3.17 Pre-Himalayan tectonics in Zanskar

The HHCS of Zanskar is composed of metasedimentary rocks (metamorphic equivalent of the Phe Formation) intruded by orthogneisses. The protolith of these orthogneisses are essentially the Cambro-Ordovician granites associated with the late Pan-African event. This is testified by several radiometric ages. Frank et al (1975) obtained a whole-rock Rb/Sr age of 495±16 Ma for the Kade orthogneiss located in the southwestern part of the studied area. Pognante et al. (1990) obtained a whole rock Rb/Sr isochron age of 549±70 Ma and a zircon U/Pb age of 472 (+9, -6) Ma for the same intrusion. Tertiary leucogranites are also frequently present at the top of the HHCS of Zanskar. It has been shown on the bases of their isotopic composition that these leucogranites probably originated from the partial melting of both the para- and the orthogneisses of the underlying HHCS (Ferrara et al. 1991). Thus the U/Pb upper intercept age of 499±235 Ma that we obtained for the monazites of the Tertiary Gumburanjun leucogranite as well as the better constrained zircon ages of 463±13 Ma and 476±12 Ma obtained by Noble and Searle (1995) from two leucogranitic intrusions in NW Zanskar are interpreted as representing relictual ages of the Cambro-Ordovician granites. Recent zircon U/Pb ages obtained by Girard and Bussy (1998) for the Rupshu area also yielded ages of 479±2 Ma for both the Polokonga La granite and the Tso Morari gneiss and of 482±1 Ma for the Rupshu granite. All these data show that the Zanskar area was greatly affected by the intrusive episode of the late Pan-African event.

Further evidences of pre-Himalayan deformations can be observed in the Zanskar region within the Tethyan Himalaya stratigraphic record. Along most of the main NW-SE trending Doda valley, the Permian Panjal Traps rest directly upon the Precambrian to Cambrian Phe Formation and then, from Padum to Tanze, the Panjal traps top the Cambrian Karsha Formation. Both the Karsha and Phe formations form large-scale undulations of decametric to kilometric wavelength which are especially well evidenced, at a distance, by the bending of the dolomitic Thidsi Member. These undulations have axes with a general NE-SW to N-S orientation and an almost vertical axial plane. As the overlying Panjal Traps truncate these folds and as the sediments younger than the Permian are not affected by this deformation phase, these deformations must be related to a pre-Permian tectonic event.

The timing of this event can be better constrained in the Tanze area, where the Karsha Formation steeply plunges towards the southeast, whereas the Panjal Traps remain at a constant altitude. The depression that forms between these two previously superposed formations is progressively, but very quickly, filled with the complete succession of Paleozoic sediments missing to the northwest. Despite the strong Himalayan deformation affecting the sediments within this syncline, it can be observed that the Cambrian Kurgiakh Formation follows the undulation of the underlying sediments, but is unconformably overlain by the Ordovician Thaple formation. Thus, in Tanze and in Spiti, the presence of an unconformity at the base of the Ordovician points towards a tectonic event that occurred at the Cambro-Ordovician boundary.

It is also in the Tanze area that Srikantia et al. (1980) claim to have found clasts of Lower Paleozoic tourmaline bearing granites within the Kurgiakh Formation (their Thango Formation). This observation was for these authors, as also for later ones (Baud, 1984; Garzanti et al. 1986), one of the arguments for the uplift and erosion of a cratonic basement related to a late Pan-African orogenic cycle in Cambro-Ordovician time. However, we have never found any granitic clasts nor any metamorphic rock fragment within the conglomerates of the Thaple Formation. This is explained by the fact that Srikantia et al. (1980) most likely mistook Quaternary deposits for the Thaple Formation, because they associate their granitic clast with the erosion of the Gumburanjun leucogranite which they thought to be of Lower Paleozoic age, but which, in reality, is of Tertiary age!. In fact, clear evidence of a marked schistosity or regional metamorphism usually associated with an orogenic process is lacking in Zanskar.

Thus, even if we agree with the interpretation that the Cambro-Ordovician boundary is marked in the Tethys Himalaya of Zanskar by the uplift and erosion of deformed early Paleozoic sedimentary sequences, we find it very difficult to determine what caused these tectonic movements. As mentioned above, the bending of the Precambrian to Cambrian formations, the unconformity at the base of the Thaple Formation and the granitic intrusions could as well be related to extensional movements as to a pre-Himalayan orogenic event.

From the Ordovician to the Permian, we believe the Tanze area to represent the rim of a basin. This is supported by the fact that all the formations from this period thin out gradually from the southeast towards the Tanze syncline where they disappear. The Zanskar region, lying northwest of Tanze, might thus have been for most of the Paleozoic a structural height with no or very little sedimentation. Several periods of non-deposition or erosion, also within the basin, are revealed by the absence of the Silurian and by unconformities at the base of most of the Ordovician to Permian formations. The more or less continuous subsidence of the basin zone until the Permian is attested by the progressive tilting of the Paleozoic formations towards the southeast. These tectonic movements are most probably related to the opening of the Neotethys and have accentuated the downwarping of the Thidsi Formation, initiated in the Cambro-Ordovician.

The subsidence or uplift movements in the Tanze area, as in the rest of Zanskar, seem to have come to an end in the "mid" Permian as is shown by the Panjal Traps which seal the underlying structures, forming what looks like a Permian peneplain. This basaltic outpouring which is related to the oceanisation of the Neo-Tethys is followed by the thermal subsidence of the whole area which is then covered by an open sea carbonate platform.