Info

o ooOOOOOs§

Figure 10.26 (a,b) Initial conditions and (c,d) results of a finite element model of the Himalayan-Tibetan orogen (images provided by Y. Yang and M. Liu and modified from Liu & Yang, 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). The Main Boundary Thrust is simulated by a weak zone that is adjusted to reflect the degree of mechanical coupling between the Indian plate and the Eurasian continent. Effective viscosity profiles in (b) correspond to the Tibetan Plateau (dashed line), the Indian plate (thick line), and the rest of the Eurasian continent (thin line). (c) Shows the predicted stresses at 10km depth. Each symbol is a lower hemisphere stereonet projection of the three-dimensional stress state, similar to that used for earthquake focal mechanisms. (d) Shows variation in predicted stresses with depth. The depth variation is related to the rheological model used in (b). Scale shown in lower right.

to the bottom of the model. Unlike many other models, this experiment also incorporates lateral variations in rheology using three crustal blocks, including a stiff Indian plate, a weak Tibetan Plateau, and an intermediate-strength Asian continent north of the plateau. In addition, six layers of material with different effective viscosities represent vertical variations in the rheology of these blocks (Fig. 10.26b).

Within this framework, Liu & Yang (2003) considered that combinations of the following forces contribute to the present state of stresses in the Himalaya and Tibetan Plateau: (i) a horizontal compressive force resulting from the collision of India with Asia; (ii) buoyancy forces resulting from isostatically compensated topography; (iii) basal shear on the Eurasia plate as India slides beneath Tibet; and (iv) horizontal forces originating from the pull of subduction zones located south and east of Asia. The stress field is constrained by GPS data, earthquake focal mechanisms, topography, and other observations. Figure 10.26c and d show the predicted stresses in the upper crust (at 10 km depth) using the velocity boundary conditions based on geodetic data: a uniform convergence rate of 44 mm a-1 toward N20°E at the Himalayan front (V1), 7 mm a-1 to the east on the east side (V2), and 10 mm a-1 to the southeast on the southeast side of the model (V3). A velocity of 20 mm a-1 to the north (VN) occurs at the western side of the model and decreases to zero on the eastern side. Higher convergent rates lead to enhanced mechanical coupling between the Eurasian and Indian plates, although this effect can be offset by a Main Boundary Thrust Zone that is mechanically weak.

The model results suggest that the surface velocity field and regime of deformation in the orogen (Section 10.4.3) reflect a mechanical balance between gravitational buoyancy, the indenting Indian plate, and the specific geometry and the boundary conditions of the plateau. Crustal thickening and topographic uplift are enhanced by the presence of the Tarim Basin, which acts as a rigid back-stop at the northern end of the model. To obtain the observed east-west extension and the high elevations of Tibet, the Tibetan crust must be very weak. The model suggests that the force balance evolves through time as the crust deforms and thickens. When the plateau is 50% lower than its present elevation of nearly 5 km, strike-slip and reverse faulting dominate the plateau region. Significant crustal extension occurs when the plateau reaches 75% of its present elevation. The model also suggests that although far field extensional forces may enhance the collapse of the plateau they are not required. Basal shear also enhances the extensional regime in the Himalaya and southern Tibet while increasing shortening in northern Tibet. This latter effect results because basal shear relieves the compressive (indentation) stresses that balance the buoyancy forces driving extension at the southern edge of Tibet. This leads to a decrease in compressive stress in the upper crust, which enhances extension. North of the Indus-Zangbo suture, the basal shear adds to the horizontal compression, resulting in increased shortening.

4 Lower crustal flow and ductile extrusion. The simple numerical and analogue experiments of indentation described above illustrate the sensitivity of deformation in collisional belts to local boundary conditions and variations in lithospheric rheology. A particularly interesting group of numerical experiments has explored the effects of weak, flowing middle and lower crust on the dynamics of continental collision. This condition of weak crust is in good agreement with geologic and geophysical observations indicating that the middle crust beneath Tibet is hot, fluid-rich, and/or partially molten (Section 10.4.5).

Royden (1996) and Ellis et al. (1998) showed that a vertical stratification of the lithosphere into strong and weak layers influences the degree of strain localization during convergence. Where the lower crust is relatively strong and resists flow, the crust tends to couple to the underlying mantle during shortening and results in a relatively narrow zone of localized strain at the surface. This effect may explain the relatively narrow width, triangular shape, and lack of a high orogenic plateau in the Eastern Alps and the Southern Alps of New Zealand. By contrast, where the lower crust is relatively weak and flows easily, the crust decouples from the mantle and results in diffuse strain. This latter effect may apply to Tibet and the central Andes where low viscosity zones have developed in the deep crust during crustal thickening and wide, steep-sided plateaux have formed above the weak zones (Sections 10.2.4, 10.4.5).

A vertical decoupling of the lithosphere as a result of ductile flow in a weak lower crust is well illustrated along the northern and eastern margins of Tibet. In these regions balanced cross-sections show that thrust faults sole out into décollement surfaces in the middle crust (Yin & Harrison, 2000). A comparison of geodetic data (Fig. 10.16a) and geologic observations has indicated that the lateral motion of the crust in the Longmen Shan region of eastern Tibet is mostly accommodated by lower crustal flow with little faulting occurring at the surface (Burchfiel, 2004). Northwest of the Sichuan Basin topography is anomalously high compared to the rest of Tibet. Clark & Royden (2000) and Clark et al. (2005) explained these relationships as a result of dynamic pressure resulting from the lateral flow of a partially molten lower crust as it encounters the strong crust and upper mantle of the Sichuan Basin. At the western margin of the basin, the flowing lower crust diverts northeastward along a rheologically weak crustal corridor that coincides with the Paleozoic-Mesozoic Qinling suture. The response of the upper crust to this flow may include dynamic uplift and strike-slip faulting, resulting in the anomalously high topography of eastern Tibet compared to its central and southern sectors.

The strain-softening effects of continental underthrusting coupled with enhanced surface erosion also can result in strain localization that alters the dynamics of orogenesis. An excellent example of this process occurs in the Southern Alps of New

Zealand (Section 8.6.3). Strain-softening feedbacks also have contributed significantly to the tectonic evolution of the Himalayan fold and thrust belt and southern Tibet where Indian lithosphere is underthrust to the north beneath Eurasia. Hodges (2000) summarized nine principal geologic and tectonic features of this relatively narrow zone that require explanation in any quantitative model of the orogen. These features include (Fig. 10.20c): (i) rapid erosion of the southern flank of the Himalaya; (ii) shortening on the Main Central Thrust (MCT) system and thrust faults to the south; (iii) extension on the South Tibetan Detachment (STD) system; (iv) high-grade metamorphism and crustal melting in the Greater Himalaya; (v) crustal melting in the middle crust beneath Tibet; (vi) juxtaposition of contrasting lithologies across the MCT; (vii) an inverted metamorphic sequence where high-grade rocks are thrust over the Lesser Himalaya along the MCT; (viii) the position of the Indus-Zangbo suture; and (ix) normal faults accommodating north-south extension in the southern Tibetan Plateau.

To determine how enhanced erosion coupled with continental underthrusting may explain these principle features, Beaumont et al. (2001, 2004) constructed thermomechanical models involving combinations of two related processes. The first process is a channel flow of ductile middle to lower crust. Channel flow involves the lateral movement of partially molten crust in a narrow zone bounded above and below by shear zones. These authors used this type of flow to explain the progressive growth of the Tibetan Plateau. The second process is the ductile extrusion of high-grade metamorphic rocks between coeval normal-sense and thrust-sense shear zones. This latter process is used to explain the exhumation of the Greater Himalaya rocks along the southern flank of the mountain range. In the models these two processes are linked through the effects of surface denudation (i.e. the removal of surface material) that is focused along the southern edge of a plateau and the presence of low viscosity, partially molten crust beneath Tibet. Variations in crustal thickness between the high plateau and the Ganga foreland, the rate of denudation and upper crustal strength also affect the style of the deformation. The models are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the modeled plateau.

The thermomechanical models of Beaumont et al. (2004) consist of a vertical plane divided into crust and mantle layers (Fig. 10.27a). A passive marker grid and numbered vertical markers track the progressive deformation of the model during convergence. The suture (S) marks the position where Indian lithosphere is subducted beneath Eurasia and descends into the mantle at a constant velocity (Vp) and a constant dip angle (0). This point is allowed to migrate during convergence. The basal velocity condition drives flow in the upper plate. The crust consists of upper and middle quartzo-felspathic layers overlying a dry granulitic lower crust that are modeled using a viscous-plastic power-law rheology. The initial thermal structure (Fig. 10.27b) shows two radioactive layers (A1, A2) that provide internal heat to the crust. The lithosphere-asthenosphere boundary is defined to be at the 1350°C isotherm. Given a basal heat flux of qm = 20 mW m-2, a surface heat flux of qs = 71.25 mW m-2, and a surface temperature (Ts) of 0°C with no heat flux through the sides of the model, the Moho temperature is 704°C. Other important model properties include an extra increment of viscous weakening in the crust, which simulates the presence of a small amount of partial melt, and surface denudation scaled to 1.020 mm a-1.

Figure 10.27c-e show the results of a model that provides an internally consistent explanation of the large-scale geometry and tectonic features of the Himalaya and southern Tibet. This model incorporates a convergence rate of 50 mm a-1 and advancing subduction, which mimics the manner in which precollisional suture zones wrap around the rigid India indenter as it penetrates into Eurasia. Surface denudation also requires the suture (S) to advance, which is modeled at a rate of Vs = 25 mm a-1. Although S moves during the model, the results in Fig. 10.27c-e are shown with a fixed point "S" to keep the size of the diagrams manageable. Advancing subduction requires the removal of Eurasian lithosphere, which also is modeled by subduction. Indian lower crust is subducted along with its underlying mantle lithosphere. No displacements occur out of the plane of the model.

As Indian lithosphere is underthrust beneath southern Tibet, channel flow initiates by the development of partially molten material in the mid-lower crust beneath the plateau (Fig. 10.27c,d). Coeval thrust- and normal-sense shear zones develop across the lower and upper parts of the channel, respectively. These shear zones are interpreted to correspond to the Main Central Thrust and the South Tibetan Detachment Fault. The channel propagates through the converging crust. Efficient erosion at the southern edge of the plateau leads to a coupling between the channel flow and surface denudation. Denudation causes the surface position of the suture (S) to migrate toward India relative to the mantle (Fig. 10.27c-e) because it creates an imbalance in the flux of crustal material through the model. The final position of the suture after 51-54 Ma mimics the position of the Indus-Zangbo suture within the Tibetan Plateau.

The coupling between channel flow and surface denudation eventually leads to the ductile extrusion and exhumation of hot material in the channel between the coeval thrust and normal faults (Fig. 10.27d,e). The exhumation exposes the high-grade metamorphic rocks and migmatite (i.e. a mixed rock consisting of both metamorphic and igneous components) of the Greater Himalaya. The provenance of the channel material is derived from two sources. Initially, melt weakening in the middle crust occurs just to the south of point "S" (Fig. 10.27c). Later, as the suture is advected southward, material is derived from

(a) Crust and marker grid

600 km India Suture Eurasia 1400 km

(a) Crust and marker grid

600 km India Suture Eurasia 1400 km

(c) t = 30 Myr Ax = 1500 km EroSion front Current rate of erosion

Figure 10.27 (a,b) Initial conditions and (c-e) results of a thermomechanical model of the Himalayan-Tibetan orogen (images provided by C. Beaumont and modified from Beaumont et al., 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Passive marker grid and mechanical layers are shown in (a). Initial thermal structure, radioactive layers, isotherms, and instantaneous velocity vectors are shown in (b). Top panels in (c-e) show deformed marker grid, bottom panels show evolved thermal structure. Heavy line with dots represents position of model suture (vertical marker 0), S is the mantle suture, whose position is tracked by a horizontal distance (Dx). Plots above profiles show the distribution and rate of slope-dependent erosion across model surface. The amount of convergence, which progresses from 1500 km to 2400 km, also is marked by horizontal distance Dx.

Figure 10.27 (a,b) Initial conditions and (c-e) results of a thermomechanical model of the Himalayan-Tibetan orogen (images provided by C. Beaumont and modified from Beaumont et al., 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Passive marker grid and mechanical layers are shown in (a). Initial thermal structure, radioactive layers, isotherms, and instantaneous velocity vectors are shown in (b). Top panels in (c-e) show deformed marker grid, bottom panels show evolved thermal structure. Heavy line with dots represents position of model suture (vertical marker 0), S is the mantle suture, whose position is tracked by a horizontal distance (Dx). Plots above profiles show the distribution and rate of slope-dependent erosion across model surface. The amount of convergence, which progresses from 1500 km to 2400 km, also is marked by horizontal distance Dx.

the Eurasian side of the suture. This process predicts that channel crust south of the Indus-Zangbo suture will show Indian crustal affinities, whereas channel material north of the suture will have Eurasian crustal affinities in a manner consistent with geologic observations (Section 10.4.4). Other similar models predict the formation of gneiss domes similar to those observed in the Greater Himalaya.

Was this article helpful?

0 0
Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook


Post a comment