The fold and thrust belt of the Bolivian Sub-Andes has been a topic of much discussion and debate for the past twenty years. Varying ideas regarding the Sub-Andean zone (SAZ) structural geology have been published, documenting conflicting ideas on the evolution of this complex region. Variations in balanced cross-sections result in a wide range of shortening estimates, thus highlighting the need for accuracy and precision when constructing balanced cross-sections.Shell Oil Corp

Bevis, Michael

Judy, K.

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I. AbstractCrustal Shortening Rates in Correlation to Structural Geology in the Sub-Andean Zone of BoliviaK. Judy, M. Bevis -The Ohio State University Department of Earth SciencesIV. Discussion V. ConclusionThe fold and thrust belt of the Bolivian Sub-Andes has been a topic of much discussion and debate for thepast twenty years. Varying ideas regarding the Sub-Andean zone (SAZ) structural geology have beenpublished, documenting conflicting ideas on the evolution of this complex region. Variations in balancedcross-sections result in a wide range of shortening estimates, thus highlighting the need for accuracy andprecision when constructing balanced cross-sections.II. IntroductionThe structural geology of the SAZ of Bolivia is thought to vary from north to south, and the debate as towhy is a geological hot topic. Various geologists have proposed a wide range of catalysts such aslithospheric delamination, lower crustal flow, crustal scale tectonics, and paleotopographical influences toexplain the geology of the Central Andes. This poster focuses on the structural and paleotopographicalaspects by presenting different balanced cross sections of the SAZ created by different geologists I haveNorthern Bolivian LatitudesSW15ºS NESAZUse same legend as seen on the 21 S section45 km15 kmIAZModified from Kley, 199915º-18º Modified from Baby, 1997Kley, 1999•Amount of Shortening: 74 km•Structure: Basal detachment found in Silurian strata underlies the IAZ and SAZ. The SAZ comprises tertiary sediments filling the Alto Beni Basin, and three main thrusts reaching the surface. The IAZ is comprised of Silurian to Devonian strata, with little folding and stacked duplexes. Note the triangle zone highlighted.•Basement Structure: Independent stacked basement thrusts.•Problems: Defining ages of strata with the terms Paleozoic, Mesozoic, Upper and Lower Crust is quite blanketing, allowing room for possible major discrepancies in interpretation of basement structure.Baby, 1997A f Sh i 74 kAlthough the amounts of crustal shortening vary by section, the differences are small. This indicates that small discrepancies in structural interpretation will slightly impact shortening estimates. Although McQuarrie’s estimates are smaller than Key and Baby’s, her data is well substantiated in her retrodeformations. Some of the trends in shortening that are expected are indeed present, with highest amounts seen in the central portion of the orocline. However, one would expect to see higher amounts in the north as compared to the south. This would reflect a structural response to the Paleozoic Basin which is more expansive in the south, allowing the basal detachment surface to propagate further eastward, accommodating compression, and reducing the need for additional shortening Therefore additional research is required tort er  li i  tit es- .chosen cross-sections from different authors, grouping them by latitude. Comparing cross-sections fromapproximately the same location illustrates that predetermined ideology can impact ones interpretation ofstructure, and how the smallest differences in section can effect crustal shortening estimates.III. Geologic BackgroundThe Bolivian Andes are located between 14 and 23 degrees South, and are generally divided into fivesections: the Western Cordillera (WC), the Altiplano (AP), the Eastern Cordillera (EC), the InterandeanZone*(IAZ), and the Subandean Zone (SAZ) (Fig 1). These regions are grouped by topographic elevation(Fig 2) and, recently, basement structure (Figs. 3 and 4) (McQuarrie 2005, 2006, 2008; J. Kley, 1997). TheSAZ of Bolivia is a thin-skinned fold and thrust belt, meaning that all deformation takes place above ai b l d t h t l i d f d b t k It f th t d f th C t lReferences15 km15º- 17º SLower CrustUpper MantleSW IAZ SAZ50 kmNEPaleozoic-MesozoicMiddle CrustCenozoicFrom McQuarrie et al, 2007• mount o  orten ng:  m•Structure: Large scale thrust sheets and broad synclines overlying a basal detachment in Ordovician strata.•Basement Structure: Large basement duplexes consisting of middle and lower crust.•Problems: Once again, general terms are used in the description of stratigraphic layers. Also, the triangle zone appears here, indicating that Kley’s cross-section may be strongly based on the work of Baby. While Baby interprets this triangle zone to be present, McQuarrie shows this same area to contain several duplexes,  structures much more likely to occur.McQuarrie  et al., 2007•Amount of Shortening: 66 km•Structure: Large wavelength synclinal basins separated by narrow zones of thrust faulted anticlines. Tertiary basin fill is segmented by a fault propagation anticline of Cretaceous rocks. Th th t f lt D i k d f b l d t h t i O d i i t t•Allmendinger, R.W., Smalley, AR., Bevis, M., Caprio,H., Brooks, B., 2005, Bending of the Bolivian orocline in    real  time,  Geology, v.33, p.905‐908.•Baby, P., Morretti,  I., Guiller,B., Limachi,R., Mendez,E., Oller,J., Specht,M., 1995, Petroleum systems of the northern and central Bolivian Sub‐Andean zone, Petroleum Basins of America:AAPG memoir 62, p.      445‐ 458.•Baby, P., Rochat,P., Mascle,G., Herail,G., 1997, Neogeneshortening contribution to crustal thickening in the back  arc of the central Andes, Geology: v.25, no. 10, p. 883‐ 886.•Boyer, S.E., and Elliott, D., 1982, Thrust systems: A i A i i f P l G l i•McQuarrie,N., Barnes,B., Ehlers,T., 2008, Geometric,kinematic and erosional history of the central  Andean plateau, Bolivia (15‐17º S), Tectonics,v. 27•McQuarrie,N., Gotberg,N., 2008, Comparison of crustal thickening budget and shortening estimates in southern   Peru (12‐14ºS): Implications for mass       balance and rotations  in the Bolivian orocline, unpublished.•Mitra, G., 1978, Ductile deformation zones and mylonites:The mechanical processes  involved in the deformation of crystalline basement rocks: American Journal of Science, v. 278, p. 1057–1084.Mi G 1997 E l i f li i f ld d h . ,      understand this data. ma n asa e ac men over y ng un- e orme asemen roc . orms e eas ern e ge o e en raAndes, and is the region of present day compressional deformation between the Nazca Plate subductingeastward under the South American Plate, which is moving westward. The differences in longitudinalextent of the SAZ are believed to be controlled by a Paleozoic basin (Sheffels, 1995; McQuarrie, 2008;Giraudo, 2001; Baby, 1997; Echevarria, 2001) which inhibits the basal detachment surface fromprorogating eastward in some areas of the SAZ. This change in paleotopography is responsible for anoticeable lack of the textbook SAZ structure in a section to the north of the Santa Cruz Bend at theChapare Basement High (Fig 1). This has caused some to group the SAZ into two separate crescents(Roeder and Chamberlain, 1995) instead of the continuous zone illustrated in Figure 1 b. The topographiclayout of the Paleozoic Basin also dictates the shape of the Andes, with the northern section of the oroclineexhibiting a width of no more than 550 km, and the southern section reaching 750 km (Kley, 1999). Theshape of the orocline is dictated by the extent of the basin, as ‘compression across a basin that varies inwidth produces a thrust belt whose variations in width reflect the basin geometry’(Sheffels, 1988).ree rus  au s carry evon an roc s upwar s rom a asa  e ac men  n r ov c an s ra a•Basement Structure: Large stacked basement megathrust of nearly equal thickness.18ºS IAZ SAZW E25 kmCentral Bolivian LatitudesModified from Kley, 1999Kley, 1999•Amount of Shortening: 90 km, shorter cross-section length.•Structure: The IAZ is comprised of Paleozoic through Mesozoic strata, segmented by large thrusts and fault propagation anticlines. The SAZ consists of Paleozoic through Tertiary rocks, with fault propagation folding among 3 thrusts.•Basement structure: Stacked independent thrusts.•Problems: Lack of deconstruction in literature allows for speculation inmer can   ssoc at on o   etro eum  eo og sts Bulletin,  v. 66, p. 1196–1230.•Barke R., Lambe, S., 2006, Late Cenozoic Uplift of the  eastern cordillera, Bolivian Andes, Earth and Planetary  Science Letters 249, p.350‐367.•Dahlstrom, C.D.A., 1969, Balanced cross‐sections:        Canadian Journal of Earth Sciences, v. 6, p. 743–757.•DeCelles, P.G., and Giles, K.N., 1996, Foreland basin systems: Basin Research, v. 8, p. 105–123.•DeCelles, P.G., and Horton, B.K., 1999, Implications of early Tertiary foreland basin development for orogenesis in the central Andes: Eos (Transactions, American Geophysical Union), v. 80, p. 1052.•Dunn, J.F., Hartshorn, K.G., and Hartshorn, P.W., 1995,Structural styles and hydrocarbon potential of the       Subandean thrust belt of southern Bolivia, in Tankard.•Echevarria, L., Herrnandez,R., Allmendinger,R., Reynolds,J., 2003, Subandean thrust and fold  belt of northwestern Argentina: Geometry and timing of the• tra,  .,  ,  vo ut on o  sa ents n a  o ‐an ‐t rustbelt: The effects of sedimentary basin geometry, strain distribution and critical taper, in Sengupta, S., ed., Evolution of geological structures  in micro‐ to      macroscales: London, Chapman and Hall, p. 59–90.•Mitra, G., and Boyer, S.E., 1986, Energy balance and      deformation mechanisms of duplexes, in Platt, J.P.,      Coward, M.P., Deramond, J., and Hossack, J., eds.,    Thrusting and deformation, Volume 8: Oxford–New York,Pergamon, p. 291–304.•Moretti,I., Letouzey,J., Otero,A., Calvo, J.C., Structures and  growing  detachment role in the sub Andean zone of  Bolivia, Caipipendi block•Pope, D.C., and Willett, S.D., 1998, Thermal‐mechanicalmodel for crustal thickening in the central Andes driven by ablative subduction: Geology, v. 26, p. 511–514.•Roeder, D., and Chamberlain, R.L., 1995, Structural geology of Subandean fold and thrust belt in northwestern Bolivia in Tankard A J Suarez R andSandbox models have been completed to replicate this statement (Baby, 1992).*Some authors do not include the Interandean zone in their publications (Sheffels, 1995; Giraudo, 2001;Moretti et. al., 1998; Sempere et. al., 1997; Barke and Lamb, 2006; Roeder and Chamberlain, 1995;Watts et. al., 1995).Figure 3 -Illustration of Kley’s (1997) interpretation of the basement structure of the Central Andes with geologic divisions.ECIAZSAZPliocene and younger sedimentsLate Miocene volcanics (10 Ma) MioceneOligocene (upper Potoco/Coniri)Eocene (Potoco)CarboniferousSilurianOrdovicianDevonianCambrianPrecambrian (map only)Mesozoic17ºS02520151055 km30Interandean zoneEastern Cordillera forethrust zone/Subandean zoneeFrom McQuarrie 2002          balancing.McQuarrie 2002•Amount of Shortening: 72 km, shorter cross-section length.•Structure:  The IAZ shows duplexing in Ordovician rocks overlaying a large basement sheet. The SAZ shows deformation of Ordovician through Miocene aged rocks, with Mesozoic rocks lying disconformably over Ordovician in the east. Three major thrusts segregate narrow propagation folds.•Basement Structure: Major flat basement thrust (highlighted).S th B li i L tit d             Andean evolution, AAPG Bulletin, v. 87, no.6, p. 965‐985.•Elger,K., Oncken,O., Glodny,J., 2005, Plateau‐style accumulation of deformation: Southern Altiplano, Tectonics, v.24.•Giraudo,R., Limachi,R., 2001, Pre‐Silurian genesis of the central and southern Bolivian fold belt, Journal of South  American Earth Sciences 14, p. 665‐680.•Gubbels, T.L., Isacks, B.L., and Farrar, E., 1993, High‐levelsurfaces, plateau uplift, and foreland development,Bolivian central Andes: Geology, v. 21, p. 695–698.    •He´rail, G., Baby, P., Lo´pez, M., Oller, J., Lo´pez, O., • Salinas, R., Sempere, T., Beccar, G., and Toledo, H., 1990,  Structure and kinematic evolution Subandean   thrust system of Bolivia, in Extended abstracts: Paris,    ORSTOM 1st International Symposium on Andean    geodynamics ,p. 179–182.•Isacks, B.L., 1988, Uplift of the central Andean plateau and bending of the Bolivian orocline: Journal of   ,    ,  . .,  ,  .,   Welsink, H.J., eds., Petroleum basins of South America: American Association of Petroleum Geologists Memoir 62, p. 459–479.•Schmitz, M., and Kley, J., 1997, The geometry of the central Andean backarc crust: Joint interpretation of cross‐section balancing and seismic refraction data: Journal of South American Earth Science, v. 10, p. 99–110.•Sempere, T., Herail, G., Oller, J., and Baby, P., 1989,  Geologic structure and tectonic history of the Bolivian      orocline: International Geological Congress, 28th,Washington, D.C., Abstracts, v. 3, p. 72–73.•Sheffels, B.M., 1988, Structural constraints on structuralshortening  in the Bolivian Andes [Ph.D. thesis]:  Cambridge, Massachusetts  Institute of Technology, p.   167•Sheffels, B.M., 1990, Lower bound on the amount of crustal shortening  in the central Bolivian Andes: Geology, v. 23, p. 812–815.Figure 1-Topographic map of Bolivia. The five main geologic provinces have beenf i i i iChaco BasinSAZFigure 4-  The lack of the SAZ in the Southern Andes has been attributed to the absence of abasement structure for that region.MIAZ SAZ50 km21º-22ºSW EIAZ SAZW E20 km35 km21º STertiaryMesozoicBasement/Middle CrustPaleozoicLower Crustou ern o v an a u esModified from Kley, 1999Kley, 1999•Amount of Shortening: 79 km•Structure: Several smaller, tightly packed thrusts and backthrusts in the IAZ and western SAZ and three large thrusts segregating strata in the eastern SAZ (highlighted).•Basement Structure: Large individual thrust sheets.•Problems: Generalization of strata dating.Baby, 1997•Amount of Shortening: 86 km•Structure: Fault propagation folds, fault bend folds, and passive roof duplexes overlying a main detachment in Silurian dark shales. This detachment is covered by 2000m of Paleozoic and Mesozoic sandstones. Geophysical Research, v. 93, p. 3211–3231.•Kley, J., 1996, Transition from basement involved to      thin skinned thrusting in the Cordillera Oriental of  southern Bolivia: Tectonics, v. 15, p. 763–775•Kley, J., 1999, Geologic and geometric constraints on akinematic model of the Bolivian orocline: Journal ofSouth American Earth Science, v. 12, p. 221–235.       •Kley, J., and Monaldi, C.R., 1998, Tectonic shortening and crustal thickening in the central Andes: How good is the correlation?: Geology, v. 26, p. 723–726.•Kley, J., Gangui, A.H., and Kruger, D., 1996, BasementInvolved blind thrusting  in the eastern Cordillera   Oriental, southern Bolivia: Evidence from cross‐sectional balancing, gravimetric and magnetotelluricdata: Tectonophysics, v. 259, p. 179–184.•Kley, J., Mu¨ller, J., Tawackoli, S., Jacobshagen, V., andManutsoglu, E., 1997, Pre‐Andean and Andean agedeformation in the Eastern Cordillera of southern Bolivian Journal of South American Earth Science, v.     •Sheffels, B.M., 1995, Is the bend in the Bolivian Andes an orocline?, in Tankard, A.J., Suarez, R., and Welsink,H.J., eds., Petroleum basins of South America: American Association of Petroleum Geologists Memoir 62, p. 511–522.•Welsink, H.J., Franco, M.A., and Oviedo, G.C., 1995,      Andean and pre‐Andean deformation, Boomerang Hills area, Bolivia, in Tankard, A.J., Suarez, R., Welsink,H.J., eds., Petroleum basins of South America: American Association of Petroleum Geologists Memoir 62, p. 481–499.•Wigger, P.J., Schmitz, M., Araneda, M., Asch, G., Baldzuhn, S., Giese, P., Heinsohn‐Wolf, D., Martinez,          E., Ricaldi, E., Roewer, P., and Viramonte, J., 1994, Variation in the crustal structure of the southern central Andes deduced from seismic refraction investigations,  in Reutter, K.J., Scheuber, E., and Wigger, P.J., eds., Tectonics of the southern central Andes: Structure and evolution of an active added, as well as the extents o the Paleozo c bas n. The Ben Bas nnarrows northward, while the Chaco Basin extends further westward into Brazil and Paraguay. ( Modified from McQuarrie , 2002)Figure 2-Topographic cross-section  corresponding to  A-A’ on Figure 1. This section illustrates McQuarrie’s (2002) categorization of the Andes based upon topographic elevation.Illustration of McQuarrie’s (2002) structural basement interpretation. Notice the complete continuation of the IAZ to the south, and the exact correlation of basement megathrusts and topographic elevation of the EC and IAZ. Pliocene and younger sedimentsLate Miocene volcanicsMioceneEocene-OligocenePaleocene (Santa Lucia and Cayara)Cretaceous (El Molino)MesozoicSilurianOrdovicianCarboniferousDevonianLate Miocene intrusivesModified from Baby, 1997From McQuarrie 200220ºS•Basement Structure: Large thrust sheets of varying thickness.•Problems: Generalization of  strata dating.McQuarrie, 2002•Amount of Shortening: 67 km•Structure: Several imbricate duplexes in Silurian rocks, broad synclines. Basal detachment in Ordovician shale shallows in the IAZ. •Basement Structure: One large, flat Megathrust sheet.•Problems: McQuarrie reassesses her hypothesis of this basement structure in her 2007 publication. 10, p. 1–19.•Lamb, S.H., and Hoke, L., 1997, Origin of the high plateau in the central Andes, Bolivia, South America: Tectonics,  v. 16, p. 623–649.•McQuarrie, N., and DeCelles, P.G., 2001, Geometry andstructural evolution of the central Andean backthrustbelt, Bolivia: Tectonics, v. 17, p. 203–220.•McQuarrie,N., 2002, The kinematic history of the central  Andean fold‐thrust belt, Bolivia: Implications for building  a high plateau : GSA Bulletin,2002; v. 144, p. 950‐963•McQuarrie,N., Horton,B., Zandt,G., Beck,S., DeCelles,P., 2005, Lithospheric evolution of the Andean fold‐thrust   belt, Bolivia, and the origin of the central Andean  plateau, Tectonophysics 399, p. 15‐37.continental margin: New York, Springer‐Verlag, p. 23–48.

Crustal Shortening Rates in Correlation to Structural Geology in the Sub-Andean Zone of Bolivia

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Crustal Shortening Rates in Correlation to Structural Geology in the Sub-Andean Zone of Bolivia

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