Regional Geologic Framework
The base of the Kathmandu Nappe is defined by the Mahabharat thrust, which is widely but not universally interpreted as the southern trace of the Main Central thrust (e.g., Stöcklin, 1980; Fuchs, 1982; Pêcher and Le Fort, 1986; Pandey et al., 1995; Arita et al., 1997; Upreti and Le Fort, 1999; Johnson et al., 2001; cf. Searle et al., 2008). Early Proterozoic Lesser Himalayan Sequence rocks occur in the Mahabharat thrust footwall, whereas the hanging wall is dominated by the Late Proterozoic Bhimphedi Group and overlying Ordovician-Devonian Phulchauki Group (Figure 2) (e.g., Stöcklin, 1980; Pearson and DeCelles, 2005; Gehrels et al., 2006). The Bhimphedi and Phulchauki Groups are deformed in an elongated bowl-shaped synform, the Kathmandu synform, which forms the bulk of the Kathmandu Nappe.
The Phulchauki Group is generally dominated by unmetamorphosed (anchizone) limestone, shale, and sandstone, with a conglomeratic horizon and associated unconformity at or near the base. These rocks are generally accepted as correlative to age-equivalent Tethyan Himalayan Sequence rocks exposed to the north (e.g., Upreti, 1999). The Bhimphedi Group consists of pelites, quartzites and carbonates which are locally intruded by Cambro-Ordovician granitoids (Stöcklin, 1980; Gehrels et al., 2006). The Bhimphedi rocks display a progressive increase in grade with structural depth, from chlorite phyllites at the top to garnet schists at the base (Figure 4) (Johnson et al., 2001).
Figure 4. Geological map of the lower Mahesh Khola
Geological map of the lower Mahesh Khola from Johnson et al. (2001). MCT/MT = Main Central thrust / Mahabharat thrust.
The Bhimphedi Group forms the immediate hanging wall of the Mahabharat thrust / Main Central thrust throughout the Kathmandu synform, except in the north. There, the Sheopuri gneiss occurs directly along the thrust (Stöcklin and Bhattarai, 1982; these rocks are alternatively named the Gosainkund gneiss). The Sheopuri gneiss consists of kyanite- / sillimanite-bearing paragneisses, orthogneisses, and migmatites (e.g., Rai et al., 1998). The Sheopuri gneiss appears contiguous with the Greater Himalayan Crystalline complex to the north, and thus may be the same tectonic unit.
The nature of the contact between the Sheopuri gneiss and the adjacent Bhimphedi Group is disputed. The Sheopuri gneiss may transition laterally to lower metamorphic conditions, such that the Bhimphedi Group is the low temperature equivalent and there is no sharply defined contact between these units (e.g., Stöcklin, 1980). Alternatively, a fault zone may separate the units: (1) the Main Central thrust may place Sheopuri gneiss above Bhimphedi rocks (with the Mahabharat thrust interpreted as a structurally lower splay of the Main Central thrust) (Rai et al., 1998; Upreti and Le Fort, 1999), or (2) the South Tibet detachment may place the Bhimphedi Group rocks atop the Sheopuri gneiss (Webb et al., 2011).
Such uncertainties are the essence of the Lesser Himalayan Crystalline Nappe problem, i.e., what are the tectonic positions of the Bhimphedi and Phulchauki Group rocks? If the Phulchauki Group is accepted as Tethyan Himalayan Sequence, then is there a South Tibet detachment-type contact separating them from the metamorphic rocks of the Bhimphedi Group below? If the Bhimphedi Group rocks are taken as frontal equivalents of the Greater Himalayan Crystalline complex, which famously has an inverted metamorphic field gradient, then why does the Bhimphedi Group display a right-way-up gradient?
Below, we discuss the implications of different Lesser Himalayan Crystalline Nappe interpretations for tectonic models for the assembly of the main three Himalayan units (i.e., the Lesser Himalayan Sequence, the Greater Himalayan Crystalline complex, and the Tethyan Himalayan Sequence). Next, we outline the geology to be observed throughout the field trip.
+ (Himalayan.tectonics) Himalayan tectonic models vs. the lesser Himalayan Crystalline Nappes
Early models for the construction of the Himalayan orogen suggested that tectonic units were juxtaposed by in situ thrusting of Indian basement and cover sequences (Argand, 1924; Heim and Gansser, 1939; Dewey and Bird, 1970; Le Fort, 1975). However, top-north shear structures recognized along the gently north-dipping Greater Himalayan Crystalline complex – Tethyan Himalayan Sequence contact at the range crest (e.g., Caby et al., 1983; Burg et al., 1984) have been interpreted by most workers as evidence for major normal faulting during Himalayan orogenesis. The contact is now generally structurally defined as the South Tibet detachment, and is commonly interpreted as a top-north low-angle normal fault system with 10s or even 100s of km of slip (e.g., Searle, 1986; Burchfiel et al., 1992). Current models for the assembly of the Himalayan units focus on the emplacement of the Greater Himalayan Crystalline complex along the South Tibet detachment and the Main Central thrust. These models exclude consideration of the Lesser Himalayan Crystalline Nappes, perhaps because (1) these are a volumetrically modest component of the range, and (2) as discussed above, the Nappes are commonly interpreted as contiguous with one of the three major tectonic units (Upreti and Le Fort, 1999; Webb et al., 2011). In this section, the three major models for the emplacement of the Greater Himalayan Crystalline complex are reviewed and then discussed in the context of the Lesser Himalayan Crystalline Nappes.
Tectonic Models
The first kinematic model proposed after the discovery of the South Tibet detachment is wedge extrusion. In this model, the Greater Himalayan Crystalline complex extruded southwards between the other two units as a northward-tapering wedge (Figure 5A) (Burchfiel and Royden, 1985). Recently, these kinematics have been understood in the context of critical taper – Coulomb wedge theory (e.g., Robinson et al., 2006; Kohn, 2008; Zhang et al., 2011), which suggests that normal faulting may occur during collapse of over-thickened thrust wedges (e.g., Davis et al., 1983; Dahlen, 1990).
The second kinematic model is channel flow – focused denudation. The Greater Himalayan Crystalline complex represents partially molten lower/middle crust that tunnels southwards during the Eocene-Oligocene (Figure 5B) (e.g., Beaumont et al., 2001; 2004; Godin et al., 2006). Subsequently, the channel is exhumed by enhanced erosion across a narrow zone where precipitation is focused along the topographic front of the orogen (e.g., Beaumont et al., 2001; Hodges et al., 2001). In both wedge extrusion and channel flow – focused denudation models, the Main Central thrust and South Tibet detachment are active, surface-breaching faults during Early-Middle Miocene emplacement of the Greater Himalayan Crystalline complex.
Figure 5. Tectonic models for the emplacement of the Greater Himalayan Crystalline Complex
Tectonic models for the emplacement of the Greater Himalayan Crystalline Complex (GHC) modified from Webb et al. (2013). A. Wedge extrusion (e.g., Burchfiel and Royden, 1985). B. Channel flow – focused denudation (e.g., Beaumont et al., 2001). C. Tectonic wedging (e.g., Webb et al., 2007). ITS = Indus-Tsangpo suture; THS = Tethyan Himalayan Sequence; LHS = Lesser Himalayan Sequence
The third kinematic model is tectonic wedging. The South Tibet detachment is interpreted as a backthrust splaying off of the Main Central thrust (Figure 5C) (Yin, 2006; Webb et al., 2007). Motion along these thrusts accommodated Greater Himalayan Crystalline complex emplacement below the Earth surface, with exhumation resulting from subsequent footwall duplex development (Yin, 2006; Webb et al., 2007). Kinematic models of channel flow – focused denudation and tectonic wedging are distinguished by two criteria: timing and extrusion. In the first model, channel tunneling occurs in the Eocene-Oligocene, preceding Miocene surface emplacement of the Greater Himalayan Crystalline complex via channel flow coupled to extrusion (e.g., Beaumont et al., 2001; Hodges et al., 2001; Godin et al., 2006). In contrast, proposed tectonic wedging occurs in the Miocene and accomplishes emplacement of the Greater Himalayan Crystalline complex at depth, without extrusion.
Structural Geometry of the Lesser Himalayan Crystalline Nappes
Four interpretations are advanced to explain the structural geometry of the Lesser Himalayan Crystalline Nappes (Figure 6): A. The three layer-two fault Himalayan tectonic pattern of the Himalaya is maintained across the Lesser Himalayan Crystalline Nappes. The South Tibet detachment contact occurs between the Bhimphedi Group and the Phulchauki Group, but has yet to be identified because of poor exposure (e.g., Yin, 2006). The Sheopuri gneiss and Bhimphedi Group are contiguous. B. The South Tibet detachment cuts upsection to the north of the Lesser Himalayan Crystalline Nappes. The Phulchauki Group was deposited on the southerly Greater Himalayan Crystalline complex rocks (e.g., Gehrels et al., 2003; Johnson, 2005). The Sheopuri gneiss and Bhimphedi Group are contiguous. C. The Main Central thrust cuts upsection along the northern margin of the Lesser Himalayan Crystalline Nappes. The Sheopuri gneiss forms the Main Central thrust hanging wall; the Bhimphedi and Phulchauki Groups are restricted to the footwall (e.g., Rai et al., 1998; Upreti and Le Fort, 1999; Hodges, 2000). The Mahabharat thrust at the base of the Bhimphedi Group is not interpreted as the Main Central thrust, but rather as a relatively minor synchronous thrust deforming the Lesser Himalayan Sequence. The Bhimphedi Group rocks are likewise interpreted as Lesser Himalayan Sequence rocks. The Phulchauki Group rocks are interpreted to represent the southernmost extent of the Tethyan basin, deposited south of the Cenozoic Main Central thrust. D. The South Tibet detachment merges with the Main Central thrust along the northern margin of the Lesser Himalayan Crystalline Nappes. The Sheopuri gneiss forms the South Tibet detachment footwall; the Bhimphedi and Phulchauki Groups are the hanging wall (Webb et al., 2011).
Figure 6. Cross-sections of the Lesser Himalayan Crystalline Nappes
Cross-sections with different interpretations of the Lesser Himalayan Crystalline Nappes (the Bhimphedi and Pulchauki Groups) taken from Webb et al. (2011). THS = Tethyan Himalayan Sequence; GHC = Greater Himalayan Crystalline complex; LHS = Lesser Himalayan Sequence.
The first three interpretations are generally compatible with all models for Greater Himalayan Crystalline complex emplacement, although model A requires these rocks to taper to the south, contrary to predictions of wedge extrusion models, and model B requires them to taper to the north, contrary to predictions of tectonic wedging models. However, model D limits the leading edge of the Greater Himalayan Crystalline complex to the northern margins of the Lesser Himalayan Crystalline Nappes, where it is locally preserved. This precludes extrusion models.