s two-way traveltime [t.w.t.]). Extension was generated by reactivation of the earlier Permian fault system. Interpretation. Detailed logging of 8,000 m of Purilactis Group in some 50 localities Figure 2. Landsat MSS image of the Holocene Salar de Atacama fore-arc basin, annotated with (Hartley and others, 1992) has elucidated the our interpretation of key structural and depositional features. Critical points include the position sedimentology of the unit. The Tonel Mem- of the Cordillera de la Sal (intrabasinal uplift), the thrusted Cordillera de Domeyko (which forms ber represents a playa complex, overlain by the western basin margin), and the Andean volcanic arc to the east. See text for discussion. an incised valley sequence of braided fluvial sandstone, representing a significant fall in base level. Above this unit, ubiquitous, small- deposits have not been studied in extensive measured section) continental clastic and scale, fining-upward cycles are interpreted as detail sedimentologjcally, but ongoing work evaporite rocks. The unit includes both the products of individual flood events, whereas by Breitkreutz (1991, personal commun.) in- Purilactis and Salinas de Purilactis formations large-scale, coarsening-upward cycles may dicates a lacustrine environment for the silt- (Tonel Formation of Dingman, 1963), as orig- represent longer-term alluvial-fan prograda- stone. We interpret the sequence in Figure 7 inally defined by Briiggen (1942). In outcrops tional episodes (Fig. 9) (Leeder and Gaw- as a playa/shallow lacustrine environment in- along the western margin of the Cordillera de thorpe, 1987). Extensive paleocurrent data terrupted by major fluvial flood events that Domeyko, the unit lies with angular uncon- and provenance studies indicate that Purilac- introduced thin, high-energy distal alluvial- formity on the El Bordo Formation (Fig. 3), tis alluvial-fan systems prograded from the fan sheetflood sandstone event beds into the whereas seismic interpretations in the Salar west (Hartley and others, 1988,1992), and it system. Small-scale fining-upward cycles in basin (Fig. 6) indicate that the Purilactis has been postulated that progradational epi- the alluvial-fan sandstone and conglomerate Group unconformably overlies Permo-Trias- sodes were responses to thrust movements in may represent autocyclic fan lobe switching sic red beds of the Peine Formation. the westerly Cordillera de Domeyko (Hartley processes associated with seasonal flood The uppermost unit of the Purilactis Group and others, 1988). Our seismic interpreta- events. is 200 m of andesitic lava and ignimbrite, with tions, however, show major thickening of Pu- subordinate interbedded continental con- rilactis strata into normal faults (Fig. 6b), Fill Unit 2: Purilactis Group glomerate and sandstone (Cinchado Forma- which we take as convincing evidence for tion of Ramirez and Gardeweg, 1982; Figs. 3 extensional faulting during Purilactis deposi- Description. The Purilactis Group (Charrier and 5). Recent isotopie dating of upper tion (Fig. 6). Moreover, our recent outcrop and Reutter, 1990), exposed along the west- Cinchado volcanic rocks by Ramirez and studies and isotopic dating suggest that all ern margin of the Llano de la Paciencia Gardeweg (1982) (40 ± 3 Ma, K/Ar) and by active thrusting in the Cordillera de Domeyko (Figs. 3 and 4), comprises thick (>4.1-km Reutter and others (1991) (44 ± 1 Ma, Ar/Ar) had ceased by Purilactis time.

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Figure 3. Simplified geological map of the western Salar basin margin showing the outcrop pattern of the main tectono-sedimentary units and the dominant eastward-verging thrusted margin. Simplified from the combined works of Ramirez and Gardeweg (1982) and Marinovic and Lahsen (1984). Profiles A, B, and C are key stratigraphie logged sections. A = Quebrada Seilao; B = Que- brada del Diabalo; C = Baque- dano Road. Profile B-B ' along the Calama-San Pedro road is location of Figure 4.

Fill Unit 3: Paciencia Group Fig. 10b) and consists of a 2-km-thick suc- Purilactis Group (41 Ma) and was probably (Oligocene-Earliest Miocene) cession of continental sedimentary rocks deposited during a regionally recognized Oli- (Tambores and San Pedro Formations of gocene phase of magmatic quiescence and Description. The Paciencia Group overlies Briiggen, 1942), exposed in the area from San cessation of contractional tectonic activity the Purilactis Group with angular unconform- Bartolo to the southern Cordillera de la Sal (Coirà and others, 1982; Jordan and Alonso, ity (Marinovic and Lahsen, 1984; Flint, 1985; (Fig. 3). The unit is younger than the upper 1987). Ashes in the upper part of the se-

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KEY TO FIGURE 3 a relatively constant thickness sequence hav- ing complex lateral reflector terminations and Quarternary/Holocene alluvium Paciencia Group (Oligocene) amplitude shifts (Fig. 6). Notably, this is the first unit that crosses all the earlier exten- Vilama Formation (Pleistocene) Cinchado Fm. sional faults with unchanged thickness, sug- Purilactls Group gesting cessation of extension by early Mio- Tonel and Purllactis (U. Cret.-Eocene) Neogene strato-volcanoes/lavas Fms. cene time. Interpretation. During the early Miocene Loa Formation (U. Miocene- Agua Dulce/Peine Formations Pliocene) (Permo-Trlassic) epoch, the Andean volcanic arc was estab- San Bartolo Gp. & other ignlmbrites lished in its present-day position (Lahsen, Paleozoic basement (Late Miocene-Pleistocene) 1982). Thus the Salar basin has been geo-

Other Middle Tertiary continental ++++ graphically an intermontane fore-arc basin ++++ Late Cretaceous-Paleocene sediments +++++ arc Intruslves throughout the last 20 m.y. Sedimentation over this period has been controlled by a com- Margin of modern salar bination of volcanic activity and thin-skinned thrusting (Hooper and Flint, 1987; Jolley and Anticline others, 1990; Fig. lOf). The ash flows were all Syncline derived from the east and north (Holling- Thrust Strike-slip Fault worth and Rutland, 1968), during activity of Normal Fault the early Miocene-Holocene Andean volcanic arc. Local ignimbrite thicknesses and geometries were controlled by paleotopogra- quence, dated at 28.6 ± 6 Ma (K/Ar, material that could have been deposited. phy. During Pleistocene time, climatic Travisany, 1979), indicate the initiation of a Thus the basin was probably permanently un- cyclicity induced by glaciation (Caviedes and new volcanic edifice. The minimum age for derfilled, leading to an underestimation of Paskoff, 1975) resulted in pluvial periods, the Group is restricted by a lava at the base of subsidence rate. characterized by the braided fluvial and la- the San Bartolo Group (K/Ar, 17 ± 2 Ma, There was no active arc in this area during custrine sequence of the Vilama Formation. Ramirez, 1979a) that unconformably overlies the Oligocene epoch; however, regional strat- Frequent minor volcanic eruptions contrib- the Paciencia Group (Fig. 10c). Thus the Pa- igraphic relations indicate that the ash-fall ac- uted significant ash-fall sediment to the basin. ciencia Group was deposited in —10 m.y., tivity that was recorded in the upper (proba- although our age on the initiation of the basin bly earliest Miocene) part of the Paciencia Fill Unit 5: Holocene Deposits is only as good as "post-Eocene." The cal- Group is consistent with the establishment of culated subsidence rate is 200 m/m.y. the Andean arc in its present position to the In the main Salar basin, Holocene deposits (Fig. 8b), similar to the lower Purilactis east of the basin (Coira and others, 1982). are dominated by claystone and evaporite Group. Provenance and paleocurrent studies (Flint, with minor aeolian dune systems and inter- Facies associations include playa siltstone, 1987) indicate that this sequence was derived tonguing lahars on the eastern margin. The evaporite and lacustrine claystone, marginal dominantly from the uplifted Purilactis Group final, Holocene segment of the constructed sand-flat deposits (Figs. 10c and 11), and al- to the west. burial history curve shows a marked increase luvial-fan conglomerate in the west of the ba- in subsidence to —0.7 km/m.y. (Fig. 8b). sin (Flint, 1985). Interpretations of seismic Fill Unit 4: San Bartolo Group data (Fig. 6) indicate that minor extensional and Vilama Formation SUMMARY OF KEY OBSERVATIONS faults were active in the Llano de la Paciencia area during Paciencia time, but the main Pu- Description. Fill unit 4 is dominated by vol- The Salar de Atacama basin is presently a rilactis extensional faults appear to have be- canic rocks and includes the Miocene San fore-arc basin and has had this configuration come inactive. Paleomagnetic studies indi- Bartolo Group (Hollingworth and Rutland, since early Miocene time. Late Tertiary-Qua- cate discrete rotation of the Purilactis Group 1968; Ramirez, 1979b), seven younger ternary depositional sequences have been prior to deposition and magnetization of the Pliocene-Pleistocene ignimbrites and inter- controlled by microclimatic fluctuations and Paciencia Group (Hartley and others, 1991). bedded alluvial gravel and sandstone the growth of the Cordillera de la Sal as a These data imply an oblique component of (Ramirez and Gardeweg, 1982), and the Pleis- thin-skinned contractional intrabasinal uplift, movement on the post-Purilactis faults. tocene Vilama Formation (Moraga and oth- which dies out southward (Figs. 2 and 3). Interpretation. The depositional architec- ers, 1974). The Vilama Formation is com- Outcrop and particularly seismic reflection ture of the Paciencia Group differs from the posed of some 60 m of poorly consolidated data indicate that the dominant control on Purilactis Group in not exhibiting clear coars- gravel, sandstone, mudstone, lacustrine lime- basin formation and deposition of the bulk of ening-upward cycles (Fig. 11). Our previous stone, and diatomite, interbedded with vol- the Salar basin fill was extensional to oblique work (Flint, 1985) indicated clear fluctuations canic ashes (Jolley, 1991, personal commun.). extensional faulting. Our seismic stratigraph- between shallow lacustrine environments and This composite basin-fill unit is only 1 km ic analysis indicates 1,500 m-l- of pre-Puri- hyper-arid salt-pan systems. We believe that thick, yet it spans as much as 8 m.y. (averaged lactis Group sedimentary rocks, character- facies distribution was controlled by a com- subsidence rate of 125 m/m.y.; Fig. 8b) and is ized by discontinuous reflectors. This earliest bination of high-frequency microclimatic the best-dated part of the whole Salar basin fill basin-fill unit has large thickness variations fluctuations and lower-frequency tectonic ep- (Fig. 5). Unit 4 lies with angular unconformity across the Salar de Atacama area; these vari- isodes. During this period of high aridity, sed- on the Paciencia and Purilactis Groups. Unit ations coincide with the positions of several iment supply may have limited the available 4 is interpreted on seismic reflection lines as large faults. Thickening of stratal units toward

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B CERROS DE PURILACTIS B'

iiimmiiiiiinniiim

• PA' •• &4)' Reactivated Frontal Thrust Ramp

Possible Location of pre-thrusting fault ramp V= H I

KEY STRATIGRAPHY QUATERNARY-RECENT 0 Villama Fm Gravels Approx Thrust Timing TERTIARY Pliocene San Bartolo Gp. Ignimbrite © Post Quaternary Oligo-Miocene Paciencia Gp. (PA) Evaporite & CD Pliocene Conglomerate mudstone © U. Miocene CRETACEOUS L. Eocene Paleocene Purilactis GP (PR) 0 Cretaceous Continental clastic rocks Direction of Tectonic Transport I I TRIASSIC Agua Dulce/Peine Formations WBT Western Back Thrust FDT Frontal Domeyko Thrust 0 1 2 3 4 5 km I BT Ignimbrite Back Thrust Scale

Figure 4. West-east outcrop structural cross section from the Cerros de Purilactis/western basin margin into the central Salar de Atacama, showing the dominant structural elements and timing of Neogene thrusting. See Figure 3 for line of section. From Jolley and others (1990).

the faults indicates synsedimentary fault ac- ment high, separating the easterly Salar ciencia Group sequences, whereas their tivity that defined several sub-basins. The ' 'failed rift" from the western Domeyko basin Sequence 5 equates to the San Bartolo Cordon de Lila (Fig. 2) was a basement high (see below). The Salar basin thus received Group to Holocene stratigraphy. during this time. We think that the whole some continental detritus during Late Trias- Permo-Triassic succession represents a ma- sic time but had probably filled to depositional REGIONAL CORRELATIONS jor episode of rifting; the internal unconform- base level by Jurassic time. ities interpreted from seismic data and in the A seismic interpretation of lines across the Although much of the Triassic-Holocene limited outcrops are interpreted as the prod- northern Salar basin (Macellari and others, stratigraphic record of the Central Andes is ucts of discrete extensional faulting events. 1991) defined five unconformity-bounded represented by continental sedimentary Our data also indicate that Permo-Triassic ex- depositional megasequences, but the inferred strata with no significant fauna/flora, the in- tension here did not continue into the Juras- stratigraphy of their units 1 and 2 and ties to creasing number of radiometric dates on in- sic, as in the case of the Domeyko basin to the western outcrops differ from ours for the terbedded volcanic rocks, good biostratigra- west, because marine strata were not depos- Paleozoic through Purilactis Group. Se- phy of the Domeyko basin marine limestones ited. We thus conclude that the El Bordo area quences 3 and 4 of Macellari and others (Chong, 1977), and chronology of the late Ter- (Figs. 2 and 3) represents a Paleozoic base- (1991) correspond to our Purilactis and Pa- tiary ignimbrites allow reasonable dating of

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Lithostratigraphy, Salar de Atacama Basin melting of the upper mantle (Worsley and others, 1984). Geochemistiy of regionally important syenogranite, monzogranite, AAAAAAAA and rhyolite is consistent with an exten- oooooooo Undifferentiated Quaternary gravel, AAAAAAAA oooooooo evaporate and volcanic rocks sional regime (Kay and others, 1989; vvvvvvvv AAAAAAAA Mpodozis and Kay, 1992). /WW \ A A A A Vilama Formation > (fore-arc basin) vvyyvvw •Y y y Y V Y V. y The Domeyko Basin y.y'v y y y'yv San Bartolo Group vvvvvvvv 17Ma Rocks of equivalent stratigraphic position to the Peine Formation crop out extensively Paciencia Group in the Cordillera de Domeyko (Fig. 1) and Extensions! phase 3 were first named the Agua Dulce Formation (inter-arc basin) by Garcia (1967). The unit is exposed at El O V V V V V V V V Bordo on the inverted western Salar de Ata- 41 Ma cama margin (Fig. 3), where > 1 km of tra- Figure 5. Lithostratigraphy chytic lava passes upward into alluvial clastic of the Salar de Atacama basin rocks and andesitic lavas (Montano, 1976). Purilactis Group fill, as exposed on the inverted Farther south, the succession is overlain by basin margin and interpreted marine mixed clastic/carbonate cycles includ- Extensional phase 2 (back-arc basin) from seismic data. ing Norian-aged patch reefs (Chong, 1991, personal commun.). This flooding is coinci- 64 Ma K/T Boundary 6- dent with an inferred global eustatic highstand (Haq and others, 1987), indicating an important eustatic component to relative sea level at this time. Local, fault-related differential subsidence Â'A ÀAÀAAA AAAAAAAA resulted in intra-Triassic disconformities and a diachroneity of the transgression (Chong vvvvvvvv and Hillebrandt, 1985; Groschke and Wilke, vvvvvvvv 1985). Active extension in northern Chile may vvvvvvvv Peine Formation have ceased by Late Jurassic time, because 9- Extensional phase 1 there is evidence for a basinwide regression, (foreland rift) producing a marine evaporite unit (Chong, vvvvvvvv 1977; Fig. 12). This relative fall in sea level took place during the Kimmeridgian, contig- Palaeozoic sedimentary uous with a global eustatic highstand (Haq •^•ttt+1 rocks and granite and others, 1987) and therefore must be of tectonic origin. During the whole of this period, the Salar basin was above depositional major unconformities and regional structural trast, postulated an intraplate origin for the base level (Fig. 12). development. Previous authors isolated re- whole Carboniferous-Triassic succession of The presence of a marine extensional back- gional tectonic events (for example, Stein- northern Chile, based on a combination of arc basin during Late Triassic-middle Creta- mann, 1929; Coira and others, 1982) and igneous rock geochemistry, stratigraphy, and ceous time is well known through the length attempted to link these regional events to first- interpretation of fore-arc-trench geometry. of South America; this basin formed by con- order plate-tectonic evolutionary changes Noble and others (1978) described rift-related tinued extension of the Triassic rift system, (Jordan and others, 1983; Pilger, 1984; Pardo- undersaturated igneous rocks from the Mitu now in a back-arc setting (Coira and others, Casas and Molnar, 1987; Daly, 1989). We Group of central Peru. In a synthesis of the 1982). In Peru (Atherton and Webb, 1989) and now attempt to place the Salar de Atacama geodynamics of southern South America, Uli- southern Chile (Dalziel, 1986), the basin basin evolution into the regional development ana and Biddle (1988) also argued for a series evolved into a true marginal basin with pro- of the Central Andes (Fig. 12). of north-northwest-south-southeast-oriented duction of oceanic crust. rift and thermal-sag basins along the Andean Regional Permo-Triassic Evolution margin. The origin of the extensional basins Middle Cretaceous Contraction and Uplift has been linked to large-scale plate dynamics. The early history of the Andean cycle in Gondwana is inferred to have been stationary During the middle Cretaceous, the opening northern Chile is poorly understood, although from Late Permian to Middle Triassic, with of the South Atlantic ocean (Rabinowitz and some authors postulated that the margin has no related subduction at its western margin Labrecque, 1979) resulted in the westward been convergent, with continued subduction (Valencio and Villas, 1976). It is thought that movement of the South American continent since early Paleozoic time (Coira and others, such static conditions under large conti- relative to Africa (Pilger, 1984; Daly, 1989). 1982). Breitkreutz and others (1988), by con- nents results in heat build-up and partial This resulted in a middle Cretaceous period of

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Figure 6. Interpreted versions of seismic lines Z1F012 (A) and ZIF012. Note the clear evidence for the Permo-Triassic and Late Cretaceous-early Tertiary basins being controlled by extensional faults. Position of lines shown in Figure 3. See text for fiill discussion.

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Ignimbrite

Upper Member

(Ignimbrites)

Ephemeral lake

Middle Member

(Playa deposits]

Lacustrine delta

Distal sheetfloods

Figure 7. a. Detail of abrupt transition from playa/lacustrine red beds to ignimbritic volcanics, Peine Formation, b. Log through part of the middle member of the Peine Formation (basin-fill unit 1) showing playa/lacustrine siltstones with interbedded distal alluvial-fan sheetflood sandstones.

Cohesive debris flow. Andesite clasts Andesite O ±; w « rn o o ¡75^-0 u O-O

KEY TO FIGURE 7b

Climbing ripple Asymptotic cross strata cross lamination Desiccation cracks S Fining upward sequence

Planar-tabular cross Rip-up clasts Bioturba tion bedding Sè, Coarsening upward sequence Paleocurrent Trough cross bedding Branching burrows t direction

Ripple lamination Massive strata XX Halite Dm Maximum clast diameter

Isolated vertical Convolute lamination Parallel lamination ft •¡¿A, burrows Channel geometry

Geological Society of America Bulletin, May 1993 611

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Permo-Triassic

Km 4 " Purilactis Group

6 -

Incaic Shortening y, i

8 -

Holocene

12 (a) (b) Figure 8. a. Burial history plot (uncorrected for compaction) for the main basin-fill units of the Salar de Atacama basin, within the limited chronostratigraphic control available, b. Similar burial history plot showing Purilactis Group-Holocene deposits in more detail. See text for discussion.

regional contraction (Fig. 12), with the clo- into the Central Andes probably encroached depth through the Salar basin. We infer that sure of the Domeyko back-arc basin (Coira from the Atlantic along rift systems such as one strand of this fault system is the Permo- and others, 1982). The net result was produc- the Salta basin of northwest Argentina (Salfity Triassic normal fault, bounding the Cordon tion of a basement-cored thrust belt (proto- and others, 1985; Grier and others, 1991). de Lila (Figs. 2 and 6), which was later reac- Cordillera de Domeyko; Fig. lOd) and very tivated as an oblique-slip fault during the limited middle Cretaceous deposition (Ric- Late Eocene Transpression Eocene period of oblique convergence (Figs. cardi, 1987). Linked to this shortening was an 2 and 12). This deformation may have pro- eastward movement of the Andean volcanic The Incaic orogeny (Steinmann, 1929) rep- duced the distinctive regional asymmetric arc, relative to its Jurassic position, to the resents crustal shortening throughout the Cen- syncline in the northern section of the Puri- western Cordillera de Domeyko (Fig. 1; tral Andes (Coira and others, 1982; Mégard, lactis Group outcrop (Reutter and others, Reutter and others, 1988). This arc migration 1984) and has been attributed to high rates of 1991; Fig. 2). Paleomagnetic studies (Hartley has been related to tectonic erosion of the convergence (Fig. 11) between the Farallón and and others, 1991) indicate that the Purilactis margin (Rutland, 1971; von Heune and South American plates (48-25 Ma; Pilger, 1981, Group was rotated 19° clockwise prior to dep- Lallemand, 1990). 1984; Pardo-Casas and Molnar, 1987). Pro- osition of the Paciencia Group. Identification During the Late Cretaceous, a further east- jected plate reconstructions for the Eocene in- of oblique slip during the Salar history is im- ward jump of the Andean arc is apparent, to dicate a period of oblique convergence, imply- portant, but as we see no clear major regional a basin margin position coincident with the El ing right-lateral strike slip of the margin (Pilger, strike-slip fault system to the north and south, Bordo Dorsal (now the uplifted eastern mar- 1981,1984; Beck, 1988). Northwest-southeast- we do not have evidence to suggest that the gin of the Domeyko range). Arc plutons are striking strike-slip faults have been mapped basin as a whole is a strike-slip basin. represented by diorite bodies such as Cerro along part of the western Salar basin margin Quimal (64.6 ± 1.1 Ma; K/Ar biotite by (Dingman, 1963, 1967) and have produced in- Early-Middle Miocene Contraction Ramirez and Gardeweg, 1984) (Fig. 3). The tense folding within the Purilactis Group (Char- dominance of fresh boulders of granodiorite rier and Reutter, 1990; Reutter and others, The Paciencia Group was folded prior to in the upper conglomeratic members of the 1991). This deformation is confined to a rela- the deposition of the San Bartolo Group Purilactis Group reflects the unroofing of the tively local zone and preferentially developed in (Ramirez, 1979b; Travisany, 1979). This Mi- Cretaceous arc. Major, continent-wide flood- fine-grained siltstone and evaporite (Figs. 3 and ocene deformation (Figs. 10c and 12) may ing took place in Late Cretaceous time (Uli- lOe) of the lowermost Purilactis Group. relate to the regional Quechua 1 phase of ana and Biddle, 1988), but marine incursions The major oblique-slip fault system runs at Mégard (1984), which has been linked to the

612 Geological Society of America Bulletin, May 1993

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Paciencia Group

44 +/-1 Ma 4000 »y red Ephemeral lake margin Cinchado Formation Distal sheetflood 225 Playa lake

Lake margin 3000 •

TI 220- c 33 Purilactis r~ Multi-storey channel/sheetflood > Formation system lining up into playa lake O CD 64 +/-10 Ma 2000 CD 215 Playa lake E O Lake margin 33 O C TI 210

Multi-storey channel/sheetflood 1000 system lining up into playa lake

Debris flow 205i Tonel Hyperconcentrated flood flow Formation Playa 202 J ^OTU.q'-'QOÜ C5 .o s vcpb Tuina / El Bordo a Formation

Figure 9. a. Schematic log through the Purilactis basin fill, modified from Hartley and others (1988). See Figure 7b for key. Note the lower fine-grained sequence (Tonel Formation) and the complex coarsening- and fining-upwardtrends . Paleocene-aged lava in the middle section of the stratigraphy. Clast composition and paleocurrent data reflect synsedimentary uplift of the Domeyko basin fill to the west and nearby establishment of a Late Cretaceous volcanic arc. b. Detailed logs showing the dominance of small-scale (5-15 m) fining-upward cycles in the upper half of the Purilactis Group, Baquedano Road section.

fragmentation of the Farallon plate into the of the basin (Llano de la Paciencia) was ef- the Rio San Pedro, which hugs the uplift fea- Cocos and Nazca plates. By this time the fectively partitioned from the rest of the Salar ture for some 10 km (Figs. 2 and 3). Andean arc was fully established in its as a thrust sheet-top basin (Fig. 4), thus trap- present-day position east of the Salar basin. ping all coarse-grained sediments. The DISCUSSION change in depositional gradient resulted in a Quaternary Contraction swing in drainage from a lateral west-east sys- Basin Classification Problems and the Role tem, into an axial north-south trend. Further of Contractional Tectonics During Quaternary time, the emergence of emergence of the Cordillera de la Sal resulted the intrabasinal Cordillera de la Sal as a func- in westward back-shedding of slightly In a review of Tertiary Andean basins, tion of thrust-front propagation (Jolley and younger gravel into the Llano (Jolley and oth- Jordan and Alonso (1987) drew attention to others, 1990) resulted in a significant change ers, 1990). Westward tilting of the main Salar problems in basin classification at convergent in basin-fill architecture. The western quarter toward the Cordillera de la Sal is indicated by margins. Because of the complex and cur-

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CORDILLERA DE LA SAL

rently poorly understood interplay between belt between the arc and the basin and Group) indeed represent "foreland basins" if tectonic and thermal processes in arc-related whether or not flexural loading (Beaumont, the above definition of Jordan and Alonso is basins, these authors described basins as 1981; Jordan, 1981) contributed to subsidence. followed. However, our data indicate that, foreland basins if they lay to the cratonic side Within the Salar de Atacama basin, fillunit s although flexural loading may have been a of an arc, whether or not there was a thrust 2 and 3 (Purilactis Group and Paciencia factor in controlling subsidence, the dom-

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Figure 10. a. View east from El Bordo from the gravel plain of the Llano de la Pa- sional faulting. Important but short-duration (Fig. 2) across the lower Purilactis Group and ciencia (Figs. 3 and 4), changing the sediment contractional episodes do link to known first- into the Llano de la Paciencia. The Cordillera dispersal system. Modern drainage in the ba- order plate margin changes, but their strati- de la Sal (latest position of frontal Domeyko sin is still controlled by neotectonic contrac- graphic effect appears to be restricted to up- thrust) is visible in the middle distance. The tional fault scarps. lift/erosion rather than creation of significant main Salar de Atacama and the Holocene fiexural subsidence. volcanic arc (High Andes) are visible in the Causes of Dominance of background. Extensional Tectonics b. Unconformity between the Late Creta- ceous-Paleocene Purilactis Group (left) and We consider that the dominance of exten- Oligocene gravels of the Paciencia Group sional tectonics in the late Paleozoic-early 1000 (right). Miocene of the Central Andean belt is due to c. Playa siltstones and sheetflood sandstones several causes. A A A [ A A of the Paciencia Group overlain with angular (1) The Late Permian hosted the splitting of unconformity by ignimbrites of San Bartolo Pangea, due to thermal doming and rifting Group, Rio Grande valley, San Bartolo. Cliff (Mpodozis and Kay, 1992). Thus the early 900 A A A 1 AA ^ .V: -;AJ, A ..VI is —80 m high. Salar basin extension was driven by stretch- A A d. Stratigraphic relations in the Cordillera ing in a setting with no direct evidence of A A A 1 A A , de Domeyko, southwest of the Salar de Ata- subduction or volcanic arc activity. Contin- '•'A ' A'- •< A' A M cama. Eastward view of imbricate backthrust ued extension and thermal subsidence took AA system; early Tertiary strata in foreground place through the Triassic and Early Jurassic. mmtTK (IT), overlain by Jurassic carbonate sequence (2) Following middle Cretaceous contrac- (J), Triassic Agua Dulce Formation (T), and tion and uplift of the proto-Cordillera de Paleozoic basement granite (P). Domeyko (driven by the opening of the South e. Highly deformed playa sequence of the Atlantic ocean), Purilactis Group deposition Lower Purilactis Group (Tonel Member) on was controlled by extension, now in a back- the western margin of the Llano de la Pacien- arc basin setting. This extension re-used the cia. This localized zone of deformation (com- Permo-Triassic fault systems. 600 pare with Fig. 9a) lies within an Eocene strike- (3) Extension and oblique slip during Pa- slip fault zone (see text for discussion). ciencia Group time (Oligocene) was partially f. Plio/Quaternary thrusting of Paciencia controlled by collapse of the Cordillera de Group (Oligocene) over late Miocene ignim- Domeyko, which had been uplifted again dur- brites of the San Bartolo Group. White ignim- ing late Eocene transpression. brite is —20 m thick. Regional gravity data indicate a north- northwest-south-southeast-trending positive gravity anomaly that runs from the northwest of the Salar de Atacama through to the Ar- \ gentinian Puna (Gotze and others, 1988). Al- inant control was extensional faulting. The though the gravity data were not modeled in migration of the Andean volcanic arc to a site detail, these authors proposed a fragment of \ adjacent to the basin margin prior to Purilactis dense, Paleozoic or Precambrian metamor- deposition may have superposed thermal phic basement to explain the anomaly. The I»

modifications on the basin-margin tectonic coincidence of such dense basement with the -fì character. Our subsidence curve for the Pu- Salar basin might have contributed to the suc- rilactis and Paciencia basins does not have cessive formation of basins in this site, but the the common foreland basin pattern, with a extension of the gravity anomaly for more - n sudden increase in subsidence rate related than 200 km south-southeast under the high » to thrust-sheet emplacement (for example, Puna questions the genetic linkage between n Homewood and others, 1986). Indeed, we subsidence in the Salar area and this dense lack evidence that thrusting was still active in crust. More-detailed gravity modeling is re- the Cordillera de Domeyko during Purilactis quired to better assess the role of basement time. We do have a clear signal through prov- heterogeneity on basin development. enance evolution that the Late Cretaceous We therefore propose that the Salar de Ata- arc chain was being actively unroofed in the cama stratigraphy preserves a series of "ba- BASE NOT SEEN early Tertiary section of the basin fill. sins" that, owing to the longevity of the An- m s fs ms cs The late Tertiary fore-arc basin phase, rep- dean margin and hemisphere-scale tectonic Figure 11. Schematic log through part of the resented by fill units 4 and 5, was controlled evolution, have evolved from a continental Paciencia basin-fill unit, modified from Flint by contractional tectonics. Emergence of the rift, through a back-arc basin and possible (1985). Note the early derivation of material Cordillera de la Sal as an intrabasinal uplift on interarc stages, to a late Tertiary-Holocene from the east, replaced by dominant supply an eastward-propagating thrust system (Jol- fore-arc basin. Accumulation of the sedimen- from the inverted Purilactis Group to the west. ley and others, 1990) separated the main Salar tary succession was mainly due to exten- See Figure 7b for key.

Geological Society of America Bulletin, May 1993 615

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Coastal Salar Nazca Plate Central Domeyko Igneous Activity Cordillera Depression Basin Basin Convergence Rate N w-4—e mm/yr i g o O o O o o o m o m o ín , Atacama Fault Zone Analen ? ? y 7 , Ma W \ Ma 0- Establishment 0 of modern Andean Arc (Narrow belt) h 20 No arc 5 Dontraction/strike slip(2) 40 Arc Magmatism (Broad discontinuous o 60 o belt) / o o "B. o 80 E. Arc Magmatism CAI Discontinuous belt QO I- 100 n>" 120 vvvvvvvv vvvvvvvv Arc Magmatism VVVVVVVVÌ r« vvvvvvvvs 140 3. vvvvvvvvx AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA No deposition (Narrow continuous o vvvvvvvvv P5 • vvvvvvvv v| 03 vvvvvvvvv belt) e vvvvvvvvv Extension/ -160 vvvvvvvvv . vvvvvvvvv thermal vvvvvvvvv vvvvvvvvv vvvvvvvvvv subsidence - 180

vvvvvvvvvvv1 -200 200 - V vvvvvvvvv vvvvvvvvv - - . vvvvvvvvvvvJ ( VVVVVVVVVVVVVVVVVVV ' ' ~ vyyvvvvvvvv/ Rift Magmatism 1>J VVVVVVVVVVVVVVVVVVVV vv V vv V vvV vvvvvvvvvv'j I v.v. v v. v. vv v.v v.v.v v v v v.y.y V-v v.y y y y. y y v. v. VVVVVVVVV jj I- vvvvvvvv] Extension -220 220- vvvvvvvv'

A A A A A V V V V V 1 Peruvian orogeny A A A A A Evaporites V V V V V Volcanic rocks A A A A A V V V V V

+ + + + +I 2 Incaic orogeny Sandstone + + + + + Intrusive rocks + + + + +

3 Quechua orogeny Shale Conglomerates

4 Diaguita orogeny Marine limestone STTST

Figure 12. Chronostratigraphic diagram for the Andean fore arc, northern Chile, showing all phases of basin filling and inversion. Magmatic and tectonic activity are plotted alongside convergence rate/angle data and show a good correlation between plate dynamics and continental margin evolution. Data on igneous activity and plate kinematics are integrated from Coira and others, 1982; Pardo-Casas and Molnar, 1987; Pilger, 1981, 1984; Rabinowitz and Labrecque, 1979; Reutter and others, 1988.

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ACKNOWLEDGMENTS tofagasta Province, Chile: Geological Society of London Jour- tures in central and northern Perú: Geological Society of Lon- nal, v. 141, p. 533-546. don Journal, v. 141, p. 893-900. Flint, S., 1987, Diagenesis of Tertiary playa sandstones of northern Montaño, J. M., 1976, Estudio geológico de la zona de Caracoles y Chile: Implications for Andean uplift and metallogeny: Sed- áreas vecinas, con énfasis en el sitema Jurásico, Provincia de This paper synthesizes field work under- imentology, v. 34, p. 17-31. Antofagasta, II Region, Chile [M.S. thesis]: Santiago, Chile, taken during the last eight years. Field dis- Flint, S., Hartley, A. J., Rex, D. C., Guise, P., and Turner, P., 1989, University of Chile, 168 p. Geochronology of the Purilactis Formation of northern Chile: Moraga, A., Chong, G. D., and Fortt, M. A., 1974, Estudio geo- cussions with Guillermo Chong and Phil Implications for early Tertiary basin dynamics in the Central lógico del Salar de Atacama, provincia de Antofagasta: San- Andes: Revista Gcologica de Chile, v. 16, p. 241-246. tiago, Chile, Instituto de Investigaciones Geológicas Bulletin, Scanlan have considerably clarified our ideas. Garcia, F., 1967, Geologia del norte grande de Chile: Proceedings, v. 29, 59 p. Symposium on Geosyncline Andes, Society Geologica Chile, Mpodozis, C., and Kay, S. M., 1992, Late Paleozoic to Triassic Financial support by the U.K. Natural Envi- v. 3, 138 p. evolution of the Gondwana margin: Evidence from Chilean ronment Research Council, the Royal Soci- Gotze, H. J., Strunk, S., and Schmidt, S., 1988, Central Andean frontal cordilleran batholiths (28°S-32°S): Geological Society gravity field and its relation to crustal structures, in Bahlburg, of America Bulletin, v. 104, p. 999-1014. ety, Liverpool University, and Birmingham H., and others, eds., The southern Central Andes: Lecture Noble, D. C., Silbermann, M. L., Mégard, F., and Bowman, H. R., notes in earth scicnces, Number 17: Heidelberg, Germany, 1978, Comendite (peralkaline rhyolite) in the Mitu Group, University is gratefully acknowledged. Em- Springer-Verlag, p. 199-208. central Perú: Evidence of Permian-Triassic crustal extension presa Nacional del Petróleo (ENAP), San- Grier, M. E., Salfity, J. A., and Allmendinger, R. W., 1991, Andean in the Central Andes: U.S. Geological Survey Journal of Re- reactivation of the Cretaceous Salta rift, northwestern Argen- search, v. 6, p. 453-457. tiago, is thanked for access to seismic data by tina: Journal of South American Earth Sciences, v. 4, Pardo-Casas, F., and Molnar, P., 1987, Relative motion of the Nazca p. 351-372. (Farallón) and South American plates since Late Cretaceous Jolley in 1987 and Flint in 1991 and for release Groschke, H., and Wilke, H. G., 1985, Investigaciones sedimento- time: Tectonics, v. 6, p. 233-248. of the seismic lines for publication. Discus- logicas en el Jurasico de la Precordillera entre 22° and 24°S, Pilger, R. H., Jr., 1981, Plate reconstruction, aseismic ridges and low region de Antofagasta, Chile: Actas, IV Congreso Geologico angle subduction beneath the Andes: Geological Society of sions with Francisco Townsend of ENAP Chileno, Antofagasta, v. 1, p. 1/385-1/396. America Bulletin, v. 92, p. 448-456. Hamilton, W. B., 1988, Plate tectonics and island arcs: Geological Pilger, R. H., Jr., 1984, Cenozoic plate kinematics, subduction and clarified aspects of the seismic stratigraphy, Society of America Bulletin, v. 100, p. 1503-1527. magmatism: South American Andes: Geological Society of although all interpretations are our own re- Haq, B., Hardenbol, J., and Vail, P. R., 1987, Chronology of fluc- London Journal, v. 141, p. 793-802. tuating sea levels since the Triassic: Science, v. 235, Rabinowitz, P. D., and Labrecque, J., 1979, The Mesozoic South sponsibility. Thank you to the crew of Hunt p. 1156-1166. Atlantic ocean and evolution of its continental margin: Journal Hartley, A. J., Flint, S., and Turner, P., 1988, Lithostratigraphy and of Geophysical Research, v. 84, p. 5973-6002. Oil Toconao No. 1 for their hospitality in sedimentology of the Cretaceous Purilactis Formation, An- Ramírez, C. 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