From 1989 to 1995 I mapped in the Annapurna region of the Nepal Himalaya, and collected an extensive set of large, oriented samples from the Main Central Thrust Zone (MCTZ). The aim of this project is to evaluate in detail the structural history of this zone. In addition, the samples I have collected can eventually be used for detailed thermochronology and thermobarometry studies. Below is a slightly modified abstract that I published, which outlines some of the interesting features of the MCTZ. I have added links to diagrams and photos. In several of the maps, mineral elongation lineations are shown as black lines; the rest is self-explanatory. This project has been funded by small ARC grants to me, mainly through Macquarie University.
The Himalayan chain was caused by collision between the Indian and Eurasian plates. The Main Central Thrust Zone (MCTZ) is a major ductile shear zone along the southern base of the Himalaya Mountains. The MCTZ: (1) is defined by a zone, commonly exceeding 10 km in thickness, of highly-strained schists and gneisses with a gently-dipping foliation; (2) is traceable for more than 2000 km along strike; (3) displays a throw that is consistently around 100 km; and (4) has a strong mineral elongation lineation along its entire length. For these four reasons, the MCTZ is one of the most impressive geological structures exposed at Earth's surface. An on-going project investigating deformational and metamorphic evolution of the MCTZ in the Annapurna-Manaslu-Ganesh region of the Nepal Himalaya shows how little is actually known about kinematics and deformation paths in the MCTZ.
In the Annapurna region, the upper portion of the MCTZ is composed of kyanite and sillimanite gneisses of the Tibetan Slab, whereas the lower portion is composed of chlorite, biotite and garnet schists of the Lesser Himalayas (click here for a map showing my sample distribution).The MCTZ is generally considered to have accommodated north-over-south displacement, bringing the higher grade gneisses over the lower grade schists. A "thrust plane" lying at the contact between schists and gneisses has been accepted by many workers, and is known as the Main Central Thrust. The MCTZ might be expected to have a relatively simple deformation history involving a significant component of simple shear along one main foliation, possibly with development of shear bands or S & C planes. Although such features are present, the MCTZ has a more complex deformational history involving the development of crenulation cleavages at both high and low angles to the main, gently-dipping foliation. In fact, in most samples collected from the schists within the MCTZ, the main, gently-dipping foliation is a differentiated crenulation cleavage at various stages of development. This gently-dipping crenulation cleavage is continuous with sigmoidal inclusion trails in some chloritoid, garnet and staurolite porphyroblasts, and therefore formed during prograde metamorphism.
The MCTZ might also be expected to contain kinematic indicators showing unambiguous evidence for north-over-south displacement. Although many indicators are consistent with such displacement, there are complexities and contradictions in the kinematic history. In the Annapurna region, flat-lying shear bands overprint the main, gently north-dipping foliation, and in 85% of rocks containing shear bands they indicate north-over-south displacement. The other 15% contain conflicting shear bands that generally form conjugate sets, indicating local coaxial deformation. There is no systematic increase in development of shear bands as the MCTZ is approached from the south. The best development of the bands is at the base of the gneisses, just above the thrust plane, and in local horizons in the schists below. Thus, deformation and shear-band development were heterogeneous throughout the MCTZ. The shear bands formed during retrograde metamorphism, as shown by their close association with chlorite that replaces garnet porphyroblasts, and muscovite that replaces kyanite porphyroblasts. Thus, the shear bands post-date development of the prograde mineral assemblage and the main, gently-dipping foliation. Some previous workers (e.g. Brunel, 1986, Tectonics, 5, 247-265) have postulated that the late-stage movement forming the shear bands was probably responsible for most of the nappe transport, but little is known about the kinematics of the MCTZ during the earlier prograde metamorphism.
Past workers have referred almost exclusively to garnet porphyroblasts with spiral-shaped inclusion trails to support the idea that the prograde kinematics involved north-over-south movement (e.g. Brunel, 1986). Apart from the fact that the mechanism of spiral-shaped inclusion-trail formation is controversial (Johnson, 1993, J. Met. Geol., 11, 635-659), use of these porphyroblasts as shear sense indicators assumes that they grew during formation of the MCTZ. This assumption can be tested by determining the 3-D orientations of spiral axes in garnet porphyroblasts (Johnson, 1993, J. Met. Geol., 11, 621-634). If the following three criteria can be met, it may be reasonably argued that the porphyroblasts are contemporaneous with the MCTZ: (1) the matrix foliation is continuous with the inclusion trails in the porphyroblasts, (2) the spiral axes lie in the matrix foliation, and (3) the spiral axes are perpendicular to the mineral elongation lineation. If these three criteria cannot be met and there is no evidence for more than one rotational axis within the porphyroblasts, it is likely that the porphyroblasts are not contemporaneous with the MCTZ. Garnet porphyroblasts in most samples so far tested are probably not contemporaneous with the MCTZ, and are therefore not useful as indicators of shear sense or deformation paths within the MCTZ. Numerous samples still need to be evaluated.
Other prograde microstructures include symmetrical strain shadows around opaque (magnetite or ilmenite) grains, and foliation-parallel boudinage of metamorphic minerals where the two halves of bouninaged minerals have not rotated relative to one another. These observations may suggest that, at least locally, the deformation was coaxial, but they do not elliminate the possibility of an overall non-coaxial deformation path.
Bouchez & Pecher (1981, Tectonophysics, 78, 23-50) conducted a strain analysis on quartz porphyroclasts in the lower part of the MCTZ in the Annapurna region. Very low K values (0.12-0.35) place the rocks in the flattening field on a Flinn diagram, and may provide further evidence that, at least locally, pure shear was important. These authors argued that shear strains probably reached higher values towards the thrust plane, but the strain profile remains unknown, owing to lack of suitable markers.
Bouchez & Pecher (1981) have also used quartz c-axis fabrics to study MCTZ kinematics. They noted that 80% of their samples indicated north-over-south displacement, and suggested the other 20% indicated local heterogenieties in either the strain gradient or strain path. It is now generally accepted that c-axis fabrics within the MCTZ probably largely reflect the later deformation associated with shear bands and retrograde metamorphism (Brunel, 1986). Thus, quartz c-axis fabrics appear to be of little use in unravelling the prograde kinematics.
Given the above information regarding the MCTZ, some questions that need to be answered include the following.
(1) Are the main, gently-dipping foliation and later shear bands products of a deformational continuum that spanned different metamorphic conditions, or products of separate deformation events?
(2) Why is the main, gently-dipping foliation most commonly a crenulation cleavage in the schists below the thrust plane? Was the deformation that formed this foliation highly non-coaxial on the scale of the MCTZ, as is generally assumed?
(3) If most of the nappe transport occurred during the late-stage deformation that produced the shear bands, what was the role of the MCTZ during its earlier history?
(4) If spiral-shaped inclusion trails in garnet porphyroblasts are not contemporaneous with the MCTZ, they provide no evidence for non-coxial deformation within the zone. When did these garnet porphyroblasts grow? Radiometric dating may help solve this mystery.
