New structural, petrographic, and 40Ar/39Ar data constrain the kinematics of the ASRR (Ailao Shan‐Red River shear zone). In the XueLong Shan (XLS), geochronological data reveal Triassic, Early ...Tertiary, and Oligo‐Miocene thermal events. The latter event (33–26 Ma) corresponds to cooling during left‐lateral shear. In the FanSiPan (FSP) range, thrusting of the SaPa nappe, linked to left‐lateral deformation, and cooling of the FSP granite occurred at ≈35 Ma. Rapid cooling resumed at 25–29 Ma as a result of uplift within the transtensive ASRR. In the DayNuiConVoi (DNCV), foliation trends NW‐SE, but is deflected near large‐scale shear planes. Stretching lineation is nearly horizontal. On steep foliations, shear criteria indicate left‐lateral shear sense. Zones with flatter foliations show compatible shear senses. Petrographic data indicate decompression from ≈6.5 kbar during left‐lateral shear (temperatures >700°C). 40Ar/39Ar data imply rapid cooling from above 350°C to below 150°C between 25 and 22 Ma without diachronism along strike. Along the whole ASRR cooling histories show two main episodes: (1) rapid cooling from peak metamorphism during left‐lateral shear; (2) rapid cooling from greenschist conditions during right‐lateral reactivation of the ASRR. In the NW part of the ASRR (XLS, Diancang Shan), we link rapid cooling 1 to local denudations in a transpressive environment. In the SW part (Ailao Shan and DNCV), cooling 1 resulted from regional denudation by zipper‐like tectonics in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests left‐lateral rates of 4 to 5 cm/yr from 27 Ma until ≈17 Ma. DNCV rocks always stayed in a transtensive regime and do not show cooling diachronism. The similarities of deformation kinematics along the ASRR and in the South China Sea confirms the causal link between continental strike‐slip faulting and marginal basin opening.
We describe a new method to estimate directly ductile strain rates at an outcrop scale from the deformation of dikes emplaced within a shear zone. The method is tested in a well‐constrained shear ...zone: the Ailao Shan–Red River shear zone, for which global strain rates can be calculated from published fault rates. The strain rate was determined by measuring independently the shear strain (γ) recorded by the dikes and the age (t) of dikes emplacement. The shear strain was quantified by three different methods that take into account either the stretching of the dikes or their angle variations during deformation or both of them. The values of minimum shear strains range between 0.2 and 9.7 for the less to the most deformed dikes, respectively. The ages of dike emplacement were obtained by Th‐Pb sensitive high‐resolution ion microprobe (SHRIMP) dating of monazites. We obtained three groups of ages: the younger age is 22.55 ± 0.25 Ma, the intermediate age is 26.81 ± 0.66 Ma, and the oldest ages are 29.89 ± 0.46 Ma and 29.93 ± 0.38 Ma. The geochronological data are in agreement with the structural data, the most deformed dikes being the oldest. The minimum strain rates deduced from these measurements are 3 to 4 × 10−14 s−1, which is consistent with previous estimates of geological strain rates in ductile shear zones.
Thrusting implication in the crustal thickening history of eastern Tibet is highly debated. The ∼250 km‐long Muli thrust of the Yalong thrust belt in SE Tibet is a major Miocene structure with a ...pronounced topographic step (∼2,000 m). Using thermo‐kinematic modeling based on thermochronology data, we constrain the crustal geometry of the thrust as being steep (>70°) at the surface, in agreement with field observations, and flattening at depth (≥20 km) on an intra‐crustal décollement. Thrusting motion on the fault shows a velocity of 0.2 ± 0.06 km/Ma since 50 Ma, followed by an acceleration at a rate of 0.6 ± 0.08 km/Ma starting at 12.5 ± 1 Ma, yielding a total of ∼15 km of exhumed crust. Deeper, deformation may be localized through a ductile shear zone, and be related to the ∼15 km Moho step and shear wave velocity contrast imaged by tomography beneath the Yalong thrust belt.
Plain Language Summary
The India‐Eurasia collision (∼50 million years ago Ma) led to the formation of the Tibetan Plateau, the world's largest and highest orogenic plateau. The formation and evolution of such a unique geological feature has been one of the main controversies in Earth Sciences for decades, especially regarding the role of faulting in the thickening of the crust. Here, we present 3D thermo‐kinematic models of thermochronology data allowing to constrain the exhumation history of the Muli thrust fault, a ∼250 km‐long major structure of the SE Tibetan margin, linked to significant steps in surface topography and in crustal boundary at depth (Moho). We constrain a steep fault (>70°) within the upper crust, consistent with field observations, that flattens at depth (≥20 km). The Muli thrust presents rapid thrusting motion (0.6 ± 0.08 km/Ma) that initiated at ∼12.5 Ma, following a slower phase (0.2 ± 0.06 km/Ma) since 50 Ma, with total rock exhumation of ∼15 km. This underlines the important role of thrust faulting in the thickening of the SE Tibetan crust.
Key Points
Thermo‐kinematic modeling of Muli thrust, a major thrust fault of SE Tibetan Plateau
15 km crust exhumation in 50 Ma on a high‐angle (>70°) ramp—décollement fault linked to thickening of SE Tibetan crust
Fault related to significant Moho step and shear wave velocity contrast in deep crust suggests entire crust implication
The Indus‐Yarlung suture of southernmost Tibet marks the initial collisional zone, the ongoing India‐Asia collision, and yet more than ~30 million years after the onset of collision, a thick detrital ...sedimentary unit was deposited just north of the suture: the Kailas Formation. The mechanism permitting subsidence of the deep intracontinental Kailas basin in a compressional tectonic regime remains uncertain. We present new apatite (16–11 Ma) and zircon (24–19 Ma) fission track (AFT and ZFT) ages from the Gangdese batholith just north of the Kailas basin. ZFT analysis of modern‐river sand from the northern Gangdese magmatic arc indicates an exhumation at 27.3 ± 1.3 Ma. Thermal modeling indicates that the batholith experienced reheating between 28 and 20 Ma, coeval with deposition in the Kailas basin (between 26 and 21 Ma), followed by overall rapid cooling between 20 and 17 Ma. We interpret this thermal history as a phase of regional Oligocene‐Miocene sedimentary burial followed by exhumation. By modeling mantle dynamics in the geodynamic framework of the India‐Asia collision, we show that transient dynamic topography over the relative southward folding of the Indian slab is consistent with burial and exhumation of the Gangdese magmatic arc during Oligocene‐Miocene time. The northward migration of the Indian continent relative to its own stati onary slab created a wave of dynamic topography that caused subsidence in the overriding plate north of the Himalaya, followed by a phase of surface uplift since ~27 Ma of the northern Gangdese magmatic arc. During latest Oligocene‐early Miocene time, the dynamic deflection center was in the Kailas area, and it progressively relocated southward to its present position at the Ganges basin.
Key Points
Gangdese batholith experienced reheating between ~28 and 20 Ma, followed by rapid cooling between 20–17 Ma at a rate of ~50 °C/Myr
The northward migration of the Indian continent created a wave of dynamic topography that caused the successive subsidence and uplift
The Kailas basin is a unique occurrence of a perched basin that owes its existence to dynamic deflection within a mountain belt
The presence of ~NS‐trending rifts within the Tibetan Plateau attests that it is undergoing ~EW extension. In southern Tibet, the total extension rate, distributed across seven main rifts over a ...distance of ~1,000 km, has been inferred to amount to about half of the shortening rate across the Himalayas. Quantifying the late Quaternary extension rates across the largest rift (Yadong‐Gulu rift YGR) is important to understand Tibetan deformation and to discuss the high plateau evolution during the later stages of continental collision. We performed 10Be surface‐exposure cosmogenic nuclide dating of 57 samples from three fluvial surfaces and two moraines that are vertically offset by the normal faults bounding the northern YGR. After carefully assessing individual ages at each site, to elucidate scatter in the age distributions, we obtained ~EW extension rates of up to 3–6 mm/yr near the northern end of the rift (Gulu) and of only 1.3 ± 0.3 mm/yr in the south (Yangbajing). The fast rates in the north may be influenced by dextral slip along the Beng Co fault, whose rate ought to be at least 6.0 ± 1.8 mm/yr. The total late Quaternary extension rate of 9 ± 2 mm/yr we infer across southern Tibet between ~81°E and 92°E, assuming similar rates across each rift, is similar to earlier, qualitative inferences and consistent with recent geodetic results. Distinct deformation rates north and south of the Bangong‐Nujiang suture may reflect significant differences between the extensional kinematics and mechanisms across the Qiangtang and Lhasa blocks.
Key Points
Extension rates along Yadong‐Gulu rift decrease from north (3–6 mm/yr) to south (~1.3 mm/yr) due to Beng Co dextral faulting
The post‐250 ka extension rate across the ~1,000 km stretch of South Tibet north of Nepal is 9 ± 2 mm/yr, consistent with geodetic rates
The causes and mechanisms of eastward extension south and north of the Bangong‐Nujiang suture are different
Using multispectral SPOT images and 1/100,000 topographic data, we present an improved map of the active Red River fault zone between Midu (Yunnan, China) and Hanoi (Vietnam). The fault zone is ...composed of parallel strands, one of which, the Yuanjiang fault was previously undetected. There also appears to be a component of extension all along the fault zone. Such extension increases toward the SE, from Yunnan to the south China sea coast, and the vector describing the motion of south China relative to Indochina points within the N45°–135°E quadrant. We attempt to assess the Plio‐Quaternary dextral slip rate on the Red River fault (RRF) by restoring large river offsets and searching for the largest, plausible one. Across much of Yunnan, the fault is perpendicular to local catchments that drain into the Red River. From precise mapping of the river courses on SPOT satellite images and on 1/100,000 topographic maps, numerous multiple offsets along the fault can be detected and reconstructed. The lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred. We thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m, up to an offset of 50 km. For each increment we measure the misfits (root mean squares, RMS) between the upstream and downstream channels. The best fit and smallest RMS are obtained for an offset of 25±0.5 km that we interpret to represent the clearest, large right‐lateral displacement recorded in the geomorphology along the active Red River fault. Since dextral motion is likely to have started around 5 Myr, the most probable average Plio‐Quaternary slip rate on the fault is of order of 5 mm/yr. We attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes. Our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets.
New U–Pb and Rb–Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan–Garzê fold belt, west Sichuan, China. The Ma Nai granite ...was probably syn-kinematic with the main deformation and yields an age of 197±6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195±6 and 181±4 Ma). The post-orogenic Markam massif gives two ages of 188±1 and 153±3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and décollement tectonics in the Songpan–Garzê fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India–Asia collision.
Exposures of high‐grade, midcrustal rocks within the Red River shear zone (RRSZ), which separates the Indochina and South China blocks, exhibit clear evidence of left‐lateral, ductile deformation. ...Assuming that the South China Sea represents a pull‐apart basin formed at the southeastern termination of the RRSZ, it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the RRSZ between ∼32 and 17 Ma at a rate of ∼4 cm/yr. While 40Ar/39Ar thermochronometry indicates that left‐lateral slip occurred along the RRSZ between 25 and 17 Ma, the timing of earlier high‐temperature deformation has not been directly constrained. In situ Th‐Pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite‐grade metamorphism and synchronous left‐lateral shearing. Results from northern segments of the RRSZ in Yunnan, China, indicate that synkinematic garnet growth occurred between 34 and 21 Ma and are the first to document late Oligocene metamorphism and left‐lateral shearing. Data from the southern RRSZ within Vietnam are complicated by Tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the Indosinian orogeny (∼220 Ma). The period during which sinistral deformation is now constrained to have occurred along the RRSZ (i.e., 34–17 Ma) is essentially coincident with spreading of the South China seafloor (32–17 Ma). This temporal and kinematic link between left‐lateral shearing along the RRSZ and opening of the South China Sea supports the view that Indochina was extruded from Asia as a block along lithospheric‐scale strike‐slip faults.
The total offset, lifespan and slip rate of the Karakorum fault zone (KFZ) (western Tibet) are debated. Along the southern fault half, ongoing oblique slip has exhumed dextrally sheared gneisses ...intruded by synkinematic leucogranites, whose age (∼23 Ma, U/Pb on zircon) indicates that right-lateral motion was already in progress in the late Oligocene. Ar/Ar K-feldspar thermochronology confirms that rapid cooling started around 12 Ma, likely at the onset of the present dextral normal slip regime. Correlation of suture zones across the fault requires a total offset greater than 250 km along the active – northern – fault branch. An average long-term slip rate of 1±0.3 cm/yr is inferred assuming that this offset accrued in a time span of 23–34 Ma. Southwest of the Ladakh-Karakorum Range, the large-scale boudinage of ophiolitic units suggests that an offset of several hundreds of kilometers exists along another – southern – branch of the KFZ. Towards the southeast, in the Mount Kailas region, the fault zone does not end at Gurla Mandatha, but continues eastwards, as a transpressive flower structure, along the Indus–Tsangpo suture. Our new data thus suggest that the KFZ contributed to absorb hundreds of kilometers of India–Asia convergence.
... Searle (2006) claims that all the deformed granitoids found within the shear zone predate left-lateral shear and that their crystallization ages should thus be interpreted to provide an upper ...limit for the onset of deformation, rather than a lower limit, thus suggesting a maximum age of 21 Ma for this deformation. Because 39Ar/40Ar ages generally reflect cooling below a closure temperature (from c. 510 ±50 °C for amphiboles to <200°C for the less retentive domains of K-feldspars), the cooling and exhumation can be dated, yielding further indirect constraints on the timing of shearing.\n There remain no accurate offset data for the Red River shear zone, unlike the precise pinning points of well-mapped and dated granites along the Karakoram fault (Phillips et al. 2004; Phillips & Searle 2007; Searle & Phillips 2007).
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