Chamba Syncline: fabric development 25/03—06/04/2012

Prograde fabric development in metasediments of the Chamba syncline in the NW Himalaya: a neat example of the changes seen with structural depth and increasing metamorphic grade from the first slaty cleavage in the lower chlorite zone to garnet-zone schist with a complex fabric developed from a crenulation cleavage.

First, five large-field images taken at the same scale, to show macroscopic changes in style with grade. Then, details of microstructure in sequence of increasing structural depth and metamorphic grade.

These photos formed the basis for an examination question in 2012. The samples were collected by J.D. Walker (D.Phil 1998).
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Sample 1 (JDW223b) Sample 1 (JDW223b). Lower chlorite zone (lowest-grade sample). Banded silty mudstone from the core of the large-scale syncline. This is from the hinge zone and...
Sample 1 (JDW223b)
Sample 4 (JDW221) Sample 4 (JDW221). Chlorite zone. A graphitic mica-rich phyllite. Lithological darker-lighter banding (S0) just visible at a small angle to the prominent S1...
Sample 4 (JDW221)
Sample 6 (JDW180) Sample 6 (JDW180). Upper chlorite zone metapelite. Sedimentary layering (S0) visible at the top of the photo. The obvious planar fabric is at 10-15° to...
Sample 6 (JDW180)
Sample 7 (JDW230) Sample 7 (JDW230). Biotite zone metapelite. Clearly a spaced cleavage with fairly broad and somewhat lenticular quartz-rich microlithons separated by...
Sample 7 (JDW230)
Sample 8 (JDW226) Sample 8 (JDW226). Garnet zone. No garnet in this specimen but it occurs nearby. A much coarser rock now with well-differentiated mica-rich and quartz-rich...
Sample 8 (JDW226)
Sample 1 (JDW223b) Sample 1. Detail of slaty cleavage crossing silty layer. Field of view 3.5 mm.
Sample 1 (JDW223b)
Sample 1 (JDW223b) Sample 1. Further detail at lower left of previous photo, shows narrow micas aligned in the slaty cleavage, trains of opaque oxide along the cleavage, and...
Sample 1 (JDW223b)
Sample 2 (JDW220) Sample 2 (JDW220). Lower chlorite zone. Metagreywacke. Tectonic fabric defined by chlorite and muscovite between and around clastic quartz and feldspar. Note...
Sample 2 (JDW220)
Sample 2 (JDW220) Sample 2 (JDW220). Lower chlorite zone. Metagreywacke. Same as previous. Note here the varied patterns of undulose extinction and subgrains in the quartz...
Sample 2 (JDW220)
Sample 3 (JDW179) Sample 3 (JDW179). This rock has an almost-continuous fine schistosity but there are still scattered chlorite-mica stacks with mineral cleavage at high angles....
Sample 3 (JDW179)
Bani Hamid Quartzites
Moine Thrust Zone