The Takena Formation of the Lhasa terrane, southern Tibet: The record of a Late Cretaceous retroarc foreland basin

Abstract

Our understanding of the processes involved in the Indo-Asian collision and the construction of the Tibetan Plateau are, in part, predicated on our understanding of the tectonic setting and crustal conditions of southern Tibet in the time period immediately preceding the Indo-Asian collision (Late Cretaceous). Several hypotheses have been proposed that describe the middle to Late Cretaceous tectonic and paleogeographic evolution of southern Tibet, each with different implications for when and how the Tibetan Plateau was uplifted. We examined the mid-upper Cretaceous Takena Formation of the Lhasa terrane in southern Tibet in order to reconstruct the middle to Late Cretaceous tectono-sedimentary history of this area and test the competing tectonic hypotheses. The Takena Formation consists of >2 km of sedimentary strata that include a lower marine limestone member (Penbo Member) and an upper member of clastic red beds (Lhunzhub Member). The Aptian-Albian Penbo Member consists of ~250 m of orbitolinid-limestone beds that were deposited in a shallow-marine seaway during a time when there was little regional subsidence. The overlying Lhunzhub Member (>1500 m) consists primarily of fl uvial strata that were deposited during a period of increased subsidence. Overall, the Lhunzhub Member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base, to multistory-multilateral braided stream deposits near the top. The fl uvial sandstone units are lithic-rich and contain abundant plagioclase and volcanic grains, and paleocurrent data record northwest-directed transport; these data indicate that the sediment was derived from the Gangdese volcanic arc that developed along the southern margin of the Lhasa terrane. Following deposition, but prior to ca. 70 Ma, the beds of the Takena Formation were folded and partially eroded. The sedimentary and stratigraphic characteristics of the Takena Formation are most consistent with deposition in a retroarc foreland basin setting. The limestone beds of the Penbo Member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the Gangdese volcanic arc. The low subsidence rate recorded in the lowermost strata of the Takena Formation, including the Penbo Member and the paleosol-rich interval of the Lhunzhub Member, is interpreted to be associated with the passage of a fl exural forebulge. The overlying, upward-coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin. The Late Cretaceous folding of the Takena Formation indicates that the foreland basin strata were eventually incorporated into a fold-and-thrust belt. The upper Cretaceous sedimentary strata of the Lhasa terrane of southern Tibet predict that a north-verging fold-and-thrust belt existed along the southern margin of the Lhasa terrane during Late Cretaceous time, prior to the Indo-Asian collision. Collectively, the evidence indicates that the southern mar...

Late Cretaceous (~81Ma) high-temperature metamorphism in the southeastern Lhasa terrane: Implication for the Neo-Tethys ocean ridge subduction

“…The influx of asthenospheric mantle through the slab window may have enhanced early uplift of the southern Lhasa terrane (Guillaume et al, 2010;Zandt and Humphreys, 2008). This provides an explanation for the cessation of deposition of the Takena Formation, which was deposited in a retroarc foreland basin, and its subsequent uplift, deformation and erosion during Late Cretaceous (Leier et al, 2007).…”

“…Volkmer (2010) suggested that the formation of eclogitic lithospheric root resulted from shortening of Gangdese arc that caused the lull in magmatic activity. Regionally, the 69-40 Ma Linzizong volcanic rocks in the Gangdese unconformably overlie strongly deformed Late Cretaceous and/or older rocks (Burg and Chen, 1984;England and Searle, 1986;Kapp et al, 2007;Leier et al, 2007), suggesting that significant crustal shortening and uplift occurred prior to 69 Ma. The Xigaze fore-arc basin is located at the southern margin of the southern Lhasa terrane, immediately north of the YarlungTsangpo suture zone, extends from Xigaze in the east to Saga in the west with a length of~550 km (west of our study area).…”

“…5c) (Ji et al, 2009;Wen et al, 2008b). Although Volkmer (2010) contemporary HT metamorphism recorded in the Nyingchi Complex (Zhang et al, 2010c; this study); (2) Wen et al (2008a) reported a suite of epidote-bearing adakitic rocks (80-82 Ma) in the southeastern Lhasa terrane, and suggested that these rocks originated from partial melting of underplated mafic lower crust; (3) Guo et al (2011) reported a Late Cretaceous granite (~81 Ma) which resulted from binary mixing between juvenile crustal materials (or mantle-derived magma) and an old crustal component, suggesting crustal anatexis at that time; (4) A major regional unconformity across southern Lhasa terrane has been documented, with gently dipping Palaeocene Linzizong volcanic rocks above folded Late Cretaceous and/or older rocks, showing that significant crustal uplift occurred prior to the India-Asia collision (Burg and Chen, 1984;England and Searle, 1986;Kapp et al, 2007;Leier et al, 2007); and (5) Gahalaut and Kundu (2012) analyzed the Gangdese arc morphology and earthquake ruptures, and suggested that the buoyant seismic/aseismic ridge subduction influenced the Gangdese arc morphology by causing a cusp in the southeastern Lhasa terrane. Taking all these independent and diverse studies into account, we propose that Late Cretaceous tectonothermal phenomena (the magmatic gap, adakitic rocks, crustal anatexis, fore-arc HT metamorphism, crustal uplift) were strongly influenced by the subduction of a Neo-Tethys oceanic ridge.…”

Cenozoic Evolution of Hinterland Basins in the Andes and Tibet

“…Estimating the onset of shortening is complicated by pre-collisional deformation and uplift. Prior to collision, north-dipping subduction of the Indian oceanic slab accounted for the Trans-Himalayan (Gangdese-Ladakh-Kohistan) magmatic arc and associated retroarc shortening (Burg et al, 1983;England and Searle, 1986;Kapp et al, 2007;Leier et al, 2007a). Based on structural restorations, total shortening during the India-Asia collision is estimated at 600-750 km for the Himalayas (DeCelles et al, 1998, 2002Robinson et al, 2006) and roughly 500-750 km across the Tibetan plateau (Murphy et al, 1997;Yin and Harrison, 2000;Kapp et al, 2005).…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…The youngest marine sedimentary rocks in the Lhasa block are Aptian-Albian shallower marine limestones of the middle Takena Formation (Leier et al 2007). After this time only continental red-bed deposition occurred, indicating that the entire region was subaerial.…”