Page 345 - Global Tectonics

328 CHAPTER 10

presence of low viscosity, partially molten penetrates into Eurasia. Surface denudation
crust beneath Tibet. Variations in crustal also requires the suture (S) to advance, which
−1
thickness between the high plateau and the is modeled at a rate of V s = 25 mm a .
Ganga foreland, the rate of denudation and Although S moves during the model, the
upper crustal strength also affect the style of results in Fig. 10.27c–e are shown with a

the deformation. The models are relatively fixed point “S” to keep the size of the
insensitive to channel heterogeneities and to diagrams manageable. Advancing subduction
variations in the behavior of the mantle requires the removal of Eurasian lithosphere,
lithosphere beneath the modeled plateau. which also is modeled by subduction. Indian
The thermomechanical models of Beaumont lower crust is subducted along with its
et al. (2004) consist of a vertical plane divided underlying mantle lithosphere. No
into crust and mantle layers (Fig. 10.27a). A displacements occur out of the plane of the
passive marker grid and numbered vertical model.
markers track the progressive deformation of As Indian lithosphere is underthrust beneath

the model during convergence. The suture southern Tibet, channel flow initiates by the
(S) marks the position where Indian development of partially molten material in
lithosphere is subducted beneath Eurasia and the mid-lower crust beneath the plateau (Fig.
descends into the mantle at a constant 10.27c,d). Coeval thrust- and normal-sense
velocity (V p ) and a constant dip angle (θ). shear zones develop across the lower and
This point is allowed to migrate during upper parts of the channel, respectively.
convergence. The basal velocity condition These shear zones are interpreted to
drives fl ow in the upper plate. The crust correspond to the Main Central Thrust and
consists of upper and middle quartzo- the South Tibetan Detachment Fault. The
felspathic layers overlying a dry granulitic channel propagates through the converging
lower crust that are modeled using a viscous- crust. Efficient erosion at the southern edge

plastic power-law rheology. The initial of the plateau leads to a coupling between
thermal structure (Fig. 10.27b) shows two the channel flow and surface denudation.

radioactive layers (A 1 , A 2 ) that provide Denudation causes the surface position of
internal heat to the crust. The lithosphere- the suture (S) to migrate toward India

asthenosphere boundary is defined to be at relative to the mantle (Fig. 10.27c–e) because
the 1350°C isotherm. Given a basal heat fl ux it creates an imbalance in the flux of crustal

−2
of q m = 20 mW m , a surface heat fl ux of q s material through the model. The fi nal
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= 71.25 mW m , and a surface temperature position of the suture after 51–54 Ma mimics
(T s ) of 0°C with no heat fl ux through the the position of the Indus–Zangbo suture
sides of the model, the Moho temperature is within the Tibetan Plateau.
704°C. Other important model properties The coupling between channel fl ow and surface
include an extra increment of viscous denudation eventually leads to the ductile
weakening in the crust, which simulates the extrusion and exhumation of hot material in
presence of a small amount of partial melt, the channel between the coeval thrust and
and surface denudation scaled to 1.0– normal faults (Fig. 10.27d,e). The
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20 mm a . exhumation exposes the high-grade
Figure 10.27c–e show the results of a model metamorphic rocks and migmatite
that provides an internally consistent (i.e. a mixed rock consisting of both
explanation of the large-scale geometry and metamorphic and igneous components) of
tectonic features of the Himalaya and the Greater Himalaya. The provenance of
southern Tibet. This model incorporates the channel material is derived from two
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a convergence rate of 50 mm a and sources. Initially, melt weakening in the
advancing subduction, which mimics the middle crust occurs just to the south of point
manner in which precollisional suture zones “S” (Fig. 10.27c). Later, as the suture is
wrap around the rigid India indenter as it advected southward, material is derived from