3.16 Synthesis of stratigraphic observations

The presence, since the Upper Precambrian, of an elongated and narrow intracontinental sea between the Indian continent and the Cimmerian Superterrane is documented by the sedimentary series of the Phe Formation. The sediments of this 5000 to 10'000 metres thick formation (Fuchs and Linner, 1995; Wyss, 1999; Frank et al.,1995; Steck et al., 1998) are mainly derived from the erosion of the relief fringing this trough to the north. To accommodate such a thick series of shallow water sediments either a continuous sea level rise, or more likely a major subsidence must have affected this basin.

From the Lower to Middle Cambrian, the rate of detrital input and subsidence diminished, giving way to the formation of the dolomitic horizons, stromatolitic colonies and platform carbonates of the Karsha Formation.

In the Upper Cambrian, the presence of turbidites within the Kuru Member of the Kurgiakh Formation indicates a return to a deeper sedimentation environment and of tectonic subsidence exceeding the sedimentation rate.

At the boundary between the Cambrian and the Ordovician, a major tectono-magmatic event affected a large part of Gondwana. This so-called late Pan-African event is marked by numerous ~500 Ma old granitic intrusions forming a wide belt stretching from the Alps (Bussy et al. 1996) over the Arabic peninsula, Afghanistan, Africa, India, Australia and down to Antarctica (Le Fort, 1986 and references therein). In parallel with these intrusions, some of the above mentioned areas show evidences of tectonic activity marked by an angular unconformity at the base of the Ordovician sediments.

Evidence for this late Pan-African event is also frequently observed within the Himalaya. Not only are ~500 Ma granites quite common within both the Lesser Himalaya and the High Himalayan Crystalline Sequence (see Le Fort, 1986 and Valdiya, 1995 for a review), but an angular unconformity at the base of the Ordovician Thaple Formation has long been known to exist in the Spiti region (Hayden, 1904). Moreover, the conglomeratic lithologies of the Thaple Formation testify for the uplift and erosion of parts of the underlying formations, as these conglomerates incorporate fragments of the Phe, Karsha and Kurgiakh Formations.

The nature of the late Pan-African event in the Himalaya is still poorly understood. Several authors (Srikantia et al., 1980; Baud et al., 1984; Garzanti et al., 1986; Fuchs, 1987; Valdiya, 1995) associate this event with an orogenic episode, as can be observed at the same period in Antarctica with the Ross orogeny, while others (Le Fort et al. 1986; Miller and Frank, 1992; Vannay, 1993) consider this event to be associated with an episode of vertical movements associated to extension and crustal thinning.

After the Early Ordovician continental conglomerates, the sedimentary environment in the Himalaya is essentially littoral (Middle to Upper Ordovician members of the Thaple Formation) or coastal (Devonian Muth Formation). Most of the Silurian is missing, possibly as a consequence of erosion.

Following this rather uneventful period, the rifting between the Indian continent and the Cimmerian micro-continents, followed by the opening of the Neotethys, will of course greatly affect the sedimentary record within the Tethys Himalaya. An early transtensive stage of rifting is observed since the Early Carboniferous in the Lipak Formation, where synsedimentary extension faults were described by Vannay (1993). In the Lower Permian, an episode of thermal uplift of rift shoulders is suggested in many areas by the absence, to various degrees, of the Paleozoic formations (Ordovician to Trias), whilst in adjacent graben areas the stratigraphic record is complete. A magmatic event also occurred at the boundary between the Carboniferous and the Permian, as documented by the granitic intrusion of the Yunam, in the Sarchu region. These granites yield a zircon U/Pb age of 284±1 Ma (Spring et al. 1993).

The opening of the Neotethys starts in the Middle Permian with the formation of oceanic crust. This event is marked in many regions of the Himalaya by the outpouring of the Panjal Traps continental flows which, by sealing the underlying formations, sometimes reach considerable thicknesses (2000 metres as in the Kashmir basin).

Following the rifting of the Neotethys, the transgressive Upper Permian Kuling Formation testifies to the progressive subsiding of the passive Indian margin. The Triassic Lilang Group then corresponds to the formation of a carbonate platform on this flexural margin. The evolution of the northern Indian margin from the Triassic to the Eocene was described by Bassoullet et al. (1980, 1981; 1984), Baud et al. (1984), Gaetani et al. (1986, 1987), Gaetani and Garzanti (1991), Fuchs (1982a, b) and Spring (1993).