Sedimentary record of late paleozoic rift and break-up in northern gondwana: a case history from the thini chu group and tamba-kurkur formation (dolpo tethys himalaya, nepal)

Abstract

Quantitative compositional data from selected sandstone samples in the Upper Paleozoic to lowermost Triassic succession of the central Dolpo Tethys Himalaya (Thini Chu Group and base of the Tamba-Kurkur Formation) are relevant to understand the tectonic and climatic evolution of the northern margin of Gondwana from continental rift to break-up and spreading in the Neotethys Ocean. In central Dolpo, where the Upper Permian is thicker than in other sections of the Northern India and Nepal Tethys Himalaya, and sandstones occur both just below and above the Permian/Triassic boundary, the Thini Chu Group rests discon- formably over Lower Carboniferous carbonates. A terra rossa paleosoil documents a major hiatus that possibly spans the Visean to Early Bashkirian (“rift unconformity”). Another major unconformity overlain by the Murgabian-Midian? “Costiferina arenites” is interpreted as indicating final break-up and initial spreading in Neotethys (“break-up unconformity”). In fact, arenite lenses mantling the unconformity yielded euhe- dral grains of Cr-rich chromian spinel, indicating enhanced partial melting of the asthenosphere rising beneath the Northern India-Nepal “upper plate” margin. Regional correlations indicate a late Early Permian age for this event. LithoZones recognized within the Thini Chu Group are here correlated with formations recently established to the east in Manang (Nepal), largely according to their petrofacies. The Atali Quartzarenite (Petrofacies 1; “white quartzarenites”) best corresponds with the Bangba Fm. of Manang, whereas the “Costiferina arenites” (Petrofacies 2a), the “ochre pelites” and “estuarine quartzarenites” (Petrofacies 2b), and the “black shales and glauco-phosphorites” (Petrofacies 3, 4 and 5) are broadly coeval with Member C of the Puchenpra Formation. Detritai feldspars appear in the middle part of the Thini Chu Group (Petrofacies 2) and sharply and progressively increase upwards (Petrofacies 3), until they peak around the Permian/Triassic boundary (Petrofacies 4 to 5). This trend may be explained with continued uplift of rift shoulders after breakup, or rather with rapidly increasing aridity towards the close of the Permian, while Gondwana was shifting northwards toward the Southern Tropic. Arkosic composition around the Permian/Triassic boundary may be also consistent with an arid episode at global scale. © Masson, Paris, 1996.