TECTONICS, VOL. 21, NO. 1, 10.1029/2000TC001246, 2002
[39] The inference of denudation rate from thermochronologic data is often complicated by time-dependent thermal regimes whose evolution depends on the advection of heat during denudation [e.g., Moore and England, 2001]. However, the regional extent and remarkably slow rate of cooling between the mid-Mesozoic and the Cenozoic suggests the establishment and persistence of a stable thermal regime following Mesozoic tectonism. If we make the reasonable assumption that isotherms during this time were subhorizontal and that cooling reflects the motion of rocks through this thermal structure because of erosional denudation, we can translate cooling rates into estimates of denudation rate. For geotherms typical of stable continental regions (15°–25°/km), cooling rates of <1°/m.y. imply that denudation rates must have been extremely slow (<0.1 mm/yr). These rates would have persisted over much of the region until at least the early Eocene and, importantly, over the position of the present margin until the latest Miocene (Table 2 and Figure 9).
[40] Rapid late Cenozoic cooling observed in samples from the margin of the plateau (Table 2) implies that denudation rates increased substantially in the late Miocene. However, an estimate of their magnitude is complicated by the advection of heat during rapid denudation [Mancktelow and Grasemann, 1997]. We can, nevertheless, estimate the depth of the sample prior to the onset of rapid cooling and obtain a mean denudation rate over the period of the rapid cooling event. Both samples from the topographic margin record the onset of rapid cooling at ~200°C, implying ~8–10 km of denudation during the late Cenozoic (for nominal geotherms). For sample 93-4 (Longmen Shan) this translates into mean denudation rates on the order of 1 mm/yr, while for sample 97-14 (Min Shan), denudation rates appear to have been ~1–2 mm/yr.
[41] In contrast, samples from the Songpan-Garze terrane appear to record lesser degrees of late Cenozoic denudation. The youngest ages recorded by feldspars range from 50–70 Ma and are consistent with zircon (U-Th)/He ages of 55–85 Ma. Together, these indicate that rocks now exposed at and near the plateau surface had cooled below ~200°C by the early Tertiary. Furthermore, the presence of (U-Th)/He apatite ages between 8 and 20 Ma indicates that Miocene-Recent denudation on the plateau has been limited to a few kilometers. Thus the data suggest that late Cenozoic denudation has been heterogeneous across the eastern margin of the plateau adjacent to the Sichuan Basin, with high rates focused in a narrow zone along the present topographic front. These results are broadly consistent with the distribution of denudation inferred from apatite and zircon fission track ages in the Longmen Shan region [Arne et al., 1997] and along the margin of the plateau south and west of the Xianshuihe fault [Xu and Kamp, 2000].
[42] The increase in cooling rate experienced by samples from the plateau interior during the early to middle Tertiary (~50–20 Ma; Table 2 and Figure 9) suggests a concomitant increase in denudation rate. As noted in section 4.5, the form of the cooling path, and thus, the timing of the increase in denudation rate, during this interval is not constrained by our data, and we can only estimate mean denudation rates during this interval using a linear extrapolation between data points. Mean denudation rates appear to have been on the order of 0.1–0.2 mm/yr (for nominal geotherms). Whether this apparent increase in denudation rate is tectonically significant will be considered in section 7.1.
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