The Thermal Evolution and Slip History of the Renbu Zedong Thrust, southeastern Tibet




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The Thermal Evolution and Slip History of the Renbu Zedong Thrust, southeastern Tibet


Xavier Quidelleur, Marty Grove, Oscar Lovera, T. Mark Harrison, An Yin

Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics

University of California, Los Angeles, CA 90024

and

F.J. Ryerson

Institute of Geophysics and Planetary Physics

Lawrence Livermore National Laboratory, Livermore, CA 94550


Thermochronological results from 100-70 Ma granitoids of the Gangdese batholith in southeastern Tibet show evidence of thrusting. The granitoids were buried during the Miocene beneath the north-directed (~30° dip) Renbu Zedong thrust (RZT). Near Lian Xian, its hanging wall consists of Tethyan metasediments derived from the Indian shelf that have experienced upper greenschist to lower amphibolite facies metamorphism. The footwall granitoids and associated wallrocks adjacent to the RZT exhibit considerable recrystallization to greenschist facies assemblages. Footwall samples from a northeast-southwest traverse extending 15 km away from the trace of the RZT all yield 9-12 Ma ages for the initial ~20% of gas release. Biotite and K-feldspar 40Ar/39Ar ages increase systematically away from the RZT. All possible thermal histories consistent with the measured K-feldspar age and kinetic properties were computed using a variational method and contoured plots of probability density calculated from the best fit solutions constrain the full range of temperature-time histories afforded by the sample. Samples far-removed from the RZT are well described by regional slow cooling from 300 to 275°C until about 10 Ma, but thermal histories for samples adjacent to the thrust show evidence for rapid cooling between 19 and 11 Ma which we interpret as indicating the timing of thrusting along the RZT. In conjunction with a numerical thermal model, these results constrain the minimum average slip rate and displacement along the ramp during this period to be 2 mm/yr and 12 km, respectively. A cooling episode recorded by all the K-feldspar age spectra beginning at ~10 Ma may either reflect denudation following regional uplift due to displacement along the ramp of the Main Himalayan Thrust or topographic collapse following cessation of RZT thrusting.


Introduction

Knowing the timing and sequence of thrusting within the Himalaya and southern Tibet is pivotal to understanding the evolution of that unique mountain system. It has long been suspected [Gansser, 1964] that thrusting within the Himalaya is a relatively late response, forestalled for perhaps 20-30 m.y. [Harrison et al., 1992; Yin et al., 1994] following the onset of India’s collision with Asia, which began at about 50 Ma [e.g., Burbank et al., 1996]. The south-directed crustal-scale thrusts within the Himalaya, including the Main Central Thrust (MCT), Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT) [Gansser, 1964; LeFort, 1975; Bouchez and Pêcher, 1981; Mattauer, 1986; Burbank et al., 1996] all appear to sole into a common decollement, termed the Main Himalayan Thrust (MHT) by Zhao et al. [1993]. In general, the MCT places high-grade Indian gneisses atop medium grade schists, the MBT juxtaposes those schists against unmetamorphosed Miocene molasse, and the MFT is presently active. Farther north, within the Tethyan Himalaya, south-directed thrust imbricates generally juxtapose greenschist grade rocks suggesting relatively small displacements [Wang et al., 1983; Burg, 1983; Burg et al., 1984; Yin et al., 1994]. North of the Indus-Tsangpo suture, the south-directed, Gangdese Thrust (GT) has placed Late Cretaceous-Early Tertiary granitoids representing a pre-collisional magmatic arc (the Gangdese batholith) of Asian affinity over both Late Cretaceous-Early Tertiary forearc strata and a collisional melange including ophiolite and related suture zone assemblages [Yin et al., 1994]. Zircon U-Pb results and 40Ar/39Ar thermochronology indicate that the GT was active during the Late Oligocene-Early Miocene [Yin et al., 1994; T.M. Harrison, unpublished data].

The relatively late discovery of the south-directed GT is owed to the fact that a younger, north-directed thrust, termed the Renbu Zedong Thrust (RZT), generally obscures its exposure [Yin et al., 1994].

The RZT truncates the GT, burying both the GT and its hanging wall granitoids beneath the RZT hanging wall, which consists of predominately low-grade Tethyan shelf rocks derived from the northern margin of India (Figure 1). The RZT was first mapped by Chinese geologists [Wang et al., 1983] and later appeared locally on regional compilations of southern Tibetan geology [Burg et al., 1983; Liu et al., 1988; Kidd et al., 1988]. Our own field work has consolidated these observations and confirms that, with the exception of the Zedong window (Figure 1), the RZT defines the ‘suture’ between rocks of Indian and Asian affinities from Renbu to the eastern syntaxis (Figure 1). Both regional geologic relationships and mesoscopic structures including asymmetric folds and thrusts directly above the RZT confirm that the thrust is north-directed. South-dipping cleavage and north-verging isoclinal folds in the Tethyan metasediments immediately above the fault also indicates northward thrusting along the RZT [Yin et al., 1994].

Although the RZT is the dominant structural feature along the suture zone in southern Tibet [Wang et al., 1983; Burg, 1983; Liu et al., 1988; Kidd et al., 1988; Yin et al., 1994], its timing and kinematics have not previously been well studied. The RZT is clearly younger than the GT as its hanging wall strata are locally thrust over the trace of the GT, putting both the hanging wall and footwall rocks of the GT in the RZT footwall. This relationship thus places an upper age limit on the RZT of ~23 Ma [Yin et al., 1994]. Because the RZT appears not to be offset by the north-south trending Yadong-Gulu rift, dated at 8±1 Ma [Harrison et al., 1995a], we infer that its activity is restricted to the interval 23 to 8 Ma. A possible indication of the time of displacement along the RZT occurs near Renbu (Figure 1), where Ratschbacher et al. [1994] obtained a 17.5±0.9 Ma K-Ar age from white mica filling a syn-kinematic tension gash in the RZT shear zone that potentially dates mica growth during low-grade metamorphism.

Reheating of the footwall beneath a major overthrust potentially allows the slip history of the fault to be estimated through the combined use thermochronology and numerical heat flow models. Numerical heat flow calculations indicate that relatively high displacements rates (>15 mm/yr) are required to bring about significant temperature increase (>100°C) within a stationary footwall beneath a major overthrust. This estimate also depends upon associated effects, such as shear heating, fluid circulation within the fault zone, and topographic effects. In this paper, we present 40Ar/39Ar thermal history results from granitoid samples collected in a traverse from the trace of the RZT into its footwall. In addition we report 238U-206Pb zircon ages which constrain the timing of prior pluton emplacement. These results show clear evidence of a thermal imprint that we relate to thrusting and permit us to evaluate models describing the RZT slip history. This analysis has led us to conclude that the RZT was active between 19-10 Ma. Although the RZT is out-of-sequence thrust with respect to the GT and the Himalayan thrusts, this estimate of its timing fills an otherwise unrepresented interval in the Oligo-Miocene history of thrusting in the Himalaya and southern Tibet.

Geologic Setting

Between Renbu and the Zedong window (Figure 1b), the nature of the RZT is obscured due to both structural complexity (e.g., thrust imbricates) and burial beneath the wide Tsangpo river channel deposits. East of the Zedong window however, the geological relationships are straightforward and the RZT is well exposed. During fieldwork in 1994, we encountered geological relationships that permitted us to systematically sample the granitoids within the footwall of the RZT at progressively greater distances from the fault. Near Lang Xian (Figure 2a), the Tsangpo river turns abruptly north providing excellent exposure of the RZT (Figure 3). Here, the fault places Tethyan phyllites directly atop a ~100 m thick section of deformed Kailas-type molasse [Harrison et al., 1993] which is itself deposited nonconformably on granitoids of the Gangdese batholith (Figure 2b). Farther east, the Tsangpo turns northeast into the footwall of the RZT (Figure 2a). Here we collected twelve granitoid samples from exposures along the Tsangpo river. Our 25 km sampling traverse along the river valley occurs along a trend that diverges at ~40° from the surface trace of the RZT (Figure 2 inset; Table 1). Assuming a constant 30° dip to the south for the fault (Figure 3), our samples are situated between 0-9 km beneath the RZT.

At the beginning of the traverse, the faulted contact between the metasediments and Gangdese batholith granitoids is clearly exposed with intrusive rock below the RZT penetratively deformed and recrystallized to greenschist facies. Close to the RZT, recrystallization in the footwall includes replacement of biotite by chlorite±prenhite±sphene, reaction of amphibole to chlorite+epidote, and alteration of plagioclase to albite+quartz+epidote±calcite. This greenschist facies alteration abruptly decreases in intensity over a distance of 1 km and is imperceptible at ~5 km from the RZT. The spatial association of retrograde overprinting to the RZT in the footwall implies that mild heating and/or access to fluids occurred in response to overthrusting of the Tethyan metasediments.

Rocks within the hanging wall are characterized by perceptibly higher grade (upper greenschist to lower amphibolite facies) metamorphism than is typically found in the Tethyan metasediments farther west [e.g., Ratschbacher et al., 1994; Yin et al., 1994]. Directly above the RZT for example, biotite-grade schists are exposed and locally derived float blocks contain garnet-bearing, epidote amphibolite mafic schist are present. Existence of these assemblages indicates that temperatures at the base of the hanging wall were potentially as high as ~450°C at the time of thrusting.

Results

Below we present 40Ar/39Ar (biotite and K-feldspar) and ion microprobe 206Pb/238U (zircon) analysis of materials sampled as a function of distance from the surface trace of the RZT. Interpretation of 40Ar/39Ar thermochronologic results from granitic rocks in the footwall is facilitated by knowledge of their emplacement ages. In addition, 40Ar/39Ar analyses obtained from hornblendes obtained from the hanging wall immediately adjacent to the RZT provide high-temperature thermochronologic constraints for the base of the upper plate. Details of the analytical approaches employed are outlined in Appendixes A and B respectively. Thermal history modeling of the 40Ar/39Ar K-feldspar results is then described. Additional details appear in Appendix 3. Finally, numerical heat flow calculations performed to evaluate the thermochronologic results in terms of the slip-history of the RZT are discussed.

RZT Hanging Wall

Hornblendes extracted from hanging wall epidote amphibolite facies mafic schists near the RZT yield isochron ages of 17.9±0.4 Ma and 17.6±0.4 Ma (Figure 4). We interpret these amphibole ages as approximating the time that the presently exposed hanging wall had cooled below ~ 450-550°C [Baldwin et al., 1990]. Inasmuch as the peak temperature of recrystallization recorded by metamorphic mineral assemblages within these rocks indicate that they are unlikely to have exceeded ~ 450-550°C, we regard it likely that the hornblende ages record the time of initial motion of the RZT hanging wall towards the surface due to slip along the thrust.

RZT Footwall

U-Pb Zircon Ages. Single-grain, 206Pb/238U apparent ages calculated from ion microprobe analyses of granitoids from the footwall are dominantly Late Cretaceous (Table 2; Figure 5). Because of the low radiogenic 207Pb levels (~50%) of these relatively young, U-poor zircons, there is a large uncertainty associated with the calculation of 207Pb/235U ages limiting our ability to assess the concordance of the U-Pb system in these materials. We evaluated crystallization ages by plotting the 206Pb/238U and 207Pb/235U ratios uncorrected for common Pb on a concordia plot. This generally yields an array with a slope which corresponds to the common 207Pb/206Pb ratio (~1.1) and an intersection that is indistinguishable from the corrected 206Pb*/238U age. This indicates that the U-Pb ages are concordant and that the Pb*/238U ages date the time of crystallization. With the exception of a few restitic grains yielding Early Cretaceous or older ages, the majority of samples including those in close proximity to the RZT yield 206Pb*/238U ages between 80-100 Ma (Table 2). From these results we conclude that: (1) the emplacement ages of rocks we have examined from the RZT footwall overlap with the time of large-scale granitoid intrusion that formed the Gangdese batholith [Allègre et al., 1984; Schärer et al., 1984]; and (2) the materials examined are derived from multiple intrusions all of which are Pre-Tertiary in age. GR-13 and GR-15 appear to be somewhat older (Late Early Cretaceous) while the youngest (GR-20) is latest Cretaceous.

Biotite and K-feldspar 40Ar/39Ar Ages. Biotite was dated using the laser fusion 40Ar/39Ar method. Total fusion ages for biotite are provided in Table 1 and plotted as a function of distance from the RZT in Figure 5. Apparent ages increase regularly from 12.3 Ma at the RZT to ~60 Ma at horizontal distances greater than 5 km from the fault (2.5 km beneath the fault assuming a constant 30° dip). At distances greater than 5 km biotite ages remain fairly constant (GR-20 is an exception). The maximum biotite ages are ~5-10 younger than the youngest 206Pb/238U zircon age (GR-20) and typically more than 20 Ma younger than the 206Pb/238U ages of the zircons with which they coexist. This disparity in apparent age for samples more than 5 km from the RZT (excluding GR-20) likely reflects protracted Ar loss due to elevated geothermal gradients and/or episodic Ar loss due to multiple intrusion during emplacement of the Gangdese batholith.

K-feldspar 40Ar/39Ar ages also increase away from the fault (Table 1). Age spectra for each of our K-feldspar samples are presented together in Figure 6. Each sample was analyzed using a step-heating procedure that included contiguous isothermal steps for the initial temperature increments. This procedure allows us to correct K-feldspar 40Ar*/39ArK ratios for Cl-correlated excess radiogenic argon (40Ar*) derived from decrepitation of fluid inclusions [Harrison et al., 1994. Most samples (GR-04, GR-11, GR-13, GR-14, GR-15, and GR-18) yielded correlated relationships between DCl/K and D40Ar/39Ar determined for successive isothermal steps that permitted age corrections to be performed. After correction, the ages obtained for the initial gas released from all samples (0-20% cumulative 39Ar released) was found to range from 8 to 15 Ma. Although maximum ages in the age spectra generally increased as a function of distance from the fault, the relationship is complicated by somewhat younger ages obtained for the most northerly samples(Figure 6i, 6j).. It is likely that the significantly lower ages obtained for the most northerly K-feldspars (GR-19 and GR-20) may reflect thermal effects associated with an unrelated process.

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