/ JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. BIO, PAGES 22,965-22,984, OCTOBER 10,199i" Vertical axis rotations across the Puna plateau (northwestern Argentina) from paleomagnetic analysis of Cretaceous and Cenozoic rocks - - Isabelle Coutand, Annick Chauvin, Peter Robert Cobbold, - --- - i and Pierre Gautier l l GCosciences Rennes, CNRS (UPR 4661), Rennes, France Fonds DOI Pierric Roperch d i UIUU I IYGI: IRD and $epartkento de Geología y Geofísica, Universidad de Chile, Santiago i . - - 1- _- -- Abstract. Between loosand 3OoS, the central Andes are marked by both a major topographic anomaly, the Altiplano-Puna plateau, and a westward concave geometry whose origin remains controversial. The arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the central Andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc (throughout Peru, northernmost Chile, and northern Bolivia) and clockwise rotations on the southern limb (throughout southern Bolivia, northwestern Argentina, and northern Chile). To fill a gap in data from northern Argentina and to contribute to the ongoing debate on the origin of rotations in the central Andes, we have undertaken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into seven localities). The samples are from sediments and lava flows of Cretaceous to Tertiary age located in intermontane basins of the Puna plateau in northwestern Argentina. Vertical axis rotations, calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central Andes. However, significant variations in the amount of rotation occur from one locality to another, suggesting that they are, at least in part, influenced by local tectonics. As most faults in the Puna plateau have reverse dip-slip components, we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults. Whether or not this mechanism has operated across the entire area of thickened crust in the central Andes remains to be demontrated. Even if such faulting has locally influenced rotations, Cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central Andes, Fonds Documentaire ORSTOM Cote :4gVC339 i. 1. Introduction Hoke, 1997; Schmrtz. and Kley, 19971, but also ecaÛse of magmatic additions to the crust [Sheffels, 1990; Francis and The central Andes, a noncollisional orogen, have resulted Hawkesworth, 19941. from subduction of the oceanic Nazca (or Farallon) plate In map view, the central Andes are arcuate (Figure 1). beneath continental South America (Figure 1). The relative Between 6" and 1S0S, major structures trend NW-SE, whereas, convergence vector, trending N77"+12O, has been almost south of Arica, they adopt a N-S trend. These trends are steady since 50 Ma [Gripp and Gordon, 1990; Pardo-Casas ana' associated with a remarkable pattem of rotations about vertical Molnar, 19871. It is generally assumed that the central Andes axes. Paleomagnetic studies have demonstrated were uplifted mainly between the lower Oligocene and the counterclockwise rotations on the northern limb of the arc, present [Jordan et al., 1983a, b; Jordan and Alonso, 19871. ' throughout Peru, northernmost Chile, and northern Bolivia Between 15" and 27"S, the central Andes form a plateau, the [Heki et al., 1983, 1984, 1985; Kono et al., 1985; May and Altiplano-Puna (Figure 1). It lies at about 3800 m, between Butler, 1985; Mitouard et al., 1990; Macedo-Sánchez et al., two mountain ranges, the Eastern Cordillera and a volcanic arc 1992a, b; Roperch and Curlier, 1992; P. Roperch et al., to the west (Figure 1). Under the Altiplano the continental Tectonic rotations within the Bolivian Altiplano: crust is up to 70 km thick [Jaines, 1971; Wigger et al., 1993; Implications for the geodynamic evolution of the central Dorbath et al., 1993; Zandt et al., 1994; Beck et al., 19961, Andes during the late Tertiary, submitted to Joumal of mainly as a result of tectonic shortening [Jordan et al., 1983a, Geophysical Research, 1998, hereinafter referred to as b; Lyon-Caeiz et al., 1985; Allinendinger, 1986; hacks, 1988; Roperch et al., submitted manuscript, 19981, and clockwise Roeder, 1988; Baby et al., 1992a, b; Schmitz, 1994; Lamb and rotations on the southern limb throughout southem Bolivia, northwestem Argentina, and northern Chile [MacFadden et al., 1990, 1995; Butler et al., 1995; Roperch et al., 1997; Prezzi Copyright 1999 by the American Geophysical Union. and Vilas, 19981. At the hinge of the arc, along a transect from Paper number 1999JB900148. Santa Cruz de la Sierra (Bolivia) to the Peru-Chile trench at 0148-0227/99/1999JB900148$09.00 about 2OoS, rotations are absent or too small to be measured. 22,965 t THE 22,966 COUTAND ET AL.: VERTICAL AXIS ROTATIONS ON PUNA PLATEAU 10"s 15"s 20"s 25"s 30"s S0"W 75"W 7Q"W 65"W 60"W Figure 1. Digital relief map of the central Andes. Data are from the U. S. Geological Survey (USGS). Each pixel is 30' wide. Artificial lighting is from the east at an inclination ol' 30°. Initially, paleomagnetic results were obtained only from the on a subduction zone and strike-slip motions on continental forearc, near the continental margin, but more recent studies faults, parallel to the margin. On applying this model to the have shown that rotations also occur further to the east, within central Andes, currcnt convergence should resuli in (1) left- Go thickened continental crust of the Altiplano-Puna and Eastern lateral motions on faults trending NW-SE, belween and 18"s Cordillera [Butler et al., 1995; MacFauíiett et al., 1990, 1995; and (2) right-lateral motions on faults trending N-S,south of Roperch et al., submitted manuscript, 19981. the Arica deflection [Beck, 1987; Dewy und Lurnb, 19921. Although the pattern of vertical axis rotations is striking, However, this pattern of fault motions is not simple to there is no consensus as to its origin. The simplest model is reconcile with the observed block rotations. that of an orocline. Rotations are assumed to reflect simple One of the simplest models of block rotation between bending of the western continental margin of South America, parallel faults is the domino or bookshelf model [Freud, in the manner of a single strut. A plausible mechanical 19701. On applying this model to the central Andes, assuming explanation for bending has been given by Isacks [1988]: It is that strike-slip faults are parallel or nearly parallel to the due to a variation in horizontal shortening along strike, during margin, we f-ind that clockwise rotations are associated with the Quechua phase (Miocene) of the Andean orogeny. Indeed, left-lateral motions and counterclockwise . rotations are shortening reaches a maximum along the Anca - Santa Cruz associated with right-lateral niotions. These senses of strike- transect, where the Andes are widest. Beck [1987] interpreted slip are exactly opposite to the ones predicted by the model of the arcuate shape of the margin as a primary feature, predating Fitch [1972]. Andean deformation. In this view, rotations are confined to One way out of this dilemma is to associate domino minor crustal blocks, separated by faults. Later, Beck ef al. rotations with strike-slip faults that are not parallel to the [I9941 accepted that both block rotations and oroclinal margin. For example, in the forearc of northern Chile, al. bending may have occurred, although in succession. Forsythe and Cliisholni [ 19941 and Randall et [ 19961 have Fitch [I9721 suggested that oblique convergence at an associated clockwise rotations with left-lateral motions on active margin can lead to partitioning between dip-slip motion faults that strike NW-SE, oblique to the margin. Further work .<.:> COUTAND ET AL.: VERTICAL AXIS ROTATIONS THE PUNA PLATEAU ON 22,967 22"OO' - 23"OO' - 24"OO' - 25OOO' 68 "00' 67"OO' 66'00' 65"OO' Figure 2. Geological and structural map of the Puna plateau (modified after Aiuengual et al. [1979]), showing paleomagnetic sampling sites. of this kind would be necessary to demonstrate the generality 2. Geological Setting of such an association. Another unsolved problem is the origin of differential In detail, the Puna plateau is not flat,, Small mountain rotations between neighboring blocks. From the ranges, made of basement rocks, alternate "with sedimentary paleomagnetic database for the central Andes, it is evident that basins formed in a compressional setting (Figure 2). Between such differential rotations are common. In contrast, according the ranges and basins are reverse faults and thrusts [Rimer aid to the domino model (in its purest form, based on parallel Méndez, 1979; Coira et al., 1982; Marrert et al., 19941. Most of these trend N-S to NNE-SSW. straight faults), rotations should be identical in sense and magnitude among neighboring fault blocks. In terms of subsurface structure, the Puna differs from the Bolivian Altiplano. Several fundamental changes occur across Thus the origin of rotations in the central Andes is still a NW-SE zone between in the magmatic arc and subject to debate. We believe that expanding the database will 2Oo-21"S 23'- on the eastern margin of the plateau [Alltnendinger et al., help provide explanations. In the Puna plateau of northwestern 24"s From north to south the dip of the subducted slab Argentina, there has been an obvious gap in paleomagnetic 19971. shallows progressively [Callill and hacks, the sampling until now. fill this gap and to contribute to the 19921, To lithosphere thins [Chitin arld hacks, 1983; Whitman et al:, ongoing debate on the origin of rotations in the central Andes, 19961 and the amount of horizontal shortening decreases, we have obtained and analyzed new paleomagnetic samples directly influencing tectonic style, kinematics, and the from sedimentary and volcanic rocks of Cretaceous and distribution and timing of deformation (for a review, see Tertiary ages on the Puna plateau.