Geodinarnica Geodynamique

lnstitut de recherche pourle developpernent

Geodynamique andine Andean Geodynarnics

Geodinarnica Andina

Resumes etendus Extended abstracts Resumenes expandidos

'IVI!RSI IH

PAUl.

SABATIER

lDlJ l.OlJ SIIII

Organisateurs

Institut de recherche pour le developpement

Organizers Organizadores Universite Paul Sabatier

Toulouse

lnstitut de recherche

pOUfle de\leloppement

2002

GEODYNAMIQUE ANDINE ANDEAN GEODYNAMICS

GEODINAMICA ANDINA

UNIVERSITE

PAUL

•

5th International Symposium on Andean Geodynamics

Universite Paul Sabatier, Toulouse, France, 16-18 Septembre 2002

Resumes etendus

Extended abstracts

Resumenes ampliados

Organisateurs / Organizers / Organizadores

Institut de recherche pour le developpement Universite Paul Sabatier

IRD

INSTITUT DE RECHERCHE POUR LE DEVELOPPEMENT

Paris, 2002

COMITE D'ORGANISATION COMITE ORGANIZADOR ORGANIZING COMITTEE

P. Baby (IRD-Toulouse), I Darrozes (Univ. Paul Sabatier-Toulouse), I Deramond (Univ. Paul Sabatier-Toulouse), B.Dupre (CNRS-Toulouse), I-L. Guyot (IRD-Toulouse),G.Herail (IRD-Toulouse),

E. Jaillard (IRD-Quito),A.Lavenu (IRD-Toulouse), H. Miller (Univ. Munchen), T. Monfret (IRD-Geoscience Azur), G. Warner (Univ. Gottingen)

Comite scientifique et representants nationaux Comite Cientitico y Representantes Nacionales Scientific Advisory Board and National Representatives

R Armijo (IPG, Paris), I-P. Avouac (CEA, Paris), R Charrier (Univ. Chile, Santiago),

I-Y. CoBot (IRD, Geoscience Azur), L. Dorbath (IRD, Strasbourg), S. Flint (Univ. Liverpool),B.France-Lanord (CNRS, Nancy), L. Fontbote (Univ. Geneve), Y. Gaudemer (Univ. Paris VII), R Gaupp (Univ. Jena),

F.Herve (Univ. Chile, Santiago), T.E. Jordan (INSTOC, Cornell), I Mojica (Univ. Bogota), O. Oncken (Univ. Potsdam), L. Ortlieb (IRD, Bondy), RI Pankhurst (Brit. Antartic Surv.),

V.Ramos (Univ. Buenos Aires), P. Ribstein (IRD, Paris),

C.Robin (Univ. Clermont-Ferrand), S. Rosas (Univ. Lima), F. Sabat (Univ. Barcelona), M. Schmitz (FUNVISIS, Caracas), R Suarez Soruco (YPBF, La Paz),

M. Rivadeneira (Petroproducci6n), W.Winkler (ETH, Zurich).

APPUIS FINANCIERS FUNDINGS APPOYO FINANCIERO

L'organisation de l'ISAG 2002 et les bourses accordees

a

(notamment de la Delegation

a

a

©IRD,2002

SOMMAIRE / SUMMARY / SUMARIO

Thin-skinned tectonics in the Cordillera Oriental, Chorornoro Basin, NW, Tucuman, Argentina 11 Liliana ABASCAL

Low temperature thermochronology and tectonics in the Chiloe Region Southern Chilean Andes (41°-43°S; 72°-74'W) 15 AlbertoADRIASOLA, Bernhard STOCKHERT and Francisco HERVE

Compositional features and geodynamic evolution of volcanic and plutonic components in the lower cretaceous magmatic province 19 (coastal range of central Chile)

Luis AGUIRRE, Miguel PARADA, Diego MORATA, Gilbert FERAUD and Francisco FUENTES

A carboniferous volcanic arc along the coastal Cordillera? 23

Antenor ALEMAN and Waiter LEON

Tectosedimentaryevolution of Triassic Jurassic extensional basins in the high Andes of Argentina and Chile (32'_34' SL)

Pamela ALVAREZ, Laura GIAMBIAGI, Estanislao GODOY and Victor RAMOS 27

Structure of the precordillera along the 30' latitude cross-section (Jachal River, NW Argentina) 31 Joaquina ALVAREZ-MARRON, Roberto RODRIGUEZ-FERNANDEZ, Nemesio HEREDIA, Pere BUSQUETS and Ferran COLOMBO Mid-cretaceous crustal shortening, uplift and exhumation: evidence from a regional-scale ductile shear zone in the coastal range of 33 central Chile (32'S)

Gloria ARANCIBIA

Tabular plutons from the coastal Cordillera ofCoplapo -Vallenar (27'00' -28'30'S) and an approach to their emplacement mechanisms, 37 Atacama Region, Chile

caries AREVALO, John GROCOTT, Justiniano VALENZUELA and Daniela WELKNER

Cretaceous to Paleogene compressional tectonics during deposition of the purilactis group, salar de Atacama CesarARRIAGADA, Peter COBBOLD, Constantino MPODOZIS and Pierrick ROPERCH

Late cretaceous to tertiary events in the western cordillera of Ecuador John ASP DEN and Bill Mc COURT

41

45

Stability of quaternary Morainic complexes cut by the aocono fault, Merida Andes, Western Venezuela 49 Franck AUDEMARD, Christian BECK, Victor CANO, Eduardo CARRILLO, Raymi CASTILLA, Michel COUSIN, Franc;:ois JOUANNE, Luis MELO and Thierry VILLEMIN

Neotectonics on the western flank of the Domeyko Cordillier and central depression (Northern Chile) 53 Laurence AUDIN, Gerard HERAIL, Rodrigo RIQUELME, Jose DARROZES, Joseph MARTINOD and Manuel MOREIRA

Cathodoluminescence of quartz as provenance indicator for late palaeozoic metasediments of southern Andean Patagonia 57 Carita AUGUSTSSON and Heinrich BAHLBURG

Factors controlling partition of deformation during plateau formation 61

Andrey BABEYKO and Stephan SOBOLEV

The 1999-2000 seismic experiment of the swarm of Macas (Ecuador) in relation with a subandean unexpected wrench fault system 65 Patrice BABY, Denis LEGRAND.sevenneBES DE BERC, Francis BONDOUX and Marco RIVADENEIRA

Oltantaytarnboformation: a probably arc-derived volcaniclastic succession of Cambrian to Ordovician age in the Cordillera Oriental of 69 southern Peru

Heinrich BAHLBURG, Victor CARLOTTO, Jose CARDENAS and Eva WESTERVOSS

Volcanogenic evidences of North Andean tectonic segmentation: volcanoes Sumaco and El Reventador Ecuadorian subandean zone 73 J. Roberto BARRAGAN and Patrice BABY

.Concordant titanite U-Pb ages of Cambrian to Silurian high T metamorphism at the western edge of Gondwana (Southern Puna and 77 Western Sierras Pampeanas, Argentina, 26-29'S)

Raul BECCHIO and Friedrich LUCASSEN

Nature and origin of hydrothermal fluids during the low-grade metamorphism in the Jurassic - Tertiary volcanic successions Chilean 81 Andes (35'S) preliminary results

Mauricio BELMAR, Susanne SCHMIDT and Joseph MULLlS

Uplift and active deformation of the Pastaza alluvial fan (Subandean zone of Ecuador) 85 Severine BES DE BERC, Patrice BABY, Jean-Claude SOU LA, Marc SOURIS and Jaime ROSERO

Assessment of gis Andes: predictive mapping of neogene gold-bearing magmatic-hydrothermal systems in the Central Andes 89 Mario BILLA, Daniel CASSARD, Laurent GUILLOU-FROTTIER, Andor LIPS and Bruno TOURLlERE

The southern Andes between 36' and 40'S latitude A 3-D image of the lithospheric structure inferred from local earthquaker Data 93 Mi~amBOHM, GOnter ASCH, Klaus BATAILLE, Carsten BRUHN, Andreas RIETBROCK and ISSA working group

Assessement of ground deformation measurements on Andean volcanoes using radar interferometry and gps data 97 Sylvain BONVALOT, Dominique REMY, Germinal GABALDA, Andres PAVEZ, Michel DIAMENT and Patricia MOTHES

Thrust kinematics and foreland basin dynamics of the southern subandean zone of Bolivia: new insight from apatite fission track 101 analysis

SlE~phaneBRUSSET, Philippe RaC HAT, Patrice BABY and Joan FLINCH

The Upper Carboniferous in frontal cordillera (San Juan Province - Argentina): Regional implications 105 Pedro BUSQUET, Ferran COLOMBO, Nuria SOLE de PORTA, Nemesio HEREDIA, Roberto RODRIGUEZ FERNANDEZ and

Joaquina ALVAREZ-MARRON

ChronoStratigraphy of the San Juan de Oro basin and its tectonic implications for the northern Puna during the Miocene 109 Pablo Jorge CAFFE and BeatrizL. COIRA

Biotites⁴⁰ARJ³⁹AR and zircon fission track dating in the zaldivar porphyry copper, northern Chile 113 Eduardo CAMPOS, Jan WIJBRANS and Paul ANDRIESSEN

Paleogeographic control on the evolution of Tertiary basins in the western cordillera and altiplano of southern Peru (Condorama - 117 Cusco- Ayauri)

Victor CARLOnO, Gabriel CARLlER, Etienne JAILLARD, Jose CARDENAS, Michel FORNARI and Luis CERPA

Geochemical aspects of host-rocks of polymetallic veins Paramillos de Uspallata, Mendoza, Argentina 121 Silvia Irene CARRASQUERO

Sedimentary disturbances in late Quaternary f1uvio-lacustrine deposits of the Los Zerpa moraine induced by the Bocono fault, Merida 125 Andes, Venezuela

Eduardo CARRILLO, Franck AUDEMARD, Christian BECK and Michel COUSIN

Brittle deformation and fluid transport in magmatic arcs: a case study from the Atacama fault zone 129 Jose CEMBRANO, Gabriel GONZALEZ, lvo AHUMADA and Gloria ARANCIBIA

Geodynamic evolution of the Miocene Descanso - Yauri basin, Cusco region, Peru 133

Luis CERPA, Victor CARLonO, Thierry SEMPERE and Paola MEZA

Rapid tectonic and paleogeographic evolution: the Chucal anticline, altiplano of Arica, northern Chile 137 Reynaldo CHARRIER, Andres CHAVEZ, Sara ELGUETA, Gerard HERAIL, JohnFLYNN, Darin CROFT, Andres WYSS and

Marcelo GARCIA

Structure of the subduction channel at the Ecuador convergent margin from wide Angle seismic modeling and inversion 141 Philippe CHARVIS, Audrey GAILLER, Valenti SALLARES, Jean-Yves COLLOT, David GRAINDORGE, Alcinoe CALAHORRANO

and Ruth VILLAMAR

Lead isotope evidence for latitudinai mantle heterogeneity beneath the Andes 145

Massimo CHIARADIA and L1uis FONTBOTE

The loading and relaxation processes in the northern Chile subduction zone using SAR interferometry and GPS measurements 149 M. CHLlEH, J.B. de CHABALlER, J.C. RUEGG, R ARMIJO, K. FEIGL and J. CAMPOS

Phases of Andean deformation, foothills of the neuquen Basin, Argentina 153

Peter COBBOLD and Eduardo ROSSELLO

Correlations between crustal structures of the northern, Ecuador southern Colombia margin from MCS data, and rupture zone of 1942, 157 1958 and 1979 great subduction earthquakes

Jean-Yves COLLOT, B MARCAILLOU, Marc-Andre GUTSCHER, Philippe CHARVIS, W. AGUDELO and F. MICHAUD Study of intraplaque seismicity triggered in northern Chile by the june 23, 2002 Peruvian subduction earthquake Diana COMTE, Catherine DORBATH, Jaime CAMPOS, Louis DORBATH, Henri HAESSLER

161

Assimilation of mafic-ultrmafic cumulates by arc Basalts may be an important process in modifying major and trace element signatures: 163 evidence from the tatara-san Pedro complex (TSPC) 36'S Chilean Andes

Laurie CORTESI, Michael DUNGAN, Jon DAVIDSON, Fred FREY, Fidel COSTA and Andrsa MARZOLl

Petrology of the Hualpen stock: evidences of late Triassic crustal epizonal plutonism at the western margin of Gondwana 167 (36'45' S - 73'1O'W)

Christian CREIXELL, Freidrich LUCASSEN, Gerhard FRANZ, Paulina VASQUEZ and Oscar FIGUEROA

Intracontinental seismicity and neogene deformation of the Andean forearc in the region of Arica (18.5°S-19.5°S) 171 Claire DAVID, Joseph MARTINOD, Diana COMTE, Gerard HERAIL and Henri HAESSLER

Quaternary minor volcanic centres in southern Peru: volcanology, petrology and geochernistry 175 Adelie DELACOUR, Perrine PAQUEREAU, Marie-Christine GERBE, Jean Claude THOURET and Gerhard WORNER

Venezuelan Andean tectonics revealed by Sar Jers imagery 179

Damien DHONT, Guillaume BACKE, Yves HERVOUf:T, Leonardo GONZALES and Jose CASTRILLO

Paleozoic evolution of the central Andes: A non-accretionary model Gondwana's active margin 183 Enrique DIAZ-MARTINEZ

Seismotectonics on the central region of the Colombian eastern cordillera 187

Cristina DIMATE, Alfredo TABOADA, Luis RIVERA, Armando CISTERNAS and Domenico GIARDINI

The neotectonic evolution of the gulf of Guayaquil: a tail structure between converging plates 191 Jean Francois DUMONT, Essy SANTANA, W. VILLEMA, Martha ORDONEZ, Nelson JIMENEZ, I. ZAMBRANO and K. PEDOJA

Observations and speculations pertaining to regional magmatic patterns in the Andean southern volcanic zone 195 Mickael DUNGAN, Wes HILDRETH, Fred FREY, Jon DAVIDSON, Leopoldo LOPEZ-ESCOBAR, Daniel SELLES and Carolina

RODRIGUEZ

Second partial melting stage of a slab-melt metasomatized mantle at Sumaco Volcano (northern volcanic zone, Ecuador) 199 Jean Philippe EISSEN, Erwan BOURDON, Bernardo BEATE, Silvana HIDALGO and Joseph COTTEN

The pattern of deformation related to growth of the southern altiplano plateau (Bolivia) 203 Kristen ELGER and Onno ONCKEN

Navidad formation: deltayc sedimentation that reflects the tectonic change during the lower Miocene in South America 207 Alfonso ENCINAS

Actual erosion by Rivers in the Bolivian Andes 211

Naziano FllIZOLA, Pascal FRAIZY, Jean Loup GUYOT, FrE§derique SEYLER, Patrice BABY, Gerard HERAIL

Cenozoic Andean paleoenvironments and tectonic history: evidence from fossil mammals 215 John FLYNN

Instability of southern Andean strain during the last 25 Ma 219

Andres FOLGUERA and Victor RAMOS

AR-AR dating of late Oligocene-early Miocene volcanism in the altiplano 223

Michel FORNARI, Eddy BALDELLON, Felix ESPINOZA, lvanoff IBARRA, Nestor JIMENEZ and Miriam MAMANI

The high pressure ultramafic sequence of the El Toro formation (El Oro metamorphic complex, SW Ecuador): characterisation and 227 metamorphic evolution

Piercarlo GABRIELE, Giovanni PICCARDO, Giorgio MARTINOTTI and Jean HERNANDEZ

Spatial and temporal variations of the quality factor (Q) before and during the 1999 Guagua Pichincha Eruptions 231 Alexander GARCIA-ARISTIZABAL, Alvaro Pablo ACEVEDO and Mario RUIZ

Oligocene-Neogene tectonic evolution of the Altiplano of northern Chile (18-19"S) 235 Marcelo GARCIA, Gerard HERAIL, Reynaldo CHARRIER, Georges MASCLE, Michel FORNARI and Carlos PEREZ de ARCE

U-Pb Dating with sub-million year precision of early Andean plutonic rocks in the coastal cordillera of Chanaral, northern Chile 239 Sergio GELCICH, Donald DAVIS and Edward SPOONER

Numerical modelling of deformation processes in the Andes 243

Muriel GERBAULT, Gerard HERAIL and Joseph MARTINOD

Defonnational history of the Andes between 33" and 34" south latitude, Chile and Argentina 247 Laura GIAMBAGI, Victor RAMOS, Estanislao GODOY and Pamela ALVAREZ

Tectonic implications of Permian to Jurassic paleomagnetic results from the Peruvian cordillera 251 Stuart GILDER, Sonia ROUSSE, Daniel FARBER, Brendan MC NULTY, Victor TORRES and Oscar PALACIOS

The rnud-diapir trend in the eastern Venezuelan basin 255

Carlos GIRALDO, Eduardo ALVAREZ and Marco ODEHNAL

Geochronological constraints on material cycling velocities, structural evolution and exhumation of a paleo-accretionary wedge: the 259 Bahia Mansa complex, south central Chile

Johannes GLODNY, Jo LOHRMAN, Wolfgang SEIFERT, Kristen GRAEFE, Helmut ECHLTER and Oscar FIGUEROA

Does crustaI thinning trigger subsequent building of the Andes? 263

Estanislao GODOY

Neogene to quaternary deformation of the salar Grande area, costal cordillera of northern Chile implications for forearc tectonics 267 Gabriel GONZALEZ, Alejandro MACCI, Daniel CARRIZO and Jose CEMBRANO

Polyphased tectonics in the northern Venezuelan Andes 271

Leonardo GONZALEZ, Yves HERVOUET, Damien DHONT and Jose T.CASTRILLO

Apatite fission track thennochronology of granitoids at the south Chilean active continental margin (37"S - 42"S): implications for 275 denudation, tectonics and mass transfer since the cretaceous

Kristen GRAEFE, Johannes GLODNY, Wolfgang SEIFERT, Matthias ROSENAU and Helmut ECHLTER

Shallow melting of oceanic crust during spreading ridge subduction: origin of near-trench Quaternary volcanism at the Chile Triple 279 Junction

Christele GUIVEL, Yves LAGABRIELLE, Rene C. MAURY, Nicolas ARNAUD, Jo COTTEN, Jacques BOURGOIS, Serge FOURCADE and Herve MARTIN

Tectonic-magmatic evolution of the mineral district agua de Dionisio (YMAD) Argentina 283 Adolfo GUTIERREZ, Shoji KOJIMA and Sergio ESPINOZA

The structure and mechanical properties of the Andean lithosphere from gravity-based studies 287 Ron HACKNEY, Hans-JOrgen GOTZE, Sabine SCHMIDT, Zuzana TASAROVA and Susann WlENECKE

The obliquely subductinq Nazca Ridge at the Peruvian active margin: the present collision zone and a re-evaluation of its migration 291 history

Andrea HAMPEL, Nina KUKOWSKI, Joerg BIALAS and Udo BARCKHAUSSEN

Foreland basin system evolution of the Peruvian Andes: New insights from mass balance computation 295 Wllber HERMOZA, Patrice BABY, Stephane BRUSSET, Frederic CHRISTOPHOUL and Wily GIL

Low-grade early Permian (?) metasedimentary complexes at the western entrance of the strait of Magellan, Southern Chile 299 Francisco HERVE, John BRADSHAW and Robert J.PANKHURST

The TIPA shear zone (NW - Argentina): Isotopic dating and geochemical processes 303 Martin HOECKENREINER, Franck SOLLNER and Hubert MILLER

A groundwater genesis for the Giant Quebradas of the Atacama Desert, Northern Chile 307 Gregory HOKE, Teresa JORDAN and Bryan L1SACKS

Altiplano uplift from gravity driven channel flow 311

Laurent HUSSON and Thierry SEMPERE

Crustal thicknesses and general isostatic balance of the Peruvian Andes from observed and predictive shortening 315 Antonio INTROCASO and Iris Rosalia CABASSI

Structural characteristics of the Incapuquio fault system, southern Peru 319

Javier JACAY, Thierry SEMPERE, Laurent HUSSON and Adan PINO

Evolution of the Cretaceous Celica-Lancones Forearc basin (NW Peru-SW Ecuador) 323

Etienne JAILLARD, Raynald ETHIEN, Henrietle LAPIERRE, Cedric REYNAUD, Marc MAMBERTI and Piercarlo GABRIELE

Geochemical characteristics of the cretaceous basement of western Ecuador 327

Diego JAYA, Liliana TRONCOSO and Arturo EGUEZ

Orogenic-scale ground water circulation in the Central Andes: evidence and consequences 331 Teresa JORDAN, LV. GODFREY, N. MUNOZ,R.ALONSO, T. LOWENSTEIN, Gregory HOKE, P. PANNANONT, P.

PERANGINANGIN, B.L. ISACKS and L. CATHLES

Late Pleistocene and Holocene tephrostratigraphy and chronology in southern Peru 335 Etienne JUVIGNE, Jean Claude THOURET, Jersy MARINO, Marcela MOSCOL,A.LEGELEY-PADOVANI and Isabelle LOUTSCH Endogeneous arc-related processes and their fingerprints in lake sediments lonquimay basin south-central Chile 339 Helga KEMNITZ

Tectonics of the tres cruces basin, eastern cordillera Northern Argentina: Seismic and outcrop evidence of inverted cretaceous normal 343 faults

Jonas KLEY, Eduardo ROSSELLO, Cesar MONALDI and Bjorn HABIGHORST

Determination of erosion rates in a decoupled river and pediplane system in the central Andes, northern Chile 347 Florian KOBER, Fritz SCHLUNEGGER,R.WIELER, S. IVY-OCHS and G SIMPSON

Geophysical investigations of the central Chilean Continental margin 351

Heidrun KOPP, Ernst R.FLUEH, Peter THIERER, Dirk KLAESCHEN, Frederik TILMANN and Christoph GAEDICKE

Subduction processes off Chile (SPOC) combined offshore-onshore experiments in central Chile 355 Charlotte KRAWCZYK, Onno ONCKEN, SPOC and IMTEQ working groups

Long-term and short term tectonic erosion at the Peruvian active margin between 9"S and 15"S: evidence from bathymetry data and 359 3D Sandbox analogue modelling

Nina KUKOWSKI, Andrea HAMPEL, Joerg BIALAS, Anne BROSER, Christian HUEBSCHER, Udo BARCKHAUSEN and Jacques BOURGOIS

Sedimentary provenance and tectonic regime of the depositional basin of the Duque de York metamorphic complex, Magallanes, Chile 363 Juan Pablo LACASSIE and Francisco HERVE

Margin-Parallelvismargin-orthogonal compression along a convergent margin: the Andes Central-southem Chile and Western 367 Argentina

Alain LAVENU, Jose CEMBRANO and Cartes COSTA

Maximum and minimum volume estimates of an ash fall layer from the august 2001 eruption of MT Tunguahua (Ecuador) 371 Jean-Luc LE PENNEC, Patricia MOTHES, Minard L. HALL, Patricio RAMON and Gorki A RUIZ

A Miocene submarine Canyon in North Central Chile: implications for Andean tectonism 375 Jacobus P.LE ROUX, Carolina G6MEZ, Heather MIDDLETON, Juliane FENNER, Binyamin BUCHBINDER and Kathryn M. GREGORY- WODZIKI

Stress tensor analysis of the 1998-99 tectonic swarm of northern Quito related to the volcanic swarm of Guagua Pichincha Volcano, 377 Ecuador

Denis LEGRAND, Alcinoe CALAHORRANO, Bertrand GUILLlER, Luis RIVERA, Mario RUIZ, Darwin VILLAGOMEZ and Hugo YEPES Forearc extension: tectonics, sedimentologic and stratigraphy evolution of the East Pisco Basin 381 Waiter LEON and Antenor ALEMAN

Preliminary comparison between Antuco and Sierra Velluda Volcanoes (southern Andes) 385 Silke LOHMAR, Leopoldo L6PEZ-ESCOBAR and Hugo MORENO

Progress report on a petro-geochemical and mineralogical study of the "El Teniente Belt", central Chile: implications of lower 389 crustal melting processes and the linkS to giant Andean porphyry Cu-Mo mineralization

Leopoldo LOPEZ-ESCOBAR, RobertW.KING, Osvaldo M.RABBIA and Laura B.HERNANDEZ

Crustal recycling and juvenile additions: the Paleozoic to recent tectonic and compositional evolution of the Chilean continental margin 393 (36"-40"S)

Freidrich LUCASSEN, Helmut ECHTLER, Robert TRUMBULL, Gerhard FRANZ, Rolf L.ROMER and Oscar FIGUEROA

Subduction processes off Chile (SPOC) N-S variations of crustal structure from offshore - onshore wide-angle seismic observations in 397 Chile (36'-39'S)

Stefan LOTH, James MECHIE, Peter W1GGER andSP~Cwor1ling group

Neogene ignimbrites in the area of Arequipa, southern Peru: Paleomagnetic correlations and flow direction 401 Orlando MACEDO, Perrine PAQUEREAU, Pierrick ROPERCH, Jean-Claude THOURET and Michel FORNARI

Andean adakites from slab melting, crustal thickening, and forearc subduction erosion 405 Suzanne MAHLBURG KAY

Late quaternary slip rates of the Mejillones fault, Northern Chile (23'S), using10be dates 409 Carlos MARQUARDT, Regis BRAUCHER, Jean-Francois RITZ, Herve PHILlP, Didier BOURLES, Alain LAVENU, Bertrand DELOUIS and Luc ORTLlEB

Tectonics from the northern segment of the Iiquine-ofqui fault system (37'-39'S) Patagonian Andes 413 Daniel MELNICK, Andres FOLGUERA, Matthias ROSENAU, Helmut ECHTLER and Susanne POTENT

Environment, provenance and paleogeographic setting of two late aptian-early Albian open-sea sedimentary sequences in the western 417 Piedmont of the central cordillera south Pijao, Quindio department, Colombia

Jairo MOJICA and Oscar AREVALO

Structural complexities associated with lateral ramps in the eastern cordillera, central Andes of Argentina 421 Ricardo MON, Cesar Ruben MONALDI and Jose SALFITY

Structural style and timing of deformation in the Piedras-Girardot fold belt, Colombia: early tranpressional deformation in the northern 425 Andes

Camilo MONTES

Evolution of the Pichincha Volcanic Complex (Ecuador) 429

Michel MONZIER, Pablo SAMANIEGO, Claude ROBIN, Bernardo BEATE, Joseph COTIEN, Minard HALL, Patricia MOTHES, Daniel ANDRADE, Erwan BOURDON, Jean-Philippe EISSEN, Jean-Luc LE PENNEC, Andres Gorki RUIZ and Theofilos TOULKERIDIS

Early Pliocene magmatism and high exhumation rates in the Patagonian Cordillera (46'40'S) K-Ar and fission track data 433 Diego MORATA, Luis BARBERO, Manuel SUAREZ and Rita DE LA CRUZ

Magmatic basic events during the late cretaceous, in the Colombian eastem mountain range 437 Manuel MORENO, Ana CONCHA, Pedro PATARROYO and Angela NAVARRETE

The Ouebrada Turbia sequence: a snapshot of magmatic development within the long lived Tatara - San Pedro Volcanic complex 441 George A.MORRIS and Michael A.DUNGAN

Tertiary tectonic evolution of the southwestem edge of the Puna Plateau: cordillera Claudio Gay (26'-27'), northern Chile 445 Constantino MPODOZIS and Jorge CLAVERO

Tectonic relation of metallogenic provinces in the Chilean Lake region, Southern Andes (39-44'S) 449 Jorge MUNOZ-BRAVO, Pedro CRIGNOLA, Alexandro ORDONEZ and Paul DUHART

ZWB&thermal structure beneath the Andes 453

Miguel MUNOZ

Seismic stratigraphy and tectonics of the Tucuman basin (cretaceous continental rift of South America) 457 Maria PACHECO, Nelida Y. MANSILLA and Ricardo MON

Neogene ignimbrites in the area of Arequipa southern Peru: correlations based on field observations, geochemistry and geochronology 461 Perrine PAQUEREAU, Jean-Claude THOURET, Gerhard WORNER and Michel FORNARI

Magnetic fabric and field evidence for the construction of the caleu pluton by multiple injections and doming. Coast range of central 465 Chile

Miguel A.PARADA, Pierrick ROPERCH and Claudio GUIRESSE

Flat-Slab subduction zone in central Chile - Argentina: seismotectonic and body-wave tomography from local data 469 Mario PRADO, Tony MONFRET, Emilio VERA, Alfredo EISENBERG, Stephane GAFFET, Emilio LORCA and Adriana PEREZ

Eastem cordillera fold-thrust belt, structural framework of the central Andes in northem Argentina 473 Gonzalo Javier PENALOZA, Marcos J.JIMENEZ and Ricardo MON

The volcanic rocks of the Corrales Negros, Cordillera principal (33'45'LS) Mendoza Province, Argentina 477 Daniel PEREZ

Erosion tracing of the Bolivian-Chilean orocline by heavy mineral and major and trace elements discrimination in sediments of neogene 481 basins

Luisa PINTO, Gerard HERAIL, Francois FONTAN, Bernard DUPRE and Reynaldo CHARRIER Orthogonal deformation in the eastern Andes of Ecuador

Warren PRATI, Pablo DUQUE and Miguel PONCE Rheology and the evolution of the central Andes Claudia PREZZI

The structural evolution of the central Andean fold-thrust belt, Bolivia Nadine QUARRIE (Mc)

485

489

493

Ridge Collision and topography: Foreland deformation in the Patagonian Andes 497 Victor RAMOS

Secondary gold deposits in the Upez region (Bolivia) 501

Wilfredo RAMOS and Raul FERNANDEZ

New paleomagnetic data from the western Puna of Argentina: some tectonic speculations on its early Paleozoic evolution 505 Augusto RAPALlNI, M.SVELASCO and M.KOUKHARSKY

Adakitic magmatism at the Los Pelambres giant porphyry copper deposit, Chile 509

Martin REICH, Miguel Angel PARADA, Carlos PALACIOS, Andreas DIETRICH and Bernd LEHMANN

Modelling the hydrological changes during the Holocene in theAltiplano 513

Anne RIBSTEIN-COUDRAIN, Thomas CONDOM, Francois DELCLAUX, Alain DEZETTER, Michel LOUBET, Laurent L1, Alain GIODA, Edson RAMIREZ

Petrography, mineralogy and geochemistry of tholeiitic gabbroic rocks of Coastal Range at northern Antofagasta, Chile 517 Alejandra RIOSECO, Oscar FIGUEROA, Silvino CORDOVA, Bernard DERUELLE and Christian CREIXELL

Nenogene tectonics of the central depression and precordillera (north Chile) from the analysis of a drainage basin evolution 521 Rodrigo RIQUELME, Jose DARROZES, Gerard HERAIL and Reynaldo CHARRIER

Petroleum geology of the eastern edge of the oriente basin 525

Marco RIVADENEIRA

Foreland basin dynamics in western Amazonia inferred from forebluge evolution: the case study of the Arch of Iquitos (Peru) 529 Martin RODDAZ, Patrice BABY, Stephane BRUSSET and Wilber HERMOZA

Gravity and magnetic modelling across the Guarico Sub-basin, Espino Graben, Venezuela 533 Inlda RODRIGUEZ, Josmat RODRIGUEZ

Sedimentologic evolution of the Tinajani intermontane basin (late Oligocene middle Miocene southeast Peru) 537 Rildo RODRIGUEZ, Tomasa FLOREZ, Rene MAROCCO, Victor CARLOTTO and Jose CARDENAS

Transpressional motions along the cordillera occidental-coastal belt boundary since the late cretaceous (southern Peru, 16" S) 541 Darwin ROMERO, Agapito SANCHEZ, Maria CRUZ, Pedro TICONA and Rildo RODRIGUEZ

Counterclockwise rotation of the south Peruvian forearc and implications for the formation of the Bolivian orocline 545 Pierrick ROPERCH, Thierry SEMPERE, Orlando MACEDO, Cesar ARRIAGADA and Michel FORNARI

Acua Mahuida (Neuquen basin, Argentina): A quaternary shield volcano on a hydrocarbon-producing substrate 547 Eduardo ROSSELLO, Peter COBBOLD, Marc DIRAISON and Nicolas ARNAUD

Paleomagnetic evidence for late Miocene vertical-axis rotations in the Peruvian Cordillera 553 Sonia ROUSSE, Stuart GILDER, Daniel FARBER, Brendan Mc NULTY, Victor TORRES and Oscar PALACIOS

Regional aspects of the southern end of the central volcanic zone (between 27" - 28"S) Argentina

Daniel RUBIOLO, Eduardo ZAPPETTINI, Antonio L1ZUAIN and Catherine HICKSON 557

Coseismic and aseismic slip observed from continuous gps measurements for the 2001 southern Peru earthquake (Mw

=

J.C. RUEGG, M. OLCAYand D. LAZO

Detrital provenance and exhumation in the Ecuadorian sub-Andean zone: a key region leading to the understanding of Andean geodynamics 565 Geoffrey RUIZ, Diane SEWARD, Wilfried WINKLER and Richard SPIKINGS

Wide-Angle seismic constraints on the evolution of Galapagos hotspot-cocos Nazca spreading center interaction 569 Valent; SALLARES, Philippe CHARVIS and EmstR. FLUEH

Cafe-alkaline to adakitic transition in the northern volcanic zone (Ecuador) 573

Pablo SAMANIEGO, Michel MONZIER, Claude ROBIN, Herve MARTIN, Jean-Philippe EISSEN, Erwan BOURDON and Joseph COTTEN

The San Lorenzo Fault, a new active fault in relation to the Esmaraldas-Tumaco seismic zone 577 Essy SANTANA and Francois DUMONT

Mesozoic and Cenozoic rl1eological evolution of the lithosphere of the Eastern Cordillera and hypothesis about its deep structure 581 Luis Fernando SARMIENTO-ROJAS, F. ROURE and S. CLOETINGH

Aridisation and enhanced rates of fluvial incision in the Andes of northern Chile 585 Fritz SCHLUNEGGER, Florian KOBER, Guy SIMPSON and Heinz SCHNEIDER

Crustal thickness in the eastern Venezuelan basin from seismic wide angle data 589

Michael SCHMITZ, Ana MARTINS and Jesus CASTILLO

Low Paleozoic extensional tectonic evidence in the Quebrada de Humahuaca Cordillera Oriental Argentina 593 Raul SEGGIARO and Edwardo F. GALLARDO

Nevado de Longavi: anomalous incompatible element-poor compositions in the SVZ of the Andes (36-25"S) 597 Daniel SELLES, Carolina RODRIGUEZ and Michael DUNGAN

Lithospheric-scale transcurrent fault systems in Andean southern Peru 601 Thierry SEMPERE, Javier JACAY, Michel FORNARI, Pierrick ROPERCH, Harmuth ACOSTA, Conrado BEDOYA, Luis CERPA.

Alexander FLORES, Laurent HUSSON, Ivanoff IBARRA, Omar LATORRE, Mirian MAMANI, Paola MEZA, Francis ODONNE, Yovani OROS, Adan PINO and Rildo RODRIGUEZ

Trace elements Geochemistry in the Bolivian province of Amazon basin 605

Patrick SEYLER, Jean-Loup GUYOT and Laurence MAURICE-BOURGOIN

Magnetic fabric in the antena granodiorite Chuquicamata area northern Chile 609

Silvia SINGER, Ruben SOMOZA, Andew TOMLlNSON, M6nica LOPEZ DE LUCHI and Irene BARTHOLOMIEU RAPOSO

Estimating the age of the flat slab beneath the Central Andes 613

RubenSOMOZA

Terrane accretion and Ororgenic growth in Ecuador 617

Richard SPIKINGS,W.WINKLER, D. SEWARD, G. RUIZ andR.HANDLER

Subduction processes of Chile (SPOC) First results of the onshore seismic reflection experiment in Chile (38'S) 621 Manfred STILLER, Charlotte KRAWCZYK and SPOC working group

Nazca-southAmerica interaction: Review of a 2D flexural analysis along the central Andean Forearc 625 Andres TASSARA

Magnetic and gravity anomalies in the central part of the Tierra del Fuego Island. Argentine: evidences of transtensive tectonics 629 Alejandro TASSONE, H. L1PPAI,A.COMBA, E. LODOLO, M. MENICHEDI, J.L. HORMAECHEA and J.F. VILAS

New paleomagnetic results from the coastal cordillera/precordillera boundary northern Chile: implications for plate margin deformation 633 Graeme TAYLOR, Mark GIPSON and John GROCOD

An extensive new fission-track thermochronological database from the coastal Patagonian Andes (4rS to 53'S): a record of Cenozoic 637 denudation and magmatism

Stuart THOMSON and Francisco HERVE

Valley evolution. uplift. volcanism, and related hazards in the central Andes of southern Peru Jean-Claude THOURET, Gerhard WORNER,A.FINIZOLA and A. LEGELEY-PADOVANI

641

Paleogeographycevolution of the southwest gondwana Boundary during the late Palaeozoic.News studies in the pre-Andean volcanism 645 of the San Raphael Block, Mendoza Province, Argentina

Renata TOMEZZOLl, Laura KLEIMAN, Julio SALVARREDI, Carta TERRIZZAN and Ernesto CRISTALLlNI

Determination of the present stress tensor for the North segment of the Andes from local major Earthquakes mechanisms 649 Alejandra TORO-RAMIEZ, N.J.A. OSORIO

Provenance of the upper cretaceous to middle Eocene clastic sediments of the western cordillera of Ecuador 653 Jorge TORO ALAVA and Etienne JAILLARD

K-Ar geochronology of the San Luis Gold belt, Central Argentina: A review 657

Nilda URBINA

Particulate organic matter analyses and oil potential of the Napo group, sub-Andean zone, Ecuador 661 Cristian VALLEJO, Peter HOCHUll and Wilfried WINKLER

Neoproterozoicdiamictites from four different cratons: provenance and paleotectonic selling - first results 665 Anelda VAN STADEN and Udo ZIMMERMANN

State of strain and stress in Norwestern of Southamerica 669

Cartes VARGAS-JIMENEZ and Juan Pablo DURAN-TOVAR

Radiochronology in a marine sequence of northern Chile, for high resolution paleosismological and paleoclimatic reconstructions 673 Gabriel VARGAS, Luc ORTLlEB and Jorge VALDES

Mineral composition variations as effect of Contact-Metamorphism in the Metapelites of the eastern series, Nahuelbuta-Mountains, 677 South central Chile

Paulina VASQUEZ ILLANES, Gerhard FRANZ, Friedrich LUCASSEN and Oscar FIGUEROA

Current movement expression along the Northern Andes boundary in SW Colombia 681

Francisco VELANDIA, Roberto TERRAZA and Henry VILLEGAS

Numerical modelling of plateau kinematics in the central Andes 685

Tim VIETOR and Onno ONCKEN

Plio-quaternary sedimentary and tectonic evolution of the central inter-Andean valley in Ecuador 689 Diego VILLAGOMEZ, Arturo EGUEZ, Wilfried W1NKLER and Richard SPIKINGS

Anomalous lower Miocene forearc magmatism after an extensional regime in central Andes (33'S) Chile 693 Renate WALL and Luis LARA

The Carrizal Bajo Breccias: indications of contemporaneity between two end member magmas in a late Tnassic extensional selling 697 Northern Chile (2r45'- 28'20'S)

Daniela WELKNER and Cartes AREVALO

Sequence stratigraphy of the cretaceous Ecuadorian oriente basin: Tectonic and sedimentary evidences of an early foreland 701 deformation

Howard WHITE, Roberto BARRAGAN, Frederic CHRISTOPHOUL, Patrice BABY and Marco RIVADENEIRA

The Chota basin and its significance for the formation of the inter-Andean valley in Ecuador 705 Wilfried WINKLER, Richard SPIKINGS, Diego VILLAGOMEZ, Arturo EGUEZ, Peter ABEGGLEN and Stephan TOBLER

Preliminary fission-track data on the effects of the sudbucling Nazca Ridge on the geomorphology in south central Peru 709 Martin W1PF, D. SEWARD and Fritz SCHLUNEGGER

Viscous modeling of the dynamic trench topography along the central and South American active margin: implications on interplate 713 rheology and degree of coupling

Gonzalo YANEZ and Jose CEMBRANO

Prestack depth imaging of deep seismic reflection datasets: Ancorp&Precorp 717 Mi-Kyung YOON, Stefan BUSKE and Stefan LUTH

The provenance of the Ordovician deposits of the Famatina range (northwestem Argentina): first results 721 Udo ZIMMERMANN, Susana ESTEBAN and Heinrich BAHLBURG

INDEX DES AUTEURS I AUTHOR INDEX IINDICE DE LOS AUTORES .725

THIN-SKINNED TECTONICS IN THE CORDILLERA ORIENTAL, CHOROMORO BASIN, NW TUCUMAN, ARGENTINA

Liliana del Valle ABASCAL(l)

(1) Universidad Nacional de Salta. Sede regional Tartagal.Warnes 890. TartagaJ. Salta. Argentina (abascall @netscape.net)

KEY WORDS: Cordillera Oriental, Argentina, thin-skinned tectonics, structural geology

INTRODUCTION

The Choromoro Basin in northwestern Tucuman province, Argentina, is a tectonic depression that lodges about 6000 meters of Cretaceous and Tertiary sedimentary strata resting unconformably on a low-grade, Proterozoic, metamorphic basement exposed in anticlinal structures (Fig. 1). The sedimentary column comprises, at the base, strata of the Salta Group (Upper Cretaceous-lower Tertiary), deposited in the Salta Cretaceous Rift, overlain across a very low angle unconformity by beds of the Choromoro Group (Miocene-Pliocene) which comprises two major formations, the Sali Formation, consisting of red siltstones with intercalated layers of gypsum and oolithic limestones, and the India Muerta Formation, composed of thick-bedded reddish conglomeratic sandstones. Unconformably above the Choromoro Group rest conglomeratic beds of the Ticucho Formation, deposited in extensive early Pleistocene alluvial fans.

The Choromoro Basin forms part of the Cordillera Oriental geologic province (Bossi, 1969; Baldis et aI., 1976; Fernandez Garrasino, 1992), characterized by a thick-skinned tectonic style. In this province the low- grade Proterozoic basement is intensely involved in the Andean folding and forms the nuclei of the anticlines developed along the western and eastern borders of the basin. The western boundary ofthe basin is constituted by a large anticline topographically represented by the Cumbres Calchaquies ranges, and a smaller anticline that stems from the northeastern end of the former and underlies the Altos de la Totora ranges. Both anticl ines are related to high angle reverse faults with eastward vergence and offsets greater than 3 km. The eastern border is given by the Medina Ranges, which underlie an asymmetric anticline with a steep, faulted eastern limb. The faults responsible for the uplift of the blocks of metaporphic basement along the western and eastern borders of the Choromoro Basin are related to a detachment at a depth of about 20 km (Mon, 1999).

Within the Choromoro Basin a series of narrow and symmetric anticlines and synclines have been mapped. The fold axes generally plunge northwards and strike NS to NNE-SSW. The folds commonly show eastward vergence, with low dipping western limbs and eastern limbs intersected by eastward vergent reverse

faults. An analysis of the geometry of the structures indicates that they mostly originated as fault bend folds and have offsets smaller than the thickness of the Choromoro Group (approximately 2 km) as nowhere do beds of the Salta Group crop out in the hanging walls.

A second, shallower detachment level is postulated for these faults (Fig. 2). This detachment would be located in shaly, ductile beds of the Lumbrera Formation (uppermost Salta Group) at a depth of about 2 to 2.5 km. A YPF, east-west seismic line across the central Choromoro Basin shows evidence of this detachment.

In the Choromoro Basin the thick-skinned tectonic style is manifest in the structures underlying the Cumbres Calchaquies and Medina ranges, along the western and eastern borders of the basin, respectively. In contrast, within the Choromoro Basin the structural style is thin-skinned.

CONCLUSIONS

Mapping in the Choromoro Basin revealed a series of eastward vergent structures related to a detachment level located in shales of the Lumbrera Formation (Salta Group), at a depth of 2 to 2.5 km. This shallow detachment allowed the deformation of the Choromoro Group strata separately from that of the metamorphic basement. Thus, during Andean compression, a thin-skinned tectonic style developed locally in the Cordillera Oriental that is regionally characterized by a thick-skinned tectonic style.

REFERENCES

Baldis, B., Gorrofio, R., Ploszkiewicz, J. and Sarudiansky, R. 1976 Geotectonica de la Cordillera Oriental, Sierras Subandinas y comarcas adyacentes.Aetas6° CongresoGeologicoArgentino, I:3-22, Buenos Aires.

Bossi, G.E. 1969 Geologia y estratigrafia del sector sur del valle de Choromoro. Acta Geologica Lilloana, v. 10, p.19-61.

Fernandez Garrasino, C. 1992 Aspectos geologicos de algunas serranias del Noroeste argentino, lIanuras orientales tucumanas y valle de Trancas-Choromorro, temas exploratorios, (Provincias de^Tucuman,Catamarca y La Rioja, Argentina).Boletinde Informaciones Petroleras (terceraepoca),Buenos Aires.

Mon,R. 1999 Cordillera Oriental. Relatorio XIV CongresoGeologicoArgentino, v.I, p. 426-431.

Stla",,.plt1c legMd

.I!!I

• C1tnIJ

Neos-

..No

. . . -CTntt

hleogene E3 ^L_f'I'II)

Cfetllce_

rza -- ...,

C_1IrIaitI 1!:3 ..- ^(SI!

""'le,

o

! 20

!

Figure 1. Geologic and structural map of the Choromoro Basin in northwestern Tucuman. Shown is the trace of a YPF seismic line mentioned in the text.

km

w

€umbres Cal:haquies

Ohotorn oro Bas in

E

Sierra de Medina

crys alline basement

o

20

i\

o ^{km 10}

Figure 2. Schematic cross-section for the Choromoro Basin. The section shows the two detachment levels. For cross-section location see line AB in Figure 1.

LOW TEMPERATURE THERMOCHRONOLOGY AND TECTONICS IN THE CHILOE REGION, SOUTHERN CHILEAN ANDES (41°-43°S; 72°-74°W)

Alberta ADRlASOLA M (1),Bernhard STGCKHERT(1), Francisco HERVE(2)

I) Institut fur Geologie, Ruhr-Universitat Bochum, Universitatstr. ISO, D-44780 Bochum, Germany.

([email protected]) ([email protected])

2) Departamento de Geologfa, Universidad de Chile, CasillaJ3518, COITeo 21, Santiago, Chile.

(therve@cec. uchile.c I)

KEYWORDS: Liquifie-Ofqui Fault, Fission Track Analysis, Microfabrics, Erosion Rates

INTRODUCTION

One of the main important structural features controlling the late development the South-central Andes is the Liquifie Ofqui Fault (LOFZ). In the Chiloe Region, its activiy has been well documented (Herve, 1994;

Lavenu et al. 1994; Cembrano et aI, 2000), but the question of uplift/erosion remains unknown. Fission track (FT) dating of zircon and apatite combined with microfabric analyses on deformed rocks are applied in this study to determine the low temperature history and uplift/erosion rates from the South-central Andes in relation to the latest movements of the LOFZ.

GEOLOGY OF THE STUDY AREA

The most prominent geological unit in the Chiloe region is the Meso-Cenozoic North Patagonian batholith. This batholith intrudes a Late Paleozoic metamorphic basement, interpreted as the remnant of an accretionary complex, the provenance of which is still poorly understood. These units are overlain by a few Meso-Cenozoic volcanosedimentary sequences, distributed as basin deposits or as roof pendants of the batholith. The Liquifie Ofqui Fault Zone (LOFZ) extends N-S for ea. 1000 km, and has displaced dextrally all of the mentioned units since the Cretaceous (Herve, 1994; Cembrano et aI., 2000). It defines a regional weakness zone which controls the location of the main volcanic arc related to present day subduction of the Nazca Plate under the South Central Andes. Additionally climate has played an important role in the development of the Andean landscape within the region. Big differences in rainfall across the Andean range have resulted in a complex interplay between the topography and uplift/erosion rates.

FIRST RESULTS AND INTERPRETATION

37 new apatite FT ages and length measurements are compared to available Rb-Sr, K-Ar and Ar-Ar ages on tectonically juxtaposed Miocene and Cretaceous plutons from the Reloncavi Sector (figure 2). Along the Fault zone itself ages vary between 5.6+/- 2.0 and 3.3 +/- 1.2 Ma, although older ages with large errors were obtained from samples with low Uranium concentration. Apatite FT length distribution patterns from 34 samples imply increasing cooling rates from west to east on the western side of the LOFZ, whereas along the fault zone itself very rapid cooling rates are implied. Microstructural studies on deformed tonalites from L1ancahue Island- Cholgo sector show quartz grains with undulose extinction, development of low angle grain boundaries, and in rare cases recrystalIisation. Feldspar and micas show abundant microfractures and development of mechanical induced twinning. These microstructures imply that strain hardening processes occurred mostly under low temperature conditions (300°C, Jaw-temperature plasticity regime for quartz). Together with the time- temperature profiles, this evidence indicates high rates of late Cenozoic cooling and hence uplift/erosion due to activity of the LOFZ. Further results concerning zircon FT ages and microstructures will be presented at the Symposium in September.

ACKNOWLEDGEMENTS

This project was partly funded by the DFG grants StoI96/11-1 and 196/11-2. A.Adriasola is funded by the DAAD exchange program. Field trips were in part sponsored by the Depto. de Geol., U. de Chile and by the SERNAGEOMIN office from Puerto Varas.

REFERENCES

Cembrano J, Schermer E, Lavenu A, Sanhueza A. (2000): Contrasting nature of deformation along an intra-arc shearzone, The Liquifie Ofqui Fault Zone, Southern Chilean Andes. Tectonophysics, 319: 129-149.

Gallager K., Brown R; Johnson C. (1998): Fission Track Analysis and its applications to geologic problems.

Annu. Rev. Earth Planet. Sci., 26: 519-72.

Herve, F. (1994): The Southern Andes between 39° and 44°S latitude: the geological signature ofa transpressive tectonic regime related to a magmatic arc. Tectonics of the Central Andes, p. 243-248.

Lavenu A; Cembrano l(l999): Compressional- and transpressional-stress for Pliocene and Quarternary brittle deformation in fore arc and intra-arc zones (Andes of Central and Southern Chile). lStruct. Geol., 2: 1669-1691.

Munizaga, F., Herve, F., Drake, R., Pankhurst, RJ., Brook, M., Snelling, N.(1988): Geochronology of the Lake Region of south-central Chile (39°-42°S): Preliminary Results. South Am. Earth Sci., 1: 309-316.

SERNAGEOMIN (1994): Carta Metalogenica de la X Region.

Total Fusion ages on biotite K-Ar ages on biotite Samples for IT Dating

D

A •

Figure 1

Geology of the Keloncavi sector of the Liquiiie-Ofqui Fault (simplified after

SERNAGEOMIN,1994). Available radiometric ages compiled after Cembrano, et al.

(2000) and Munizaga et al. (1982).

~ Quaternary sediments

B Quaternary volcanic rocks

w

w

D Upper Faieozoic metamorphic basement ."",. L1quiite Ofqui Fault Zone (LOfZ)

"I

" : 80

Figure2:

Apatite FT ages and length distributions in the Keloncavi Sector of the WFZ.

• Location of Samples for FT Dating

16 •2+/-

0.71

COMPOSITIONAL FEATURES AND GEODYNAMIC EVOLUTION OF VOLCANIC AND PLUTONIC COMPONENTS IN THE LOWER CRETACEOUS

MAGMATIC PROVINCE (COASTAL RANGE OF CENTRAL CHILE)

Luis AGUJRREfI), Miguel Angel PARADA(I), Diego MORATA(l), Gilbert FERAUD^(2), Francisco FUENTES(1)

(I) Departamento de Geologia, Universidad de Chile. Chile, [email protected]

(2) Unversite de Nice-Sophia Antipolis 06108, Nice, cedez 02, France, [email protected]

KEY WORDS: Chile, Coastal Range, Lower Cretaceous, magmatism, ages.

INTRODUCTION

A Lower Cretaceous belt, c. 1200 km long and mainly composed of magmatic rocks, is exposed along the Chilean Coastal Range between latitudes 25°and 36°S. At the La Serena area, c. 29-30^{0 S} (Fig. lA), the Lower Cretaceous volcanism is represented by the Arqueros Formation which consists of basaltic andesites and marine limestone. Between 32° and 33°30'S (Fig.1B) the main units of this belt are the basic volcanic Veta Negra Formation and the Las Chilcas Formation, both largely accumulated in a subsiding basin thicker than 10 km. A pervasive and non-deformative very-Iow grade regional metamorphism (prehnite-pumpellyite facies) affects the volcanic products of both formations (Aguirreet aI1989).

Plutons belonging to this Lower Cretaceous magmatic belt are numerous. The Caleu pluton, located in the Coastal Range of central Chile c. 40 kmnorthwest from Santiago, is an excellent example (Parada et aI., 2002). Ithas good exposures covering an area of about 340 krrr'and consists in three N-S elongated zones, which define an across-pluton compositional variation characterized by a westward increasing in Si0₂content. These three zones are: Gabbro-Diorite Zone Tonalite Zone and Granodiorite Zone.

COMPOSITIONAL FEATURES OF VOLCANIC AND PLUTONIC COMPONENTS

The magmatic development of this province is marked by two contrasting events: an older volcanic episode represented by the Veta Negra and Arqueros formations and a younger plutonic event characterized by granitoid magmatism. From latitudes 33°30' to 29°, the basic lavas are chemically homogeneous contents classifying as high-K to shoshonitic basaltic andesites and andesites (Levi et aI., 1988, Vergara et aI., 1995, Morata et al., 2000). N-MORB normalized trace element patterns both for volcanic and plutonic rocks are characterized by L1LE enrichment, systematic HFSE decrease and a marked Nb-Ta depletion, typical of subduction related magmatism. In spite of some chemical similarities between the volcanic and plutonic products (e.g.high Ah03 and K20and low MgO), conspicuous differences between them can be recognized (Table I).

±

QTertlmy granitoids

CJCretaceous gnmitoids

o

o

o

WiiIiI volcanic andsedimentmy

• LowerCtctaceous volcanic and sedimenlary rocks .,.., Jurassic"oIcanic

L....J and sedimenlary rocks

E::J PoIeozoic basement

Tres Cruces

,'OAr/l'f_Ar;,~):= 11TO+/- 0.6Ma Sr;= 0.70362+/-0.00000

eNd

=

Arqueros

WArrArt,,)= 114,3+/- 0.7Ma SI',==0.70354+/- 0.00012

eNd

=

Chacana

4°Ar/'YJAr(I"l

=

4QAr/9Ar!m"···97.6+/- 0,3 Ma Sr,=0.70372 +/- 0.00007

aNd:=5.3+/-1.1

Caleu

llJ<ipbfllsu(uotj= 94.5 +/- 2.2

JOAr

f9

Sr.> 0.70341 +/- 0.00008 eNd

=

Bustamante

·!DAr/r'JAr_rvll= 119.4+/-1.2Ma.

JOAr/9Arr.<c = 93.1 +f- O.6Ma Sri = 0.70374+/-0.00005

eNd

=

data.

FIGURE 1.- Geological map of the studied area showing some geochemical and geochronological

It is of particular interest to compare the Veta Negra volcanic products with the spatially related Caleu pluton that put in evidence the fact that the Veta Negra flows are more basic and less isotopically depleted than the Caleu granitoids.

Table 1.- Summary of geochemical features and ages of the tectonic processes of the Lower Cretaceous magmatic rocks. Average and standard deviation with number ofsamples in parenthesis.

Data from Morataet al.2001

(2)Data from Paradaet al.2001, 2002.

Tres Cruces'" Arqueros'!' Chacana'!' Bustamante" ) Caleu'"

Latitude S 29°20' 29°50' 32°30' 33~5' 33°

Formation/pluton Arqueros Arqueros Veta Negra Veta Negra Caleu

%Si0₂ 54.09± 1.87(6) 53.73± 1.87(8) 58.45± 1.95(5) 52.39± 0.80(2) 60.33±5.72(43)

%Na20 3.05±0.41(6) 3.96±0.80(8) 3.49±0.17(5) 3.22±0.64(2) 3.90±0.47(43)

%K_2O 4.1 O± 1.00(6) 2.61±0.87(8) 4.24±0.97(5) 2.33±0.05(2) 2.59± 1.13(43)

%MgO 3.78±0.70(6) 3.05± 1.29(8) 2.50±0.48(5) 3.67±0.31 (2) 2.59± 1.16(43)

%A1_{20 3} 17.64± 1.12(6) 18.16±0.87(8) 16.04± 1.39(5) 18.98± 1.25(2) 16.90± 1.43(43)

Sri 0.70362± 0.70354± 0.70372± 0.70374± U.70341±

0.00000(2) 0.00012(4) 0.00007(2) 0.00005(2) 0.00008(13)

ENdlCHUR 3.0±0.1(2) 3.9±0.7(4) 5.3± 1.1(2) 4.2±0.7(2) 6.1±0.5(13)

Age (Ma) 117.0±0.6 117-115 118.7±0.6 119.4±1.2 94.5±2.2

Rate of subsidence 20.25 mm/year 0.18 mm/year

Age of

metamorphism (Ma) 97.6±0.3 93.1±0.6

Age of

exhumation (Ma) 94-90

(I)

GEODYNAMIC CONSlDERAnONS

The geochronological record shows the existence of a magmatic gap between 115 and 94.5 Ma, which could be explained by a decrease in intensity of the decompression melting mechanism at the origin of the Veta Negra volcanism as a consequence of progressive infilling and subsidence of its basin. Subsidence was interrumpted by fast exhumation at 94-90 Ma (Table 1) which would have been triggered by a mechanism of crustaI density inversion between upper crustaI basic and denser rocks, and lower crustal, felsic and lighter basement such as advocated by Martinezet al.(2001), to explain the presence of metamorphic core complexes in the Papua New Guinea region.

A coincidence in age between plutonism and low-grade metamorphism of the volcanic rocks (see Table 1) suggests that this last was not solely produced by burial but was related to a regional increase in thermal gradient associated with the magmatic event at the origin of the Caleu pluton. The compositional differences between the Caleu granitoids and the Veta Negra flows, already mentioned, are attributed to a lower melting

degree of deeper asthenospheric protoliths in the genesis of the former, This process would result from renewal of decompression melting associated with crustal root attenuation during the period of fast exhumation.

ACKNOWLEDGEMENTS

This work was funded by the FONDECYT Projects 1961108 and 1990980, the DID Project WO 1-99/2 and the CNRS-CONICYT Projets 1999,2000 AND 2001, all ofthem are duly acknowledged.

REFERENCES

Aguirre, L.; Levi, 8. and Nystrorn,1.0. 1989. The link between metamorphism, volcanism and geotectonic setting during the evolution of the Andes. In: 1.S. Daly, R.A.Cliff and 8.W.D.Yardley (Editors), Evolution of metamorphic belts. Geological Society of London Special Publ.ication 43, pp. 223-232.

Levi, 8., Nystrom, lO., Thiele, R. and Aberg, G., 1988. Geochemical trends in Mesozoic-Tertiary volcanic rocks from the Andes in central Chile and tectonic implications. Journal of South American Earth Sciences, 1:

63-74.

Martinez, F.; Goodliffe, A.M. and Taylor, 8. 2001. Metamorphic core complex formationby density inversion and lower-crust extrusion. Nature, 411,930-934.

Morata, D.; de la Rosa, 1. and Aguirre, L. 2000. A preliminary LAM-ICP-MS study on plagioclase phenocrysts from Lower Cretaceous volcanic rocks from La Serena region (Chile). IX Congreso Geol6gico Chileno, Aetas vol. 1,649-653.

Morata, D.; Aguirre, L.; Feraud, G.; Fuentes, F.; Parada, M.A. and Vergara, M. 2001. The Lower Cretaceous volcanism in the Coastal Range of central Chile: geochronology and isotopic geochemistry. III South American Symposium on Isotope Geology, Extended Abstracts (CD), 321-324.

Parada, M.A.; Feraud, G.; Aguirre, L.; Fuentes, F.; Morata, D.; Vergara, M. and Palacios,C.2001. LT-Pb, 4°Ar_

39Ar and fission-track geochronology of the Early Cretaceous Caleu pluton and its volcanic envelope, Coastal Range of central Chile: tectonic and metamorphic implications. III South American Symposium on Isotope Geology, Extended Abstracts (CD), 612-615.

Parada, M.A.; Larrondo, P.; Guirresse, C. and Roperch, P. 2002. Magmatic gradients in the Cretaceous Caleu Pluton (Central Chile): injections of pulses from a stratified magma reservoir. Gondwana Research, 5,307-324.

Vergara, M., Levi, B., Nystrom, J. and Cancino, A., 1995. Jurassic and Early Cretaceous island arc volcanism, extension, and subsidence in the Coast Range of central Chile. Geological Society of America Bulletin, 107:

1427-144.

A CARBONIFEROUS VOLCANIC ARC ALONG THE COASTAL CORDILLERA?

Antenor M ALEMAN(J) and Walther LEON(2)

(I) 2807 Stoney Wood, Houston TX, [email protected] (2) INGEMET [email protected]

KEY WORDS:Western Gondwana, arc magmatism, petrofacies

INTRODUCTION

Late Paleozoic Magmatism along the Gondwana's Pacific margin has been documented from Antarctic to northern Chile (Mukasa and Dalziel, 2000, Ramos and Aleman, 200, Bell, 1987), to southern Peru as documented by the late Silurian to early Devonian San Nicolas Batholith. However, there is an apparent absence of radiometric ages to infer the persistence of this arc-trench system throughout the Carboniferous.

The Lower Carboniferous in southern Peru is represented by the Ambo Group (Newel1 et aI, 1953), which consists of interbedded sandstones and shales with abundant coal seams. The only direct evidence of Carboniferous igneous activity is in the Shira Mountains where Megard (1978) and Dalmayrac and others, (I980) reported interbedded andesitic to rhyoJitic lavas and tuffs; however, there is little comprehension about the nature and tectonic setting of this volcanic activity in eastern Peru. Recently, indirect evidence of volcanic activity has been reported from petrofacies studies in the northern Bolivian Altiplano (Diaz Martinez, 1995). He documented significant percentage of volcanic rock fragments in the uppermost unit of the Ambo Group.

Four sections of the Ambo Group in southern Peru have been studied using Dickinson's (1983) discrimination triangles. The Paracas peninsula section plots on the undissected to transitional arc (QtlsF9L76 and Qm13F9Lt78), however, southward, the Chala section plots in a more mature transitional to dissected arc (Qt31F26L43 and Qm24F26LtSO). In this section the Ambo group contains thinly bedded tuffs are interpreted to suggest the presence of a contemporaneous volcanic arc rather than the erosion of an older arc massif. Although the arc-derived debris is more abundant along the coast, near the Carboniferous forearc and interarc basins, the influence of the magmatic arc also reached the retroarc basins such as documented in the Pongo de Mainique (QsoF13L37 and Qm4sFI3Lt39) and Cofiec(Q64F9L27 and Qm61F9L30) in the Ucayali and Madre de Dios basins respectively.

Qt Qm

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litJ

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15

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Along the coastal sections, the abundance of coal seams and low Q/F ratio (1.2 to 1.7) is interpreted to represent a warm and humid climate supporting the interpretation from the fossil flora. This climate may account for the relatively poorly preserved volcanic rock fragments and partially altered feldspars to chlorite and sericite.

Variable, but relatively high feldspar content and high Lv/Lt supports a magmatic arc provenance with significant arc dissection, and episodic unroofmg of batholith roots. Eastward, the Q/F ratio significantly increases in the Pongo de Mainique (3.8) and Pongo de Cofiec (7.4) and is interpreted to reflect the source from recycled transitional orogen and quartz rich lithologies. The vertical modal analysis at the Pongo de Mainique section indicates random variation controlled by the source area.

This petrologic evidence provides new lines of evidence to suggest the presence of an Upper Paleozoic arc- trench system in the western margin of Gondwanaland, which may have persisted since the late Silurian to early Devonian emplacement of the San Nicolas Batholith (Mukasa and Henry, 1990). This high K calc-alkaline magmatic arc developed during closing of the Lower Paleozoic extensional basin.

flood plain

lagoon offshore

bars

~r

influenced~

delta front CONCLUSIONS

The already accreted Grenvillian Arequipa terrane was extended during the early Paleozoic ensialic and formed deep troughs where thick marine sequences interbedded with basic lava flows and sills were deposited. This facies are well documented in Bolivia and northern Argentina. Closing of this lower Paleozoic basin took place by collision and re-welding of the extended Arequipa terrane and was earlier in the south (Ocloyic Orogeny) than in the north (late Silurian to meandering early Devonian Atico event of Shackleton and others, belt 1979). This event was coeval with the emplacement of

the San Nicolas Batholith (Mukasa and Henry, 1990).

An arc-trench system continued throughout the lagoon/marsh

Carboniferous with different kinematics and subduction ebb tidal delta angle. Low angle subduction may account for a volcanic lull during the Carboniferous; however, the late Permian has been ascribed to insipient back-arc extension (Megard, 1978).

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Pongo de Mainique •Ambo Group

% 50

200 800

600

400

/lIc, E~ ^mts F!(5 1000

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From Antarctica to southern Peru, the Gondwana's Pacific margin underwent a prolonged subduction- related magmatism at least since the early Devonian, which persisted through Carboniferous. The San Nicolas Batholith, formed during the closing of the early Paleozoic extensional basins, supports late Silurian to early Devonian magmatism. Petrofacies and the occurrence of thinly bedded tuffs in the Ambo Group suggest the presence of an arc-trench system along southern Peru during the Carboniferous. The lack of direct evidence for an arc may be related to a low angle subduction similar to the present day Peruvian margin. Regional analysis seems to support the presence of an arc-trench system in southern Peru at least since early Devonian.

REFERENCES

Bell, C. M., 1987, The Late Paleozoic evolution of the Gondwanaland continental margin in northern Chile. AGU Geophysical Monograph, 40, p. 29-48.