Chapter 25

Structural, geo orphic, and depositional characteristics of contiguous and broken forel d basin : examples from the eastern flanks of the central A des in Bolivia and NW Argentina

MANFRED R. STRECKER<. GEORGE E. HILLEyt, BODO BOOKHAGENt and DWA DR. SOBEL§

"Institut filr Geowt senschaftetl, Universitiit Potsdam, Potsdam tDepartment of Geological and Environmental Sciences. tanford Univemty, Stanford. USA 'Geogrophy Departm 'lit. University of California, Santa lJarbara. USA iJnstitut fur GeowisSBns haften. Universitiit Potsdam, Potsdam. Gennany

ABSTRACf

In s chapter, we contrast tho elements of 8 typical, contiguous foreland basin systom in the Bolivian Andes with the broken fo land farthersouth innorthwestom Argentina. We illustrate dIlIeran in d position d geomorphic shape that ise from the s ctu:ral conditions to which these two syst s ore subjected. G9nerally. th principaJ al ments offoreland-basin systems result mainly from commodation space created by the Jlaxural response of the eru. t to th topographic load of a fold-end-thrust belt. This 1 ads to the formation of four distinct dopoumes~ the wedge-top. foredeep. forebulge. and backbulg . In coulrast, brokan foreland b 18 ore fanned ill area whore troare can ergen is accommodated primarily along re-aetivated. bigh-angLe structures. Rath then creating a broad of eOlllilstant1y sloping mean topography, rock uplift along these structures is often displlDlt in space and time, I a . g to the formation of d screte rJIDg of limited along-strike xtent that aceill' fur tnba d of the maiD topographic front of the orogen. The potentielly high rock-uplift rotes accommodated by at p. reactivated reverse mulls ors isolation of the b dw ter basins of such syst ms from the downs m fluvial nelwork, leading to ediment ponding b lind the . ing mountain ranges. In addition, the limite flexural response associat d wilh ShOIt

wavelength, t lly lrkted topography may il to create large amounts ora .co • modstion space een in th foredeep depo2on that typifies foreland basin systems. Thus. inst ad of the generally continuous. laterally extensive foreland basin system associated with continentaJ-scal cmstBl Oexure. brokJ.n foreland basins. such as ill t northw08lam Argentine Andas. COIl.'list ora et ofvarinbly conn ctad,l t.arally r tricted depoe tEl1'S. Those evolve behind eeti ely rising topograpby or small basins that form within the limited acallnmodation apac created ound dividual uplifted mountain ranges. Th se ty as of depositional systems lackmallY of the laments that ar typical of recent fOleland bnsius, includilJg well·developed foredeep, for bulge, d ba hulg depo iti0l1111 systems. Inst ad of appl "Dg fOIaland basin mod 1s to hroken foreland basin sys ms, it is important to vi w these s lams in the conlaxt of their di tinct strue.tu:re1, topographic, d geodynamic circumstancas.

Keywords: contiguous fore nd basin: hrok n foreland basin; fold and thrust belt; re-activated structures; central Andes

Tectoni of Sedimental}' Basins: Recant Advances. First Bditio . Edited by Cathy Busby and Anlmlio Azor. e 2012 Blaclcwell Pub' hing Ltd. Puhliahed 2012 by Bl8cIcwell Publishing Ltd.

508

ken fumland- in systmnmodsla. (AJ FOI1I1and basin $)'Slams coosist syn-dafounational l>edbnoolS deposit.edin Lhewodge-mp a fore­deep dapocllotet created by fleXW'll in front of an advancing fold..and-thrust belt, a forebulgo

(B) __ Brokan Foreland BasIn System --_ depocoOllll' lhat is locatad on Lhe n xural

--PIal au ----.. Ponded and FlexUral BaslllS bulge and the back.bulge basin local d behind Lhe noxurel bulge Im0di.6.ed alter DeCall'" and GillIS, 1996). £BI Broken rom­land sins amsi of sedimant daposilod within spa 'ally Umited flexural basins and sedimenl pondingbehind dsinglopograpby.

INTRODUCTION

The architecture of foreland basins and the thick­ness f their sedimentary fills are primarily deter­mined by the creation of structurally controlled accommodation space associated with the lateral and vertical growth of fold-and-thrust belts (e.g., Dickinson, 1974; Jordan, 1981, 1995). Typically, four ganeml lectomcJdepositional domains char­acterize the e environments. reflecting the inter­play between shortening. crustal loading. climate. sediment production, and deposition (Fig. 25.1A) (e.g., DeCelles and Giles. 1996): (1) piggy-back basins. formed within and blind thrusting out­board of the e tent of the fold-end-thrust belt define the boundaries of wedge-top basins, whose sediments are deposited within tectonically

clive portions of the orogen (e.g., Jordan, 1995; DeCelles and Giles, 1996); (2) flexural subsiden.ce outboard ofthe fold-and-thrust beltprovides space for foredeep sediment that thick.en and coarsen toward the thrust wedge (e.g.. Dickinson end Suczek. 1979; Schwab. 1986; DeCelles and Hertel, 1 89: DeCelles and Giles, 1996); (3) the forebulge, a distal upwarped sector outhoard of the foredeep. results from c tal-s Ie flexure (e.g., Jacobi, t981; Karner and W Us. 19 3; Quinlan and Beaumont, 1984; Crampton d Allen, 1995; Turcotte and Sc ubert. 2002); and (4) the backbulge basin is a second flexural basin created' the wake of the forebulge. defining a broad. low-energy eposi-. LianaI en ironment (e.g.. DeCelles and Giles. 1996; Horton and DeCelles. 1997: DeCelles and Horton. 2003). A th fold-an -thrust belt a ­vances into the Ioreland, these different tectonic and associated depositional domains also migrate systematically basinward. often leading to vertical

(A) .. FareIand Basln System - OrogenlcWedge --

WlIdge Top FoI9deep ForebuI

Contiguous arid BroDm Foreland Basins 509

stacking of sediments created within these domains (e.g., DeCelle and Horton, 2003). The fold-and-thru t bell geometry approximates that of a seU-simllarly growing tectonic wedge that progressively entrains foreland-basin sediments by fran 1 accretion and motion along detach­menl horizon. such as currently observed in the Subandean bell of Bolivia in the central Andes (e.g., Seropereetal., 1990' Kennan et al.. 1995; Baby etal.• l 7; Horton and DeCelles, 1997; M uarrie. 2002; Uba et al., 20(6).

This standard. model successfully explains the primary features of both ancient and modem peripheral ( ensu Dickinson, 1974J and retroarc foreland basins (e.g., Jar n, 1981. 1995; DeCelles and Giles, 1996). The model may be inappropriate [or depositional basins in th e orogens where contraction is accommodated by spatially dispa­rale. iachronous,revel'Se-fault bounded basement uplifts. rether than a thin-skinned fold-and-thrust belt (e.g., Jordan and llmendinger, 1986). Modem examples ofsuch foreland basins include e Santa Barbara system of northwestem Argentina. adj ­cenlto the Subandes fold-end-thrust belt. and the

iarras Pampeanas morphotectonic provinces far­ther south (Fig. 25.2; Stelzner. 1923; Gonzalez­Bonorino, 1950; Allmendlng [etal., 1983; Jordan and Allmendinger. 1986; Ramos et al .• 2002). The Laramide foreland of the western United States (Steidlmann tal.,l 83) may erve as n ancient

log of such a b in ystem (TIi inson and Snyder, 1978; Jordan and Allmendinger. 1986). Although being related to a different geodynamic etLing. the Ti Shan fKyrgyzstan and Chlnaor

theQilian Shan uplifts north ofthe Tibetan Plateau comprise imllar settings, where ongoing shorten­ing excises end uplifts basement hlocks in the

•

BacIdluIge. Fig. 25.1. SdJemaUc (Al rondami lIlld CDI br0­

510 Pan 4: ConV4lg nl Margins

FIg.15.2. Topography ofthl! central Ide ith el vations in m asl. Locations of longitudinal topographic swaths in Figu 25.4C and 25.4D idenlicalto the llXtents repra­

nt in Figures 2S.3A and 25.3B, rasp ·valy. Mll8Il. lopograp c swaths (cakulaled I rally) used in the Bex­ural modeJs in Figwes 25.7A. 25.78. and 25.7C e also indicated. SBS SP denote SantJI Barbam and Sian' Pampeans mnrphoslru tum! provine . raspuc: ·valy. Solid white line delIlllI.'Cat intemally drained ltiplana-Puna plateau in the interior of the orogen.

fore I nd (e.g., Metivier et aI., 1998; Sobel at 1., 2003). This mode of forelan basin evolution eventually compartmel tali.zes a formerly conligu­ou secUmenlary basin and often causes trends in rock up ift. and edimentation patt rns that are not predicted by slandar foreland basin models (e.g., Jordan and Allmendinger, 19S6). This accommo­dation of deformation within ba ement ranges and the formation of a broken foreland may oc r well inboard of the primary topographic .margin of the orogen (e.g., Ramos at al .• 2002; S der el aI., 2009; laffa et a1.. 2011). Importantly, the uplift of such basement range leads to a different flexural

response than the one expected in typical for land basin systems related to a growing orogenic wed a. Consequently, the evolution of associated deposj­tionalsystems may be radically different com cad to foreland asins adjacent to fold-and-lhrust belts.

Here, we fo on the topographic, geomorphic, and depositional development ofsuch broken fore­land systems. We use the depositional lam adja­cent to e Puna Plateau and the Eastern Cordillera in north e tern enlina to contrast the geody­namic conditions and development f this etting wilhthed posilionalsy te preservedintheChaco foreland basin ofBotivia to the north (Fig. 25.2). Key differences between the structural development of theBolivianandthenorthw sternArgen . eAndes largely result from the differing pre-Ceno:l:oic geo­10' istory oft.hese areas (e.g., Allmendinger et al.. 1983; Kley taL, 2005), which ha. a profound impact on e fl ural sponse of the e two differ­enlareas to deformation, geomorphic development, and deposition in the s rounding basins.

RETROA C TOPOG APHY. DEFORMATlON, AND DEPOSITION IN THE CE T ES

Retroarc lOp phy, deformation, and deposition in th cenual Andes between 15° and 35°S (Fig. 25.2) results from convergence between the Nazca and South Amarican plates and e geom­etry of the downgol.ng oc anic slab (e.g., Barazangi and I ads, 1976; Be is and Isacks, 1984). orth of ....24°8, the subdu ling plate dips relatively st ply (""3 ) beneath South America, while south of ",,26°8, the lab shoals lo a ub-horizontal geometry (e.g., Cahill and Isacks, 1992). There is a general correlation between this subduction geom­etry and the xtent fhigh elev lion in the Andean realm (Gephart, 1994). he internally drained Altiplano-Puna plates spans outhern Bolivia an' northwestern Argentina betw en 50 S and 27"8 a d h average elevation of 3.7k.m (e.g., Allmendingereta1.,19 7). bisregioncompris sa onttae . nal basin and range topa raphy, eha

terize by' alated, somelim caales ed sedimen­tary basins l host lon-thick evaporitic and clastic sequences (e.g.. Jordan and lonso. 1987; Alonso eta1., 1991; Vandervoort tal .. 1995). These basins are bound d by high-angle reverse faults or volcanic a ifices that either constitute the western margin of th plaleau or furth ,r compartmentalize the basins in a transverse manner (Alonso al al ..

1984; Riller t I., 2001). To the east of the plateau and the Eastern Cordillera in Bolivia. a wedge­shaped "'250-km-wide fold-and-thrust belt defines the eastern border of the orogen. However, uth· ward in northwestern rgentina, thi fold-and­thrusl bell vanishes and' replaced by 8 region of thick-skinned deformation within the Santa Bar and Sierras Pampeanas range (e.g., Allmendinger et aI.. 1983; Mon and Salfity. 1995). The patial extent of the fold-and-thrust bell in Bolivi co late with thick Paleozoic units. in which a series of Silurian, Devonian. and Carbon­iferous detachment surfaces define the basal decol­lement of the orogenic wedge (e.g., Sempe:re et a1.. 990; M uattie. 2002; Elger el aL. 2005). However, appr ximately south of 24°S. these mechani Iy weak layers become significantly thinner and vantually disappear. and the thin­skinn d style of deformation terminates (e.g.• Allmendinger et aI., 1983; Mon and alfity,1995; Cristallini e a1.. 1997; Kley and Monaldi. 2002). Instead, inverted n rID I fi ults and transfer struc­tures rei ted lo e Cretaceous Salta Rift have often accommodated shortening during the C nozoic Andean orogeny (Grier tal.. 1991; Kley nd Monaldi, 2002; Carrera et aI. 2006; Hongn lat. 2007; Hain el aI.. 2011). In addition, the metamor­phiC fabrics generated during the early P leozoic (Ordovi .ian) Ocloyic orogeny follow the western margin of the broken foreland uplifts· the tran­sition to the Puna Plateau and are inferred to have affecled the patial characteristics of contractile reactivation during Andean shortening (e.g.• Mon and Hongn, 1991). The reacti led, inherited. e ensional and contractional anisotropies have pr duced discrete rang that occur both along the eastern flank of the plateau as well as fur to the east of the plateau margin within the otherwise wldefonned foreland, as might be e ected from the mechanics of reaclivation of pre-existing weak structures in the crust (Mon and Salfity, 1995; Hilley et ai., 2005). This is virtually identical La inverted extensional structure in the Eastern Cordille of Colomb' (.g.. More et al., 2006; Parra et aI., 2009) and be rese lance to the p II rn of re ctiv ted . tructm:e in the North

merican La mide province (Marshak et aI., 2000). Thu., the pre-Andean paleogeography of nol1hw tern Argentina 'has left imprint on the manner In which ozoic ,'hOltenlng is accommo­dated, resultin in a broad zone of deformation with­out a \' ell-defio d, tectonicall active orogenic nt

ConIiguous and Broken Foreland Bo.tin 511

and unsystemat'c lateral growth (e.g" Al1men­din er et aL. 1983; Grier et al.. 1991; Man nd Hongn 1991;Reynold etal.,2000;Carrera tal.• 2006; Ramos el aI.• 2006; Hongn et aI., 2007; Hilley and Coutand. 2010). The en foreland in Argentina therefore constitutes a morphotectonic province with spatially and temporally dispanI1e range uplifts and intervening basins, a setting akin to the hydrologically i olated. adjacent basins of the Altiplano-Puna in the orogen interior (Alonso et aL, 1984, 1991; Jordan and Alonso. 1987; Kraemer et al. 1999; Canapa el aL. 2005).

Th differences in deformation style in the cen­tral Andes have fundamentally impacted the flu­vial an deposit'onal ystem. Within, and to th. easlofthe Boli . n fold-and-thrustbeh, deposition occurs within a wedge top that transitions i to the und formed Chaco foredeep deposits to the ea t (Horton and DeCelles. 19 7). These foredeep deposits tb.i towards the east and are inferred to thick n again to the east of the forebulge high (Horton and DeGell ,1997), s redicted by the fl ure in urred by lopographic loading of the advancing fold-and-lhrust belt (e.g., DeCelle and Giles. 1996). Inswk ontra t. the depositional system within northw tern Argentina generally consis of variably connected. patti lly separate depoe nters, whose ins are primarily defined by ba ement uplift long crustal anisotropies. Unlike th foreland basin ystem within Boli . or in the northemmost part of the Argentine S b­andean province. Lh.is region clcs a deep and lateraD extensive foredeep and only co tams subdued back- and forebulg areas. For example. In the Santa Barbara system the foreland deposits have a thickness ofappro imale. y 3000 m (Crista!­lini el al.. 1997; Bossi et al.. 2001). In the ition to the Suband an ranges. sedimentary thie e increases (Reynolds et aL, 2001) and reaches more than 7000 m in the Argentine and southern Boivian s tors of the Subandean fold-and-the t belt (E .havarria 1 I., 2003; Dba et aI., 2006). The hydro ogic connectivity of basins within this broken foreland has ded over geologiC time. The generall arid to sub-humid basins vir­tually all show e idence of several transi all

between internal and external drainage (Strecker et aI., 1989, 2007. 2009; Mortimer et 1.. 2007). uggesting complex relationship et een tec­

tonic movements. drainage adjustments. and cli­mate (Starck and Anz6tegui, 2001' Hilley and Str er, 2005).

512 Part 4: Convergent Margins

TOPOGRAPHIC A D GEOMO PHIC CHARACTERISTICS OF THE EASTER ANDEAN MARGI IN BOLIVIA AND NW ARGENTINA

The contrasting deformational style observed within Bolivia and northwestern Argentina is clearly revealed in the topography of ilie e two

. Deformation within the Bolivian fold-and­thrust belt is characterized by laterall continuous thrust faults iliat have 6Ulfaced from the mechan­ically w decollement horizons. The along-strike continuity of thrust-fault geometry likely results from lateral homogeneit a the nderlylng trata (e.g., Allmendinger et al., 1 83; Sempere et a1.. 1990; Hilley and Coutand, 2010). creating virtually uninterrupted ridges oriented perpendicular to the r ional shortening direc 'on (Figs. 25.2 and 25.3A). This pattern of topography can be mo t clearly seen by examining the va . bility of eleva­tions between adjacent points. In Bolivia, eleva­tions perpendicular to the shortening clirec .on or alan strike of the orogeu are well orreiated in space (over length scales of >50km; Fig. 25.4A). Elevations parallel to the shortening dire. ion or parallel to the lopograp'c dient are far less consistent. Thus, following a canto line in south­ern Bolivia allows OIle to move long the trend of l e orogen over great distanc • with interruptions only DC rring in antecedent river valleys CUlling through the lopography. In addition, the mech ies of fol -and-thrust belt deformation favors a geom­etry in which the surlace of the orogenic edge thicken toward the back of the belt and tapers toward its fronl (e.g., Dahlen, 1984). Thus, while

these fold-and t-faull related ridges form sig­nificant topograp 'c variations along the direction of shortening, the mean topographic slope along -300 kIn of the fold-and-thru l bel is quit consis­tent (Fi . 25.4C).

In contrasl, within northwestern Argentina, ele­valio are much I sCalisi lenl over long I ngth scales as shortening is accommodated within dis­crete basement ranges. In these areas, elevations are typically correlated 0 er distances of -20km in the dire tion perpendicular to conv rgence {FIg. 25.481. The reverse-fault-bounded blocks are ar mora discontinuous than stru tura in the fold­

and-th lhelt, resulti g in the prevalenc ofslruc­tural aps throu 1 which drainage may be routed (Trauth et al., 2000; Strecker and Marrett, 199 ; Sobel tal., 2003). In addition. easterly moisture­bearing win may penetrate along large al eys into the int.erior of the arid orogen during naga 've ENSO years (Strecker et aI., 2007; Bookhagen and Streck C, 2008, 2010). Mean elevations profiled perpendicular to shortening Ii ow much greater variability and are characterized by the short wav ­length lopogra bic feature e, tending into the fore­land, which represent discrete uplifts along re livated structure (Fig. 25.4D).

The geomorphic development of basins within the deforming portion f thes areas reflects the different ways in which shortening is accommo­dated in the two d1ffenmt foreland types. In Bolivia, large, regional-scale drainage traverse the fold­and-thrustb !t. and a lrell' drainage pau rn route runoffbelween ridges (Fig. 25.5A). Al the orogenic fronl. megafans store sediment supplied from the orogen d are transitional with braidpIains ~ rther

Fl .25.3. Detailed topography and !Itru in the Subandean Cold-and-thrust belt in Bolivia (Al and the Dssocia Chaco Coreland basin system.. fB) Puna Plat u margin and Santa BarbaralSilm'1lll Pamp . morpho tructwal provinces.

Contiguous and Broken Foreland Basin 513

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Fia.1S.4. Variognuns oftopographywilhin (A) theSu dean fold-and-thru I belt. and (13) AI ipJano..Punamargln in no:rt.hw entina. showing the spatialcolT8lalion oCelwtionasa fun.ctionofdlrecti.onallagdislBnCa. his Iation function was computed by comparing the COlT8lalion bill y the ele atiODll of'ea.d1 pixel' the DEM and the correspondiDg pixel whose location is offset a specified diredlon and magnItude (die linn I lag dislllnce). In ' whlll8 elevations 8I1l similar at large distancm &omone Mother, co arillDcB values will be large. In addltion, the dia ams show hn elevations covary 8 n lion afdirectioo fromagiven point. Far pIe. covariance between elevations thai plll'Slst in 8 -S di on relative to th E-W dinlcti. at r distan s 8I1l n b the -5 elangati of large covariance v ues. The extent of tho lopograph U89d 10 ·ulate· iognuns in (Al an [Blis idllntl to thaI s in Figures 25.31. and 25.3B. respectivel .ID the Subandes. elllVations 8I1lCOI:I1llalucl in -S direction 0 r long (> 50-kmllength cal •whlll:llllS lopography Is correlated over distances of only ~20 kIn io the bto foreland. This illuslr8tes the discontiuuous nature ofilie mpography alo08 the Puna margin relative to thaloh rved within the )u fo!d-and-thrust bell. eel and (D) show Inngitu.dln topographic swath proB! aero s the Subandoan fold-and-thrust bait aod the AJti lano-Punn margin, re9pect'ively. d Jinas represent themllllJ1 topography, while dottedenvelopes bounding the top and bottnmof the pmfi1e denola the maximum and minimum topograpby within swath, l'lI5pectively.

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east (Horton and DeCelles. 2001; Wilkinson et al., 2010). egafan dep Us also constitule older sedimentary rocks t were deposited begin­ning at 8 Ma, now fomli.ng all integral part of the delomli.ng orogenic wedg (e.g.. Uba at al., 2006, 2007). In contras ,in Lhe regi n oftha Santa Barbara and ierras Pampeanas, evera! generations oItran­sient basin fills were deposited and re-excavated in all of the intermontane basins that are near the headwaters of rivers currenJ.1y draining the broken foreland basin system (Hilley and Strecker. 2005; Mortimer et aI., 2007; Strecker el al., 2 07). These sediments often onlap and ar sometim over­thrust by downstreambasement ranges that accom­modate convergence in this area (Bossi e aI.,2oo1). The transient basin fill have typic ly funned where easterly moi ture-bearing winds are pre­vented from reaching the basin headwaters by downwind basement ranges, which form efficient orographic barriers (Sobel and Strecker, 2003; Hil­ley and tracker, 2005; Coutand et aI.. 2006; Streckeretel, 2007; BookhagenandStrecker. 2008). Based on the results of analytical models (Sobel et el. 2003; Hilley and Strecker. 2005). the forma­tion oC int.em.a1 drainage and sediment ccu.mula­lion appears to be a threshold process in hieh active uplift of downstream lopographic barriers steepens channels thatlraverse these ranges, while aggradation behind the uplifting range must keep pace with the rise 0 the channel bed as rock upl'ft occurs. Internal drainage is favored as rod uplift rates increase. and bedrock erodibility and/or pre­cipitation decrease. In addition. in the case that channels l:r8versing the bedrock range prior to its uplift: are steep. external drainage is favored com­pared to a settingin wWeh initial channel slopes are low (Fig. 25.6: Sobel et aI., 2003). Conversely. low rock-uplift les in the downstream basement ranges. a high degree 0 erodibility of exp ed rock types, and pronounced rainfall gradients all promote incision. headward erosion, and basin capture. All these processes are conducive to sustaining fluvial connectivity with foreland (Hilley and Streck . 2005).

In contrast to the along-strike consistency of deformation in the Bolivian fold- d-thrust belt. deformation within the basemen range to the south is disparate and the location at which deformation occurs strongly depends on inher­ited crustal zones of weakness and the evolvin topographic load above the reactivated faults (Bossi et aI., 2001; Hilley et 0.1.. 2005; Kley et aI., 2005; Mortimer et aI., 2007; Jaffa et aI.,

ConUguou and BroJcell Foreland Basins 515

Externally DraIned

Increaslng lniti Channel Slope

Vis- ~.B. Sc.hamatic dspictiau oftha factors thatcontml th threshold between intema.l wuI external dmInage. The InCf'B8Sing val . along !he x-axis lflpnllM!1lt graatar initial channel lopes prior to uplift of d0WDtllJ189 topography. while the y-axis d plct3 the elli of increasing rack uplift rate,l'8Sistance ofbedrock to Iluvia! im:iaion, and!or arldlty. The oUd line shOWli the combinations 0 Initial channel slope and rack uplift tlIlll1aroslonal istancBIclImals that are raqui to produQl t-ins that are at the verge 0 internal draina Basins with rondltions that plot aboVll thB line are expected to eVUlltually become in1emally drained u uplift pl'OC8M1s doW115lrBaDl, hile thosB that plot belo the threshold !!De will maintain a connection with the foreland. How each of th8fllllBctars may be wed to predict cond.llions res.ultiDg in inten:La1 drainage. is pre­sented. in Sobel at al (2003)lIDd Hi1.Isy md Strecbr 12(05).

2011). In addition. the high-angle contractional structures in this area produc larger amounts of uplift for a given increment ofshortening than the low-angle thrusts in Bolivia. This may facilitate the rapid removal of cover sediments Crom atop the rising basement-cored mountain ranges. thus exposIn lithologies resistant to erosion (e.g.• 'obel and Strecker. 2003). Finally, owing to the almo phericci.rculation patterns in thi partof the

nde • the basement- orad range may concen­trate moisture on their windward sirles. Forexam­pIe. the easte flanks of these b sement ranges receive between 1.5 and 3 miyr of rainfall, while the intermontane buins often ree ive < 0.3 miyr (e.g.. Bookhagen andSIseck.er, 2008). This leads to the aridification of the headw tars ofbasins in the lee of the rising topographic barrier. This may conspire to cause a transient (Hilley d Strecker. 2005; Strecker et aI .• 2009) or permanent (Alonso et al, 1991; Vandervoort et aI., 1995; Hor­ton et aI., 2002) decrease in erosional power of the rivers d.ralning these basins. redUcing the likeli­hood of fluvial connection with the foreland

The discontinuous nature of deformation within northwestern Arge tina ba. produced spatially disparate and diachronous ·tarage of sedim nts upstream of the uplifting ranges (Bossi and Palma. 1982; Seer et aI., 1989, 2009; Hille and Strecker. 2005; Carrapa at aI.. 2009). Within basins flanking the margins of the Puna Plateau. conglomerates SOllrc d from the Puna margin and ranges su· unding the e ba ins a sometimes intercalated itb lacustrine sediments s well as tephms that constrain the timing of deposition of these sediments. The diapar la nature of the ba in

Is suggests that local condi 'ons, such as rock uplift within the surrounding ranges, rock erod­ibility. microclimate surrounding the indi 'dual basementranges, and/or diIferlngbasin geometries may playa larg r role in controlling basin filling than regional factors. such as mesoscale climate change that are correlated in space and time (Stre :ker et a1.. 2007).

At first sight. the short strike lengths of the faults bounding the uplifts io the broken foreland may be expected to aid the fonnatit1n ofan external drainag system as ri eIS would have to flow a ortar dis­tancealongstrik inord rtocircumventthegrowing inverteds c.tmes. Conve) ely, in the farel dfol­and-thrust belt extensive ridges paralletlo the trend of the orogen should favor internal dminage condi­tions. owever, rainfall and discharge amounts as well as the loci of maximum precipitatiorl in the t morpbotectonic provinces are very different and' .uenc emsion processes . 1 a fundamental way. Whereas pre 'pitation is higher and much more focused in an elevation sector between 1500 and 2000 ill along the orogenic wedge in the fold­and-thrust belt, it is more wid ly d' tributed in the broken foreland in north e tern. Argentina

ookhagen and Streck ,2008). This infiuences erosional efficiency, an assessment hich is sup­ported by differing specific lre -power v lues in both regions (Bookhagen and trecker. 2010). Specific strea power is generally bigher in the regions the orth, where high erosion mles cor­relate with areas of high rainfall, high relief. and steep uvial channels (Safran el al.• 2005), wh reas 10 ar value characterlz th broken foreland are . This trend may be e cerbated during nega . e

SO years. when a threefold increase in rainfall may occur (Bookbagen and Strecker, 2010). As the

SO system has ex' te at least smca the Pliocene (e.g.. Warn et a!.. 2005). a protracted influence of the associated precipitation and rasion character­istics on landscape evolution and edlmentary

processes is to be expected. Thus, the combined effectE of dilferences in climat and efficiency of erosion, a ell as the contrasting natllte of defor­mation between the fold-and-thrust belt and the broken foreland cause the topography and geomor­phic de elopment of the southern region to be far more discontinllous than the northern region.

DlFFEK.E TIAL FLEXURAL ESPONSE TO TOPOGRAPHIC LOADING ALO G THE CB TRAL A DES

The de alopmeot of foreland basin systems pri­marily results from the accommodation space cre­ated by crustal- cale flexure (e.g.• Jordan. 1981). Th.I crustal-scale flexure s caused largely by long­wavelength topograp ic loading within the orogen's inLerior. which is transmitted to the adJa­cent forelan through elastic stresses within the crust. In for 1 nd hasins. topographic loading within a wide fold-and-thrust belt can deform the cr I at greal distances from the orogenic front. leading to the formation of foredeep, fore­bulge. and backbulge accommodation zones. How­ever, in broken foreland basi uch as those dis ased bere. rock uplift within areas bound d by weak, reactivated structures may form steep mountain ranges restricted in both their alon ­and across-strike dimensions. Because the spatial extent and amount of accommodation spac cre­ated by flexurall ading scales ·th the wa etength ofule load (e.g., Turcolle and Schubert 2002). we might pect that ese types of shurt-wavelength features c ate less accommodation space than their longer-wavelen th counterparts.

Figur 25.7 illustrates a model of the flexural accommodation that might be ca ed by the mod­ern mean topograph acros..<; a typical longitudinal transect of tlle Bolivian fold-and-thrusl belt (Fig. 25.7A) and the central ndean broken foreland (Fig. 25.7B). The flexure created by the topography that lies abuve aselevel i denoted by d hed lines for a the case of a stiff lithosphere (8e ral rigidll • D=7 X 1023 N m; Horton and DeCelles, 1997) and by dott d lines for the case of a soft lithosphere (D =2 )( 1012 m). This flexural model is one­dimensional, where the out-ol:'plane extent f the mean topography is assumed to be large compared

ith the swath width. While this as umption is likely reasonable for the laterally continuous old­and-tllIust belt. the along-strike discontinuities of the broken foreland lead to an overestimation of the amount of accommodation space that is created by

Contiguous and Broken Fore/and Basin 517

4km (A)

o

4km (8)

.. ­ _.. ~--_::"': -_ ..........•..... ----_ .. -.-.

--4

a 200 400 600 llOOkm

(e) 4 k.m - - - 0" 7ll1()Z1 N m

..--.- D,,2lC10Z"Nm

0 ..... ,.'

.'1....."

--4

o 200 km

Distance AJoog Profile

FiS- ~.1. FI ural aa:mn.modatian. respons hued the observed mean topography along the (A] SuhaD:dean fold t.luust. bait and associa fomlan basin systam. m.d (B) Puna mtugin and ociatsd broken foreland in syst.enl. and Ie) aC'OSS Sierra cooquija at approxi.mately 2 Slat. 1n A. Band C ;It axis COJ:reSponds to dist.lm.ce &l0l18 proWe and y axis corresponds to elavation of ll8D8ects indicated iII Fig. 25.2. We use the topography ext.nlcted from topographic swaths through th e two (ahownlnFig. 25.2A) with implici finita-dJ.fference scheme that calculates the daflsction of the crust for a given mantla an crustal dllll5ity (33OOkglm~ and 27ookglm3. respectivel.) and D rigidity (Eh3/ (12(1- ,,2), wb.e.re E. h, md " the Young's mod \lB, elastic thick:nBss, and Paisson's l1Itio of the lit. respectively (Turcotte and Schubert, 2002). W ums E = 50 CPa, h = 49 an 15 km, and n =0.25 for illustratio.l1). while B.xing dispLacem9D.l.ll and rotations at the!igh l!.lld of the model to :tBto. The left boundary was we:nded by a facLor of 0

from the location shown, 8.Ilda topographlcloadequal to that exerted atth.sleft ide ofthe profile was u.nilinmI applied to the area. 'I'Ws iJlsured that any lsost.a1icaJ.ly uncompensated porliO.ll of the Puna-Altiplano plat88WI' I ds tmnsm1tt.ed into the foreland. The Ihill-plale l1exu.nI BqWl.UO.ll (wilh.1lQ and load) was solved using an Implicit finite-d.iffereoce 5Chem u.sing the topographic load abo hese-1svBl (defined by the al vatiO.l1 at the flll.9tllm ends of the m88ll ~les). The flexural rigidities thet result from different ·wned aJastic thickn are D= 7lC 10n Nm md D=2>< loU m.. The mean topography is shown IIlI a solid Una iII (A...{:;). while the calculated ~ of the sed.imeIlts in th subsiding in is hown lIS a dashed and dotted line for the still'and soft crustsamarlGl5. respectively. In the Suband .1l1lJCWll! accommodatinn

nwc.hes farthar into th foreland and Is deeper than in th Santa Ba1bant and the Sierras Pamp8llllllll sysllllDS, owiug to the broad gllOlIl8Uy of th £oId..and-thrw t be.1t.

flexural loading. onetheless. the two end-member topographic load that is transmitted far into the mode from the Andes shO\ n here clearly illus­ South 'can craton. Fl.exmal ub idence adja­

te the key difference in the creation of accom­ cent to the fold-and-thrust belt forms accommoda­modation space in these two structural settings. tion space for the foredeep depozane. wb.i.le uplift of

In the C8.S8 of the Bolivian Subandes. the broad the [orebulge .....450 fromlbefronLoflheorog nie gently tape fold-and-thrust bell creaLes a wedg creates an area where either mat:eri' is

518 Part 4: Con~t'li nt Margins

eroded or depositi niB limited. F e. ml subsidence within lha baclchulge basin may create some addi­tional accommodation space for sediments. although deeper geodyna . processes such as manUe down­we . Ig within the asthenospheric w dge y augment subsidence in this case (Mitrovica et a1.. 1989; DeCelles and Giles, 1996). A the lithosphere becomes less rigId. the fore eap becom s more reslricted in its spatial extent as deflection of the basin bollom flU from the top graphi front reflee isostatic, ratber than flexural compensation of the gen sloping topography.

In e case ofthe NW Argentine broken foreland basin, the flexural response Is fat more subdued for both rigidities investigated (Fig. 25.7B) due to the short- a elength nature of the topography east of the plateau margin. The tapered load in Bolivia imparts a larger force over the wave­lengths that are nexurally compensated than the more abrupt load of the steep Argentine margin. Thls leads to a foredeep that is more re tricted in Argentina than in Bolivia. In additi n. flexural subsidence may occur adjac nt to individual basement ranges, sue a that adjacent to Sierra Aeonquija [Bossi et al.. 2001; Cristallini et al.. 2004; Fig. 25.7C) or imIDe iately to the north alongCumbres Calchaquies (de Urrei2tieta, 1996). However, the spatial extent of the sur­roundIng depocenters is limited re ti e to the foredeep (Fi . 25.7B). In the case of . tiff crust, little ac ommodalion space i created b the lopO­graphiC load of these features. The abrupt topo­graphic feature can locany create accommodation space ofup to -4 Lan when the rigidity ofthe ernst is low, Se' mic reflection data from the Sierra Aconquija area suggest that Cenozoic sediments may reach a thickness of -3.5 kID adjacent to the range (C . tallini at aI., 2004), uggesling that in these area , crustal rigidity n y in fa t e 10 . Indeed,low elastic thiclmess alues stimated for this region upport thi inferenc [Tas ara at al., 2007). For such a weak crust, the principal pattern of accommodation space mimics that of the d formation. In the n rthw tern Argentine Andes, flexural ccommodation may thus be sp ­Li lly limited and di continuous, and the location of basins may ultimately be controll d by the distribution of reaell vated zone of ustal weak­ness (Hilley et al., 2005; Kley et al" 2005; Hongn et al., 2007; Hain et a1.. 2011). In addition, due to the link between cru tal heterogeneities and th ir compressional reacti ation. the evolution of the broken foreland is unsystematic. diachronous.

and may adcUtionally depend OIl the local litho­static stresse exerted by the neighboring range uplifts. The individual depocenters created b. flex­ure maybe connected to up tream b sins fonne by secUment ponding behind ri ng basement ran es and the downstream regional baselevel b a system of fluvial channels that traverse the broken fore­land. However. a pronoun d fo bulge and back­bulge is subdued In such settings, as deflection of the crust primarily results from isostatic compen­sation over long wav 1 ngths, rather than the flex­ural ompensation that produces uplift in the forebulge.

In the central Andes, it is likely that both the geometry of the topography and the rigidity of the lithosphere play an important role in controlling the ere tion of accommodalio space. In geneml, in the Bolivian Andes, the Suba.llde fold-and­tbru. t belt is ch ra.cterized by a continuously, and gently, tapered topography. Additionally, the fact that deformation here i accommodate along sub­horizontal detachment horizons in the sedime ­tary basin suggests that crustal weaknesses may be Ie s prOIlOuoced than farther. uth in Argentina. In contrast, the steep Andean front in Argentina i aecon panied by basement uplifts suggesting that pre- xisting reactivated structures weaken the cru. l. Both the differing topo phle loading and the likel lower flexural rigidity oCthis se tor oIthe

des favors the creation of local accommodation spac in a broken forel nd Lting. rather th n a can . u us foreland basin system as farth north in Bolivia.

CO CLU ION

The accommodation sp ce wilhi for land basin systems that ultimately serves as a ediment depo­centers is created primmil b the flexural response of the crust t th topographic load of a fold- d-thrust belt. 1n bra en foreland sellings, deformation may be accommodated far inlo the foreland by reactivation of pre-existing crustal weaknesses, producing sleep but short-wave­leng topography within both the orogen and the foreland. The discontinuous nature of this defo Lion and potentially rapid rock uplift rates r llve to those within fold-and-thrust belts favurs sediment pondlng b bind active

ountain ranges buill atop old. reactivated geo­ogic structures. Inde d. many sedlmentary basins

located within the headwaters of fluvial systems draining the rna 'n of the Altiplano-Puna Plateau

ow clear evidence ofmultiple episodes of filling as downstream ranges experience rock uplift accommodated alon reac' ted extensional structures. Rower, due 0 the proximity of these basins to mountain flan 'S with high precip­itation and efficienl headward erosion. the e b in: are often r -excavaled and attain fluvial connectivity with the remainder of the foreland.

The limit d long-suO e extent of horl-wave­length topog pby produced by upllft I ng old structure in the central Andean broken foreland syslem of north e tern Argentina produces spa­tially restrict d accommodalio space relative lo that observed farther north in the Bolivian fold­and-thrust bell. Instead. flexural basins occur directly adjacenl to individual ranges and are limited in spatial exte t relative to the foredeep depozone 0 uninterrupted foreland basins. In addition. the limited flexural response lo short­wavelength topographic I ads may fail to creale a welL·defined [or bulge or backbulge depozon . Thus, the application of existing foreland basin models to broken foreland hal in systems i limited. The e systems should be viewed in a context that considers their distinct deformation style and topography relative to typical foreland basi settings.

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