University of Portland Pilot Scholars Environmental Studies Faculty Publications and Environmental Studies Presentations 2004 Vertical-axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata within the Bolivian Andes David R. Richards Robert F. Butler University of Portland, [email protected] Thierry Sempere Follow this and additional works at: http://pilotscholars.up.edu/env_facpubs Part of the Environmental Sciences Commons, and the Geophysics and Seismology Commons Citation: Pilot Scholars Version (Modified MLA Style) Richards, David R.; Butler, Robert F.; and Sempere, Thierry, "Vertical-axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata within the Bolivian Andes" (2004). Environmental Studies Faculty Publications and Presentations. 21. http://pilotscholars.up.edu/env_facpubs/21 This Journal Article is brought to you for free and open access by the Environmental Studies at Pilot Scholars. It has been accepted for inclusion in Environmental Studies Faculty Publications and Presentations by an authorized administrator of Pilot Scholars. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B07104, doi:l0.1029/2004JB002977, 2004 Vertical-axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata of the Bolivian Andes David R. Richards 1 and Robert F. Butler Department of Geosciences, University of Arizona, Tucson, Arizona, USA Thierry Sempere3 Institut de Recherche pour le Developpement (IRD), Lima, Peru Received 16 January 2004; revised 15 April 2004; accepted 6 May 2004; published 17 July 2004. [1] Thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization (ChRM) directions from 256 sites at 22 localities in Mesozoic and Cenozoic sedimentary strata of the Bolivian Altiplano and Eastern Cordillera. An inclination-only fold test of site-mean ChRM directions from Cenozoic units (principally the Santa Lucia Formation) indicates optimum unfolding at 97.1 % unfolding, consistent with a primary origin for the ChRM. For Mesozoic strata, optimum unfolding occurred at 89.2%, perhaps indicating secondary remagnetization at some locations. For Cenozoic units, comparison of locality-mean directions with expected paleomagnetic directions indicates vertical-axis rotations from 33° counterclockwise to 24° clockwise. Euler pole analysis of along-strike variation in crustal shortening within the Subandean and Interandean zones indicates 18° clockwise rotation south of the axis of curvature of the Bolivian Andes and 6° counterclockwise rotation northwest of the axis during the past 10 m. y. Along-strike variation of shortening within the Eastern Cordillera indicates 8° clockwise rotation south of the axis and 8° counterclockwise rotation northwest of the axis from 35 to 10 Ma. These vertical-axis rotations produced by along-strike variations in crustal shortening during development of the Bolivian fold-thrust belt agree well with observed rotations determined from paleomagnetism of Cenozoic rocks in the Eastern Cordillera and in the Subandean and Interandean zones. However, local rotations are required to account for complex rotations in the Cochabamba Basin and within the Altiplano. The curvature of the Bolivian Andes has been progressively enhanced during Cenozoic fold-thrust belt deformation. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8102 Tectonophysics: Continental contractional orogenic belts; 8164 Tectonophysics: Stresses-crust and lithosphere; 9360 Information Related to Geographic Region: South America; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: tectonics, paleomagnetism, South America, Cenozoic, Bolivia Citation: Richards, D.R., R. F. Butler, and T. Sempere (2004), Vertical-axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata of the Bolivian Andes, J. Geophys. Res., 109, B07104, doi:I0.1029/2004JB002977. 1. Introduction Gephart, 1994 ]), the width of the orogen between tht Pern-Chile trench and the Subandean front reaches 850 kn [2] The Andean Cordillera has been built along an active and the crustal thickness is locally over 70 km [Beck et al. continental margin by processes directly related to the 1996; Yuan et al., 2002] (Figure 1). The second larges subduction of the oceanic Nazca plate below the South continental plateau on Earth, the Altiplano of southern Pen American continent. Along the length of the mountain chain, and Bolivia and the Puna Plateau of northern Chile anc the Andes present variations in width, <!ltitude, geology, and northwestern Argentina, is found in the Bolivian Orocline crnstal thickness. In the curved segment known as the There is broad consensus that the Bolivian Eastern Cordi!len ··Bolivian Orocline" (12°-28°S latitude; [!sacks, 1988; and Subandean belt resulted from crustal shortening, wherea~ the origin and structure of the Altiplano is debated [Bahy e1 1Now at Midland Valley Inc., Golden, Colorado. USA. al., 1997; DeCelles and Horton, 2003; McQuarrie, 2002a; 2Now at Department of Chemistry and Physics, University of Portland. Sempere et al., 2002; Husson and Sempere, 2003]. Portland, Oregon, USA. [3] Deformation within the Central Andes has included 1 Now at Laboratoire des Mecanismes et Transferts en Geologie. vertical-axis rotations on a range of scales. Syntheses oJ Universite Paul Sabatier, Toulouse, France. vertical-axis rotations documented by paleoinagnetism have Copyright 2004 by the American Geophysical Union. been presented by !sacks [1988], Beck et al. [1994], Randall 0148-0227/04.200418002977$09.00 [1998], and Somoza [1998]. Both large-scale bending ofthe B07104 1 of 21 97104 RICHARDS ET AL.: VERTICAL-AXIS ROTATIONS IN BOLIVIAN ANDES B07104 66°W Argentina Figure 1. Major geologic structures of the Altiplano, Eastern Cordillera, and Subandean and Interandean zone of Bolivia. Geologic contacts are shown in gray lines; fold axes of synclines are shown decorated with cross marks; anticlinal fold axes are undecorated and shown in heavier lines. Inset map shows central Andes with elevations > 3000 m shaded. Geologic data are from l: 1,000,000 scale Mapa Geol6gico de Bolivia geographic information databases from Servicio Nacional de Geologia y Mineria and Yacimientos Petroliferos Fiscales Bolivianos. Central Andes and ball-bearing or rotating-panel vertical­ and Somoza and Tomlinson [2002]. The number of paleo­ axis rotations of crustal blocks (10-20 km dimensions) have magnetic results from the forearc region has increased affected the forearc region of southern Peru and northern substantially during the past decade. These data clearly Chile [Beck et al., 1994; May and Butler, 1985]. Recent indicate the importance of local block rotations perhaps studies of vertical-axis rotations in these coastal regions resulting from distributed shear of the South American include Beck [1987, 1988, 1998], Randall (1998], Randall continental margin by oblique subduction [McQuarrie, et al. [2001], Rousse et al. [2002], Somoza et al. [1996, 1999] 2002a]. 2 of 21 B07104 RICHARDS ET AL.: VERTICAL-AXIS ROTATIONS IN BOLIVIAN ANDES B07Ili [4] A complete analysis of the Central Andes of Bolivia, certainty whether plunge corrections are required even llf1 northern Chile, northwest Argentina, and Peru is beyond the what appear to be straightforward structural geologic set.· scope of this paper. Instead we focus on the Bolivian Andes. tings. In presentation of paleomagnetic data and discussion The Bolivian Orocline is not an orocline in the sense of an of results below, we provide cautionary notes about vertical. originally straight mountain system that has been bent into a axis rotations determined from localities with steeply dip. curved map pattern as discussed by Carey [1955]. The ping strata. Bolivian Orocline is essentially a megasalient in the orogen [7] Paleomagnetic sampling was done using methods that displays along-strike variations in the amount of crustal described by Butler [1992]. At each paleomagnetic site shortening [Marshak, 1988]. Analysis of vertical-axis rota­ oriented samples were collected either from a single sedi~ tions in the Bolivian Orocline as the result of enhanced mentary bed or several adjacent beds with cumulative curvature of the mountain belt associated with crustal stratigraphic thickness <l m. At most sites, at least eight shortening and building of the Altiplano and Puna plateaus oriented samples (mostly cored samples) were collected at was initiated by lmcks [I 988]. Analyses of paleomagneti­ each site. Exceptions include the four localities collected cally determined vertical-axis rotations within the Bolivian during magnetostratigraphic studies (Laguna Umayo, La Andes have been presented by MacFadden et al. [1990, Palca, Sucusuma, and Tiupampa) where four oriented 1995], Butler et al. [1995], Roperch et al. [2000], and Lamb samples were collected at each site [Butler et al., 1995; [2001 a, 2001 b]. In this paper, we present paleomagnetic Sempere et al., 1997; Sige et al., 2004]. Site-mean direc­ data from Paleogene and Mesozoic sedimentary rocks at a tions from those localities were illustrated by Butler et al. large number of localities within the Altiplano and Eastern [1995] but were not tabulated in that publication; site-mean Cordillera of Bolivia. We then analyze resulting determi­ directions from Laguna Umayo have subsequently been nations of vertical-axis rotations within the Bolivian fold­ tabulated