The Danba antiform (DA) exposes the highest grade metamorphic rocks in eastern Tibet. The metamorphic grades characterizing the DA evolve from sillimanite-migmatite grade to greenschist grade over a relatively short distance of ∼20 km from core to limb. This metamorphic event indicates an important Mesozoic to Cenozoic doming and exhumation history. However, the Cenozoic history of the antiform is poorly constrained due to a lack of data. Consequently, we used fission track dating on zircon and apatite from 22 samples collected throughout the DA. The zircon fission track (ZFT) data show a transition from Cenozoic non-reset (202 Ma), to partially reset (53–37 Ma), to totally reset (24–8 Ma) ages from the periphery to the core of the DA. The oldest totally reset ZFT ages are ca. 25 Ma and likely indicate the onset of Cenozoic folding of the DA. Compared to the apatite fission track (AFT) ages of ca. 10 Ma in the peripheral region, the youngest AFT ages are younger than 3 Ma in the core of the DA, suggesting that folding could be ongoing. Based on these multithermochronometer data, the cooling rate increases from ∼8 °C/m.y. on the periphery to ∼12–56 °C/m.y. in the core of the DA since ca. 12 Ma. The DA shares a similar cooling history with the Longmen Shan (LMS) fault belt, implying that the detachment fault of the LMS may extend to the DA, resulting in linked uplift histories. The differential exhumation among the samples in the core of the DA and the surrounding area indicates that both upper crustal deformation and crustal channel flow may have developed simultaneously (mainly since ca. 12 Ma) in the DA.
India began colliding with Eurasia ca. 50 Ma, and this collision has since accommodated at least 1400 km of north-south convergence and contributed to the build-up of the Tibetan Plateau (Yin and Harrison, 2000; Tapponnier et al., 2001). The Longmen Shan (LMS) is located at the eastern margin of the Tibetan Plateau, forming an abrupt drop in elevation of ∼4000 m between the plateau to the west and the Sichuan basin to the east, despite low short-time-scale shortening rates (Chen et al., 2000). Several studies have invoked the presence of lower crustal flow to explain the discrepancy between the high topography and the low shortening rates (e.g., Royden et al., 1997; Burchfiel, 2004; Clark et al., 2005). The Wenchuan earthquake ruptured two main faults in the LMS and produced a rupture of 6 m to locally 8 m of reverse slip (Xu et al., 2009; Shi et al., 2012). These faults are characterized by ramp and flat structures as revealed by balanced cross sections, seismic profiles, and earthquake data (Xu et al., 2009; Hubbard and Shaw, 2009; Wang et al., 2011). Currently, the regional tectonic questions include, is the crustal shortening limited to the plateau margin, or does it extend to the internal plateau? What are the relative contributions of lower crustal flow and upper crustal shortening to the uplift of the internal plateau (e.g., Tian et al., 2013, 2015)?
The Songpan-Garzê fold belt (SGFB), located in eastern Tibet west of the LMS, is dominated by highly deformed Triassic flysch with low to middle greenschist facies metamorphism (Huang et al., 2003a, 2003b; Harrowfield and Wilson, 2005; Yan et al., 2011; Fig. 1). This contrasts with the Danba antiform (DA), a northwest-trending structure ∼40 km in width and ∼90 km in length located on the hanging wall of the LMS in the interior of the plateau, that exposes high metamorphic grade Neoproterozoic basement with a complete Barrovian-type metamorphic sequence beneath Triassic flysch strata (Fig. 1) (Huang et al., 2003a, 2003b; Weller et al., 2013; Fig. 1). While the high-grade metamorphism is Mesozoic in age (ca. 190–160 Ma), Huang et al. (2003a) observed ca. 30 Ma Rb-Sr biotite ages in the DA, indicating a late Oligocene cooling event. Zircon fission track (ZFT) ages of ca. 24 Ma (Manai granite) and apatite fission track (AFT) ages of 33–4 Ma around the DA region also indicate rapid exhumation and deformation in the Cenozoic (Xu and Kamp, 2000; Clark et al., 2005; Wilson and Fowler, 2011; Jolivet et al., 2015; Fig. 2). Because the DA is located in the plateau interior, its Cenozoic deformation pattern may provide new constraints on the mode of crustal deformation during the Cenozoic (Royden et al., 1997; Burchfiel, 2004; Xu et al., 2009; Hubbard and Shaw, 2009). The existing low-temperature thermochronology data, however, are too sparse to evaluate the detailed Cenozoic exhumation history of the DA region. In this study we systemically collected samples across the DA and used ZFT and AFT, combined with previous data, to constrain the Cenozoic exhumation and deformation history in the DA and to discuss the exhumation mechanism and tectonic implications in eastern Tibet (Fig. 2).
The SGFB is located in eastern Tibet, west of the Sichuan basin, and is bounded by the LMS to the east, the Kunlun suture to the north, and the Jinsha suture to the south (Fig. 1). The Songpan-Garzê basin deposits consist of late Neoproterozoic–Paleozoic and Triassic turbidite sequences, which are underlain by Neoproterozoic crystalline basement (e.g., Zhou et al., 2008). During the Triassic Period, the basin was inverted and deformed into a series of northwest-trending tight folds with axial plane cleavage development (i.e., the SGFB), which record Barrovian metamorphism and a northwest-trending, southwest-verging regional décollement-fold belt (Huang et al., 2003b; Harrowfield and Wilson, 2005). This shortening event was accompanied by the intrusion of many granitoids such as the Manai granite (de Sigoyer et al., 2014; Fig. 1).
While much of the SGFB is covered by the Triassic Songpan-Garzê flysch, older units are exposed in the DA and the LMS (Fig. 1). In these areas, the pre-Sinian granitic basement shares many similarities with the basement of the Yangze block (Roger et al., 2010) and is exposed in bodies including the Gezong and Gongcai complexes (Figs. 1 and 2). The granitic rocks are overlain by a sequence of metamorphosed Sinian and Paleozoic marine sedimentary rocks, dominated by Sinian dolomitic marble and/or metapelite and Silurian–Devonian metapelite and paragneiss with minor quartzite, marble, and amphibolites (Chengdu Institute of Geology and Mineral Resources, 1991; Huang et al., 2003b). The basement and the cover sequence in the Danba area are separated by a southwest-verging ductile shear zone, which is thought to be part of the regional Triassic décollement that is also now exposed in the LMS (Zhou et al., 2008; Harrowfield and Wilson, 2005; Roger et al., 2010; Figs. 1 and 2).
Based on the mineral assemblage of the DA rocks, the maximum metamorphic temperatures are estimated to have been 710 ± 10 °C in the pre-Sinian granitic basement in the core of the DA, decreasing to 470 °C in the Triassic rocks in the limb (Huang et al., 2003a, fig. 1 therein). Huang et al. (2003b) identified three thermotectonic events. The first event (M1 and D1) in the DA region is characterized by Barrovian metamorphism, which peaked at kyanite-grade conditions of 5.3–8 kbar (equal to ∼22–29 km burial depth) and 570–600 °C ca. 210–205 Ma. The second event (M2 and D2) only occurred in the northern part of the DA with sillimanite-grade metamorphism and local migmatization at pressure-temperature (P-T) conditions of 4.8–6.3 kbar and 640–725 °C ca. 164 Ma. D3 developed mainly with northwest-oriented thrusts and strike-slip shear zones in a transpressional setting associated with the deformation of the DA during the India-Eurasia collision. Weller et al. (2013) used a pseudosection approach to calculate P-T histories and determined that peak metamorphic conditions ranged from 5.2 kbar and 580 °C at staurolite grade, to 6.0 kbar and 670 °C at sillimanite grade, and peak conditions were reached ca. 191–184 Ma (equal to ∼26 km burial depth). They identified two deformation events (D1 and D2): D1 is associated with the fabric S1, in which all metamorphic index minerals align, and D2 is associated with a crenulation cleavage fabric (S2) during low-temperature (280–400 °C) metamorphism.
In the SGFB, the Jurassic–Cretaceous Periods are usually thought to be a time of little tectonic activity, with no significant heating or cooling events (Kirby et al., 2002; Wilson and Fowler, 2011; Roger et al., 2010).
During the Cenozoic (primarily since Miocene time), the SGFB was reactivated by the India-Asia collision. Although deformation has been recognized along the LMS, the Xianshuihe strike-slip fault and other active faults (e.g., Xu et al., 2009; Ren et al., 2013; Fig. 1), the regional uplift history, and mechanism of deformation are debated (Wang et al., 2012; Tian et al., 2013). Low-temperature thermochronology data for eastern Tibet indicate accelerated cooling since the Miocene (Arne et al., 1997; Cook et al., 2013; Xu and Kamp, 2000; Clark et al., 2005; Wilson and Fowler, 2011). In the plateau interior, cooling histories obtained on Mesozoic granites from the Songpan-Garzê region show very slow and regular cooling between ca. 150 and 30 Ma (Xu and Kamp, 2000; Kirby et al., 2002; Huang et al., 2003a; Roger et al., 2004, 2010; Zhou et al., 2008). In southeastern Tibet, Clark et al. (2005) found slow cooling (<1 °C/m.y.) between ca. 100 and ca. 20–10 Ma and a change to rapid cooling after ca. 13 Ma with initiation of rapid river incision at 0.25–0.5 mm/yr between 13 and 9 Ma. The low-temperature thermochronology data [fission track and (U-Th)/He] show abrupt differences in ages across the faults in the LMS, suggesting significant uplift along the faults (Godard et al., 2009; Tian et al., 2013; Tan et al., 2014, 2015).
For the DA, several research groups have measured Cenozoic ages using biotite and muscovite Ar-Ar dating and biotite Rb-Sr dating (Huang et al., 2003a; Zhou et al., 2008; Wallis et al., 2003), which suggest differential cooling and exhumation from the core to the limb during the Cenozoic. However, the limited available data do not provide adequate constraints on the Cenozoic deformation of the DA, which remains poorly constrained (Wilson and Fowler, 2011).
METHODS AND RESULTS
The ZFT and AFT dating methods record the time at which the rocks cooled through the annealing zone, which is ∼240 °C for ZFT (Brandon et al., 1998) and ∼110 °C for AFT (Gleadow and Duddy, 1981; Donelick et al., 2005). For radiation damaged zircons, Brandon et al. (1998) considered the temperature limits of the ZFT partial annealing zone (PAZ) to be at 180–240 °C for time scales of 107 yr.
Previous studies have found a ZFT age of 202 ± 34 Ma outside the DA (Xu and Kamp, 2000) and several biotite Rb-Sr ages of ca. 30 Ma within the antiform (Huang et al., 2003a). The 202 ± 34 Ma ZFT age corresponds to the Mesozoic deformation event and indicates that the amount of Cenozoic cooling is less than the ZFT partial annealing temperature (∼180 °C) outside the antiform, while the Cenozoic cooling amount is ∼300 °C (closure temperature of biotite Rb-Sr) inside the antiform. This suggests that, from the outside to the core of the antiform, the ZFT ages decrease from Mesozoic non-reset ages to partially reset ages and then to completely reset ages. The oldest Cenozoic reset ZFT age indicate the onset of Cenozoic deformation. For this purpose, we collected 22 samples widely spread from the core to the periphery of the DA, and measured 17 ZFT ages and 16 AFT ages (Fig. 2; Table 1). Compared with ZFT, the AFT system has a lower closure temperature, allowing us to use this system to evaluate the more recent cooling history. All samples were taken from bedrock outcrops along the bottom of the valley to reduce the influence of elevation, hence the samples have similar elevations and can be compared along a cross section.
The sample preparation and experimental processes follow the methods of Liu et al. (2000, 2001). We used grain by grain and mica external detector techniques to obtain individual grain ages (Wagner and Van Den Haute, 1992). Zeta values (Green, 1985; Hurford and Green, 1983) for the standard glasses CN-5 (apatite) and SRM-610 (zircon) were 340 ± 12 (1σ) and 27.5 ± 1.0 (1σ), respectively. Neutron irradiation was carried out at the National Tsing Hua University Reactor of Taiwan. Errors were calculated using the conventional analysis given by Green (1981). In order to confirm the age accuracy, two apatite samples (Ki-1 and Bo-15) were analyzed by Apatite to Zircon, Inc. (www.apatite.com/; A2Z) as well as by the low temperature thermochronology laboratory at the University of National Chung-Cheng. The ages for Ki-1 are 9.7 ± 0.9 Ma in our laboratory and 9.2 +1.28/–1.12 Ma by A2Z, and those for Bo-15 are 3.5 ± 0.3 Ma in our laboratory and 3.77 +0.55/–0.48 Ma by A2Z. The two samples (Ki-1 and Bo-15) were analyzed with a Cf-252 source to reveal more horizontal confined tracks in the A2Z laboratory (Donelick and Miller, 1991). The consistency of these ages indicates that our ages are reliable (Table 1).
The summarized ZFT and AFT ages and sample conditions are shown in Table 1 and Figure 2. The detailed dating results are shown in Tables 2 and 3. In the eastern limb of the DA, the ZFT pooled ages are 52.7 ± 3.0 Ma (Bo-8) and 37.8 ± 1.9 Ma (Ki-4), decreasing to 22.1 ± 1.3 Ma (Bo-6) and 20.8 ± 1.4 Ma (Bo-9) (Fig. 2). In the northeastern limb the oldest ZFT pooled ages are ca. 24 Ma (114 in Xu and Kamp, 2000) to ca. 21 Ma (Ki-10). In the core, the ZFT ages are younger than 12–10 Ma. Generally, our results combined with previous studies show ZFT ages decreasing from ca. 202 Ma, 53 Ma, 37 Ma, 24–20 Ma, to 14–12 Ma, and finally to 10–8 Ma from the limb to the core area (Fig. 2). AFT ages are generally ca. 12–10 Ma outside of the DA and decrease to ca. 7–3 Ma in the antiform limbs and 4–2 Ma in the core area (Table 3; Fig. 2). The AFT ages are younger than 10 Ma and the track density is generally too low to measure enough confined track lengths (100+) to evaluate the thermal history. The 2 samples analyzed with Cf-252 by A2Z (Ki-1 and Bo-15) yielded enough track lengths, and the mean length of 15.02–14.51 mm indicates relatively rapid cooling through the apatite PAZ (Wagner and Van den Haute, 1992).
Onset of the Cenozoic Cooling
The Ar/Ar ages from the DA are widely scattered; Zhou et al. (2008) attributed this to slow cooling from ca. 166 Ma to ca. 47 Ma. The Rb/Sr biotite ages concentrate between ca. 34 and 24 Ma in the DA; Huang et al. (2003a) related this to their D3 deformation and the onset of Cenozoic rapid cooling in the DA. Compared to the Ar/Ar and Rb/Sr data, the ZFT data provide additional constraints on the onset of Cenozoic cooling. In the eastern part of the DA, the ZFT ages increase progressively from 12–8 Ma in the core to 24–20 Ma at the limb, and to ca. 202 Ma to the northeast of the DA (Fig. 2). This Mesozoic age, obtained far from the DA, corresponds to a domain that has recorded a metamorphic temperature of ∼300–400 °C (Wang et al., 2013), higher than the ZFT closure temperature (∼240 °C) (Brandon et al., 1998), suggesting that at this location only a Late Triassic deformation event is recorded by the ZFT data. This implies that the post–Late Triassic exhumation amount should be less than the burial depth of the ZFT partial annealing zone (PAZ) (with temperatures of ∼180–240 °C; Brandon et al., 1998), and indicates a non-reset post–Late Triassic zone on the periphery of the DA. In the northeastern margin of the DA, the ZFT pooled ages are widely distributed and decrease from 53 Ma to 37 Ma, indicating that these samples were located within the PAZ for a long period of time (Galbraith, 2005). In order to estimate the oldest Cenozoic reset age we use BinomFit (Brandon, 1992) to decompose the grain age spectrum, yielding younger group ages of ca. 36 Ma and 27 Ma (for Bo-8 and Ki-4, respectively) (Fig. 3; Table 1). The oldest reset zircon ages are ca. 24 Ma in the northeastern limbs (sample 114 from Xu and Kamp, 2000). Although the 27 Ma group age is a maximum for the onset of cooling, the close proximity between sample Ki-4 and the 24 Ma completely reset sample suggests that the onset of Cenozoic cooling could be constrained to ca. 25 Ma (between ca. 27 and 24 Ma) (Fig. 4). This is consistent with a biotite Rb-Sr age of ca. 26 Ma (Huang et al., 2003a) found near the boundary of the total reset ZFT zone (Fig. 2).
Accelerated Cooling of the DA Since the Middle Miocene
The use of multiple low-temperature thermochronometers with different closure temperatures (∼300 °C biotite Rb-Sr, ∼240 °C ZFT, and ∼110 °C AFT) and ages affords an opportunity to constrain the pattern of Cenozoic cooling (Fig. 4). The cooling rate varies in time and space across the DA. In the core of the DA, the biotite Rb-Sr ages are between 34 and 24 Ma, close to the onset of the Cenozoic cooling, while the ZFT ages are younger than 12 Ma, indicating a slow average cooling rate of ∼3 °C/m.y. between ca. 27 Ma and 12 Ma. The cooling rate then increases to ∼12–25 °C/m.y. between 12 Ma and 3 Ma and then ∼29–63 °C/m.y. from ca. 3 Ma to present, which indicates a strong acceleration since the middle Miocene (Fig. 4).
In the limb of the DA, the cooling rate is ∼8 °C/m.y. between ca. 24 Ma (∼240 °C) and ca. 7 Ma (∼110 °C), according to the ZFT and AFT ages. However, considering that the timing of a change in uplift and cooling rate in the limb area is consistent with the timing in the core area, we suggest that the cooling history in the limb area should have pattern similar to that in the core, yielding a rate of ∼19 °C/m.y. from 12 Ma to present in the limb area (Fig. 4). In the periphery area, the cooling rate is ∼8 °C/m.y. since ca. 10 Ma according to the AFT ages; this is also consistent with the timing of acceleration in the core (Fig. 4).
Mesozoic to Cenozoic Folding of the DA
AFT and ZFT ages progressively increase from the core to the margin of the DA (Fig. 5). This is consistent with a folding structure, which results in higher exhumation and cooling rates in the core and lower rates in the margin area. On the southwestern limb of the DA there is a small dome with Precambrian granitoid exposed in its core (Figs. 2 and 5). The AFT and ZFT ages here are as young as the ages in the core area of the DA, suggesting that this small dome has been active simultaneously with the DA since the late Miocene. The AFT ages are younger than 3 Ma, suggesting that the folding maybe still active.
The total amount of cooling and exhumation since the peak Mesozoic metamorphism has been constrained by the metamorphic isograds in the Danba region (Huang et al., 2003b; Weller et al., 2013; Robert et al., 2010; Fig. 6A). To evaluate the Cenozoic folding history, we constructed cooling isotherm maps at 30 Ma (Fig. 6B) and 12 Ma (Fig. 6C). The Rb-Sr biotite ages are ca. 30 Ma in the DA and the oldest reset ZFT age is ca. 25 Ma in the northeastern limb. Based on these data, we roughly constrain the 300 °C isotherm in the core and the 240 °C isotherm in the limb to ca. 30 Ma (Fig. 6B). East of the DA, the AFT ages (110 °C) are 12–10 Ma, and 2 zircon (U-Th)/He ages (180 °C) in the eastern side of the SGFB are ca. 86–55 Ma (Kirby et al., 2002). These thermochronometry data reveal that cooling from 180 °C to 110 °C took place from 55 to 12 Ma, yielding a cooling rate of 0.9–1.2 °C/m.y. From this rate, we estimate that the periphery of the DA had been cooled to ∼135 °C at 30 Ma. Figure 6B shows the 300 °C, 240 °C, and 135 °C isotherms in the core, limb, and outside of the DA at 30 Ma. At 12 Ma, the 240 °C and 110 °C isotherms can be defined by the ZFT and AFT ages (Fig. 6C). The 240 °C isotherm is constrained by the ca. 12 Ma ZFT ages (Fig. 6C), and in east of the DA, we use the AFT ages of 12–10 Ma to constrain the 110 °C isotherm (Fig. 6C). With these isotherms, we made a profile of cooling temperature (D-D′ in Fig. 6A) across the DA at different times to quantitatively constrain the cooling and exhumation history from Mesozoic to Cenozoic time (Fig. 6D).
As shown in Figure 6D, the temperature differences between the core and the outside of the DA in the Mesozoic, and ca. 30 Ma and ca. 12 Ma are ∼300 °C, 165 °C, and 140 °C, respectively. These temperature differences are interpreted to correspond to the amount of differential uplift due to folding, indicating that more than half of the differential uplift occurred during the Cenozoic. Between 30 Ma and 12 Ma the relative temperature difference is small, indicating that folding was not significant during this time and that the major Cenozoic folding activity took place after 12 Ma in the DA. The Cenozoic folding reactivated the preexisting Mesozoic fold, pointing out the importance of inherited structures in partitioning deformation in the Tibetan Plateau.
Because an increase in exhumation rate will cause the advection of heat toward the surface, affecting the geothermal gradient and therefore the calculation of erosion rates (Reiners and Brandon, 2006), we used the TERRA software (www.terrasoftware.com/; Ehlers et al., 2005) to model the cooling rate since 12 Ma with various rates of steady-state erosion. The results show that the observed cooling rates are best fit when the erosion rates are ∼0.35 mm/yr and ∼0.8 mm/yr in periphery and core of DA, respectively (Fig. 4). The topographic variation is rather small across the DA, indicating that the erosion rate nearly balances with the uplift rate. The 0.45 mm/yr difference in erosion rate across the DA suggests differential uplift of 5.4 km since 12 Ma (Fig. 6D).
Tectonic Implications of DA Evolution
Recent evidence suggests that the onset of uplift of the eastern margin of the Tibetan Plateau is early-middle Cenozoic (30–25 Ma) (Wang et al., 2012; Li et al., 2012; Tan et al., 2014), instead of late Cenozoic, as previously proposed (Kirby et al., 2002; Clark et al., 2005; Godard et al., 2009). A late Oligocene cooling event in the DA was also proposed (Huang et al., 2003a). Our new thermochronology data show that an onset of Cenozoic exhumation ca. 25 Ma is consistent with the Oligocene cooling event in the DA and with the onset of uplift in the east margin of the plateau. Moreover, our study shows that the cooling rate in the DA remained relatively low from ca. 25 Ma until ca. 12 Ma (Fig. 4). The acceleration of cooling and differential uplift ca. 12 Ma is consistent with the timing of both the second phase of rapid exhumation at 15–10 Ma in the LMS (Godard et al., 2009; Wang et al., 2012; Tan et al., 2014) and the initiation of rapid river incision between 13 and 9 Ma in eastern Tibet (Clark et al., 2005; Tian et al., 2015), all of which suggest that the Oligocene and middle Miocene events are regional tectonic events in eastern Tibet.
Our thermochronology data provide evidence for Cenozoic folding in the DA, indicating that crustal shortening was not limited to the plateau margin, but extended to the interior of the plateau. Using balanced cross-section analysis, Hubbard and Shaw (2009) concluded that the 2008 Wenchuan earthquake occurred along a ramp and flat structure at a depth of ∼16–20 km, consistent with earthquake and geodetic data. The low-temperature thermochronology data of Tan et al. (2014) also suggest a ramp and flat structure under the southern LMS, as well as an onset of Cenozoic exhumation of southern segment of LMS at 30–20 Ma and one major fault with rapid exhumation since 15–10 Ma. Because the DA is located on the hanging wall of the southern segment of the LMS fault, we argue that the detachment fault of the LMS may have extended north to the Danba area (Fig. 1B). In both the LMS and the DA, Mesozoic detachment faults from different depths with top-to-the south shearing are now exposed at the surface (Huang et al., 2003b; Harrowfield and Wilson 2005), indicating that the Mesozoic folding of the DA was likely related to shortening in the LMS area and could have been associated with antiformal stacking of thrust duplexes at depth or detachment folding. A similar deformation mechanism may have occurred during the Cenozoic, causing the LMS and the DA to share similar exhumation histories (Fig. 1B).
Instead of crustal shortening in the shallow crust, recent geophysical data (including magnetotelluric and P and S wave studies) suggest that crustal channel flow may have developed on the hanging wall of the LMS fault (Bai et al., 2010; Liu et al., 2014). We compared the difference of cooling rates across the DA (Fig. 6D) and estimated the uplift due to folding to be ∼5.4 km, which cannot explain all of the measured exhumation (∼11 km, assuming a 25 °C geothermal gradient) since 12 Ma in the core of DA. In addition, on the periphery of the DA the exhumation rate increased after ca. 12 Ma (based on the 12–10 Ma AFT ages). Because the detachment fault of the LMS is nearly flat in the plateau region of the SGFB, it should not contribute to the exhumation here. Therefore the rest of the regional exhumation, which is widely distributed in eastern Tibet (Clark et al., 2005; Tian et al., 2015), could be caused by crustal channel flow (Royden et al., 1997). Our observations indicate that both upper crustal deformation and crustal channel flow may have developed simultaneously in the Danba area.
We use low-temperature thermochronology data to constrain the cooling and deformation history across the DA. The ZFT data include Cenozoic non-reset, partially reset, and totally reset ages from the periphery to the core of the DA, and the oldest Cenozoic ZFT ages of ca. 25 Ma indicate the onset of the Cenozoic cooling event. However the cooling rate is rather low from 25 Ma to 12 Ma, then abruptly increased from ca. 12 Ma. The AFT ages are as young as post–3 Ma in the antiform core, suggesting that the folding may be ongoing.
Both the LMS and the DA show similar exhumation histories since the Oligocene, suggesting that the two regions may be related and that the detachment fault of the LMS could extend to the DA and explain the Cenozoic folding. However, upper crustal folding cannot explain all of the Cenozoic exhumation in the region, suggesting that crustal channel flow may have developed simultaneously to enhance the exhumation.
This project was fully supported by the Special Projects for Basic Research Work of the Institute of Geology, China Earthquake Administration (IGCEA1518) and the National Science Council, Taiwan, ROC, under grant NSC 100-2119-M-94-002. We thank Xin-Mei Tu, Wen-Lin Tsai, Cheng-Yang Xu, Shao-Jun Wang, Chong Xu, Kang Li, and Qi Yao for their help with field work, figure editing, and experiments. We appreciate the editorial effort of A.B. Weil; three anonymous reviews provided detailed and constructive comments that improved the manuscript.