Raising the Gangdese Mountains in southern Tibet

Abstract

The surface uplift of mountain belts is in large part controlled by the effects of crustal thickening and mantle dynamic processes (e.g., lithospheric delamination or slab breakoff). Understanding the history and driving mechanism of uplift of the southern Tibetan Plateau requires accurate knowledge on crustal thickening over time. Here we determine spatial and temporal variations in crustal thickness using whole‐rock La/Yb ratios of intermediate intrusive rocks from the Gangdese arc. Our results show that the crust was likely of normal thickness prior to approximately 70 Ma (~37 km) but began to thicken locally at approximately 70–60 Ma. The crust reached (58–50) ± 10 km at 55–45 Ma extending over 400 km along the strike of the arc. This thickening was likely due to magmatic underplating as a consequence of rollback and then breakoff of the subducting Neo‐Tethyan slab. The crust attained a thickness of 68 ± 12 km at approximately 20–10 Ma, as a consequence of underthrusting of India and associated thrust faulting. The Gangdese Mountains in southern Tibet broadly attained an elevation of >4000 m at approximately 55–45 Ma as a result of isostatic surface uplift driven by crustal thickening and slab breakoff and reached their present‐day elevation by 20–10 Ma. Our paleoelevation estimates are consistent not only with the C–O isotope‐based paleoaltimetry but also with the carbonate‐clumped isotope paleothermometer, exemplifying the promise of reconstructing paleoelevation in time and space for ancient orogens through a combination of magmatic composition and Airy isostatic compensation.

Slab Breakoff of the Neo‐Tethys Ocean in the Lhasa Terrane Inferred From Contemporaneous Melting of the Mantle and Crust

“…In the southern Lhasa Subterrane, early Cenozoic mafic magma generation around 57 Ma is strikingly sparse and volumetrically minor (Mo et al, ; Yue & Ding, ; Zhao et al, ). This is distinctly different from the ubiquitous Cenozoic arc‐related intermediate to silicic rocks in the Lhasa Terrane (Ji et al, ; Mo et al, ; Zhu et al, ), which were formed by the subduction of the Neo‐Tethys Ocean from the Late Triassic to the early Cenozoic, recorded by the Gangdese batholiths. As discussed above, their intraplate‐like geochemistry (Figure ) precludes a subduction‐related setting for the ∼57 Ma mafic dikes.…”

“…Mesozoic magmatic and sedimentary rocks are ubiquitous in the central and northern Lhasa subterranes (Figure b) (Coulon et al, ; Q. Wang et al, ; Zhu et al, ). The southern Lhasa Subterrane is mainly composed of Jurassic‐Miocene Gangdese batholiths and the Paleocene‐Eocene Linzizong volcanic succession (Dong et al, ; Huang et al, ; Ji et al, ; Kang et al, ; Lee et al, ; Li et al, ; Mo et al, ; Zhu et al, ), the latter consists of Dianzhong, Nianbo, and Pana formation. The sedimentary cover in the southern Lhasa Subterrane is limited, and is mainly of Late Triassic‐Cretaceous age (Zhu et al, ).…”

“…A slab window will be formed as a result of slab breakoff and subsequently results in partial melting of various source regions, including the upwelling asthenospheric mantle, lithospheric mantle, and overlying crust. The latter process produces mafic magmatism whose geochemistry resembles within‐plate basalts, and felsic intrusive bodies (Chen et al, ; Ferrari, ; Ji et al, ; Von Blanckenburg & Davies, ; Xu et al, ; Zhu et al, ). Considerable effort has been made to unravel the slab breakoff process and the subsequent magmatic events.…”

Paleomagnetic Evidence for 25–15 Ma Crust Fragmentation of North Indochina (23–26°N): Consequence of Collision With Greater India NE Corner?

“…On the other hand, the kinematics of present‐day SE Asia deformation is clearly imaged by seismologic, geodetic, and geophysical data that image crust structure (Nelson et al, ; Socquet & Pubellier, ; Gan et al, ; Yao et al, ; Liang et al, ; Huang et al, ; Zhu et al, ; Figure ). These data show that—at a large scale—the crust of E‐SE Tibet may be viewed as a viscous fluid, floating east‐southeastward above a very ductile lower‐middle crust (Houseman & England, , ).…”

Constructing the Eastern Margin of the Tibetan Plateau During the Late Triassic

“…Reasons for the difference in crustal thickness between the eastern margin and the inner part of the Tibetan Plateau during the Late Triassic may include the following: (1) crust of the inner part was originally thin with either oceanic crust or highly thinned continental crust as mentioned above, (2) the tectonic imbrication during the Late Triassic resulting from thrusting onto the Sichuan Basin (e.g., Dirks et al, ) contributed significantly to the crustal thickness of the eastern margin, and (3) mantle‐derived magma underplating (Q. Chen et al, ; Deschamps et al, ; C. Yuan et al, ) as exemplified by southern Tibet (Zhu et al, ) also contributed to the crustal thickness of the eastern margin as the plutons are mainly exposed along the eastern margin. Note that the above explanations (2) and (3) require that the eastern margin crust was also originally thin like the inner part before the Late Triassic orogenesis.…”