TECTONICS, VOL. 21, NO. 1, 10.1029/2000TC001246, 2002
[43] The apparent focusing of late Cenozoic denudation along the present topographic margin of the plateau and subsequent exhumation of rocks from upper midcrustal depths allows us to place an upper bound on the onset of rapid cooling. Adjacent to the Sichuan Basin the close correspondence between a zircon (U-Th)/He age of ~11 Ma and the transition from slow to rapid cooling (11–12 Ma) in the feldspar thermal model suggest that this sample still resided at temperatures ~200°C at circa 11 Ma. The onset of rapid cooling is therefore bracketed between 11 Ma and 4.5 Ma (the age of the corresponding apatite). Likewise, in the Min Shan, the onset of rapid cooling in the feldspar model (5–6 Ma) corresponds almost exactly with the zircon age (~5 Ma), restricting the onset of rapid cooling to between 5 and 3 Ma.
[44] Evaluating whether or not these transitions also represent the onset of rapid denudation is problematic. For example, the advection of heat that accompanies an increase in denudation rate imparts a phase lag between the onset of rapid denudation and rapid cooling that complicates a direct interpretation of the onset of rapid cooling [House and Hodges, 1994; Stuwe et al., 1994; Mancktelow and Grasemann, 1997]. As packages of rock are translated toward the surface along with isotherms, cooling is delayed until they pass through compressed isotherms near the surface [Moore and England, 2001]. However, simple modeling of the advective/conductive transfer of heat in response to rapid denudation in extensional settings [Ruppel et al., 1988] suggests that this lag time is relatively short (1–2 m.y.) in the upper regions of the crust. Therefore our estimates of the onset of rapid cooling in the Longmen Shan are probably a close proxy for the onset of rapid denudation. We estimate that the onset of rapid denudation probably occurred no earlier than 12–13 Ma in the Longmen Shan and no earlier than 6–7 Ma in the Min Shan. It is important to note that these are maximum estimates; denudation could have begun as recently as 5–6 Ma in the Longmen Shan and as recently as 4–5 Ma in the Min Shan.
[45] The difference in cooling ages and inferred denudation rates between the Longmen Shan and the Min Shan may be an indication of diachroneity in the onset of deformation between different regions along the modern plateau margin. However, given that cooling could have begun as recently as 5–6 Ma in the Longmen Shan, we cannot say with confidence whether this apparent difference reflects true variability in the timing of plateau formation. A definitive test of this possibility awaits further sampling.
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