INTERACTION OF METAMORPHISM, DEFORMATION, AND EXHUMATION IN LARGE CONVERGENT OROGENS

R.A Jamieson1, C. Beaumont2, M.H. Nguyen1,2, and B. Lee2

   1Department of Earth Sciences, Dalhousie University, Halifax, N.S., Canada B3H 3J5
   2Department of Oceanography, Dalhousie University, Halifax, N.S., Canada B3H 4J1

   Corresponding author: R.A. Jamieson, beckyj@is.dal.ca, (902) 494-3771

  Submitted to Journal of Metamorphic Geology, 'Metamorphic Processes', 
  special volume in honour of R.H. Vernon, 8 September, 2000

  Revised version submitted 28 February, 2001
 


ABSTRACT

Coupled thermal-mechanical models are used to investigate interactions between metamorphism, deformation, and exhumation in large convergent orogens, and the implications of coupling and feedbacks between these processes for observed structural and metamorphic styles. The models involve subduction of sub-orogenic mantle lithosphere, large amounts of convergence (³450 km) at 1 cm/y, and a slope-dependent erosion rate. The model crust is layered with respect to thermal and rheological properties - the upper crust (0-20 km) follows a wet quartzite flow law, with heat production of 2.0 mW/m3, and the lower crust (20-35 km) follows a modified dry diabase flow law, with heat production of 0.75 mW/m3. After 45 My, the model orogens develop crustal thicknesses on the order of 60 km, with lower crustal temperatures in excess of 700 ºC. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 ºC in the upper crust and 900 ºC in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T ³ 600ºC. The channel flow zone is characterised by a reversal in velocity direction on the pro-side of the system, and is driven by a depth-dependent pressure gradient that is facilitated by the development of a temperature-dependent low viscosity horizon in the mid-crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post-convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust-related exhumation on the flanks. Velocities in the pro-side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro-side flank of the plateau leads to 'ductile extrusion' of the channel flow zone. The exhumed channel displays apparent normal-sense offset at its upper boundary, reverse-sense offset at its lower boundary, and an 'inverted' metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P-T-t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.

Key words: thermal-mechanical models, regional metamorphism, convergent orogens, channel flow, exhumation


   Abstract    Fig. 1     Fig. 2   Fig. 3    Fig. 4     Fig. 5   Fig. 6    Fig. 7    Fig. 8   Fig. 9     Figure captions


 

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FIGURE CAPTIONS

Figure 1. Initial conditions for reference model LHO-71 and all other models presented here. Upper panel shows undeformed, rectangular, passive marker grid for the model crust; solid vertical lines at 100 km intervals are numbered (positive on pro-side, negative on retro-side) for comparison with the deformed grids shown in Figs. 2-6. Upper crustal layer is in light grey; lower crustal layer is white. Lower panel shows thermal structure of the model crust, with isotherms at 100 ºC intervals; 700 ºC isotherm shown as a heavy line to facilitate comparison between different models. Distribution of upper crustal (A1) and lower crustal (A2) heat-producing material corresponds to mechanically weak upper crust (0-20 km) and strong lower crust (20-35 km). Velocity vectors are shown as short heavy lines. Model is started in conductive steady-state, i.e., in thermal equilibrium with VP = 0. Model boundary conditions: surface temperature = 0 oC, basal temperature, Ta = 1350 oC; basal mantle heat flux, qm = 20 mW/m2; no horizontal heat flux at the sides of the model; retro-side of model crust is fixed at x = 0 km (not shown). At the start of the model, pro-lithosphere converges with velocity VP = 1 cm/y, imposed at the base of the crust and in the mantle, and vertically through the crust as a far-field boundary condition. Similarly, the base and far-field of the retro-crust has VR = 0. Pro-mantle lithosphere is subducted beneath stationary retro-lithosphere at fixed detachment point, S. Erosion is proportional to surface slope. The model is time-stepped by alternating mechanical and thermal steps. Thermal steps solve the time-dependent heat balance equation with the current velocity field determined dynamically within the crust and by the boundary conditions. Radioactive upper and lower crust and the temperature field are advected and the updated temperature field calculated for the entire model region. In the frictional field, both the upper and lower crust deform by incompressible plastic flow with an internal angle of friction, f = 15º. In the ductile field, the upper crust deforms according to the 'wet quartzite' flow law (Gleason & Tullis, 1995) and the lower crust deforms according to 'dry Maryland diabase/10' flow law (modified after Mackwell et al., 1998). Flow law parameters: h =  effective viscosity (Pa.s), B* = pre-exponential factor (Pa.s); e. =  strain rate (s-1), n = stress exponent, Q* = activation energy (kJ/mol), R = gas constant (8.314 J/ºKmol), T = ºK. Heat balance equation parameters: r = density, Cp = heat capacity, K = thermal conductivity, k = K/rCp = thermal diffusivity (all constant); T(x,z,t) = temperature, t = time, A(x,z,t) = rate of heat production / unit volume; v(x,z,t) = tectonic velocity (dynamic in crust, kinematic in mantle); Ñ = d/dx + d/dz = gradient operator. Parameter values and changes between models are given in Table 1. 

Figure 2. Reference model LHO-71 after 450 km (45 My) of convergence. Upper panel is the 'peak grade profile', which shows the maximum temperatures (Tmax) reached by points at or near the model surface at 45 My, and the times at which these points reached Tmax, in My after the start of the model. In this and all similar figures, My (millions of years since model start) can be converted to Ma (millions of years before 'present') by subtracting the My value for any point from the My value of the time-step shown (45 My in this case). Note that outside the deformed zone, surface Tmax does not change with time because there is no exhumation, and the times of Tmax shown are not significant in detail; some of these points are slightly below the model surface (0-2 km) and in these cases Tmax > 0. Middle panel is deformed marker grid. Note development of lower crustal antiform on retro-side, asymmetric upper crustal thickening on pro- and retro-sides, and offset of originally vertical markers (compare with Fig. 1). Lower panel shows thermal and velocity fields. Note elevation of crustal isotherms in vicinity of thick upper crustal layer on the pro-side, relative depression of isotherms where upper crust is thin above lower crustal antiform, and depression and inversion of mantle isotherms above subduction zone.


Figure 3. Model LHO-66 after 450 km (45 My) of convergence, showing the effect of a mid-crustal weak zone developed at the base of the upper crust in response to an extra increment of temperature-dependent 'melt' weakening. The effective viscosity of the upper crust undergoes a linear decrease from the flow law value for the ambient T and s conditions to 1019 Pa.s over the temperature range 400-700 ºC, and the lower crust undergoes the same viscosity change over the interval 600-900 ºC. The model crust deforms according to the rheological model that yields the highest strain rate (lowest effective viscosity) for the current conditions. For T < 600 ºC, the upper crust deforms according to the wet quartzite flow law, but where mid-crustal temperature exceeds ca. 600 ºC, the linear viscosity decrease model yields a significantly lower effective viscosity. This extra mid-crustal weakening is analogous to a reduction in strength caused by a small amount of 'melt', fluid, or some other T-dependent weakening process. The resulting crustal deformation (middle panel) is very similar to that for LHO-71 (Fig. 2), except that there is a narrow zone near the base of the pro-side upper crust where material is displaced pro-ward relative to that above and below it. This 'channel flow' zone is also evident as a reversal in the mid-crustal velocity field (lower panel). Peak grade profile (upper panel) and thermal field (lower panel) are almost identical to those for LHO-71 at the same time (Fig. 2).  

Figure 4. Effect of post-convergent extension on thermal and velocity fields, and on zone of mid-crustal reverse channel flow. Upper panel of each pair shows the velocity field and 700 ºC isotherm for model LHO-77, and the lower panel shows the deformed marker grid for the same time. Note the change in scale of velocity vectors for the 60 and 90 My panels. Up to 45 My, LHO-77 is identical to LHO-66 (Fig. 3). Immediately after 45 My, VP is reduced to zero, and thick crust flows outward in response to the gravitational potential energy gradient. This causes extension (E) and thinning of the orogenic core, and produces corresponding zones of contraction (C) on the flanks of the orogen. Outward flow velocities are higher on the pro-side than on the retro-side. Between 45 and 60 My, the 700 ºC isotherm migrates upward into the crust because the cooling effect of subduction is removed; the asymmetry of post-orogenic thermal relaxation reflects both the asymmetry of thickened heat-producing material and asymmetry of syn-convergent isotherms. The overall amount of extension is about 10%. In this model, upper crustal thinning is not sufficient to exhume the mid-crustal channel.  


Figure 5. Model HT-6 after 450 km (45 My) of convergence, showing the effect of focused erosion on exhumation of the mid-crustal channel flow zone. Model thermal and mechanical parameters are the same as for LHO-66 (Fig. 3) except that erosion rate varies with time and distance as well as slope. From 0-675 km there is no erosion; beyond 725 km erosion rate increases at 30 My from 4.4 x 103 x slope m/My to 4.6 x 104 x slope m/My; between 675 and 725 km there is a linear increase in erosion rate from zero to the current value at 725 km. The lack of erosion on the retro-side of the model suppresses the formation of the lower crustal antiform and produces a thick upper crustal layer across the model (middle panel) along with a broad plateau. Mid-crustal temperatures approach 600 ºC beneath the plateau, leading to reverse channel flow on both the pro- and retro-sides of the system. The mid-crustal channel is partially exhumed at 45 My in response to erosion at the pro-ward margin of the plateau; the channel reaches the surface at c. 60 My (see Fig. 6). Exhumation of the mid-crust on the pro-side of the system leads to upward advection of isotherms in this region. The asymmetric peak grade profile reflects variable exhumation across the erosion front; Tmax increases up-section across the erosion front and declines toward the plateau. 

Figure 6. Effect of focused erosion on development and exhumation of the mid-crustal channel flow zone. On the pro-side, the reversal in the mid-crustal velocity field associated with channel flow is apparent by comparing the 30 My and 45 My panels. On the retro-side, outward channel flow and convergence act in the same direction, so that the retro-ward flow reaches a maximum in the mid-crustal channel. Thickening at the toe of the pro-side channel is evident from both the deformed grids and the velocity fields at 45 and 60 My; the channel develops a 'pip' shape during exhumation in response to removal of overlying material. By 60 My the channel is beginning to be exhumed on the pro-side of the system. The velocity field beneath the central part of the plateau after 45 My indicates uniform retro-ward displacement except near the detachment point (heavy black dot). The 700 ºC isotherm migrates upward and outward as the plateau widens. 

Figure 7. Peak grade profiles for models LHO-77 (upper panel) and HT-6 (lower panel) showing how the Tmax distribution across the model surface changes as exhumation proceeds. The profiles are shown for reference time 45 My, and for the times of the bottom panels in Fig. 4 (LHO-77, 90 My) and Fig. 6 (HT-6, 75 My) respectively. Labels are the times of Tmax for 45 My (upright) and later (italics) profiles, with the earliest and latest times in each case shown in bold. Arrows labelled P07, P13, etc. correspond to the points for which P-T-t paths and particle trajectories are presented in Fig. 9; their positions are shown at 90 My for LHO-77, and at 75 My for HT-6. 

Figure 8. Summary cartoon showing contrasting effects of extension and focused erosion on the evolution and exhumation of the mid-crustal channel flow zone. 

Figure 9. P-T-t paths and particle trajectories for selected points from models LHO-77 (top set of diagrams) and HT-6 (bottom set). Positions of points relative to peak grade profiles at 90 My and 75 My respectively are shown in Fig. 7. P-T-t diagrams show the stability fields of andalusite (a), kyanite (k), and sillimanite (s), position of muscovite + quartz = K-feldspar + Al2SiO5 + H2O reaction (m + q = kf + als  + w), and minimum melting curve for water-saturated granitoid rocks (wet melting). P-T-t paths for each point are shown as heavy line with heavy dots at 15 My intervals; labels are times in My since model start. Also shown are the values of Tmax and Pmax with corresponding P, T, and t (My since model start). Particle trajectories show changes in lateral positions and depths of the same points during the model run; symbols and labels as for the P-T-t paths, except that in some cases open circles rather than dots have been used for clarity. Note that these diagrams do not show either surface topography or Moho position, which change with time. At the start of the models, P19 and P20 are vertically separated by 7.5 km, and the initial positions of these points are the same in both models. In LHO-77, convergence stops at 45 My and is followed immediately by crustal extension. In model HT-6, both channel flow and the change to a more rapid erosion rate begin at about 30 My. 

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