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The Effects of Accretion, Erosion and Radiogenic Heat on the Metamorphic Evolution of Collisional Orogens

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The Effects of Accretion, Erosion and Radiogenic Heat on the Metamorphic Evolution of Collisional Orogens J. metamorphic Geol., 1999, 17, 349–366 The effects of accretion, erosion and radiogenic heat on the metamorphic evolution of collisional orogens A. D. HUERTA,* L. H. ROYDEN AND K. V. HODGES Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (email: [email protected]) ABSTRACT Petrological and thermochronological data provide our best record of the thermal structure of deeply eroded orogens, and, in principle, might be used to relate the metamorphic structure of an orogen to its deformational history. In this paper, we present a two-dimensional thermal model of collisional orogens that includes the processes of accretion and erosion to examine the P–T evolution of rocks advected through the orogen. Calculated metamorphic patterns are similar to those observed in the field; metamor- phic temperatures, depths and ages generally increase with distance from the toe of the orogen; P–T paths are anti-clockwise, with rocks heating during burial and early stages of unroofing, followed by cooling during late-stage unroofing. The results indicate that peak metamorphic temperatures within the core of a collisional orogen and the distance from the toe of an orogen to the metamorphic core can be related to the relative rates of accretion, erosion and plate convergence. Model orogens displaying high metamorphic temperatures (>600 °C) are associated with low ratios of accretion rate to plate convergence velocity and with high heat flow through the foreland. Model orogens with metamorphic cores far from the toe of the orogen are associated with high ratios of accretion rate to erosion rate. Calculated metamor- phic gradients mimic steady-state geotherms, and inverted thermal gradients can be preserved in the metamorphic record, suggesting reconsideration of the concept that the metamorphic record does not closely reflect geothermal gradients within an orogen. Key words: accretion; erosion; orogeny; metamorphism; radiogenic heat. observed metamorphism and in determining at what INTRODUCTION rates these processes occur. An important goal in geological research is to relate the metamorphic record of orogens to the basic CONSTRAINTS ON THE METAMORPHIC processes of mountain building. Most thermomechan- EVOLUTION OF OROGENS ical models of the orogenic process have been unable to reproduce the high-temperature, low-pressure meta- Numerous geochemical methods may be used to help morphic conditions observed in real orogens without reconstruct the temperature and pressure evolution of relying on the introduction of speculative orogenic rocks as they are cycled through an orogenic system processes (e.g. mantle delamination, very high rates of (Ghent et al., 1989; Hodges, 1991; Spear, 1993). For shear heating along the subduction contact). However, example, temperatures and pressures at final equilib- recent numerical experiments indicate that accretion rium can be determined with reasonable precision of heat-producing crust from the down-going plate (±20–50 °C and ±100–150 MPa) by applying various to the over-riding plate and surface erosion during major element thermobarometers (Hodges & collisional orogeny can greatly increase temperatures McKenna, 1987; Essene, 1989; Hodges, 1991; Kohn & in model orogens and produce thermal conditions Spear, 1991a,b); analysis of compositionally zoned consistent with the observed metamorphic grades porphyroblasts and their inclusions yields information (Huerta et al., 1996, 1998). on the earlier P–T history (Spear & Selverstone, 1983; In light of these promising results, we further explore Spear, 1993), and the temperature–time (T –t) history the effects of erosion and accretion on the P–T hist- of a sample can be constrained using isotopic therm- ories of metamorphic rocks and the distribution of ochronometers with closure temperatures ranging from metamorphic isograds in orogenic systems. Ultimately, >700 °C (U–Pb zircon) to c. 100 °C (fission track our goal is to determine whether, and to what extent, annealing in apatite) (McDougal & Harrison, 1988; pressure, temperature and cooling data can be useful Heaman & Parrish, 1991). in reconstructing the processes responsible for the Unfortunately, most thermobarometric techniques recover the high-temperature portion of a rock’s P–T *Presently at: Department of Geology, Idaho State University, path, while the closure temperatures of most isotopic Pocatello, ID 83209, USA. thermochronometers record the low-temperature © Blackwell Science Inc., 0263-4929/97/$14.00 349 Journal of Metamorphic Geology, Volume 17, Number 1, 1999 350 A. D. HUERTA ET AL . portion of the rock’s T –t history. As a consequence, it metamorphism, in millions of years before the rock is very difficult to reconstruct most of the pressure–time reaches the surface). Metamorphic field gradients are (P–t) path of a metamorphic terrane directly from displayed as a function of the horizontal distance from petrological and geochemical data. Inasmuch as the the toe of the orogen (xsur) including temperature P–t path is a valuable proxy for the burial and arrays (Tmax vs. xsur), depth arrays (zm vs. xsur) and unroofing history of orogenic belts, many attempts metamorphic age arrays (tm vs. xsur). The term have been made to use forward and inverse models to metamorphic core is used to describe the region at the compensate for the limited temperature overlap surface where metamorphic temperatures are at a local between recovered P–T and T –t paths (e.g. England & maximum (peak Tmax). Richardson, 1977; England & Thompson, 1984; The relationship between the thermal structure of Royden & Hodges, 1984; Dahlen & Barr, 1989; Molnar model orogens and their metamorphic records is & England, 1990; Royden, 1993; Ruppel & Hodges, investigated by comparing geotherms to piezotherms, 1994). In this paper, these efforts are extended to and structural thicknesses to metamorphic depths. include a new, relatively simple numerical model of Geotherms (profiles of T as a function of depth), either collisional orogenesis that emphasizes the importance transient or steady-state, display the thermal structure of accretion and erosion in the thermal evolution of of a vertical column within the orogen at any one mountain ranges (Huerta et al., 1996, 1998). We begin time, while piezotherms (profiles of Tmax as a function with the hypothesis that these processes play such a of depth at Tmax) refer to the array of metamorphic dominant role in orogenic heat transfer that the conditions observed along a surface transect (e.g. metamorphic structure of any given mountain range Richardson & England, 1979). The structural height reflects the absolute and relative rates of erosion and of a column of rocks within the orogen (Dzcol) is the accretion during mountain building. We then explore distance, measured perpendicular to the subduction how variations in these two processes should be boundary, from the top to bottom of the column manifest in the distribution of metamorphic isograds (Fig. 1, upper panel). The structural thickness of a and in the topology of P–T and T –t paths across transect along the surface of the orogen (Dzstruct) is the the orogen. distance, measured perpendicular to the subduction To avoid possible ambiguities the following termi- boundary, along the transect (Fig. 1, upper panel). nology is used to describe model results presented in Pressure–depth profiles (arrays of Dzm as a function of this paper (see Table 1 for symbols, variables, and Dzstruct) display the relationship between the preserved values used). Tmax is the maximum temperature metamorphic depths and the structural thickness of experienced by a rock as it is cycled through the model rocks along a surface transect, and are used to orogen; zm is the depth of the rock at Tmax, and tm is determine whether the metamorphic depths are consist- the time at which the rock reaches Tmax (i.e. the age of ent with a lithostatic gradient. Table 1. Definitions of variables and values used. Variable Physical meaning Value or units Comments x horizontal distance from upper-plate toe (km) z vertical distance from surface (km) t time since initiation of collision (Myr) A radioactive heat production rate (mW/m) e erosion rate (km/Myr) nc convergence velocity (km/Myr) velocity of rocks in the down-going plate relative to rocks in the upper plate a accretion rate (km/Myr) vertical component relative to subduction boundary nu velocity vector of particles in the upper plate (km/Myr) n1 velocity vector of particles in the down-going plate (km/Myr) sw maximum surface width of heat-producing wedge (km) dw maximum thickness of heat-producing wedge (km) tw time to steady state shape of heat-producing wedge (Myr) h vertical thickness of accreted slab (km) ta period of accretion cycle (Myr) T (x,y) temperature at a location within the orogen (°C) xsur location where rock surfaces measured from upper-plate toe (km) ° Tmax maximum temperature experienced by a rock ( C) zm depth at Tmax (km) tm time at Tmax (Ma) age of metamorphism, in millions of years prior to rock reaching the surface zstruct structural thickness of a surface transect (km) measured perpendicular to the subduction boundary zcol structural height of a column of rocks (km) measured perpendicular to the subduction boundary H dip of subduction zone 11.3° dr initial thickness of heat-producing layer 18 km l thickness of foreland lithosphere 126 km ° Ta temperature at base of lithosphere 1260 C K thermal conductivity 2.5 W/mK a thermal diffusivity 10−6 m2/s EFFECTS OF ACCRETION AND EROSION ON METAMORPHISM 351 ward migration of the subduction boundary at a rate of e/tan H. In this work we use a frame of reference fixed with respect to the toe of the upper plate (Fig. 1, lower panel). In this case the subduction boundary remains stationary, while the velocity of upper-plate rocks (vu) has a horizontal component (a–e)/tan H and a vertical component −e, and the velocity of rocks in the down-going plate (vl) is the sum of the upper-plate velocity and the convergence velocity.
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