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- Geological Society of America Bulletin, v. 105, p. 603-617, 12 figs., May 1993. 603 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/603/3381684/i0016-7606-105-5-603.pdf by guest on 30 September 2021 FLINT AND OTHERS 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 se- quence consists of more than 600 m of con- tinental 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 exten- sional fault (Fig. 6). The seismic facies include both laterally continuous high-amplitude re- flections 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.