Analysis of balanced cross-sections and of the chronostratigraphy of the deformed foreland basin successions allows to solve the structural kinematics of the eastern Fuegian thrust-fold belt. Based on the Coulomb wedge theory, we propose a chronologically and geometrically well-constrained growth history for the frontal Fuegian Andes orogen. The mountain front behaved in a critical manner during the Early Eocene, characterized by forward thrusting. The wedge became subcritical after that, until renewed forward propagation of the thrust front in the Early Miocene. The long period as a subcritical wedge enhanced retrodeformation, recorded by out-of-sequence thrusting and backthrusting. We suggest that backthrusts should be considered an important mechanism for accommodating shortening in subcritical orogenic wedges.Fil: Torres Carbonell, Pablo Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; ArgentinaFil: Dimieri, Luis Vicente. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Olivero, Eduardo Bernardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentin

Dimieri, Luis Vicente

Olivero, Eduardo Bernardo

Torres Carbonell, Pablo Juan

Trabajos de Geología

English

CONICET Digital

The Fuegian thrust-fold belt is the thin-skinnedfrontal expression of the Andean orogen in Tierra delFuego, Argentina. Along the Atlantic coast, an almostcontinuous cross-section of the eastern thrust-foldbelt is exposed. The contractional structures affectsedimentary rocks from the Upper Cretaceous to theMiocene, which compose the clastic fill of theAustral-Malvinas foreland basin system (Olivero andMalumián, 2008).Construction of balanced cross-sections provides usefulinformation for the analysis of the kinematic evolutionof the thrust-fold belt. We divided the eastern thrust-fold belt in two sections (Fig. 1), south and north of theFagnano transform system (the boundary fault betweenthe Scotia and South American plates), which pro-duced a left-lateral offset of almost 50 km to the thrust-fold belt after cessation of thrusting (Torres Carbonellet al., 2008a). According to a well-calibrated chronos-tratigraphy of the foreland basin sedimentary units andtheir relationship with the evolution of tectonic struc-tures, we determine age constraints to the kinematics ofeach cross-section. On the basis of the Coulomb wedgetheory, it is possible to infer a chronologically and geo-metrically well-constrained history of growth of theFuegian Andes orogen, which enhances the role ofbackthrusts in mountain building. Southern cross-sectionThe unconformity-bounded units involved in thesouthern cross-section (Fig. 2) include the UpperThrust-fold belt kinematics and orogenic growth inTierra del Fuego, Argentina: implications ofbackthrusting for critical Coulomb wedge developmentAbstract: Analysis of balanced cross-sections and of the chronostratigraphy of the deformed forelandbasin successions allows to solve the structural kinematics of the eastern Fuegian thrust-fold belt. Basedon the Coulomb wedge theory, we propose a chronologically and geometrically well-constrainedgrowth history for the frontal Fuegian Andes orogen. The mountain front behaved in a critical man-ner during the Early Eocene, characterized by forward thrusting. The wedge became subcritical afterthat, until renewed forward propagation of the thrust front in the Early Miocene. The long period asa subcritical wedge enhanced retrodeformation, recorded by out-of-sequence thrusting and backthrust-ing. We suggest that backthrusts should be considered an important mechanism for accommodatingshortening in subcritical orogenic wedges.Keywords: thrust-fold belt kinematics, orogenic growth, Coulomb wedge, backthrusts, ArgentineFuegian Andes.1Centro Austral de Investigaciones Científicas (CADIC), CONICET. Bernardo A. Houssay 200, 9410 Ushuaia, Tierra delFuego, Argentina.2Departamento de Geología, Universidad Nacional del Sur and INGEOSUR, CONICET. San Juan 670, 8000 BahíaBlanca, Buenos Aires, Argentina.*e-mail: polmacleod@hotmail.comP. J. TORRES CARBONELL1*, L. V. DIMIERI2 AND E. B. OLIVERO1Trabajos de Geología, Universidad de Oviedo, 30 : 273-277 (2010)Cretaceous-Danian (KuD), Upper Paleocene (Pu),Lower Eocene (El), Lower Middle Eocene (Elm),Upper Middle Eocene-Upper Eocene (Emu),Oligocene (O), and Miocene (M). The structures haveevolved since the Early Eocene with the formation ofa fault-bend in the basal detachment of the thrust-foldbelt, which ramps up from a basal flat withinCretaceous units to the upper flat approximately at theKuD-Pu contact. An anticline forms over the detach-ment ramp in the Cretaceous-Danian rocks, andshortening is accommodated in front of it by thedevelopment of a fault-propagation fold (Cabo Leticiaanticline), which affects the Pu and El successions.Partial erosion of the early structures gives birth to anunconformity (u1), which is later covered by the Elm.After that, an out-of-sequence thrust forms (PuntaAncla thrust), branching from the basal detachmenthinterlandwards from the Cabo Leticia thrust. ThePunta Ancla thrust affects the Pu to Elm successionswithin the back syncline of the Cabo Leticia anticline,uplifting the KuD. This last deformation stage favorsthe development of a major unconformity (u2) ontop of which a thick Eml to O succession is deposit-ed. Continued contraction leads to the formation of aS-vergent thrust (Malengüena backthrust) in responseto delamination of the Cenozoic sequence (Pu to O)above the basal décollement. Renewed deformation generates a new thrust splaybelow the basal detachment, within the Cretaceous-Danian (La Chaira thrust). The splay branches fromthe lower flat of the Early Eocene ramp in the detach-ment and rejoins it at the KuD-Pu contact, forming aduplex horse. An anticline is formed above the LaChaira thrust, partially deforming the overlayingsequence synchronous with deposition of the Mcoarse sediments. The resultant cross-section geome-try corresponds to a duplex within the Cretaceous-Danian, and opposing vergence thrusts affecting thePu-M over its roof thrust. Northern cross-sectionThe unconformity-bounded units affected in thenorthern section (Fig. 3) include the ?Pu, El, Emu, O,and Lower Miocene (Ml). The kinematics of this sec-tion start with the deformation of the Pu-El succes-sions (Punta Torcida deformation), which accommo-dates shortening above the basal décollement blindtermination. The folded beds are later covered by theEmu succession after development of a major uncon-formity (u2). A backthrust forms (Campo del Mediobackthrust), branching from the basal décollement atthe rear of its tip line, folding the Pu-Emu synchro-nously with the deposition of the basal O succession(Torres Carbonell et al., 2008b). After that, a series offoreland vergent thrusts build up, branching from theblind termination line of the basal detachment andaffecting the Pu-El to upper O successions. Thesethrusts form an imbricate system (Punta Gruesaimbricate fan) coeval with the deposition of the Mlsediments at the thrust-fold belt leading edge(Ghiglione, 2002; Torres Carbonell et al., 2008b).P. J. TORRES CARBONELL, L. V. DIMIERI AND E. B. OLIVEIROFigure 1. Location map of Tierradel Fuego in South America (A)and its main geological features(B). The trace of the cross-sec-tions of figures 2 and 3 are indi-cated. FTS: Fagnano transformsystem.274Orogenic growth historyAccording to the Coulomb wedge theory (Davis etal., 1983), orogenic wedges are expected to behavecyclically in response to changes in their critical taperangle, which is the sum of the topographic slope αand the basal detachment dip β for given mechanicalparameters. The thrust sequence recorded at thestudied sections can be summarized as (Figs. 2 and3): foreland propagation of the basal detachmentaveraged at the KuD-Pu contact, accommodatingthrusting of the Cretaceous-Danian, with coevalforeland vergent thrusting of the Cenozoic above thebasal detachment in Early Eocene times (CaboLeticia thrust); forward propagation of the detach-ment with a blind termination line (Punta Torcidadeformation); out-of-sequence thrusting and upliftof the KuD at the Middle Eocene (Punta Anclathrust); backthrusting of the Cenozoic during theOligocene, branching from the basal detachmentnear its terminal line, and continuing with piggybackpropagation in a hinterlandward direction (Campodel Medio and Malengüena backthrusts); and finallyforeland and upward propagation of the sole faultand frontal thrusting during the Early Miocene(Punta Gruesa imbricates), possibly coincident withrenewed thrusting of the Cretaceous-Danian belowthe basal detachment (La Chaira thrust).THRUST-FOLD BELT KINEMATICS AND OROGENIC GROWTH IN TIERRA DEL FUEGO, ARGENTINAFigure 2. Southern balanced cross-section. Considering its position before the Fagnano fault offset, this cross-section is located 15 kmsouthwards from the northern cross-section. Numbers indicate thrust sequence. Pin line is active only for times of thrust 4. POBD:Paleocene to Oligocene basal detachment; MBD: Miocene basal detachment; CLT: Cabo Leticia thrust; PAT (OS): Punta Ancla thrust(out-of-sequence); MB: Malengüena backthrust; LCT: La Chaira thrust; BD: basal detachment. See location in figure 1.Figure 3. Northern balanced cross-section (modified from Torres Carbonell et al., 2008b). Numbers indicate thrust sequence. BD: Basaldetachment; CDMB: Campo del Medio backthrust; PTD: Punta Torcida deformation; PGIF: Punta Gruesa imbricate fan. See loca-tion in figure 1.275The cyclical behavior of the Fuegian thrust-fold beltcan be understood in terms of critical taper mechan-ics as shown in figure 4. After initial formation of thesole fault and forward thrusting in the Early Eocene(Figs. 4A and 4B), the wedge taper is too low for con-tinued movement over the detachment and furtherpropagation of the thrust front. A higher taper angleis therefore attained by internal out-of-sequencethrusting southwards from the basal detachment tipline during the Middle Eocene (Fig. 4C), simultane-ously with the clastic infill of the foreland basin. Thetectonic load concentrated at the rear of the wedgeleads to subsidence and a slight increase in the detach-ment slope β towards the south. This is followed bythe deposition of a thick sedimentary succession dur-ing the Late-Middle to Late Eocene (Fig. 4D). Sincethe wedge continues to be subcritical, further contrac-tion results in backthrusting (Fig. 4E), which affectsthe clastic successions during deposition of theOligocene package. Backthrusting evolves due todelamination above the basal detachment in a piggy-back fashion, forming a hinterlandward leadingimbricate system. The first structure formed is theCampo del Medio backthrust, which can not accom-modate further deformation after its branch linemigrates to the basal detachment termination line,thus resulting in the formation of the second structure(Malengüena backthrust). After the Oligocene back-thrusting stage, the wedge becomes critical and slidesforelandwards with deformation localized at the lead-ing edge. This causes the development of the PuntaGruesa imbricate fan, recorded by syntectonic LowerP. J. TORRES CARBONELL, L. V. DIMIERI AND E. B. OLIVEIROFigure 4. Schematic diagram showing evolution of the orogenic wedge in relation to critical taper, see text for explanation. Heavy linesmark active faults at each stage. Abbreviations are the same as for figures 2 and 3. Schematic wedges are light shaded light for geome-try at each stage, while dark shading indicates previous stage geometry. Scale and angular relationships are approximate.276Miocene strata (Fig. 4F), and probably related togrowth of the wedge with accretion below the sole byemplacement of the La Chaira thrust, as a way ofmaintaining the taper. This last structural stageenhances erosion and coarse clastic deposition atopthe orogenic wedge.Discussion and conclusionsOn the basis of previous work, the Fuegian Andes havebeen interpreted as evolving in three cycles: LateCretaceous, Paleocene-Early Eocene, and MiddleEocene-Oligocene, each involving several stages of sub-critical, critical and supercritical wedge behavior, withthrusting in the study area limited to the late Paleoceneand the Middle to Late Eocene (Ghiglione and Ramos,2005). We propose here a different model, in the lightof our new balanced cross-sections and an improvedchronostratigraphy. We found that the frontal part ofthe Andean orogen at eastern Tierra del Fuego behavedas a critical wedge during the early propagation of thesole detachment, in the Early Eocene. After that, thewedge became subcritical, being affected by internaldeformation until the next stage of foreland thrust-front propagation in the Early Miocene. The retrode-formation of the wedge during the subcritical stage isrecorded by out-of-sequence thrusting and backthrust-ing. These structures are strongly linked to thickunconformity-bounded clastic successions depositedwithin the wedge-top depocenter of the Austral basin(see also Torres Carbonell et al., 2008b). We highlight herein that backthrusting should beconsidered a very important way in thrust-fold beltsto accommodate internal wedge deformation that,during subcritical stages, cannot be propagated fore-landwards. In this sense, backthrusting plays animportant role in building up the wedge to acquire acritical taper angle.AcknowledgementsThis work is part of P. Torres Carbonell’s PhD studies, granted byCONICET. It is financed by CONICET PIP 5100 (E. Olivero),and CONICET PIP 5506-ANPCYT PICTO 841 (L. Dimieri)research projects.THRUST-FOLD BELT KINEMATICS AND OROGENIC GROWTH IN TIERRA DEL FUEGO, ARGENTINAReferencesDAVIS, D., SUPPE, J. and DAHLEN, F. A. (1983): Mechanics offold-and-thrust belts and accretionary wedges. J. Geophys. Res., 88:1153-1172.GHIGLIONE, M. C. (2002): Diques clásticos asociados a deforma-ción transcurrente en depósitos sinorogénicos del Mioceno inferi-or de la cuenca Austral. Rev. Asoc. Geol. Argent., 57: 103-118.GHIGLIONE, M. C. and RAMOS, V. A. (2005): Progression ofdeformation and sedimentation in the southernmost Andes.Tectonophysics, 405: 25-46.OLIVERO, E. B. and MALUMIÁN, N. (2008): Mesozoic-Cenozoicstratigraphy of the Fuegian Andes, Argentina. Geol. Acta, 6: 5-18.TORRES CARBONELL, P. J., OLIVERO, E. B. and DIMIERI, L. V.(2008a): Constraints on the magnitude of strike-slip displacementalong the Fagnano Transform System, Tierra del Fuego,Argentina. Rev. Geol. Chile, 35: 63-77.TORRES CARBONELL, P. J., OLIVERO, E. B. and DIMIERI, L. V.(2008b): Structure and evolution of the Fuegian Andes forelandthrust-fold belt, Tierra del Fuego, Argentina: Paleogeographicimplications. J. S. Am. Earth Sci., 25: 417-439.277

Thrust-fold belt kinematics and orogenic growth in Tierra del Fuego, Argentina: implications of backthrusting for critical Coulomb wedge development

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Thrust-fold belt kinematics and orogenic growth in Tierra del Fuego, Argentina: implications of backthrusting for critical Coulomb wedge development

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