NOTE TO USERS The original manuscript received by UMI contains pages with print exceeding margin guidelines, and slanted print. Pages were microfilmed as received. This reproduction is the best copy available UMI hlagnetic polarity stratigraphy and geochronological constraints on the sequence of thrusting in the Principal and Frontal cordilieras and the Precordillera of the Argentine central Andes (33"slatitude) by Maria Veronica Irigoyen A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfilment of the requirernents for the deçree of Doctor of Philosophy Department of Earth Sciences Carleton University Ottawa, Ontario October 1997 O 1997, M.V. Irigoyen National Library Bibliothèque nationale I*I of Canada du Canada Acquisitions and Acquisitions et BibIiographic Services services bibliographiques 395 Wellington Street 395, rue Wellington OttawaON K1AON4 Ottawa ON KIA ON4 Canada Canada The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/film, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or othewise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son pemiission. autorisation. -- - . Carleton University I!l.lY- I!W2 Ottawa, Canada K 1S 5J7 @ Thesis contains black and white and/or coloured graphs/tables/photographs which when microfilmec3 may lose their significance. The hardcopy of the thesis is available upon request from Carleton University Library. - - The University Library Ta n?y teachers of all thes Frontispiece: Photograph looking westward to the Marino Formation and in the distance the Cordon del Plata of the Frontal Cordillera, Mendoza Province, Argentina. The Marifio Formation is the oldest unit of the Neogene foreland basin deposits that accumulated in response to deformation and exhumation in the Principal and Frontal cordilleras. The photo is taken from a dry riverbed, Rio Seco de Cacheuta; the road in the foreground leads south from the international route (connecting Chile and Argentina) at a point between Cacheuta and Potrerillos. This research focuses on the reconstmction of the Cenozoic thmst history of the Andes Mountains at 33"s by anaiyzing the foreland basin strata that record the eastem migration of the Andean thmst front since the Neogene. About 4000 m of nonmarine deposits crop out in the eastem flank of the Andes at the southernmost portion of the Mendoza Precordillera and accumulated in response to exhumation and deformation of the western Principal and Frontal cordiiieras. The detrital clastic strata comprise five Miocene and Pliocene formational units that record fluvial, eolian and alluvial deposition. To link the sequence of deformational events in the western mountain belts with the sedimentary record, all units except the youngest one were dated using magnetic polanty stratigraphy calibrated with JO~r-3g~risotopic dates of interbedded tephras. Precise depositional ages in conjunction with facies, textural patterns, and sandstone petrography allowed detemination of rates of sedimentation in the foreland, variations in depositional environment through time and timing of changes of detntus provenance. The multiple data set reveals details of tectonic activity, volcanism and deposition, which are summarized as follows. The oldest unit, the Mariiio Formation (-15.7-12.2 Ma), records two phases of thrustinç in the Principal Cordillera. The earlier phase occurred pnor to 15.7 Ma and is tied to deformation and exhumation of the Mesozoic andesitic volcanic complex cropping out in the western part of the Aconcagua fold-and-thnist belt (AFTB). The Purple conglomerates at the base of the unit could record the phase of deformation in the AFTB whose onset is known to have occurred at -20 Ma. The initiation of the second phase of thnisting is interpreted to have occurred at -1 5.5 Ma and coincides with the accumulation of the eolian middle member, interpreted as a major retrograding facies developed as a result of thmst- induced subsidence. The first appearance at -14.7 Ma of fossiliferous lirnestone clasts in deposits at the base of the fluvial upper member documents the eastward migration of the deformation front and involvement of the Mesozoic marine sequences of the central part of the AFTB. The overlying La Piiona Formation (-1 1.7-9.0 Ma) is thought to record the initiation of exhumation of the Frontal Cordillera just prior to -10.9 Ma and an important phase of deformation, highlighted by a marked increase in subsidence rates, at -9.7 Ma. Provenance and paleocurrent data are consistent with clast derivation from northwestem highIand sources which provided abundant rhyolite clasts and low-grade metarnorphic and sedimentary rocks, a petrologic assemblage that typifies this morphostnictural province. Deposition of the Tobas Angostura unit (8.9-8.7 Ma) primarily reflects the influence of large volcanism-induced sediment Ioad rather than tectonism. This unit is correlative with the largest and latest extrusive episodes occumng in the Aconcagua region, which produced large volumes of volcanic debns that ovenvhelmed the basin. The overlying Rio de los Pozos Formation, at least as young as -7 Ma, records the reestablishrnent of normal stream-flow processes where drainage of hi11 slopes with different lithologies from the Frontal and Principal cordilleras gradually regained precedence. Exhumation of the Frontal Cordillera overlapped in time with thnisting in the eastem part of the AFTB, in which the latest phase of defoimation is known to have occurred at -8 Ma. Tectonic activiq in the Frontal Cordillera shifted through time to the south dong the eastern dope of the Cordon del Plata, as recorded by the proximal synorogenic deposits of the Mogotes Formation, which unconformably overlies the Rio de los Pozos Formation. Deposition of this unit at the study site probably started at -3 Ma and ceased prior to 0.4 Ma, according to temporal constraints provided by ash chronology on Quatemary deposits, and most probably before or at -1 Ma. The thick Neogene succession of the Mendoza Precordillera was defonned into a series of north-trending anticlines and synclines in the Late Pliocene - Early Pleistocene?. The present location of the thmst front near the city of Mendoza indicates that deformation has progressed eastward in the last -3 Ma. The eastern border of the Precordillera is still active as Uiferred &om easthquake focal mechanisms, intense compressive deformation and evidence of neotectonic activity in the study area and farther east in the foothills near the city of Mendoza. Fieldwork was supported by a Natural Sciences and Engineering Research Council of Canada grant to Dr. Richard Brown. The Argentine govemment through the Geological Survey contributed substantially to the success of this project and provided logistic support during field work. 1would iike to list those geologists to whom 1 am especially gratefùl. Victor Ramos inspired me to study tectonics over the years and encouraged me to initiate this project in Argentina. In one field trip to the Andes, he introduced me to Richard Brown, who took over the sponsorship and support of this research. 1 would like to thank him for his teachings and for giving me the opportunity to corne to Canada, which is like a second home to me now. Roberto Page and Emesto Mendia, my mentors at the Geological Survey of Argentina, were especially supportive. Gabriel hsta, Guiliermo Re and Hector Colli assisted me in the fieId either in 1992 or 2994. 1wish to thank the Gugiieho family from the Camping Santo Domingo, the Veas and Velez families from the Potrerillos town for their hospitality. A very special thank you to Pabio G. Guerstein, who assisted me in the field, helped with preparations, provided expertise in sandstone petrography and, most irnportantly, supported this work with his encouragement and companionship for years. In particular, the times he shoveled snow in the rnoniings or prepared dinner for me at night when 1 retumed from a long day's work at Carleton corne to mind. On the technical side, thanks go to several people for help in acquinng and processing laboratory data: Chris Roddick, Randy Pamsh, Mike Villeneuve and Pat Hunt of the Geochronology Section of the Geological Survey of Canada (GSC); Sharon Carr of Carleton University for instruction in minera1 separation and for the use of the geochronology laboratory; Ken Buchan for supeMsion of the paleomagnetic study and Gary Freda for technical assistance, both of the Paleornagnetic Laboratory of the GSC; Bill Amott of Ottawa University for in-depth editing of the sedimentology chapter. Wendy Lewis for help with processing paleomagnetic data; and Anett Bnggs and Ronald Codon, Carleton University, for help with prepaiing and staining thin sections, respectively. Thanks to fellow students Lisel Cunie, Brad Johnson, Jim Crowley, Ahmed Ali Hussein, David Comgan, Eric de Kemp, Dan Gibson and Laurent Godin for help in so many different ways (teaching the use of the computer, lending me a mattress, helping me in English, etc.) I like to make special mention of Lois Hardy, research assistant of Richard Brown, for helping me with the manuscript and graphics preparation. As English is not my first language, her editorial help was invaluable. Her cornmitment to my thesis, specially in the last diEcult weeks of the project, will never be forgotten. Finally, 1 would like to thank al1 the people whose paths 1 have crossed over these years whom 1 have iearned from, shared with, and been touched by, in particular Marion McMurray, Tryg Schonning and Miriam Benovich.