Lower Cretaceous Strata in the Lhasa Terrane, Tibet, with Implications for Understanding the Early Tectonic History of the Tibetan Plateau

Home , Acteonella, Lhasa terrane, Qiangtang terrane

Journal of Sedimentary Research, 2007, v. 77, 809–825 Research Article DOI: 10.2110/jsr.2007.078

LOWER CRETACEOUS STRATA IN THE LHASA TERRANE, TIBET, WITH IMPLICATIONS FOR UNDERSTANDING THE EARLY TECTONIC HISTORY OF THE TIBETAN PLATEAU

ANDREW L. LEIER,* PETER G. DECELLES, PAUL KAPP, AND GEORGE E. GEHRELS Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A. e-mail: [email protected]

ABSTRACT: Sedimentary strata in southern Tibet indicate that upper crustal deformation occurred throughout the region during Early Cretaceous time, suggesting that construction of the Tibetan plateau commenced tens of millions of years before the Late Cretaceous–early Tertiary Indo-Asian collision. Lower Cretaceous strata in the northern portion of the Lhasa terrane are characterized by lithic-rich conglomerate beds deposited in shallow marine and meandering-river fluvial environments. Sediments in these units were derived from two primary sources: volcanic rocks associated with Early Cretaceous intrusions, and sedimentary strata eroded from the northern Lhasa and southern Qiangtang terranes. The majority of detrital zircons from Lower Cretaceous fluvial conglomerate beds in northern Lhasa have U–Pb ages between 125 and 140 Ma and provide a maximum depositional age for these units of 125 6 2 Ma. Lower Cretaceous strata in the southern portion of the Lhasa terrane consist of mudstone, quartzose sandstone, and subordinate quartzite-pebble conglomerate beds that were deposited in shallow marine and fluvial environments. Populations of detrital zircons in Lower Cretaceous conglomerate beds in southern Lhasa have U–Pb ages between 140 and 150 Ma, 500 and 600 Ma, and 850 and 950 Ma, and provide a maximum depositional age for these units of 143 6 2 Ma. Both the modal composition and detrital-zircon U–Pb ages of the Lower Cretaceous conglomerate exposed in northern and southern Lhasa suggest different source areas, diachronous deposition, and possibly distinct genetic histories. Throughout most of the Lhasa terrane, the Lower Cretaceous clastic strata are overlain by a widespread limestone of Aptian–Albian age that was deposited in a shallow carbonate sea containing rudist patch reefs and muddy inter-reef zones. With respect to the tectono-sedimentary setting of the Lhasa terrane during Early Cretaceous time, the sedimentological and stratigraphic data are most consistent with a peripheral foreland basin model, which is interpreted to have resulted from the collision between the northern margin of the Lhasa terrane and the southern margin of Asia (the Qiangtang terrane). Several characteristics of the Aptian–Albian succession can be attributed to a peripheral foreland basin setting, although deposition within the region may have been influenced by a combination of mechanisms. Sedimentary characteristics of Lower Cretaceous rocks in the Lhasa terrane are consistent with recent ideas suggesting that portions of southern and central Tibet were deformed and above sea level before the Indo-Asian collision.

INTRODUCTION occurred during the Indo-Asian collision (e.g., Kong et al. 1997). In other words, understanding what has happened since the Indo-Asian collision In terms of its lithospheric-scale structure, the Tibetan plateau is not (ca. 55 Ma) is next to impossible without knowing the conditions of the a single entity, but rather an amalgamation of individual terranes that Tibetan plateau before the collision. accreted onto southern Asia in a series of collisions during Paleozoic and The Lhasa terrane is the southernmost terrane of Tibet and extends for Mesozoic time (Figs. 1, 2; Alle´gre et al. 1984; Dewey et al. 1988; Sengo¨r over 1000 km in an east to west direction and reaches 300 km from north and Natal’in 1996; Yin and Harrison 2000). Today, the individual to south at its widest point. It is bounded on the north by the Bangong terranes of Tibet are delineated from one another by , E–W trending suture and on the south by the Indus–Yarlung suture, which separates suture zones that are semicontinuous across the entire plateau (Liu 1988). crust of Asian affinity from that of Greater India (Alle´gre et al. 1984). Although recent investigations have suggested that the Paleozoic– The Lhasa terrane rifted from the northern margin of Gondwanaland in Mesozoic amalgamation of the Tibetan plateau was accompanied by Late Carboniferous time (Stampfli and Borel 2004), migrated northward, crustal deformation in the constituent terranes (Murphy et al. 1997), the and eventually collided with the southern margin of Asia during latest nature of this deformation is poorly understood. Determining these pre- Jurassic to Early Cretaceous time (Dewey et al. 1988; Stampfli and Borel Cenozoic events is crucial because the style and extent of deformation 2004). Approximately 100 Myr after the Lhasa terrane accreted onto that occurred prior to the Indo-Asian collision has direct implications for Asia, India collided with the southern margin of the Lhasa terrane, understanding the timing, location, and extent of deformation that marking the initiation of the Indo-Asian collision (Yin and Harrison 2000). Before and during its accretion onto southern Asia, the Lhasa * Present address: Department of Geosciences, Princeton University, Princeton, terrane was located between two subduction zones (Fig. 2A): one to its New Jersey 08544, U.S.A. north, where it was part of the downgoing plate, and one to the south,

FIG. 1.—Location and geology of the study area, including the locations of measured sections. Geologic map modified from Liu (1988). where it was part of the overriding plate (England and Searle 1986; along the southern margin of Lhasa that controlled the creation and Dewey et al. 1988; Yin and Harrison 2000). The nature of deformation infilling of accommodation during the Early Cretaceous (e.g., Zhang that occurred in the Lhasa terrane during this time period is poorly 2000; Zhang et al. 2004). constrained but important for reconstructing the history of deformation This paper presents new data on the composition, depositional facies, in the Tibetan plateau. detrital zircon U–Pb ages, regional relationships, and provenance of Despite widespread exposures of Lower Cretaceous strata in the Lhasa Lower Cretaceous strata in the central portion of the Lhasa terrane. This terrane, little is known of its Early Cretaceous tectono-sedimentary new information allows the paleogeography and regional history of the history. From what has been published, two very different hypotheses area to be reconstructed in far greater detail than what was previously have been proposed to account for characteristics of the Early Cretaceous possible. The sedimentary and stratigraphic data indicate that Lower sedimentary record (Fig. 2), but because the strata are largely unstudied it Cretaceous sedimentary rocks of the central Lhasa terrane were deposited has been difficult to evaluate their relative merit. The first hypothesis primarily in marginal marine and coastal-plain environments. Petro- postulates that the accretion of the Lhasa terrane onto southern Asia graphic and detrital-zircon data suggest that the Lower Cretaceous clastic resulted in crustal shortening, thickening, and surface uplift along the strata in northern and southern Lhasa were derived from different source Bangong suture zone, which ultimately produced a peripheral foreland areas and possibly have different depositional ages. Throughout the study basin in the Lhasa terrane (Leeder et al. 1988; Murphy et al. 1997). The area, an Aptian–Albian shallow marine limestone overlies the Lower alternative hypothesis acknowledges that some deformation may have Cretaceous clastic strata, indicating widespread marine inundation during occurred during the accretion of the Lhasa terrane but suggests that it was this time. With regard to the tectonic setting, the sedimentary and back-arc extension associated with subduction of Neotethyan lithosphere stratigraphic data are most consistent with a peripheral foreland basin J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 811

FIG. 2.—Regional tectonic setting and proposed Cretaceous basins. A) The Lhasa terrane moved northward during the Mesozoic and by Early Cretaceous time collided with the southern margin of Asia, which at that time was the southern margin of the Qiangtang terrane. B) An extensional back-arc basin model of the Creta- ceous Lhasa terrane (e.g., Zhang et al. 2004). C) A peripheral foreland basin model of the Cretaceous Lhasa terrane (e.g., Leeder et al. 1988). model, wherein the Late Jurassic–Early Cretaceous suturing of the Lhasa considered to be Lower Cretaceous although some deposition may have terrane onto southern Asia caused flexural subsidence in the northern taken place during the latest Jurassic (Yin et al. 1988). portion of the Lhasa terrane. The foreland basin is overprinted by arc Overlying the Duba Formation in the north are limestone beds of the volcanism that was associated not with subduction of Neotethyan oceanic Langshan Formation, and overlying the Chumulong Formation in the lithosphere along the southern margin of the Lhasa terrane. Reconstruc- south are limestone and marl beds of the Penbo Member of the Takena tions of the region during Aptian–Albian time are more ambiguous, and Formation (Yin et al. 1988). Between northern and southern areas, the although several basin models and tectonic settings can explain parts of limestone at the top of the Lower Cretaceous succession is time the depositional record, no single hypothesis can account for all aspects of transgressive (Fig. 3; Yin et al. 1988; this study). The Penbo Member in the strata. the south was deposited during Aptian–Albian time, and the Langshan Formation in the north was deposited from late Barremian to early STUDY AREA Cenomanian time (Fig. 3; Smith and Xu 1988; Zhang et al. 2004; Leier 2005). The Penbo Member is roughly 300 m thick, whereas the thickness The study area is located in the central portion of the Lhasa terrane of the Langshan Formation is poorly constrained and varies between (Kidd et al. 1988) and is broadly divided into northern and southern hundreds of meters to . 6 km (cf. Leeder et al. 1988; Zhang 2000). Based portions, with the lake Nam Co serving as a general border between the on Leeder et al. (1988) and our own observations, we use an , 1100 m two regions (Fig. 1). Lower Cretaceous strata in both the north and south thickness for the Langshan Formation in the northern half of the Lhasa contain a basal clastic succession and an upper limestone unit (Fig. 3). terrane. The Lower Cretaceous limestone units are overlain by arkosic red The clastic sediments at the base of the Lower Cretaceous succession in beds of the Takena Formation in the southern portion of the study area. the south overlie Jurassic limestone, shale, and local beds of volcanic In the north, the contact between the limestone and the Upper Cretaceous material, whereas to the north, the underlying Jurassic units consist of strata is not exposed. marine shale and sandstone (Leeder et al. 1988; Yin et al. 1988). Simplifying the stratigraphic nomenclature, the Lower Cretaceous clastic LITHOFACIES deposits in the north are referred to as the Duba Formation and those in the south are called the Chumulong Formation (Yin et al. 1988). We Lithofacies in Lower Cretaceous rocks of the Lhasa terrane are typical divide the Duba Formation into informal lower and upper divisions. The of carbonate and coarse-grained clastic rocks, and are well understood in clastic rocks of the Duba and Chumulong formations are generally terms of depositional processes. Accordingly, the following section 812 A. LEIER ET AL. J S R

FIG. 3.—Lithostratigraphy (left) and chronostratigraphy (right) of Upper Jurassic through Upper Cretaceous sedimentary strata in the Lhasa terrane of southern Tibet (modified from Yin et al. 1988). focuses on the genetic associations of lithofacies that can be more broadly Interpretation.—This lithofacies association is interpreted as deposits of interpreted in terms of depositional systems. a shallow marine shelf setting (e.g., Walker and Plint 1992; Orton and Reading 1993; Johnson and Baldwin 1996). The laminated mudstone is Fine-Grained Heterolithic Lithofacies Association (Northern Study Area) interpreted to have been deposited in a relatively quiet environment below fair-weather wave base, where fine-grained sediment was able to settle Description.—The fine-grained heterolithic lithofacies association from suspension. Sand- and pebble-size sediments were introduced into comprises the lower Duba Formation (Figs. 4, 5) and consists of black the offshore area during high-energy events by storm-induced currents laminated and massive mudstone with very thin to thick beds of gray and/or hyperpycnal flows associated with terrestrial rivers (Duke 1990; tabular sandstone and minor conglomerate. This lithofacies association is Orton and Reading 1993; Nemec and Steel 1984). Sandstone beds with present in the Duba region and in the Lunpola area (Fig. 1). The abundant oscillatory current ripples indicate that wave base was lowered mudstone contains siltstone laminae and is commonly bioturbated with contemporaneously with deposition of the coarser-grained material (e.g., horizontal to subhorizontal burrows. Sandstone beds are typically 0.15 Johnson and Baldwin 1996). Following the high-energy events, when low- meters thick but vary from 1 cm to over 1 m, and commonly have sharp energy conditions returned, mud-size particles were deposited from to erosional bases and sharp contacts with the overlying mudstone. suspension and organisms burrowed into the deposits. The coarse grain Unidentifiable bivalve fragments are present in several of the sandstone size of some of the sediment suggests proximity to topographic relief beds. Thinner sandstone beds tend to be fine to very fine grained and (Orton and Reading 1993). contain oscillatory current ripples and rare unidirectional current ripples. Those beds greater than 5 cm thick are typically fine to medium grained Red Sandstone–Conglomerate Lithofacies Association (Northern and have plane-parallel laminae and oscillatory current ripples. Locally, Study Area) thicker beds are structureless and contain granules and pebbles. Individual sandstone beds lack internal gradations in grain size but Description.—The red sandstone-conglomerate lithofacies association commonly have fine-grained caps. Vertical, subvertical, and subhorizon- consists of upward-fining sandstone and conglomerate units interstrati- tal unlined burrows are common in all sandstone beds. A few sandstone fied with massive and laminated mudstone. This association is best beds are moderately eroded by scour surfaces associated with overlying exposed in the Duba area where it is at least 1300 m thick (Fig. 5). clast-supported pebble conglomerate beds. These conglomerate beds are Sandstone and conglomerate units are generally between 6 and 10 m well organized and contain rounded and well-rounded clasts, some with thick, have lenticular shapes, and overlie basal scour surfaces (Fig. 6). poorly developed imbrication. Laterally along the outcrop, these sequences commonly divide into J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 813

FIG. 4.—Symbols used in the stratigraphic sections in Figures 5 and 7. individual sandstone beds that are separated from one another by thin 1964; Puigdefabregas and Van Vilet 1978; Smith, D. 1987; Smith, mudstone intervals (Fig. 6). The lower part of a typical sequence consists R.,1987; Miall 1996; Allen 2001). The fact that many of these accretion of a well-organized pebble-conglomerate bed (, 2 m thick) with sets dip orthogonally to the paleocurrent direction provides evidence that imbricated clasts, crude horizontal stratification, and weakly developed the channels were migrating laterally. A few of the accretion sets consist trough cross-stratification. The conglomerate beds occasionally fine of alternating beds of sandstone and siltstone, similar to inclined- upward into very coarse-grained pebbly sandstone with trough cross- heterolithic strata (IHS), which have been documented in both modern stratification. The upper part of the sequence is composed of coarse- to and ancient lower-coastal-plain fluvial deposits (Smith, D. 1987; Choi et medium-grained trough cross-stratified and horizontally laminated al. 2004). Crudely stratified pebble conglomerate was deposited by low- sandstone overlain by medium- to fine-grained sandstone with plane- relief bars along channel thalwegs (Hein and Walker 1977), and the parallel laminae and unidirectional current ripples. In some instances, the overlying trough cross-stratified sandstone beds were deposited by cross-stratification is relatively planar and has thicknesses of roughly 1 migrating 3-D dunes as the paleochannels filled and migrated laterally. meter. Internally, sandstone sequences locally contain large-scale cross- Intervals of massive mudstone that contain CaCO3 nodules are bedding (thicknesses . 2 m) consisting of sandstone and siltstone beds interpreted as paleosols that developed in a relatively well drained that dip orthogonally to the paleocurrent direction (as derived from floodplain environment (Mack et al. 1993; Kraus 1999). Thin, tabular, trough cross-stratification; see below). The uppermost parts of the rippled sandstone within the mudstone intervals are interpreted as sandstone and conglomerate sequences are commonly bioturbated or crevasse-splay deposits. massive and grade into red siltstone and claystone. The mudstone intervals are commonly mottled, and contain CaCO3 nodules, root traces, Massive Limestone Lithofacies Association (Northern Study Area) vertical burrows, and thin tabular beds of fine-grained sandstone with unidirectional current ripples and climbing ripples (Fig. 6). In the Description.—Limestone exposed in the northern portion of the Lhasa uppermost 100 meters of this lithofacies association, just below the terrane is classified here as a single lithofacies association in order to overlying limestone, the proportion of siltstone increases, conglomerate interpret the depositional environment at a regional scale, although beds are absent, and the sandstone contains marine bivalve fragments locally the lithofacies vary. The Langshan Formation in the Duba region (Fig. 5). (Figs. 1, 5) consists principally of laterally continuous, dark-gray to black orbitolinid wackestone and packstone (Fig. 5). The bulk of the bioclasts Interpretation.—This lithofacies association is interpreted as having are fossils of Mesorbitolina aperta and Mesorbitolina subconcava, but in been deposited by gravelly–sandy meandering rivers (e.g., Levey 1978; some beds there are also abundant gastropods (Tylostoma sp. and Nijman and Puigdefabregas 1978; Miall 1978, 1996). The sandstone and Acteonella sp.) and bivalve and echinoid fragments (Smith and Xu 1988; conglomerate sequences are interpreted as fluvial channel deposits, and R.W. Scott, personal communication). North of Duba, in the Lunpola the mudstone with CaCO3 nodules and thin tabular sandstone beds are area (Fig. 1), are thick-bedded, rudist-dominated, wackestone and interpreted as sediments deposited in floodplains adjacent to the channels. packstone which also contain indeterminate bivalve and cnidaria Regular, repetitive, upward-fining sequences like those in this lithofacies fragments and the calcareous algae Pycnoporidium sp. and Cayeuxia sp. association are most commonly produced by meandering rivers, as are the (R.W. Scott, personal communication). Locally, the rudist limestone is large-scale cross-beds, which are interpreted as bar-accretion sets (Allen interbedded with packstone and grainstone beds composed of ooids, 814 A. LEIER ET AL. J S R

FIG. 5.—Measured sections of the Lower Cretaceous Duba and Langshan formations in the northern part of the Lhasa terrane. Scale is in meters. J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 815 composite ooids, and oncoids. Massive peloidal and orbitolinid mudstone cation and occasional plane-parallel lamination. Subvertical to subhor- beds are the main lithofacies in the Nam Co area, although this mud-rich izontal burrows are common in the lowermost sandstone but are rare in lithofacies is present to some extent at all of the locations. the upper, trough-cross-stratified beds. The upper surfaces of the sequences are sharp, commonly bioturbated, and overlain by interbedded Interpretation.—Limestone of this lithofacies association is interpreted mudstone, siltstone, and sandstone of the overlying upward-coarsening as having been deposited within a shallow marine epeiric seaway that was sequence. dominated by low-energy lagoonal environments and localized patch reefs (e.g., Enos 1983), similar to the conclusions of Leeder et al. (1988). Interpretation.—Strata of the upward-coarsening sandstone lithofacies The common occurrence of peloidal and orbitolinid mudstone and association are interpreted as having been deposited in a wave-dominated wackestone indicate much of the region was occupied by muddy, low- shoreface environment that experienced repeated transgressions and energy, lagoonal environments (Leeder et al. 1988; Tucker and Wright regressions. The upward-coarsening sequences are interpreted as shallow 1990; Jones and Desrochers 1992). However, localized exposures of marine parasequences that were deposited during shoreline progradation rudist-dominated limestone and ooidal packstone and grainstone suggest (Van Wagoner et al. 1988; Van Wagoner et al. 1990; Posamentier and the former existence of patch reefs where energy levels were higher (Scott Allen 1999). Mudstone in the lower part of the parasequence is 1979; James and Bourque 1992; Wright and Burchette 1996). interpreted to have been deposited in an offshore transition zone, the overlying mudstone and sandstone with HCS in a lower-shoreface Bioturbated Mudstone–Sandstone Lithofacies Association (Southern environment, and the trough-cross-stratified sandstone in an upper- Study Area) shoreface environment (Van Wagoner et al. 1990; Bhattacharya and Walker 1991; Walker and Plint 1992; Johnson and Baldwin 1996). The Description.—The bioturbated mudstone–sandstone lithofacies associ- sharp, bioturbated upper surface of the sequences is interpreted as ation occurs in the Lower Cretaceous Chumulong Formation in the a transgressive surface that records a depositional hiatus as relative sea- southern half of the study area and is composed of brown, organic-rich level rose, the shoreline transgressed, and the depositional environment claystone and siltstone with subordinate amounts of sandstone (Fig. 6). returned to deeper marine conditions (e.g., Van Wagoner et al. 1990; This lithofacies association is common in many parts of the Chumulong Posamentier and Allen 1999). Formation, including the lower half of the succession and in the strata just below the Aptian–Albian limestone (Fig. 6). The mudstone is Dark Brown Sandstone–Conglomerate Lithofacies Association (Southern typically laminated and contains very fine-grained sandstone laminae Study Area) with oscillatory current ripples; the laminae are often disturbed or destroyed by subhorizontal burrows and vertical to subvertical Skolithos Description.—The dark-brown sandstone–conglomerate lithofacies burrows. Disarticulated fragments of oysters and unidentifiable bivalves association consists of interbedded siltstone, sandstone, and conglomer- are common in the mudstone intervals. Interbedded with the mudstone ate (Figs. 6, 7). Siltstone intervals are brown and massive and often are thin- to thick-bedded, very fine to fine grained bioturbated sandstone contain thin beds of very fine- and fine-grained sandstone with with oscillatory and unidirectional current ripples, flaser bedding, oyster unidirectional current ripples and burrows. Organic-rich mudstone is fragments, and small amounts of fossil wood and fossilized plant debris. present in the lowermost portion of the Lower Cretaceous succession, Some intervals within this lithofacies association contain repetitive where coal layers also have been reported (Yin et al. 1988). Interstratified packages, 2–4 m thick, composed of mudstone in their lower parts and with the mudstone are thick (. 60 m) sequences of fine- to very coarse- one or more , 0.5-m-thick beds of fine-grained, bioturbated sandstone in grained sandstone and clast-supported pebble conglomerate (Fig. 7). their upper parts. These sandstone–conglomerate sequences overlie basal scour surfaces and are composed of multistory sandstone–conglomerate packages 2–8 m Interpretation.—Sediments of this lithofacies association are inter- thick. Individual packages generally contain both conglomerate and preted as having been deposited in a relatively low-energy lagoonal sandstone beds, but those composed entirely of pebble conglomerate are environment in a clastic marginal marine setting (e.g., Kirschbaum 1989). common. Typical packages overlie a scour surface, contain lowermost Low-diversity fossil assemblages that are dominated by oysters, small beds of clast-supported, imbricated to crudely stratified, quartzite-pebble mudstone–sandstone packages, flaser bedding, and fossil plant and wood conglomerate, and are overlain by sandstone with trough cross- debris are all common characteristics of lagoonal deposits (Elliot 1974; stratification and plane-parallel lamination. The sandstone fines upward Ward and Ashley 1989; Kirschbaum 1989). within each package, and some contain relatively large (1.5 m) sets of planar, to slightly trough, cross-stratification. Siltstone and very fine- Upward-Coarsening Sandstone Lithofacies Association (Southern grained sandstone occasionally cap the upper parts of individual Study Area) packages.

Description.—The upward-coarsening sandstone lithofacies association Interpretation.—The sediment, lithofacies, and architecture of this is characterized by multiple upward-coarsening sequences of mudstone lithofacies association are most similar to those of low-sinuosity, sandy– and sandstone 5–15 m thick (Fig. 6). The lowest part of a typical gravelly braided rivers (Miall 1978; Cant and Walker 1978; Willis 1993; sequence contains mudstone and thinly bedded very fine-grained Miall 1996). The imbricated, well-organized pebble-conglomerate beds sandstone with oscillatory current ripples, occasional plane-parallel are interpreted as deposits of gravelly low-relief bars that migrated along lamination, hummocky cross-stratification (HCS), and Skolithos bur- channel thalwegs (e.g., Hein and Walker 1977). Trough cross-stratified rows. Upward within individual sequences, these heterolithic deposits sandstone was deposited by subaqueous three-dimensional dunes that become progressively more sandstone-rich, eventually giving way to white migrated within the paleochannels. The sandstone with larger and more to buff-colored upward-coarsening sandstone units that are 5–10 meters planar cross-stratification is interpreted as having been deposited by thick. The lower beds in these sandstone units are very fine- to fine- migrating two-dimensional dunes or transverse bars. The plane-parallel- grained and contain plane-parallel laminae. Overlying these beds are fine- laminated sandstone near the tops of individual packages are interpreted to medium-grained sandstone beds with abundant trough cross-stratifi- as having been deposited either in infilled channels or on bar tops where 816 A. LEIER ET AL. J S R J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 817

Cyclical Limestone Lithofacies Association (Southern Study Area)

Description.—In the southern half of the study area, Lower Cretaceous clastic strata of the bioturbated mudstone–sandstone lithofacies association are overlain by bioclast wackestone and packstone of the cyclical limestone lithofacies association (the Penbo Member of the Takena Formation; Fig. 6). The limestone contain orbitolinids (Mesorbitolina sp.), echinoids (Macraster sp., Selina sp.), and rare oysters (Ceratosreon (?)) and ammonites (Kazanskyella sp.) (Smith and Xu; 1988; R.W. Scott, personal communication). A succession of siltstone with minor sandstone overlies the wackestone, and is in turn overlain by a series of upward- coarsening limestone cycles 3–5 m thick (Figs. 6, 7). These cycles are composed, from bottom to top, of marly siltstone, orbitolinid wackestone (Mesorbitolina texana [Roemer]) and relatively coarse-grained and fossiliferous orbitolinid–oyster packstone, which also contain a subordinate amount of echinoderm debris and ostracodes. The packstone beds at the tops of the cycles contain numerous fossils, have sharp upper surfaces, and are overlain by marly siltstone of the overlying cycle. The limestone in the southern part of the study area eventually grade into siltstone and fine-grained sandstone with marine fossils (Leier 2005).

Interpretation.—Collectively, the cyclical limestone lithofacies association was deposited in a low energy, carbonate-dominated shallow-water marine environment. The carbonate cycles are interpreted as carbonate parasequences that record repeated shoaling and flooding of a shallow marine environment (e.g., Inden and Moore 1983; Jones and Desrochers 1992). The presence of siltstone and local sandstone in this lithofacies association suggests that although this location was dominated by carbonate sedimentation, it was proximal to limited clastic input, possibly associated with a nearby clastic shoreline.

PALEOCURRENT MEASUREMENTS In order to reconstruct paleocurrent directions, the limbs of trough cross-stratification sets in fluvial sandstone were measured (following method I of DeCelles et al. 1983), as were the strike and dip of imbricated pebble and cobble clasts in fluvial conglomerate beds. Trough cross-strata of Lower Cretaceous fluvial sandstone in the northern half of the study area record southwest-directed sediment transport (Fig. 5). Near the exposed base of the Duba Formation, paleocurrents are generally south-southwest directed, whereas trough FIG. 7.— Photos of Lower Cretaceous strata in southern Lhasa. A) Clast- cross-stratification orientations near the top of the Duba Formation supported quartzite pebble conglomerate of the Chumulong Formation. Clasts are indicate a more west-directed flow (Fig. 5). Accretion sets in some of the imbricated, beds are dipping to the left. Distance between the black marks on the fluvial sandstone sequences in the Duba area dip toward the northwest, Jake staff is 0.5 meter. B) Carbonate parasequences. Beds are dipping to the left. orthogonal to the mean paleoflow direction, and suggest lateral channel Cycles begin with marly siltstone, and coarsen-upward into orbitolinid packstones. Jacob’s staff is 1.5 m. migration. Imbricated clasts in a Lower Cretaceous marine conglomerate exposed in the Lunpola area record south-southeastward sediment transport. Paleocurrent data were more difficult to obtain in the southern half of the depth of flow is commonly shallow; local siltstone beds deposited atop the study area, owing to poorer exposures. Trough cross-stratification the plane-parallel-laminated sandstone are interpreted as having been and imbricated clasts indicate southward-directed flow in three fluvial deposited during the waning stages of flow (e.g., Langford and Bracken sequences, but northward-directed transport is indicated by trough cross- 1987; Bristow 1993). The mudstone intervals interstratified among the stratification in two fluvial units (Fig. 6). Paleocurrent data obtained sandstone sequences are interpreted to have been deposited in interfluvial from a fluvial sandstone interstratified with Aptian–Albian limestone floodplain environments (e.g., Cant and Walker 1978). The massive indicate northwestward-directed sediment transport, which is similar to nature of the mudstone suggests that these may be paleosols, but no the paleocurrent data derived from the Upper Cretaceous fluvial definitive evidence is present. sandstone beds that overlie the Lower Cretaceous strata (Leier 2005).

FIG. 6.—Measured sections of Lower Cretaceous strata exposed in southern part of the Lhasa terrane near the Penbo area. Upward-coarsening trends are shown in the lower half of the succession. Scale is in meters. 818 A. LEIER ET AL. J S R

FIG. 8.— Sandstone composition. QmFLt (monocrystalline quartz, feldspar, total lithic grains), QtFL (total quartz, feldspar, lithic grains), and QpLvLsed (polycrystalline quartz, volcanic lithic grains, sedimentary lithic grains) diagrams of Lower Cretaceous sandstone in the Lhasa terrane. See Appendix for parameters and raw data. Circles represent Lower Cretaceous strata in the southern part of the Lhasa terrane, triangles represent sandstone from the northern part. Means of the samples are shown with open symbols and the standard deviations are shown with light gray (strata from northern Lhasa) and dark gray (strata from southern Lhasa). Fields are from Dickinson and Suczek (1979).

PETROGRAPHY almost entirely quartz arenites and sublitharenites (Qm:F:Lt 73:0:27 and Qt:F:L 92:0:8; Fig. 8). Of the total quartz in the sandstone, the largest Petrographic thin sections were made from medium- and coarse- component is monocrystalline quartz, although in some samples poly- grained sandstone samples collected from measured stratigraphic crystalline quartz constitutes up to 15% of the total quartz. Consistent sections. The thin sections were stained for potassium and calcium with the petrographic results, the clasts in pebble-conglomerate beds are feldspar and point-counted (450 counts per slide) using a modified Gazzi- almost entirely quartzite and vein quartz. No feldspar grains are present Dickinson method (Ingersoll et al. 1984); the modification involves the in the Lower Cretaceous sandstone in the south, and lithic fragments are identification of monocrystalline quartz grains that are part of rare. The lithic fragments that are present are generally fragments of sedimentary lithic and plutonic fragments (Appendix, see Acknowl- edgments section for URL of JSR’s Data Archive). In addition, 7 phyllite and shale. Although sandstone units just below the Aptian– conglomerate clast counts (100 clasts/site) were performed in the field. Albian limestone beds are similarly quartzose, they contain a small The petrographic counting parameters and raw data are available are in amount of volcanic material and a slightly higher percentage of the Appendix. sedimentary and metamorphic lithic fragments relative to sandstone lower in the Lower Cretaceous succession. Rare zircon and muscovite are present in a few sandstone samples, but accessory minerals are largely Northern Study Area absent. Lower Cretaceous sandstone (n 5 13) consists of calcite-cemented litharenites and feldspathic litharenites with Qm:F:Lt modal composi- DETRITAL-ZIRCON GEOCHRONOLOGY tions of 42:16:42 and Qt:F:L of 57:16:27 (Fig. 8). Subangular to subrounded monocrystalline quartz grains are the dominant form of Uranium–lead age data of detrital zircons can provide valuable insight quartz. Feldspar grains constitute 10–20% of the modal composition, the into the provenance and depositional history of sedimentary successions. bulk of which is plagioclase. Potassium feldspar is typically minor or We collected two samples of Lower Cretaceous sandstone, one from an absent, although several samples contain 2–5% relative to the total number exposure in the Penbo area in the south and the other from the Duba area of grains. In a few samples, up to 30% of the quartz and feldspar occur not in the north, to determine maximum depositional ages, constrain as individual grains but as constituents of felsic plutonic fragments (these provenance sources, and better characterize the genetic relationship are still counted as quartz or feldspar, following the Gazzi-Dickinson between the Lower Cretaceous strata in northern and southern Lhasa. method). Lithic fragments are commonly the most abundant grain type in Raw data from this analysis are included in the Appendix (see Lower Cretaceous sandstone. Most common are andesitic–dacitic volcanic Acknowledgments section). grains containing microlaths of plagioclase feldspar. Sedimentary and metamorphic lithic grains such as mudstone/shale, phyllite, schist, and Methods limestone grains constitute an average of 20% of the modal composition. The two , 15 kg samples collected from the Penbo (sample CHMLN) Some of the sandstone and siltstone lithic grains have minerals and and Duba areas (sample DUBA) were processed for detrital zircon characteristics (e.g., abundant plagioclase and chloritic matrix) that are analysis using standard procedures described in Gehrels (2000). Once similar to that of Jurassic sedimentary strata exposed 50 km to the north in separated, the detrital zircons were encased in epoxy within 1˝ ring the Lunpola area (Leier 2005). Clast counts are consistent with point- counting data in that the majority of the clasts consist of metasedimentary mounts (1˝ equals 2.54 cm), which were then sanded and polished to rocks, felsic volcanic rocks, and granite. Zircon and tourmaline are the produce a smooth flat surface that exposed the interiors of the zircon most common accessory minerals, but also present are pyroxene grains. One hundred individual zircon grains were analyzed from each (omphacite), and minor amounts of chlorite and serpentine. sample. These were selected randomly from all sizes and shapes, although grains with obvious cracks or inclusions were avoided. Uranium–lead ages of detrital zircons were obtained using a laser- Southern Study Area ablation, multi-collector, inductively coupled plasma mass spectrometer Unlike the feldspar-rich sandstone exposed in the northern portion of (LA-MC-ICP-MS). The interiors of the zircon grains were ablated using the Lhasa terrane, Lower Cretaceous sandstone (n 5 13) in the south are a New Wave DUV193 Excimer laser operating at a wavelength of 193 nm J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 819 and using a spot diameter of 35–50 microns; laser ablation pits are , 20 Lhasa–Qiangtang Collision (, 145 Ma–130 Ma) microns deep. With the LA-MC-ICP-MS, the ablated material is carried via argon gas to the plasma source of a Micromass Isoprobe, which is Northern Lhasa.—The Early Cretaceous paleogeography in the configured in such a way that U and Pb can be measured simultaneously. northern portion of the Lhasa terrane is characterized by coarse-grained Measurements are made in static mode using Faraday collectors for 238U, shallow marine and coastal environments that were located south of 232Th, 208Pb, 207Pb, 206Pb, and an ion-counting channel for 204Pb. topographic relief (Fig. 10). Interbedded mudstone and sandstone beds Analyses consist of one 20-second integration with the peak centered but with bivalve fragments and oscillatory current ripples suggest relatively no laser firing (checking background levels), and twenty 1-second shallow marine environments with water depths above wave base. integrations with the laser firing on the zircon grain. At the end of each Southward-directed paleocurrents, lithic-rich sandstone units with analysis a 30 second delay occurs, during which time the previous sample chlorite, pyroxene, and sedimentary-lithic grains, and Early Cretaceous is purged from the system and peak signal intensity returns to erosional surfaces in the southern Qiangtang terrane (Kapp et al. 2005b), background levels. The contribution of Hg to the 204Pb is accounted suggest that sediment composing the Lower Cretaceous strata was, at for by subtracting the background values. Common-lead corrections are least in part, derived from the Bangong suture and the southern made using the measured 204Pb of the sample and assuming initial Pb Qiangtang terrane. Abundant volcanic grains and plagioclase in the compositions from Stacey and Kramers (1975). A fragment of a zircon Early Cretaceous sandstone units indicate centers of volcanic activity, crystal of known age (564 6 4 Ma, 2s error; G.E. Gehrels, unpublished such as the Bangoin granite belt (Harris et al. 1988), also served as data) is analyzed after every fifth zircon analysis to correct for inter- sediment sources. The paleoshoreline trend is inferred to have been element and Pb isotope fractionation. The ages presented are 206Pb*/238U approximately E–W to ESE–NNW, parallel to the east–west trending ages for grains with ages less than , 1000 Ma and 207Pb*/206Pb* ages for Bangong suture. This inference is derived from the distribution of facies, grains with ages greater than 1000 Ma. All uncertainties of individual the lack of Lower Cretaceous marine rocks in the Qiangtang terrane (Liu grains are reported at the 1-s level and include only measurement errors; 1988; Kapp et al. 2005b), and assuming that the shoreline was systematic errors would increase age uncertainties by 1–2%. Those approximately perpendicular to the paleocurrent directions in the analyses with greater than 10% uncertainty (206Pb*/238U ages) or more overlying fluvial strata. than 20% discordance or 5% reverse discordance are omitted from further consideration. The data from each sample are displayed on concordia Southern Lhasa.—During Early Cretaceous time the southern portion diagrams and age-probability plots/histograms using the programs of of the Lhasa terrane consisted of lagoonal, shoreface, and fluvial Ludwig (2001). Age-probability plots depict each age and its uncertainty depositional environments (Fig. 10). The quartzose composition of the as a normal distribution, summing all ages from the analyzed zircons of sandstone units and the textural maturity of the individual grains initially a sample into one curve, which is then divided by the total number of appear to suggest a stable tectonic setting; however, the beds of pebble analyzed zircons. Maximum depositional ages are calculated from the conglomerate and sandstone with sedimentary lithic fragments are weighted mean of the youngest cluster with three or more overlapping interpret to indicate that much of the Lower Cretaceous sediment in ages; uncertainties reported with the maximum depositional ages include the southern portion of the Lhasa terrane was recycled from quartzose both the measurement and systematic errors. sedimentary cover strata that were exposed in nearby uplifts (Fig. 10). Carboniferous strata of the Lhasa terrane were likely a primary sediment Results source for the Lower Cretaceous sandstone because: (1) strata of Carboniferous strata are thick and widespread throughout southern Sample DUBA, from the Lower Cretaceous Duba Formation, yielded Tibet (Yin et al. 1988); (2) Carboniferous strata consist of quartzose 94 usable ages (Fig. 9). Many of the zircons are euhedral and have U/Th sandstone, quartzite, and shale/phyllite beds, making them a logical values indicative of a plutonic origin, consistent with the composition of sediment source for the Lower Cretaceous sandstone and conglomerate; the sandstone and conglomerate beds (see above). The youngest cluster of (3) detrital zircon age populations . 400 Ma from Lower Cretaceous ages has a mean of 125 6 2 Ma, which provides a maximum depositional strata are similar to those in Carboniferous rocks, suggesting sediment age for the Duba Formation. Grains with ages between 120 and 150 Ma recycling (Leier 2005); and (4) at particular localities, Cretaceous rocks are the most numerous (peak at 141 Ma) and represent over 65% of the are in depositional contact with Carboniferous strata (Liu 1988), total population (Fig. 9). The remaining grains are in clusters between indicating that Carboniferous rocks were at the surface during this time. 250 and 340 Ma, 610 and 670 Ma, 700 and 900 Ma (peak at 850 Ma), 1000 and 1200 Ma (peak at 1040 Ma), and a few of Early Proterozoic age. Sample CHMLN from the Lower Cretaceous Chumulong Formation Mid Neocomian to Early Aptian (, 130–120 Ma) from the southern half of the study area yielded 93 usable ages (Fig. 9). The zircons are typically subhedral to rounded. Unlike the DUBA Northern Lhasa.—Southwest-flowing, gravelly, meandering rivers sample, the CHMLN sample is composed chiefly of zircons of Paleozoic occupied northern Lhasa during mid Neocomian to Aptian time (the age and older (66 of the 93 grains are . 250 Ma). The maximum upper Duba Formation; Fig. 10). Sandstone composition and conglom- depositional age is 143 6 2 Ma, corresponding to Early Cretaceous time. erate clasts indicate the sediment was derived from felsic volcanic and Zircons in this sample have age groupings of 140–160 Ma, 500–580 Ma igneous rocks, as well as sedimentary and metasedimentary cover strata (peak at 530 Ma), 620–710 Ma, 850–950 Ma (peak at 900 Ma), 1000–1300 exposed near the Bangong suture. Numerous pebble and cobble Ma (peak at 1130 Ma), and near 2500 Ma (Fig. 9). conglomerate beds suggest topographic relief existed in the surrounding areas. The presence of inclined-heterolithic strata and the fact that the fluvial strata are stratigraphically located between marine units suggest REGIONAL INTERPRETATIONS a lower coastal plain setting (e.g., Smith, D. 1987). The Early Cretaceous paleogeographic evolution of the Lhasa terrane is divided into three general stages (Fig. 10): (1) the Lhasa–Qiangtang Southern Lhasa.—As in the previous time period, sedimentary strata in collision to mid Neocomian (, 145 to , 130 Ma); (2) mid Neocomian to the southern part of the Lhasa terrane indicate that the area was occupied early Aptian (, 130 to , 120 Ma); and (3) Aptian–Albian (, 120 to by marginal marine, lagoonal, and lower coastal-plain environments , 100 Ma). during mid Neocomian to Aptian time (Fig. 10). Unlike the underlying 820 A. LEIER ET AL. J S R

FIG. 9.—Detrital-zircon data from sandstone samples collected from the Duba Formation (DUBA) and the Chumulong Formation in the Penbo area (CHMLN). A) Concordia diagram. B) Relative probability plots of all of the detrital zircons. Insets include only the zircons of Mesozoic age. Histogram bins in the probability plots are 50 Ma; 5 Ma for the plots of Mesozoic zircons. See text for full discussion. sedimentary units, conglomerate beds are absent within the strata, much greater than fluvial gravel fronts are typically thought to extend suggesting more subdued topography or a change in the source area. (e.g., Robinson and Slingerland 1998). Detrital-zircon age populations in conglomerates in northern and southern Lhasa also suggest different Relationship Between Lower Cretaceous Conglomerate Units—North depositional ages. The youngest cluster of detrital zircon ages is , 143 and South.—The sedimentary evidence indicates the Lower Cretaceous Ma in the southern Chumulong Formation conglomerate beds in the conglomerate beds of the Chumulong Formation in the south and those south and , 125 Ma in the Duba Formation beds in the north (Fig. 9). of the Duba Formation in the north may have had distinct sedimentary Detrital-zircon ages can be used only to infer maximum depositional ages, histories. It is difficult to reconcile the disparate compositions of the but the 20 Myr disparity between the two units is significant, particularly quartzose Chumulong Formation and the volcanic-rich Duba Formation because 140–100 Ma igneous rocks are present throughout the Lhasa with one shared provenance. Furthermore, removing Late Cretaceous terrane, where presumably they were acting as sediment sources (Kapp et and Cenozoic shortening of , 50% from the Lhasa terrane (Burg et al. al. 2005a). Thus, the conglomerate beds of the Chumulong Formation in 1983; Pan 1993), the distance between exposures of the pebble the southern part of the Lhasa terrane may be millions of years older than conglomerate units in the north and south is roughly 400 km, which is the conglomerate beds of the Duba Formation in the northern half of the J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 821

FIG. 10.—Paleogeography and tectonic setting. Diagrams illustrating the paleogeography of the Lhasa terrane during different episodes in Early Cretaceous time; tectonic processes active during Early Cretaceous are also shown. Num- bers refer to certain features or environments that have been placed in particular locations on the basis of data presented in this study or in other studies (see legend in lower right). A) The Lhasa terrane during latest Jurassic–earliest Cretaceous time. The collision between the Lhasa and Qiangtang terranes acted as the dominant control on deposition in the area (black arrows showing subsidence). B) Paleoge- ography of the Lhasa terrane between mid- Neocomian and Aptian time. The peripheral foreland basin in northern Lhasa remained the dominant factor in controlling sedimentation (black arrows show subsidence). Volcanic centers migrated to the northern part of the terrane. C) Paleogeography during Aptian–Albian time. The Lhasa terrane was an inundated peripheral foreland basin. Crustal loading in the north influenced long term accommodation (black arrows), but other factors (e.g., dynamic subsidence (?)—black arrows below subducted plate) began to influence deposition. See text for full discussion.

Lhasa terrane. Given the limited data set, however, this should be used more effectively, the zircon source areas in the Lhasa terrane will considered only a working hypothesis at this time, and it is quite possible need to be better cataloged. the conglomerate beds in southern Lhasa are younger than the youngest ages of their detrital zircons. Moreover, a population of detrital zircons Aptian–Albian (, 120–100 Ma) with ages ca. 140 Ma are present in conglomerate beds in both the northern and southern portion of the Lhasa terrane (Fig. 9), suggesting Northern Lhasa.—The fossil assemblages and geographic distribution that some of the sediment in the two stratigraphic units may have been of carbonate facies indicate that the northern Lhasa terrane was covered derived from a similar source. We emphasize that before any of this can by a shallow sea characterized by rudist patch reefs and muddy inter-reef be resolved more clearly and before the detrital-zircon evidence can be areas (Fig. 10; e.g., Scott 1979; Leeder et al. 1988). Biostratigraphic 822 A. LEIER ET AL. J S R evidence indicates that deposition began in the northern part of the Lhasa foreland basins, the depositional facies in the Upper Jurassic–Lower terrane before it did in the south (Yin et al. 1988; Zhang 2000; Leier Cretaceous strata in northern Lhasa progress from fine-grained 2005). These environments persisted in the north through earliest deepwater deposits at the base to coarse-grained nonmarine deposits Cenomanian time (Yin et al. 1988). Limestone in northern Lhasa is near the top (Miall 1995). Paleocurrent data and sedimentary grains in thicker than coeval strata exposed in the southern part of the Lhasa Lower Cretaceous nonmarine strata indicate that sediment was derived terrane (, 1100 m+ in the north versus 300 m in the south), suggesting from a northern source area near the Bangong suture and transported to greater subsidence in northern Lhasa during this time period. the south-southwest, which is consistent with the hypothesis that a foreland basin formed in the Lhasa terrane in response to the collision Southern Lhasa.—In Aptian time, shallow marine carbonate deposition between the Lhasa and Qiangtang terranes. The abundance of volcanic replaced the marginal marine clastic environments in southern Lhasa. grains in the Lower Cretaceous clastic strata is atypical of collisional Although the region was dominated by carbonate deposition, it received foreland basins but can be explained by arc activity associated with coeval an occasional influx of fine-grained clastic material derived from the subduction of oceanic crust beneath the southern margin of the Lhasa south. An east–west trending shoreline is inferred to have existed to the terrane (e.g., Coulon et al. 1986; Kapp et al 2005b). The tectonic setting south of the Penbo area, mimicking the southern margin of the Lhasa along the southern margin of the Lhasa terrane during this time period is terrane and perpendicular to the north-directed paleocurrents in less clear. Evidence indicates that subduction of Neotethyan oceanic crust interbedded sandstone. By the end of the Albian stage, the marine had commenced by this time (Chu et al. 2006), and stratigraphic data carbonate environments were replaced by a northward-prograding clastic (e.g., conglomerate beds) suggest that there may have been localized shoreline and lower coastal plain (Leier et al. 2007). uplifts in the region. From latest Jurassic through the rest of Early Cretaceous time, volcanic activity migrated northward in response to DISCUSSION shallowing of the subduction angle of the downgoing Neotethyan slab (Coulon et al. 1986; Kapp et al. 2005b). The Early Cretaceous history of the Lhasa terrane can be divided into an initial stage from approximately earliest Cretaceous to Aptian time Second Stage—Aptian–Albian that was dominated by clastic deposition, and a latter stage spanning Aptian and Albian time that corresponds to widespread deposition of Several mechanisms can be invoked to explain the change from clastic marine carbonate sediments. In the following paragraphs we discuss the nonmarine and shallow marine deposits associated with the earliest part tectonic setting and basin history of the Lhasa terrane during both of of Cretaceous time to the extensive shallow marine carbonate strata of , these stages, presenting the various basin models that have been proposed Aptian–Albian age (Fig. 10). A eustatic rise in sea level provides a simple to explain deposition in the region and highlighting data from this and way to explain widespread deposition of shallow marine carbonate within other studies that both support and contradict these hypotheses. Data the Lhasa terrane; however, a rise in sea-level alone fails to account for from Lower Cretaceous strata support the hypothesis that during earliest the south-to-north increase in thickness of limestone strata in the Lhasa Cretaceous time a peripheral foreland basin formed in the Lhasa terrane terrane, nor can it account for the thickness of these strata (assuming Airy in response to the collision between the Lhasa and Qiangtang terranes. isostasy, and accounting for . 1 km of carbonate strata). Northward The data from Aptian–Albian strata are more ambiguous. Although subduction of Neothethyan oceanic lithosphere beneath the Lhasa several aspects of the Aptian–Albian succession can be attributed to terrane during this time could have resulted in dynamic effects in the a peripheral foreland basin setting, there are some aspects that cannot, overlying terrane, thereby providing accommodation for sediment suggesting that deposition in the region may have been influenced by accumulation (e.g., Gurnis 1992; Burgess and Moresi 1999). This model, a combination of mechanisms. however, predicts that the carbonate strata should increase in thickness from north-to-south instead of the south-to-north thickening trend Initial Stage—Earliest Cretaceous observed. An additional possibility is that a crustal load developed along the southern margin of the Lhasa terrane as deformation in the Gangdese Structural and stratigraphic evidence indicates that the lowermost retroarc fold-thrust belt commenced (Leier et al. 2007). Although this Cretaceous strata (earliest Cretaceous–Aptian age) in the Lhasa terrane may have influenced sedimentation and subsidence patterns in particular were deposited in a peripheral foreland basin system. The Lhasa terrane locations, the amount of Aptian–Albian subsidence in southern Lhasa collided with the southern margin of the Qiangtang terrane during Late was minor (strata are , 300 m thick in the south) and spatially removed Jurassic–Early Cretaceous time (Dewey et al. 1988), providing a tectonic from northern Lhasa (400–600 km), implying that the subsidence setting conducive to the formation of a fold-thrust belt and a peripheral required for deposition of . 1 km of carbonate strata in northern Lhasa foreland basin (Leeder et al. 1988; Dickinson 1974). Structural data must have been derived from an alternative mechanism. Whereas indicate that Early Cretaceous crustal thickening and rock uplift occurred evidence conclusively supporting any of the aforementioned processes is in the northern margin of the Lhasa terrane as a south-verging fold-thrust lacking, it remains possible that one or a combination of these processes belt developed in response to the Lhasa–Qiangtang collision (Murphy et influenced subsidence and sedimentation in the Lhasa terrane during al. 1997; Kapp et al. in press). Stratigraphic relationships indicate that the Aptian–Albian time. Qiangtang terrane was above sea level and being eroded by at least middle Normal faulting related to extensional forces is an appealing Cretaceous time (Fig. 10; Kapp et al. 2005b). The collision-related mechanism to explain the depositional patterns of Aptian–Albian strata deformation is inferred to have provided a sediment source and created (Zhang et al. 2004), and although this postulation is consistent with some a crustal load on the northern part of the Lhasa terrane, which ultimately of the stratigraphic data, there are many aspects of the stratigraphy that produced subsidence and created the peripheral foreland basin (Leeder et contradict this hypothesis. The vertical trend in depositional facies in the al. 1988; Murphy et al. 1997; Zhang et al. 2004). The sedimentary and Lower Cretaceous succession, from coarse-grained fluvial strata (the stratigraphic data presented in this study and the derivative paleogeo- Duba Formation) to shallow marine carbonate rocks (the Langshan graphic reconstructions support the peripheral foreland basin model. For Formation) is similar to that present in other extensional basins (e.g., example, Lower Cretaceous sedimentary strata thicken to the north, Dickinson and Lawton 2001). However, this model is predicated on the suggesting that subsidence and/or sediment supply was greatest in this supposition that the coarse-grained fluvial strata of the Duba Formation area (Fig. 10). Similar to the stratigraphic successions in many peripheral represent the base of a genetic succession. On the contrary, the J S R LOWER CRETACEOUS STRATA OF SOUTHERN TIBET 823 stratigraphic and sedimentary data presented in this and other studies of several mechanisms, such as crustal loading along the southern margin (Leeder et al. 1988; Yin et al. 1988) indicate the coarse-grained fluvial of the terrane or dynamic topography, operating in conjunction with deposits do not represent the base of a new succession but rather the foreland basin subsidence. The specific factors and the relative role they uppermost part of a large-scale coarsening- and shallowing-upward may have played in influencing Aptian–Albian deposition are unclear, succession that begins with Upper Jurassic rocks (Figs. 3, 10). Thickness and more data are needed before the complete depositional and tectonic changes in age-equivalent Lower Cretaceous units have been used to history of the region can be understood. argue for basin segmentation and normal faulting (Zhang et al. 2004), although such stratigraphic trends can also be explained by basin SUMMARY AND CONCLUSIONS segmentation caused by thrust faults. Perhaps the most significant obstacle to the extensional basin model is the fact that structural Lower Cretaceous sedimentary strata of the Lhasa terrane were relationships record regional contraction during Early Cretaceous time deposited in marginal marine and coastal-plain environments. In the (Murphy et al. 1997; Kapp et al. in press). Small normal faults (, 1 m southern part of the Lhasa terrane, Lower Cretaceous rocks consist displacement) have been documented at one or two locations in strata of chiefly of shoreface and lagoonal strata with minor braided-stream Aptian–Albian age (Zhang et al. 2004), but these occur on the scale of deposits. Lower Cretaceous strata in the northern portion of the Lhasa a single bed and distinguishing these features from localized atectonic terrane are characterized by lithic-rich conglomerate beds deposited in synsedimentary deformation is required before they can be extrapolated shallow marine and meandering-river fluvial environments. The maxi- to infer a regionally pervasive extensional stress field. It should be noted mum depositional age of conglomerate in southern Lhasa is 143 6 2 Ma, that the extensional basin model developed by Zhang et al. (2004) was whereas those in the north have a maximum age of 125 6 2 Ma. All of the derived from the study of Cretaceous strata exposed to the west of our Lower Cretaceous clastic strata are overlain by a widespread Aptian– study area, and it is plausible the Early Cretaceous tectonic settings in the Albian limestone that was deposited in a shallow marine seaway that two regions differed. occupied the Lhasa terrane for , 20 Myr. Whereas subsidence histories are typically helpful in identifying basin- Sedimentary, stratigraphic, and structural data suggest that a latest forming mechanisms, the thicknesses and ages of the Upper Jurassic Jurassic–Early Cretaceous peripheral foreland basin formed in the Lhasa through Late Cretaceous strata in the Lhasa terrane are so poorly terrane in response to the collision between the Lhasa and Qiangtang constrained that little substantive information is revealed from such terranes. This peripheral foreland basin was the principal influence on analyses. Thickness estimates of Upper Jurassic strata in the northern sedimentation throughout most of Early Cretaceous time. Aspects of the portion of the Lhasa terrane vary from hundreds of meters to . 4 km, Aptian–Albian carbonate deposits can be attributed to a peripheral and thicknesses of Aptian–Albian limestone vary from hundreds of foreland basin setting, although particular characteristics of these strata, meters to . 6 km (cf. Leeder et al. 1988; Zhang 2000). Given such a wide such as their widespread spatial distribution, suggest that deposition in range of thicknesses, it is possible to derive subsidence histories that the region may have been influenced by a combination of mechanisms display both convex-upward and convex-downward profiles. Subsidence during this time. Lower Cretaceous deposits in the Lhasa terrane indicate histories were reconstructed from the limited amount of data from this that southern Tibet was tectonically active long before the Indo-Asian and other studies, but the results yielded a constant subsidence rate collision. throughout the Cretaceous (see Appendix, URL in Acknowledgments section) and therefore cannot be used to support or refute any particular ACKNOWLEDGMENTS basin model. Based on structural and stratigraphic data, we propose that the This work was supported by the U.S. National Science Foundation grant peripheral foreland basin hypothesized to have existed in the Lhasa (EAR-0309844), The Geological Society of America, and The American terrane during earliest Cretaceous time (Leeder et al. 1988; Murphy et al. Association of Petroleum Geologists student grants, and a Boren Fellowship from the U.S. Department of Defense. Support was also provided by grants 1997; Zhang et al. 2004; this study) continued to influence deposition in from ExxonMobil and Chevron. This work benefited from discussions with the region during Aptian–Albian time. Folding and thrusting continued many students and faculty members in the Department of Geosciences at the to occur in the northern margin of the Lhasa terrane during Aptian– University of Arizona. We thank Ding Lin, Shundong He, Dan Eisenberg, Albian time (Kapp et al. in press); thus it is reasonable to assume the John Volkmer, Alex Pullen, Jessica Kapp, Jerome Guynn, Matt Fabijanic, persistence of a crustal load in this region. This is supported by and our Tibetan drivers and assistants for help in the field, and Richard L. stratigraphic data showing a south-to-north thickening of strata of Hay for help with the petrographic work. Dave Barbeau provided comments Aptian–Albian age. Although carbonate rocks are not typically on an earlier draft of this manuscript. We thank Stephan Graham, William Cavazza, and an anonymous reviewer for their thorough and very helpful associated with foreland basins, these deposits do occur in such settings reviews. The Appendix can be found in JSR’s Data Archive, http:// and have been documented in several ancient foreland-basin sequences www.sepm.org/jsr/jsr_data_archive.asp. (e.g., Dorobek 1995; Allen and Allen 1990; Pigram et al. 1989). The presence of widespread carbonate deposits suggests that the relative influx REFERENCES of clastic material to the Lhasa terrane decreased during Aptian–Albian time, but the reasons for this decrease (e.g., climate change, decreased ALLE´ GRE, C.J., COURTILLOT, V., TAPPONNIER, P., HIRN, A., MATTAUER, M., COULON, C., relief, etc.) are not known at this time. 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