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Middle Miocene tectonic development of the Transition Zone, Salta Province, northwest Argentina: Magnetic stratigraphy from the Meta´n Subgroup, Sierra de Gonza´lez J.H. Reynolds* Division of Environmental Studies, Mathematics, and Natural Sciences, Brevard College, Brevard, North Carolina 28712, USA C.I. Galli R.M. Herna´ndez XR Exploracionistas y Servicios Regionales, s.r.l., Manzana N—Casa 14-1Њ Etapa, BЊ General Belgrano, Salta 4400, Argentina B.D. Idleman Department of Earth and Environmental Sciences, Lehigh University, 30 Williams Drive, Bethlehem, Pennsylvania 18015, USA J.M. Kotila R.V. Hilliard Department of Geosciences and Natural Resources Management, Western Carolina University, Cullowhee, North Carolina 28723, USA C.W. Naeser U.S. Geological Survey, M.S. 926A, National Center, Reston, Virginia 20192, USA ABSTRACT ment accumulation rates increased dramat- INTRODUCTION ically. These changes correlate with con- Magnetostratigraphy, isotopic dating, temporaneous tectonism in the west. A local The Central Andes are frequently cited as a and sandstone petrography establish age increase in basin accommodation may be type example for mountain ranges generated limits on the depositional history of ϳ2100 partly related to a zone of weakness near from the subduction of oceanic lithosphere be- m of foreland basin strata in the Neogene the eastern boundary of the Salta rift. neath a continent (e.g., Moores and Twiss, Meta´n Subgroup of northwest Argentina. Uplift in the western Cordillera Oriental 1995; van der Pluijm and Marshak, 1997). The strata were deposited between ca. 15.1 apparently began by 13.7 Ma and thrusting The foreland deformational style between and 9.7 Ma in the eastern Sistema de Santa rapidly migrated eastward. The eastern Peru and northwest Argentina may behave as Ba´rbara. The region is positioned above the Cordillera Oriental ranges began to rise be- a classic Davis et al. (1983) type of orogenic Cretaceous Salta rift basin, in the Transi- tween 25؇ and 26؇S ca. 10 Ma. As thrusting wedge, but surprisingly little is known about tion Zone between modern relatively steep migrated eastward, low-energy depositional the chronology of deformation in the foreland and flat subducting segments of the Nazca environments were overwhelmed ca. 13.7 region to confirm that the strata deformed in the classic fashion. plate. Ma. Above an erosional unconformity that Formations within the subgroup are The Subandean zone (Fig. 1) is host to hy- removed strata to an age of ca. 9.7 Ma, bas- shown to be diachronous over a 60 km dis- drocarbon production and exploration along al strata from the overlying Jujuy Sub- tance; the younger ages are in the east. most of its length in Bolivia and north of 27ЊS group were deposited beginning after 9 Ma. Changes in paleocurrent flow directions in Argentina, but few numerically dated ho- Sandstones from Rı´o Yacones suggest and the lithic clast component of sandstones rizons exist in the foreland basin strata. Pub- collected from the Arroyo Gonza´lez section that the Cordillera Oriental uplift contin- lished interpretations of structural history, tec- suggest that basal fluvial strata were de- ued for several million years longer be- tonic reconstructions, and petroleum genesis ؇ ؇ rived from the craton to the east beginning tween 24 and 25 S. Uplift of the Sistema de models that are applied today lack a concise in middle Miocene time, just prior to 15.1 Santa Ba´rbara, in the distal portion of the chronological foundation. To rectify this situ- Ma. By ca. 14.5 Ma, the paleocurrent foreland, did not begin until after ca. 9 Ma. ation we are attempting to determine the age flowed from a source in the west and sedi- of basin-filling strata from the best-exposed stratigraphic sections in the foreland region to Keywords: Argentine Andes, magnetostra- *Second address: Magstrat, LLC, P.O. Box 300, understand their relationships to the structures Webster, North Carolina 28788, USA; e-mail: tigraphy, Neogene, Salta Argentina, tecton- and to determine the provenance of their sed- [email protected]. ics, transition zones. iment. This will allow the construction of new GSA Bulletin; October 2000; v. 112; no. 11; p. 000–000; 14 figures; 3 tables. REYNOLDS et al. models that describe the paleogeographic evo- lution of the basins and mountain ranges. The work reports the chronologic develop- ment of middle Miocene sedimentation in the Sistema de Santa Ba´rbara portion (Fig. 1) of the Transition Zone at the southern end of the Subandean zone. Using dating from magnetic polarity stratigraphy aided by a fission-track age and chronologic information from other areas in the Transition Zone, crosscutting re- lations, and changes in the lithic content of the sandstones, we interpret the middle Miocene tectonic development of the region. Nazca plate subduction is characterized by significant along-strike subduction angle changes beneath the western margin of South America (Jordan et al., 1983). Foreland struc- tural provinces of northwest Argentina devel- oped in three tectonic domains characterized by differences in the subduction angle and the orientation of preexisting structural fabrics. An area of inclined subduction where the Naz- ca plate descends at an angle of about 30Њ to the east is between 18Њ and 24ЊS. Farther to the south, between 28Њ and 33ЊS, is an area of subhorizontal (ϳ5Њ) subduction (Barazangi and Isacks, 1976). The Transition Zone is sit- uated between these subduction domains (24Њ–28ЊS). TRANSITION ZONE PROVINCES Our work focuses on the middle Miocene tectonic history of the northern part of the Transition Zone (24Њ–26ЊS). Structural aspects Figure 1. Map illustrating the structural provinces in northwest Argentina (adapted from of the Neogene foreland provinces to the north Grier et al., 1991). The boundary between the Transition Zone and the inclined subduction of the Transition Zone continue along strike area to the north is located at ϳ24؇S, where the Cordillera Oriental narrows. Cross-section through the Transition Zone but undergo X–X؅ is shown in Figure 2. The area illustrated in Figure 3 is outlined by the open marked changes before terminating at the rectangle. The Arroyo Gonza´lez area (Fig. 4) is shown by the white rectangle. The Su- southern margin (Barazangi and Isacks, 1976; bandean zone boundary between the Sierras Subandinas and the Sistema de Santa Ba´r- Jordan et al., 1983; Allmendinger et al., 1983). bara follows the trend of the Santa Ba´rbara Ranges (Fig. 3B). The irregular southern Bevis and Isacks (1984) proposed that the limit of the Transition Zone is marked by the boundary with the Sierras Pampeanas. subducting plate is deformed by a broad gen- Heavy dashed lines denote the approximate boundaries of the Salta rift (white arrows on tle flexure through the Transition Zone. downthrown side). R´ıo Iruya (IR) is located in the Sierras Subandinas. Salta Rift Basin eastern boundary of the rift. The northern lim- a foreland basin developed eastward across Grier et al. (1991) demonstrated that the ge- it of the Sierras Pampeanas is located roughly the area (Com´ınguez and Ramos, 1995). ometries of Andean tectonism in the Transi- along the southern rift margin. tion Zone are consistent with the inversion of The rift is partitioned into six named sub- Puna the Cretaceous Salta rift basin and with Neo- basins: the Lomas de Olmedo, Tres Cruces, gene shortening by thrust reactivation of Cre- Sey, Aleman´ıa, El Rey, and Meta´n (Fig. 3). The Puna Plateau is situated along the Ar- taceous listric normal faults. Westward ver- The subbasins are grouped around the central gentina-Chile border between the main range gence in the western Cordillera Oriental por- Salta-Jujuy structural high (Fig. 3; Marquillas of the Andes and the Cordillera Oriental. It tion of the Transition Zone (Fig. 2) is localized and Salfity, 1988; Mon and Salfity, 1995) and represents the southern extension of the broad- near the western boundary of the Salta rift ba- are filled with Salta Group strata (Table 1). er Altiplano that is to the north in Bolivia and sin, where the Puna and the western Cordillera Compressional regional stresses were initi- Peru. The Puna-Altiplano is characteristic of Oriental meet (Grier et al., 1991). The eastern ated by Neogene time. Continental clastic the inclined subduction region. It continues part of the Sistema de Santa Ba´rbara (dis- strata of the Ora´n Group buried the rift during through the Transition Zone, tapering to its cussed in the following) is located above the the Quechua phase of the Andean orogeny as terminus at the southern margin. Mean basin Geological Society of America Bulletin, October 2000 MIDDLE MIOCENE TECTONIC DEVELOPMENT OF THE TRANSITION ZONE .(Figure 2. Balanced cross section of the Transition Zone at 25؇30؅S (modified from Grier et al., 1991 TABLE 1. STRATIGRAPHY OF THE ORAN AND Transition Zone varies from predominantly downcutting provides fresh, relatively contin- SALTA GROUPS westward verging in the west to eastward uous exposures along select streams. Group Subgroup Formation Age verging in the east (Fig. 2), resulting from re- Structures in this province exhibit affinities Ora´n Jujuy Piquete Pliocene– activation of the rift-bounding faults (Grier et with the basement block uplifts of the Sierras Pleistocene al., 1991). The Cordillera Oriental is wider in Pampeanas immediately to their south. The Guanaco Miocene–Pliocene Meta´n Jesu´s Marı´a Middle–upper the Transition Zone than it is in the inclined basement cores of the Sistema de Santa Ba´r- Miocene subduction area (Fig. 1). Grier and Dallmeyer bara ranges have not been unroofed due to a Anta Middle Miocene thick sedimentary cover provided by infilling Rı´o Seco Middle Miocene (1990) suggested that eastward progression of Salta Santa Lumbrera Paleocene– deformation in the Cordillera Oriental at these of the Salta rift basin thrusting (Allmendinger Ba´rbara Eocene latitudes was faster than that in the Sierras et al., 1983).

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