Andes Uplift

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    6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 824-827

    Cordillera and western Central depression north of 23S a hyperar id c limate may have prevailed from the lateOligocene/early Miocene (Dunai et al. 2005; Evenstar et al. 2005) . The Cretaceous and Cenozoic sedimentaryrecord from northern Chi le is dominated entireJy by sediments deposited under an arid/semiarid climate. Itappears therefore, that northern Chile has been subject to a continuously arid/serniarid climate for at l eas t the last100 Ma and possibly earlier . For much of the Cenozoic sedimentation took place in endorheic basins developedeast of the Coastal Cordillera, consequently Jittle or no sediment was supplied directly to the trench.

    Palaeolatitude

    An estimate of palaeolat itude der ived from palaeomagnetic data provides indirect support for an unchangedclimatic regime from the Cretaceous onwards (Hartley et al. 2005). Apparent polar wander (APW) paths andreference poles for the stable craton of South America from the Cretaceous to the pre sen t day (Beek, 1999;Somoza & Tomlinson 2002), indicate that s ince 80 Ma there has been no identifiable APW of South Americarelative to the present day spin axis. Palaeomagnetic data from northern Chil e indicate that there has been nolatitudinal translation of strata relative to the South American craton since the late Palaeozoic (Jesinkey et al.1987; Somoza & TomJinson 2002). Northern Chile has therefore remained at approximately the same latitude forthe Jast 80 Ma. I f as appears likeJy, the southern hemisphere deser t bel t has not moved significant ly dur ing thelast 80 Ma (Hartley et al., 2005) then western central South America is likely to have been subject to an aridclimate from at Ieast the late Cretaceous onwards.

    Evidence for high plate boundary shear stresses

    The Central Andean convergent margin is commonly regarded as a high stress and high frict ion interplate endmember for subduct ion zones and is considered ideal for generat ing high rates of subduction erosion. High shearstresses are considered to have built up at the interface between the Nazca and South American plates due tosediment starvation of the Peru-Chile trench resulting from the arid onshore climate (e.g. Lamb and Davis 2003).However evidence from multibeam bathymetry and seismic records from the north Chi lean margin indicate thatthis is not the case. von Heune and Ranero (2003) identified the presence of an interp late clastic layer tha treduces friction and shear stress at the plate boundary. The interplate layer is der ived from mass wasting of theslope and forms a frontal prism of remolded debris that elevates pore pressure to reduce interplate friction (vonHeune and Ranero 2003) . Interplate seismicity and taper analyses presented by these authors indicate that basalfriction levels in the upper plate are similar to those in sediment-rich convergent margins. Consequently, there isno requirement for elevated shear stresses to be present at sediment starved convergent margins. Lamb & Davis(2003) suggested that there would be considerable mater ial/composit ional dif ferences between the sedimentincorporated into the subduction zone at sediment s tarved versus sediment rich margins and that this wouldaffect the thermal properties of the subduction zone. However sediments at the two diff erent margins are bothlikely to be largely unlithified, fine grained and water-rich and are unlikely to display any substantial differencesthat might account for variations in shear stress between sed irnent poor and sediment rich margins.

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    6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 824-827

    Deformation, Relative Plate Convergence and Andean Uplift

    Th e detailed relationship between relative plate convergence and deformation in the Andes has yet to be resolvedalthough a general relationship between plate convergence and Andean upl if t is general ly accepted. Work byPardo-Casas and Molnar (1987) and Somoza (1998) allow constraint s to be placed on the direction and rate ofrelative plate convergence dur ing much of the Cenozoic. Between 68 and 49 Ma convergence was almostparal lel to the South American margin (Pardo-Casas and Molnar, 1987). From 49 to 30 Ma convergence wasoblique (between 45 and 35 degrees) and became largely orthogonal after 30 Ma when convergence ratesdoubled between 28 and 25 Ma (Somoza 1998).

    Th e deformation associated with uplif t of the Andes has been ex tens ively documen ted and the spatial andtemporal distribution of deformation is known to vary considerably. However sorne general comments can bemade. There is a Jimited amount of evidence for Paleogene deformation in Bolivia, however it appears that thevast majority of deformation associated with uplift and volcanic and magmati c activity took place during theMiocene. lndeed the present arc was established in the early Miocene with little or no Oligocene volcanicactivity recorded. From the albei t limi ted information ava ilable on uplift rates it is clear that at least 75% ofAndean uplift is Miocene or younger in age (e.g. Gregory-Wodzicki 2000; Hartley 2003) . It would appeartherefore that there is a close cor respondence between an increase in convergence rates and Andean uplift,although the detailed distribution of deformation and uplif t varies between individual tectonical1y defined areasand the precise relationship has yet to be established .

    Discussion and Conclusions

    Lamb & Davis (2003) proposed that the dynamics o f subduction and mountain build ing in the Andes arecontrolled by the processes of erosion and sediment deposit ion and are linked directly to climate. They suggestedthat increased Cenozo ic aridity led to sedimen t starvation at the trench genera ting increased friction andenhanced shear stress at the plate interface, resulting in uplif t and deformation of the South American Plate. Th esedimentary record and palaeomagnetic data suggest that there is no evidence for a significant c1imate change inwestern South America at or around the mid/late Eocene (when initial Andean uplift is conside red to havecommenced) and that an arid clirnate prevailed both before and after this time period. This suggests thatsediment fluxes to the trench are unlikely to have changed substantially during the Cenozoic. In addit ion thework of von Heune and Ranero (2003) indicate that even though the Peru-Chile t rench is starved of sediment thelack of sediment does not result in increased interplate friction. 1n fact they found that basal fr iction levels in theupper plate of f the coast of northem Chile are similar to those in sediment-rich convergent margins.Consequently, there is no requir ement for elevated shear s tresses to be present at sediment starved convergentmargins.

    lt is concluded that there is no evidence to support the theory proposed by Lamb & Davis that Cenozoic climatechange resulted in Andean uplift. Rather it is suggested that Andean upl if t results from a combinat ion of 1) the

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    6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 824-827

    lirnited erosion associated with the arid climate that has prevaiJed across the region from the Late Jurassic, and 2)a change to more orthogonal convergence in the Eocene and doubling of convergence rates in the late Oligoceneto early Miocene. In conclusion, the height of the Andes is strongly controlled by the prevail ing climate, but onlybecause uplift easily outpaces erosion.

    ReferencesBeek, M.E. 1999. Jurass ic and Cretaceous apparent polar wander re lative to South America : some tectonicimplications. Journal of Geophysical Research, 104,5063-5068.Chong, G. 1988. The Cenozoic saline deposits of the Chi lean Andes between 18S and 27S. ln: Bahlburg, H.,Breitkreuz, C. & Geise, P. (eds) The southern Central Andes. Springer-Verlag, Berlin, 137-151.Dunai , T.J., L6pez. G.A.G. & Juez-Larr, J. 2005. Oligocene/Miocene age of arid ity in the Atacama Desertrevealed by exposure dating of the erosion sensitive landforms. Geology (In press).Evenstar, L., Hartley, AJ, Rice, C.M., Stuart, F, Mather, A.E. & Chong, G.. 2005. Miocene-Pliocene climatechange in the Peru-Chile Desert. Abstract, International Symposium on Andean Geodynarnics, Barcelona.Gregory-Wodzicki, K.M. 2000. Uplift history of the Central and Northern Andes: A review. Geological Societyof America Bulletin, 112, 1091-1105.Hartley, A.J. 2003. Andean upJift and climate change. Journal of the Geological Society, London, 160,7-10.Hartley, A.J. & Chong, G. 2002. A late Pliocene age for the Atacama Desert : Implications for the deser ti ficationof western South America. GeoJogy, 30.Hartley, A.J., Flint, S., Turner, P. & Jolley, EJ. 1992. Tectonic controls on the development of a semi-arid,alluvial bas in as reflec ted in the s tra tigraphy of the Purilactis Group (Upper Cretaceous-Eocene), northernChile. Journal of South American Earth Sciences, 5, 273-294.Hartley, A.J., Chong, G., Houston, J. & Mather A.E. 2005. J50 Million Years of Climatic Stability - Evidencefrom the Atacama Desert, northern Chile. Journal of the Geological Society, London (In press).

    Jesinkey, c., Forsythe, R., Mpodozis, C. & Davidson, J. 1987. Concordant late Paleozoic palaeomagnetizationsfrom the Atacama Desert: implications for tectonic models of the Chi lean Andes. Earth and Planetary ScienceLetters, 85,461-472.Lamb, S. & Davis, P. 2003. Cenozoic climate change as a possibJe cause for the rise of the Andes. Nature, 425,792-797May, G., Hartley, A.J., Cheng, G., Stuart, F., Turner, P. & Kape, S. 2005

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