Brian K. Horton

The modern foreland basin system adjacent to the Central Andes

Regional variations in sediment thickness, internal structures, average elevation, and Bouguer gravity define a four-component foreland basin system adjacent to the Central Andes. In the most proximal part of the foreland basin system, the eastern Subandean zone and westernmost Chaco Plain, 1–3 km of Cenozoic deposits overlies active folds and thrusts of the frontal Andean orogenic wedge. These wedge-top deposits pass cratonward into a foredeep depozone containing a 3–4-km-thick sedimentary prism that tapers toward (and locally pinches out against) a broad-wavelength forebulge in the central-eastern Chaco Plain. The forebulge is underlain by Precambrian–Mesozoic rocks and is largely covered by a thin veneer of Quaternary alluvium. East of the forebulge, a thin (0.5 km) saucer-shaped accumulation of sediment beneath the Pantanal Wetland represents a back-bulge depozone. Ancient counterparts of these four depozones can be identified in the Central Andes, suggesting that modern basin architecture is the result of continuous, eastward migration of the coupled orogenic wedge and foreland basin system since the Late Cretaceous–Paleocene.

Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia

A >2.5-km-thick succession of Tertiary strata in the Eastern Cordillera of southern central Bolivia consists of predominantly fluvial and lacustrine deposits. The age of the base of the succession is middle Paleocene, and its upper part (which is erosionally truncated) is probably late Oligocene-early Miocene. The lower 50-130 m of the succession consists of interbedded fluvial sandstone and mudstone (including minor paleosols) of the Santa Lucia Formation. These strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the Santa Lucia Formation and lower part of the Impora Formation that may represent much of late Paleocene-Eocene time. Above the paleosol zone is a thin zone (∼10 m) of lacustrine carbonate and clastic rocks in the uppermost Impora Formation, and this in turn is overlain by a >2000-m-thick, upward-coarsening succession of clastic fluvial deposits in the Cayara, Camargo, and Suticollo Formations. Paleocurrent data indicate that fluvial channels that deposited the Santa Lucia, Impora, and Cayara Formations flowed mainly westward, whereas channels responsible for the Camargo and Suticollo Formations flowed generally eastward. Modal sandstone petrographic data show a long-term evolution from subarkosic (during Santa Lucia deposition) to quartz-arenitic (during Impora and Cayara deposition) to sublitharenitic (during Camargo and Suticollo deposition) compositions. We argue that the lithofacies and sediment-accumulation history of this succession are most consistent with deposition in an eastward-migrating foreland basin system. E -mail: decelles@geo.arizona.edu. Critical to this interpretation is the zone of extreme stratigraphic condensation in the lower Impora Formation, the inferred upper Paleocene-Eocene part of the succession. The abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous, postrift thermal subsidence resulted in a gradual, continuous decay of early Tertiary sediment-accumulation rates. Stratigraphic condensation is, however, consistent with passage of the forebulge through the region, an interpretation that also can be reconciled with the record of foreland basin development in the Altiplano to the west and gross sediment accumulation patterns in the Subandean zone and modern foreland basin to the east. Simple flexural modeling and palinspastic reconstruction of the complete Cenozoic migration history of the foreland basin system suggest that ∼1000 km of foreland lithosphere has been underthrust westward beneath the Andean orogenic belt at the latitude of southern Bolivia. The amount of middle and lower crust that would have been added to the Andean infrastructure is sufficient to explain the present crustal thicknesses in Bolivia.

Paleogene synorogenic sedimentation in the Altiplano plateau and implications for initial mountain building in the central Andes

Sedimentologic data and palynological ages from Paleogene clastic deposits of the northern and central Altiplano plateau suggest foreland basin development in the central Andes by mid-Paleocene time. The nonmarine Potoco Formation (3000‐6500 m thick) constitutes the majority of Cenozoic basin fill. The Potoco overlies the Santa Lucia Formation (50‐300 m thick), previously dated as mid-Paleocene by mammal fossils and magnetostratigraphy. New geochronologic data for the Potoco Formation include late Eocene to Oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels. These ages and published 40 Ar/ 39 Ar and K-Ar ages from overlying upper Oligocene‐lower Miocene volcaniclastic rocks indicate (1) nondeposition or greatly reduced deposition (average sediment-accumulation rates ,10 m/m.y.) from mid-Paleocene to middle Eocene time (top Santa Lucia to lowermost Potoco Formation), followed by (2) rapid deposition (average sediment-accumulation rates up to 500 m/ m.y.) throughout late Eocene and Oligocene time (majority of Potoco Formation). Lithofacies and paleocurrent data confirm both depositional phases. A 20‐100-mthick interval of superimposed paleosols in the basal Potoco supports reduced sediment accumulation and low rates of subsidence during mid-Paleocene to middle Eocene time. The overlying main body of the Potoco contains facies assemblages and stratigraphic architecture (dominantly nonero

Erosional control on the geometry and kinematics of thrust belt development in the central Andes

Long-term erosion rates in the central Andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt. A drastic along-strike erosional gradient exists in the modern central Andes, from a high-erosion region directly north of the 17.5°S bend in the Andes to a low-erosion region south of the bend. This gradient has existed since ∼10–15 Ma based on fission track analyses of middle Miocene to Holocene denudation and qualitative evaluations of the preservation potential of middle-late Tertiary volcanic edifices and synorogenic sediment. Global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17.5°S bend. Whereas present-day shortening in the thrust belt north of the bend is distributed over much of the width of the belt, south of the bend it is concentrated near the eastern frontal margin. Structural data suggest a similar kinematic situation during late Miocene to Holocene shortening: an out-of-sequence chronology of thrusting in the narrow (200 km wide) thrust belt to the north versus a forward-breaking sequence of thrusting in the relatively wide (350 km wide) thrust belt to the south. The long-term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust-wedge conditions, induced by rapid erosion rates since ∼10–15 Ma. Such conditions inhibit thrust-front advance and favor distributed deformation within the thrust-belt interior. In the thrust belt south of the bend, a progressive eastward migration of thrusting is interpreted to be the result of long-term critical thrust-wedge conditions promoted by extremely low rates of denudation since middle Miocene time.

Structural evolution of the Yushu-Nangqian region and its relationship to syncollisional igneous activity east-central Tibet

Field mapping, geochronological analyses, and cross section construction reveal a protracted deformation history and a minimum of 61 km of Cenozoic NE-SW shortening (in present coordinates) across the Yushu-Nangqian thrust belt in northern Tibet. Cenozoic contraction started prior to 51 Ma and was followed first by northwest-striking right-slip faulting and later by northwest-striking left-slip faulting. Renewed NE-SW contraction is expressed by folding of Neogene strata and thrusting, which again was followed by northwest-striking left-slip faults. Late Neogene deformation is expressed by local north-striking normal faults. Shortening across the Yushu-Nangqian belt appears to be accommodated by thin-skinned thrusting, which raises the question of how the deformation was accommodated in the lower crustal levels. To resolve this problem, we perform geochemical analysis of igneous rocks dated as 51–49 and 38–37 Ma. The rocks exhibit geochemical signatures characteristic of subduction, which implies that coeval crustal thickening in northeastern Tibet was most likely induced by continental subduction.

Paleogene clockwise tectonic rotation of the Xining-Lanzhou region, northeastern Tibetan Plateau

[1] To help understand the deformational history of the northeastern Tibetan Plateau, paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among Upper Jurassic-Lower Cretaceous to Pliocene sedimentary and subordinate volcanic rocks within the Xining-Lanzhou region (34–37� N, 101–105� E). A total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold, reversal, and conglomerate tests. Age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by Horton et al. [2004]. Analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region. Whereas 19.0 ± 7.2� to 37.8 ± 10.6� clockwise rotations are recorded consistently in all paleomagnetic localities in Lower Cretaceous to Eocene rocks, all paleomagnetic localities in Oligocene to Pliocene rocks have recorded minor to insignificant rotations, indicating that the Xining-Lanzhou region has undergone a wholesale regional clockwise rotation during late Paleogene time. Consistent with regional chronostratigraphic and thermochronologic results, this late Paleogene tectonic rotation confirms that deformation reached regions of the northern Tibetan Plateau shortly after the initial collision of India with Asia. When compared to rotational paleomagnetic results from adjacent regions, several mechanisms can be proposed to explain the clockwise rotation. On the basis of consistency with geologic data we prefer a model involving clockwise rotation of the Xining-Lanzhou region through right-lateral shear, and associated shortening, related to northward indentation of the Qaidam basin. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8102 Tectonophysics: Continental contractional orogenic belts; 9320 Information Related to Geographic Region: Asia; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: tectonics, paleomagnetism, Tibetan Plateau

Mesozoic‐Cenozoic evolution of the Xining‐Minhe and Dangchang basins, northeastern Tibetan Plateau: Magnetostratigraphic and biostratigraphic results

[1] Accurate stratigraphic ages are crucial to understanding the deformation history of the Tibetan Plateau prior to and during the Indo-Asian collision. Efforts to quantify MesozoicCenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata. Magnetostratigraphic and biostratigraphic results from the Xining-Minhe-Longzhong basin complex and Dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern Tibetan Plateau (34–37� N, 101–105� E). Analyses of 171 magnetostratigraphic levels and 24 palynological assemblages (>120 species) indicate Late Jurassic-Early Cretaceous to mid-Tertiary deposition. Although magnetic polarity zonation is incomplete, independent palynological age control partially restricts possible correlations to the Geomagnetic Polarity Timescale. The sediment accumulation record, basin provenance, structural geology, and published thermochronological data support a history of Jurassic exhumation, Late Jurassic-Early Cretaceous fault-related basin initiation, and Cretaceous-Paleogene reduced accumulation. These patterns, which are compatible with Late Jurassic-Early Cretaceous extension and Cretaceous-Paleogene postrift thermal subsidence, were disrupted at about 40–30 Ma, when shortening related to the Indo-Asian collision induced localized range uplift, vertical axis rotation, and amplified subsidence. INDEXTERMS: 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 8102 Tectonophysics: Continental contractional orogenic belts; 9320 Information Related to Geographic Region: Asia; 9604 Information Related to Geologic Time: Cenozoic; 9609 Information Related to Geologic Time: Mesozoic; KEYWORDS: tectonics, magnetostratigraphy, Tibetan Plateau, Cenozoic, Mesozoic, sedimentary basins

Paleocene-Eocene syncontractional sedimentation in narrow, lacustrine-dominated basins of east-central Tibet

Sedimentologic, stratigraphic, compositional, and structural data from four elongate basins ( 30 km long) in the Nangqian-Yushu region of east-central Tibet (near the headwaters of the Mekong and Yangtze Rivers) indicate nonmarine sedimentation synchronous with Paleocene–Eocene northeast-southwest shortening. Sedimentation in the Nangqian, Niuguoda, Xialaxiu, and Shanglaxiu basins involved (1) mud and carbonate accumulation in offshore to nearshore lacustrine environments and (2) gravel and sand deposition in fan-delta to alluvial-fan environments localized near basin margins. Growth strata in both fine- and coarse-grained deposits, primarily in upper levels of basin fill, provide evidence for sedimentation contemporaneous with motion on fold-thrust structures. Provenance data collected from 25 measured stratigraphic sections, including >1500 paleocurrent measurements and >150 conglomerate compositional analyses, show that each basin was fed sediment from several directions by proximal source areas composed of Carboniferous–Triassic rocks. The source-area proximity and a stratigraphic variability among the basins suggest that each basin evolved independently and was filled by sediment derived from relatively small drainage networks (<103 km2). Age control for basin fill is based on Paleogene fossils, 38–37 Ma 40Ar/39Ar ages from volcanic rocks interbedded with uppermost strata of the Nangqian basin, and 51–49 Ma 40Ar/39Ar ages from igneous rocks that intrude and unconformably overlie strata of the Shanglaxiu basin. Strata containing middle Cretaceous palynomorph and ostracod assemblages are present only locally in the lowermost part of the Nangqian basin. Although the tectonic setting for Cretaceous sedimentation is unclear, early Tertiary basin development was controlled by thin-skinned fold-thrust deformation. We interpret the narrow widths of Paleocene–Eocene basins to be a result of thrust spacing, which in turn may have been controlled by the depth to the decollement (∼5 km deep according to our balanced cross section) from which imbricate thrusts ramped up through the Carboniferous–Triassic section. Sedimentologic and provenance evidence for internal drainage, limited unroofing, and relatively low average sediment-accumulation rates in these syncontractional, plateau-interior basins indicates generally small drainage systems, short main-stem rivers, shallow regional slopes, and limited denudation in east-central Tibet during early Tertiary time. Such geomorphic conditions, which are similar to the modern low-relief interior of the Tibetan plateau, suggest that the deeply incised Mekong and Yangtze Rivers of eastern Tibet were not established until after the termination of Paleogene basin development in the region.

Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic–Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520–420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200–1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syn depositional magmatic sources (420–380 Ma), and distal sources of chiefl y Mesoproterozoic basement (1650–900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic–Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous–Permian (310–250 Ma) and Late Triassic–Early Jurassic (220–180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased infl uence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050–950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850–1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous–Paleocene (90–55 Ma), Jurassic (190–150 Ma), and limited Permian–Triassic (280–220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene–Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180– 40 Ma) and shield-derived sediments (mostly 1850–950 Ma). Late Miocene–Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic– Cenozoic zircons and increased proportions of 1650–900 Ma zircons emblematic of Cretaceous strata.

Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia

A large volume of Tertiary synorogenic sediment accumulated on top of the Eastern Cordillera of southern Bolivia as the Andean orogenic wedge was shortened, thickened, and uplifted. Oligocene to upper Miocene strata were deposited in five basins that were separated by active, north-trending, fold-thrust structures of the then-frontal part of the orogenic wedge. These coarse-grained deposits recorded accumulation in the most proximal sector of the Andean foreland basin system, the wedge-top depozone. Analyses of depositional systems, sediment dispersal patterns, and clast provenance of 0.6‐2.3-km-thick, Oligocene to upper Miocene wedge-top successions demonstrate that faultpropagation and fault-bend folds commonly isolated individual basins while serving as primary sediment sources. Growth strata which formed by progressive fold-limb rotation indicate thrust-fault displacement and related folding concurrent with deposition. Alluvial fans defined most basin margins, whereas braided streams or small lakes occupied basin axes. Diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins. Growth strata and crosscutting and onlapping relationships between contractional structures and wedge-top strata delineate the chronology of fold-thrust deformation in the Eastern Cordillera. Five new 40 Ar/ 39 Ar dates and previously published K-Ar dates, ranging from 30 to 8 Ma, define an Oligocene phase of west-vergent backthrusting followed by primarily east-vergent thrusting during Miocene time. Timing of displacement on two east-vergent thrusts is determined by 40 Ar/ 39 Ar ages of tuffs within adjacent growth strata sequences of the Tupiza Formation conglomerate (16.14 ± 0.06 Ma) and Oploca Formation (13.33 ± 0.15 Ma, 15.7 ± 2.4 Ma). These ages, combined with basin depositional histories, demonstrate synchronous and out-of-sequence thrust displacement during Miocene shortening. Upper-crustal contractional deformation and wedge-top deposition terminated in the Eastern Cordillera during late Miocene time as the thrust front propagated eastward into the Subandean Zone. Continued thrust-front migration produced the present-day configuration in which Eastern Cordillera wedge-top basins, originally developed above the toe of the orogenic wedge at relatively low elevations, are now >250 km west of the active thrust front and at ~3 km elevation. The modern wedge-top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrustbounded, Subandean Zone basins and sediment overlying blind structures beneath the westernmost Chaco Plain. In general, wedge-top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted. Nevertheless, Eastern Cordillera wedge-top deposits have been preserved for ~10‐30 m.y. in southern Bolivia. Such long-term preservation may reflect the inability of drainage systems to remove sediment mass from this low-precipitation region of the central Andes. Retention of sediment mass within the orogenic belt may promote critical thrust-wedge conditions in which propagation of the thrust front is favored.