Kangmar Dome Research Articles

Late Cenozoic cooling and evolution history of the Kangmar dome in southern Tibet: Insights from inverse thermal modeling

The North Himalayan Gneiss Domes, which are essential parts of the Cenozoic extensional structures in Southern Tibet, record the thermal and tectonic processes that occurred after the India-Asian collision and are thought to be effective structures regulating post-collision intracontinental deformation. However, it is still unclear how these domes are formed and how they contribute to the regulation process. Here, we performed detailed geological mapping, elevation transect sampling, low-temperature thermochronological testing, and 3D modeling on the Kangmar dome, which is located west of the N‒S treading Yadong-Gulu rift, and its core-cover contact fault is suspected to be the northern continuation of the South Tibetan Detachment System (STDS). Our analysis revealed a discrepancy in the deformation histories of the dome’s northern and southern portions. We proposed a model in which the core-cover contact fault of the Kangmar dome was a part of the South Tibetan Detachment System and the doming event that occurred at ∼12.2 Ma was dominated by thrust stacking of the southward mid-crustal channel flow. The rapid cooling following the middle Miocene was possibly influenced by the N‒S Trending Yadong-Gulu rift activity. The present landscape was shaped by the incision of the Nianchu River, which was accompanied by increased glacial activity during the Pleistocene. Our findings enhance the intracontinental deformation patterns following collisions and shed light on the numerous domes in Himalayas and other orogenic belts.

Multimineral fingerprinting of modern sand generated from the Tethys Himalaya (Nianchu River, Tibet)

The Nianchu, as the largest southern tributary of the middle Yarlung Tsangpo (upper Brahmaputra River in southern Tibet), drains mainly sedimentary rocks of the Tethys Himalaya and subordinately metamorphic and igneous rocks of the Greater Himalaya, Kangmar gneiss dome, and Indus-Yarlung ophiolitic suture zone. This study presents the first detailed compositional fingerprinting of detritus released from the Tethys Himalaya in southern Tibet. Sand carried by the Nianchu and by its major tributaries range from quartzo-lithic to litho-quartzose sedimentaclastic with a few metamorphic, volcanic, and ultramafic lithic grains, reflecting dominant erosion of Tethys Himalayan strata. Transparent heavy-mineral assemblages are moderately poor. They include a mixed recycled assemblage with amphibole, tourmaline and rounded zircon, together with first-cycle sillimanite and garnet grains from the Greater Himalaya, young titanite and monazite grains from either the Greater Himalaya or the Kangmar dome, chloritoid from the low-grade metapelites surrounding the Kangmar dome, and clinopyroxene, olivine and enstatite grains shed from forearc ophiolites. Forward mixing models indicate that ~80% of Nianchu sand is recycled from the Tethys Himalaya, which is roughly the same percentage as the corresponding exposure area. Based on previous geochronological and provenance studies, and on available suspended-load data from the Nianchu, Lhasa and Yarlung Tsangpo rivers, we assess the average erosion rate in the Nianchu catchment as ~0.10 mm a−1, about twice as that in the middle Yarlung Tsangpo catchment (0.05 mm a−1) and about five times as that of the Lhasa River catchment (0.02 mm a−1). This marked difference is principally ascribed to the higher erodibility of Tethys Himalayan sedimentary strata. Within the Nianchu basin, low precipitation in the rain shadow of the high Himalayan range may explain a lower erosion rate in the headwaters (0.07 mm a−1) than in the less steep lower reaches (0.14 mm a−1).

Kinematics and vorticity in Kangmar Dome, southern Tibet: Testing midcrustal channel flow models for the Himalaya

[1] Kinematic, kinematic vorticity (Wm), and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the Kangmar gneiss dome, southern Tibet record ductile deformation patterns of a “frozen” segment of a southward flowing midcrustal channel. Microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top-north and top-south shear in garnet zone rocks to dominantly top-north shear in staurolite/kyanite zone and deeper rocks. Kinematic vorticity values indicate an increase in pure shear component with depth from ∼48% pure shear in chloritoid zone rocks through ∼62% in garnet zone to staurolite/kyanite zone rocks to ∼68% pure shear in an orthogneiss, the deepest exposed rocks. Deformation temperatures inferred from grain-scale microstructures and quartz lattice preferred orientations increase from ∼300°C–400°C in chloritoid zone rocks to ≥600°C in the deepest exposed rocks. These temperatures are equivalent to temperatures derived from garnet-biotite thermobarometry, indicating that Wm was recorded during peak metamorphism. This ductile deformation zone was cut by the brittle southern Tibetan detachment system (STDS) that juxtaposed metasedimentary rocks upon the orthogneiss. On the basis of these relations, midcrustal rocks in the core of Kangmar Dome record: (1) general shear (vertical thinning and N–S horizontal extension) with a component of top-north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the STDS, (2) an increase in pure shear with structural depth, a consequence of an increase in lithostatic load, and (3) displacement of the high-temperature shear zone by the brittle STDS. Our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel.

Crustal structure of the Tethyan Himalaya, southern Tibet: new constraints from old wide-angle seismic data

SUMMARY A wide-angle seismic profile between Peigu Tso (85.5°E) and Pumoyong Tso (90.5°E) in southern Tibet, acquired under a joint Sino-French program in 1981, passes through the South Tibetan Detachment System (STDS, the eastern segment of the profile) and the central part of the Tethyan Himalaya (the western segment). We herein reinterpret this wide-angle seismic profile, which has a total length of ∼480 km, in order to improve our understanding of the crustal structure of the Tethyan Himalaya. We identify multiple P- and S-wave wide-angle reflections within the crystalline crust and from the Moho, but cannot identify any refracting waves beneath the Moho. We model the structure of P- and S-wave velocity throughout the whole crust, while acknowledging the significant uncertainties that are inherent in many places. A low-wave speed cover sequence may be identified with the Tethyan sedimentary cover (Vp < 5.8 km s−1), from the surface to a depth of (typically) 5–10 km, which is only absent where the profile crosses the Kangmar Dome. It may be presumed that these sedimentary rocks overlay a felsic upper-crustal basement (5.8 < Vp < 6.3 km s−1), which in places extends as far down as 35 km, and which itself overlays a 5–10-km-thick mid-crustal low-velocity zone. Between a depth of 30 and 45 km at the base of the low-velocity zone and the Moho at a depth of ∼73 km, the observed velocities gradually increase from 6.2 to 6.8 km s−1. The low-velocity layer (LVL) may consist of a partial melt due to the high geothermal gradients found in the felsic rocks, or of a fluid-rich shear zone as in the ductile channel of a channel flow model. The difference in depth of the LVL between the western (central Tethyan Himalaya) and eastern segments (nearly along the STDS) at the boundary of the Dingye-Mabjia fault, leads to a north–south dip estimate of the LVL of 8.5°, which is consistent with the estimated dip of the MHT obtained from the INDEPTH-I project, and can support the channel flow model. This north–south low dip angle of subduction slab, supports that the subducting Indian lithosphere endured a transition from steep subduction to low-angle subduction, and the subduction slab flattening may be attributed to the break-off of Indian lithosphere slab beneath the Indur-Zurpo suture.

The tectonometamorphic evolution of the Alpine metamorphic belt of the Central Pamir

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 131

Preliminary Results from the Yala-Xiangbo Leucogranite Dome, SE Tibet

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.91

The Malashan gneiss dome in south Tibet: comparative study with the Kangmar dome with special reference to kinematics of deformation and origin of associated granites

Abstract Despite the importance of Tethys Himalayan or North Himalayan gneiss domes for discussing extrusive flow of the underlying Greater Himalayan Sequence, these metamorphic domes in general remain poorly documented. The main exception is the Kangmar dome. The Malashan metamorphic complex, a newly documented North Himalayan gneiss dome, is shown to have strong similarities with the Kangmar dome, suggesting that the North Himalayan gneiss domes have the following features in common: (i) Barrovian-type metamorphism with grade increasing towards a centrally located two-mica granite; (ii) the presence of two dominant ductile deformation stages, D 1 and D 2 , with D 2 showing an increasing strength towards the granite contacts; and (iii) the development of a strong D 2 foliation (gneissosity) in the outermost part of the granite cores. In addition, field and bulk-chemical studies show: (i) D 2 is associated with a dominant top-to-the-north sense of shear (in disagreement with the most recent kinematic studies in Kangmar dome); (ii) the deposition age of associated metasediments is upper Jurassic suggesting that the Malashan dome is located not at the base, but within the middle section of the Tethys Himalaya; and (iii) in contrast to the Kangmar granitic gneiss that is interpreted as Indian basement, three granitic bodies in Malashan all formed as young intrusive bodies during the Himalayan orogeny. These results suggest that the formation mechanism of the North Himalayan gneiss domes needs to be re-evaluated to test the rigidity of the hanging wall assumed in channel flow models.

Oligocene-Miocene middle crustal flow in southern Tibet: geochronology of Mabja Dome

Abstract New U-Pb zircon, monazite, 40 Ar/ 39 Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U-Pb age of 23.1 ± 0.8 Ma, providing the first age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and monazite from a post-tectonic two-mica granite yield ages of 14.2 ± 0.2 Ma and 14.5 ± 0.1, respectively, indicating that vertical thinning and subhorizontal stretching had ceased by the middle Miocene. Mica 40 Ar/ 39 Ar ages from schists and orthogneisses increase structurally down-section from 12.85 ± 0.13 Ma to 17.0 ± 0.19 Ma and then decrease at the deepest structural levels to 13.29 ± 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica granites yield similar 40 Ar/ 39 Ar cooling ages of 13.48 ± 0.13 to 12.84 ± 0.08 Ma. The low-temperature steps of potassium feldspar 40 Ar/ 39 Ar spectra yield ages of c. 11.0–12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below c. 115°C at 9.5 ± 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from c. 40–60°C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to c. 350°C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and mica 40 Ar/ 39 Ar chrontours are domed, whereas the low-temperature step potassium feldspar 40 Ar/ 39 Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures between 370 and 200°C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above cold Tethyan sediments. The structural, metamorphic and geochronologic histories documented at Mabja Dome are similar to those of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet.

Mapping granite and gneiss in domes along the North Himalayan antiform with ASTER SWIR band ratios

A series of five gneiss domes that are exposed along the axis of the North Himalayan antiform 50 km south of the Indus-Tsangpo suture, southern Tibet, are characterized by cores of Cambrian gneiss and Neoproterozoic-Paleozoic schists and migmatites, often intruded by Tertiary granites and mantled by metamorphosed equivalents of Tethyan sedimentary successions. Images from the SWIR (short wave infrared) bands recorded by the ASTER (Advanced Spaceborn Thermal Emission and Reflection Radiometer) sensor on the Earth Observation System Terra satellite reveal with unprecedented clarity the nature of the cores of the domes and the extent of the surrounding mantles of metamorphic rock. Using band ratio methods, we distinguish Tertiary granite from Cambrian gneiss on the basis of the presence of 5%–10% muscovite in the granite. This methodology is ideally suited to the mapping of a relatively inaccessible area with excellent exposure but limited ground truth. With the exception of the well-studied Kangmar dome, our images of the cores of the domes and their associated mantles bear little resemblance to the published geological maps of the region and reveal hitherto unrecognized exposures of granites. The true disposition of the domes within the North Himalayan antiform have now been established, revealing a systematic westward increase in the granite component of Himalayan age within the domes across the study area.

Lateral variations of crustal seismic attenuation along the INDEPTH profiles in Tibet from Lg Q inversion

Fourier spectra of crustal guided Lg waves are collected to study seismic attenuation, or Q, along the INDEPTH profiles. The 1 Hz Lg Q (Q0) values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying Q0 models. Along the INDEPTH III profile in central Tibet, the Q0 model consists of low and nearly constant values of ∼90. These low values are consistent with abnormally high temperature as well as partial melts in the crust. Along the INDEPTH II profile in southern Tibet the Q0 values increase southward. They are extremely low (∼60) over the northernmost segment but increase by a factor of 2 over a distance of ∼100 km between the Indus‐Yalong Suture and the Kangmar Dome. Farther south and into the high Himalayas, Q0 values are higher than 300. Regional Rayleigh waves observed along the northern INDEPTH II profile are used to infer a low‐velocity, low‐Q layer at midcrustal depths (between ∼15 and 30 km) in southern Tibet. The aqueous fluid trapped in the upper crust and a midcrust partial melting zone, associated with the underthrusting of the Indian lithosphere, are the likely causes of the high Lg attenuation in southern Tibet. Low Q, rather then a systematic Lg conversion along a dipping Moho, is the main cause of the previously observed Lg blockage over paths crossing the southern Tibetan boundary.

Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

Field, structural, and metamorphic petrology investigations of Mabja gneiss dome, southern Tibet, suggest that contractional, extensional, and diapiric processes contributed to the structural evolution and formation of the domal geometry. The dome is cored by migmatites overlain by sillimanite-zone metasedimentary rocks and orthogneiss; metamorphic grade diminishes upsection and is defined by a series of concentric isograds. Evidence for three major deformational events, two older penetrative contractional and extensional events and a younger doming event, is preserved. Metamorphism, migmitization, and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid-crustal levels. Metamorphic temperatures and pressures range from ∼500 °C and ∼150–450 MPa in chloritoid-zone rocks to 705±65 °C and 820±100 MPa in sillimanite-zone rocks. We suggest that adiabatic decompression during extensional collapse contributed to development of migmatites. Diapiric rise of low density migmatites was the driving force, at least in part, for the development of the domal geometry. The structural and metamorphic histories documented in Mabja Dome are similar to Kangmar Dome, suggesting widespread occurrence of these events throughout southern Tibet.

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite‐in, staurolite‐in, garnet‐in, and chloritoid‐in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E‐W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N‐S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post‐D1 and pre‐ to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end‐member metamorphic core complex‐type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Midcrustal reflector on INDEPTH wide‐angle profiles: An ophiolitic slab beneath the India‐Asia suture in southern Tibet?

The wide‐angle seismic experiment of Project INDEPTH (International Deep Profiling of Tibet and the Himalayas) focused on the structure of the India‐Asia continental collision suture (Yarlung Zangbo suture) in southern Tibet. Three‐component portable seismographs recorded the explosive sources of the INDEPTH seismic reflection profile across the suture zone. We image a prominent subhorizontal reflector, the “Yarlung Zangbo reflector” (YZR), dipping ∼4°N at ∼20‐km depth beneath the outcrop position of the Yarlung Zangbo suture, with a total area of at least 90 × 80 km. Possible geological interpretations for the YZR include fault/shear zones, fluids (magmatic, metamorphic, or hydrothermal), or lithotectonic contacts. High‐amplitude reflections off the YZR out to postcritical offsets and refracted phases with a velocity of ∼7 km s−1 provide evidence that it is a solid‐solid interface and the top of a mafic to ultramafic lithological unit. We suggest that the YZR is a Tethyan ophiolitic slab involving several kilometers of mafic‐ultramafic rocks. We interpret the origin of the slab and its position in the midcrust in the context of the emplacement history of oceanic lithosphere and accreted material onto the northwestern and northern Indian passive margin and the tectonic history of the Yarlung Zangbo suture zone. The origin of the YZR body and its position in the midcrust are explained by the emplacement of an ophiolite nappe‐accretionary wedge complex onto India's passive margin prior to continental collision, the subduction of this nappe complex beneath the forearc basin and the Asian (Gangdese) magmatic arc, and its further burial by the Gangdese arc from the north and the Indian passive margin sedimentary sequence from the south. Our model of the YZR constituting an ophiolitic sheet can explain several features of southern Tibetan tectonics, namely, the pre‐collisional deformation within the northern Indian passive margin, the lack of strong deformation during collision, and the bivergent thrust belt along the suture zone and doming within central southern Tibet (e.g., Kangmar dome). Our interpretation implies that the Yarlung Zangbo suture is subhorizontal in the middle crust in southern Tibet.

Evolution métamorphique du dôme de Kangmar (Sud-Est-Xizang, Tibet): Implications pour les zones internes himalayennes

The thermobarometric study of metapelites from the Kangmar dome confirms the absence of high pressure metamorphism in the eastern part of the Himalayan belt, contrasting with the western Himalayan evolution. The shape of the P-T path and the geochronological data suggest a later evolution of the Kangmar dome (between 30 and 20 Ma), compared with the earlier evolution of the western Himalayan zone (between 55 and 45 Ma). Such a contrasted geochronological and metamorphic evolution from west to east is related to a lateral variation of the dip of the continental subduction plane and to a later India-Asia collision in the eastern part.

Crustal structure of the Himalayan orogen at ∼90° east longitude from Project INDEPTH deep reflection profiles

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near‐crustal‐thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower‐crustal subduction beneath the Himalaya.

Electrically Conductive Crust in Southern Tibet from INDEPTH Magnetotelluric Surveying

The crust north of the Himalaya is generally electrically conductive below depths of 10 to 20 km. This conductive zone approaches the surface beneath the Kangmar dome (dipping north) and extends beneath the Zangbo suture. A profile crossing the northern Yadong-Gulu rift shows that the high conductivity region extends outside the rift, and its top within the rift coincides with a bright spot horizon imaged on the INDEPTH CMP (common midpoint) profiles. The high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region.

The Kangmar Dome: A Metamorphic Core Complex in Southern Xizang (Tibet)

The Kangmar metamorphic-igneous complex is one of the most accessible examples of an enigmatic group of gneiss domes (the North Himalayan belt) that lies midway between the Greater Himalaya and the Indus-Tsangpo suture in southern Tibet. Structural analysis suggests that the domal structure formed as a consequence of extensional deformation, much like the Tertiary metamorphic core complexes in the North American Cordillera. Unlike its North American counterparts, the Kangmar dome developed in an entirely convergent tectonic setting. The documentation of metamorphic core complexes in the Himalayan orogen supports the emerging concept that extensional processes may play an important role in the evolution of compressional mountain belts.