Structural geometry, microstructural and strain analyses of L-tectonites from Paleoproterozoic orthogneiss: Insights into local transport-parallel constrictional strain in the Sikkim Himalayan fold thrust belt

Abstract

Paleoproterozoic Lingtse orthogneiss shows three dominant structural associations in the Sikkim Himalayan fold thrust belt (FTB): (a) these rocks locally define the Pelling thrust (PT) zone that lies in the footwall of the Main Central thrust (MCT), (b) they occur as linear, almost N–S trending imbricates within the PT sheet, and (c) they form a N–S trending ∼13.5km long, ∼2km wide discontinuous linear mylonitic klippen within the Daling Formation of the lower Lesser Himalayan Sequence (LHS) that lie east of the N–S trending Teesta valley. The klippe records N–S trending, strong L-tectonites that are spatially associated with LS and SL tectonites; the stretching lineations are oriented along the transport direction. These rocks are exposed along three mylonite zones. From north to south these are the Phodong mylonite zone, the Tumin Eingee mylonite zone and the Rumtek mylonite zone.All the three mylonite zones are overall folded along gently plunging, transport-parallel, N–S trending synform with the strongest L fabric developed along the hinge zone, and its dominance decreases progressively toward the eastern and western structural contacts of the klippen. The highest constrictional strain is recorded in Phodong and Tumin-Eingee mylonite zones and decreases south toward Rumtek mylonite zone. The long axes (X) of the quartz strain ellipsoids show ∼N–S trend and gentle plunge that agrees with the stretching lineations observed in the field. Additionally, the XY plane of the generated ellipsoids overall agree with the observed variation of the mylonitic foliation planes in the field from all the three locations.Microstructural analysis from the mylonite klippe suggests that quartz and feldspar have undergone ∼35% and ∼76% grain size reduction with respect to the protolith. Dislocation creep and microfracturing are the dominant deformation mechanisms in quartz and feldspar, respectively. The grain-size reduction is more prominent near the eastern and the western boundaries as they mark structural contacts between the L tectonites and the Daling phyllites. Strain is partitioned between quartz and feldspar in the mylonite zones with quartz recording constrictional strain while feldspar records flattening strain. We explain this variation due to their different deformation mechanisms. Shear-sense analysis of feldspar porphyroclasts suggest dominantly transport-parallel flow along with a component of transport-perpendicular flow in the mylonite zones.Existing models on mechanism of L tectonite formation and transport-parallel folds do not completely explain our observations. We propose the possible existence of a lateral ramp beneath the PT sheet to explain the generation of L tectonites and transport-parallel fold in the study area.