Boundary Thrust Zone Research Articles

Slope instabilities and evolution of tectonic geomorphology along the strike of the Main Boundary Thrust zone in the western Himalaya, India

The Main Boundary Thrust (MBT) and Main Boundary Fault (MBF) zones of the southeast Himachal Himalaya are susceptible to various types of mass movement. Several active and in-active landslides are observed along the strike of the MBT. The landslides are mainly controlled by the brecciated and highly fractured nature of the bedrocks, and the intersecting joint sets form wedges. The movements are taking place along the fault planes, fracture planes, and bedding planes; and the types of failure are wedge failure, planar failure, toppling, rock falls, and complex landslides. The various morphotectonic features observed in the study area include fault traces, fault scarps, strath terraces, paleochannels and structurally controlled channels. Fault traces and their associated deformed landforms are the most spectacular tectonic landforms in the area. Along the strike length of the fault traces several linearly arranged sag ponds have formed, in the western segment the fault trace is observed along the MBF and the fault scarp dips south, with a maximum height of about 34 ​m. In the eastern segment the fault trace cut across the MBT, the fault trace is also displaced by transverse faults. Paleochannels and multiple levels of strath and fill terraces collectively indicate the river channel's disequilibrium state concerning the ongoing tectonic activity. The cross-cutting relation and displacement pattern of fault traces indicate later phases of tectonic activity.

Active tectonics in the Main Boundary Thrust zone, Garhwal Himalaya, as evident from palaeoseismic signatures, morphotectonic features and PSI base ground deformation

In the Garhwal Himalaya, the steeply inclined Main Boundary Thrust (MBT) is characterized by landforms that indicate towards tectonic activity in the recent past. Linear active fault trace, sub‐recent fault scarps, strath terrace, uplifted terraces, deflected drainage, and triangular facet cones are some of the morphotectonic evidences of recent tectonic activity along the MBT. The MBT is defined by active fault trace in the form of linear depression and shows a prominent slope break across it. Thrusting of the Lesser Himalayan bedrocks over the colluvial deposits and back tilting of the colluvial deposits are also observed in the MBT zone. The formation of the sag pond and the deposition of the Quaternary lacustrine sediments followed by the deformation of the lacustrine deposits indicate phases of tectonic activities along the MBT. The later phase of deformation is represented by the folded lacustrine and colluvial deposits, where the amplitude of the fold measures about 3 m‐high and the deformed section measures about 23 m in length. The ongoing deformation patterns of the MBT zone have also been monitored with the help of Synthetic Aperture Radar‐based time series analysis. We used Sentinel‐1C dataset in ascending direction with polarization of the VV+ VH acquired between December 2, 2017 to September 7, 2021 to identify phase changes caused by ongoing active deformation. The Persistent Scatterer Interferometry (PSI) result based on Sentinel‐1A suggests that the Dehradun–Mussoorie and adjoining region has undergone an average deformation of 5.5 to −11 mm/year and the ground displacement of 1.5 to −3.8 cm. The time series analysis reveals that the Mussoorie hills in the MBT zone have higher rate of deformation.

Brittle tectonics in the western Arunachal Himalayan frontal fold belt, northeast India: Change in stress regime from pre‐collisional extension to collisional compression

The study of brittle deformation of the collisional mountains can explain its shallow crustal tectonic evolution and the palaeostress regime. The Main Boundary Thrust (MBT) zone in the western Arunachal Himalaya displays imbrication in the Permian Gondwana sequence between the MBT‐1 (/Bome Thrust/MBT‐Upper) in the north and MBT‐2 (/MBT‐Lower) in the south with consistent northerly dip. The Lower Gondwana rocks occur in the footwall of the MBT‐1 with the Proterozoic Bomdila Group in the hangingwall. The upper Gondwana rocks constitute the hangingwall sequence for the MBT‐2 with Neogene Siwalik rocks in the footwall. This article analyses palaeostress using brittle fractures in the Gondwana rocks that crop out for ~120 km2 in the study area. The fault‐bounded imbricate zone depicts eight brittle shear indicators and four sets of joints (J1 and J2: inclined and J3 and J4: subvertical). The signatures of the inherited pre‐Himalayan extensional deformation are preserved in the Lower Gondwana Miri Formation. The Bichom and Bhareli rocks exhibit brittle deformation features of the Himalayan Orogeny under strong ~N‐S compression. The palaeostress analysis of all joint sets indicates three phases of brittle deformation in the Gondwana and Siwalik rocks of the area. The subvertical joint sets and normal faults in the Miri Formation indicate a north‐northwest (NNW)‐directed extensional phase of the pre‐Himalayan origin. The inclined joint sets of the Bichom and Bhareli formations of the Gondwana sequence depict Himalayan orogeny with ~N‐S compressional phases. The third phase of brittle deformation in the Siwalik sequence depicts an east‐west (~E‐W) extension. The arc‐parallel extension in the frontal fold belt of the Arunachal Himalaya may be due to oblique India‐Asia collisional tectonics.

The role of excess oxygen for modeling high-Mn, low-Ca garnets in metapelites from the northern Central Metasedimentary Belt of the Grenville Province, Ontario, Canada

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a Fe3+/Fetot ratio of 0.33–0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 °C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 °C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P–T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction.

Reply to comments by S. K. Acharyya (2019) on “Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis, Patra, A. and Saha, D., Journal of Structural Geology, v.120, p.29–47”

Reply to comments by S. K. Acharyya (2019) on “Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis, Patra, A. and Saha, D., Journal of Structural Geology, v.120, p.29–47”

Comments on ‘Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault setup analysis’ Journal of structural geology, vol 120 (2019), 29–42, by Abhijit Patra and Dilip Saha

Comments on ‘Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault setup analysis’ Journal of structural geology, vol 120 (2019), 29–42, by Abhijit Patra and Dilip Saha

Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis

The Main Boundary Thrust (MBT) Zone in the Darjeeling foothills, Eastern Himalaya exhibits northerly dipping fault splays, named here as MBT-1 and MBT-2, with Proterozoic Daling Group in the MBT-1 hanging wall, Upper Paleozoic Gondwana rocks between the MBT-1 and the MBT-2, and Neogene Siwalik Group in the footwall of the MBT-2. On the basis of reduced stress tensors from fault-slip analysis and their grouping relative to different stratigraphic units in the MBT zone that crops out over a strike length of ∼20 km, the following successive stress regimes have been interpreted. The earliest brittle deformation recorded only in the basal part of the Gondwana succession was controlled by a pre-Himalayan NW-SE extensional regime. The Himalayan deformation under sustained N-S compressive regime developed in three stages: (a) thrust displacement along the MBT-1, juxtaposing folded Daling rocks against the Lower Gondwana rocks; (b) tilting and fracturing of Lower Gondwana succession and thrust transport over the Siwalik rocks along the MBT-2; (c) strike-slip regime with NW- and NE-trending faults affecting both the Lower Gondwana and Siwalik strata, developed possibly due to increased tectonic loading. The tensor solutions under N-S compressional regime are consistent with the modern plate convergence, supporting a foreland propagating thrust system in the Eastern Himalaya. In addition, an E-W extensional regime led to apparent orogen-parallel extension.

Geomorphologic study of the valley floor in different tectonic segments along Kosi River valley between South Almora Thrust and Himalayan Frontal Thrust: Kumaun Himalaya, India

In the Kumaun Himalaya, a portion of the Kosi River valley of ~90 km in length is chosen to study the fluvial morphology that provides first‐order information about the dynamic response of bedrock channels to tectonic impulse. The Kosi River flows across/along major tectonic boundaries such as the South Almora Thrust, Ramgarh Thrust, Main Boundary Thrust, and the Himalayan Frontal Thrust, and local transverse and longitudinal faults. Varied fluvial landforms correspond to different tectonic settings, lithologies, bedrock channels, hillslopes, large landslides, terraces, and fans. The longitudinal valleys are also the sites for thick aggradational landforms. Some portion of these valleys fall in the areas of active extensional tectonics and is characterized by one of the widest valley floor sections in the Lesser Himalaya. In contrast, the transverse valley sections are incised by deep‐cut v‐shaped valleys and the narrowest valley section. Swerving of the Kosi River is observed in the Ramgarh Thrust and Amel Fault zones and also in the Main Boundary Thrust zone. Recent tectonic activity is evident from the presence of the faulted Quaternary deposits, linear fault scarps, abandoned channels, incised meandering, and multiple levels of terraces/strath terraces. Field observations and the computed ratio of valley floor width to valley height (Vf) corroborate each other. Valleys developed parallel to the strike of faults and bedrocks have relatively broader valleys with higher Vf values whereas in contrast, the valleys developed across the bedrock strike are narrow with smaller Vf values. The results of computed stream length gradient (SL) and steepness (Ks) indices show considerable correlations between the obtained SL and Ks data and the field evidences; high values of SL and Ks are characterized by the presence of knick points observed at the prominent thrusts and faults.

Active Tectonics and Geomorphological Studies in India During 2012-2016

This article reviews the literature on tectonic geomorphological studies in India published in peer reviewed journals between 2012-2016. The northward drift of the Indian plate and its collision with the Eurasian plate results in compressive stresses in Indian subcontinent. Based on the broad morphotectonic setup the studies are categorised into the Himalaya and the Indian Peninsula. Studies in the Himalaya mainly demonstrate that most of the deformation during Holocene has occurred in the sub-Himalaya and especially along the Main Frontal Thrust (MFT). Evidences of active tectonics is also reported from several parts of the Himalaya which also includes the Main Boundary Thrust (MBT) zone. The impact of active tectonics on the geomorphology along the active faults is also investigated in several areas of the Himalaya. In the Indian Peninsula Narmada-Son fault, Tapti fault and Katrol hill fault are shown to be active on the basis of sedimentological and geomorphological investigations.

Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada

Studies of pre-Grenvillian (1.4–1.3 Ga) plutons offer insight into the dynamics of arc amalgamation and backarc rifting prior to continental collision during the Ottawan orogeny. The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a northeast-southwest–trending thrust zone consisting of metaplutonic thrust sheets enveloped in gneissic tectonites and calcitic-dolomitic marble. Tonalitic CMBbtz thrust sheets (Dysart and Redstone), located in the southern Ontario Grenville Province (Canada) are made up of upper amphibolite facies, foliation-concordant metatonalite (+ amphibole ± biotite ± accessory zircon and titanite) and amphibolite (± biotite ± clinopyroxene). These thrust sheets are thought to have formed and amalgamated onto the Laurentian margin prior to Ottawan orogeny. Major and trace element analyses show that the metatonalite rocks have calc-alkaline affinity and amphibolite rocks have both calc-alkaline and tholeiitic affinities, suggesting an arc environment. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) geochronology of zircon from the two thrust sheets yield igneous ages of ca. 1350–1300 Ma for diorite and granodiorite, ca. 1150–1100 Ma ages from Dysart tonalite interpreted to record metamorphic zircon growth, and a ca. 1086 Ma Ottawan metamorphic age from a Dysart amphibolite. The ca. 1150–1100 Ma metamorphic event has not been previously documented within these thrust sheets of the CMBbtz, but correlates well with thermal events in the allochthonous Parry Sound domain to the west, as do ca. 1350 Ma igneous ages of tonalite in both areas. These data support the hypothesis that the CMBbtz and Parry Sound domain may have been initially linked. Widespread ca. 1350 Ma crust along with distinct 1460–1400 Ma depleted mantle model ages (TDM) are also consistent with a shared genesis with the Dysart–Mount Holly suite in New York and Vermont, and support the correlation between the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt as a rifted arc. Alternatively, the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt may represent contemporaneous arc development at different parts of the convergent margin; however, we support the correlation between the CMBbtz thrust sheets and the Adirondack-Highlands–Mount Holly belt.

Landslides and Neotectonic Activities in the Main Boundary Thrust (MBT) Zone: Southeastern Kumaun, Uttarakhand

Abstract Main Boundary Thrust (MBT) zone is constituted of some of the landslide prone areas in southeastern part of Kumaun Sub-Himalaya. Role of landslides as natural hazard and hill slope modifying agent is well documented from various part of Himalayan region and southern hills of Kumaun particularly in the MBT zone, which are susceptible to various type of mass movement. The rocks making up the slopes has been put to a number of brittle deformation phases during the movement along the MBT, and are traversed by number of joint sets. In the open slope these intersecting joint sets forms wedges and are the most favorable site for initiation of rockfalls and other types of landslides. Landslides are taking place primarily due to high angle slopes, formation of structural wedges along the free steep slopes, sheared nature of the rocks due to proximity to the MBT and neotectonic activities along the MBT and other transverse faults. Wedge failure is a common type of landslides in rock slopes characterized by multiple joints and acts as sliding planes for the failed blocks. Field observations and wedge failure analysis indicates most of the landslides taking place in MBT zone of Kumaun Sub-Himalaya are joint controlled. Safety Factor analysis suggests MBT zone of Kumaun Sub-Himalayan region is prone to landslides and related mass movements. This zone is also neotectonically active as indicated by various geomorphic signatures such as structurally controlled drainage pattern, offsetting of fan by MBT and formation of number of small lakes.

Landslides along frontal part of eastern Himalaya in east Siang and lower Dibang districts, Arunachal Pradesh, India

Eastern Himalayan foothills particularly between Siang and Dibang Rivers are prone to landslides. The causative factors of landslides are steep slopes, presence of major structural discontinuities, multiple joint sets, drainage density, weathered to soft nature of rocks and heavy rainfall. Most of the landslides are taking place in steep to very steep slopes. Fault gouges and brecciated rocks developed along the Main Boundary Thrust (MBT) zone have also facilitated the initiation of many landslides. Wedge analyses for sandstones and quartzites have also been carried out to determine their influence on the slope stability and potential failure. The area is also susceptible to high rate of erosion due to structurally controlled streams and gullies resulting in profuse development of fans along the foothills. Seismic condition of the region is also a major concern for the slope stability. Landslides debris usually chokes the streams, destroy the bridges, subways and roads and affect cultivated lands particularly along the mountain front. Neotectonic activity along the MBT has resulted in the development of young and steep hill-slopes, which are prone to landslides.

Grenville Province and Monteregian carbonatite and nepheline syenite distribution related to rifting, collision, and plume passage

Stimulated by the recognition of a rifted continental margin to pre-Grenville Laurentia, we interpret the deformed alkaline igneous rocks and carbonatites (DARCs) adjacent to that margin in the Bancroft domain of Ontario, Canada, to be rocks initially erupted as alkaline rocks and carbonatites (ARCs) in an intra-continental rift. When an ocean began to form, rupturing Laurentia on the site of that rift, the newly formed Laurentian rifted continental margin contained ARCs that had erupted into the intra-continental rift. Later in a Wilson cycle of ocean opening and closing, those ARCs became DARCs during a collision that sutured the Grenville province composite arc belt and Bancroft domain rocks against the margin of Laurentia on the Central Meta-sedimentary Belt boundary thrust zone. Using this interpretation we show that DARCs of the down-dip Bancroft domain suture zone in the mantle at a depth of ~100 km are likely sources of the Early Cretaceous ARC rocks in the Monteregian province.

Cordierite-gedrite rocks from the Central Metasedimentary Belt boundary thrust zone (Grenville Province, Ontario): Mesoproterozoic metavolcanic rocks with affinities to the Composite Arc Belt

Cordierite–gedrite rocks in the southern Grenville Province occur near the base of the Central Metasedimentary Belt boundary thrust zone, interpreted by some as a crustal suture between the 1.29–1.24 Ga Composite Arc Belt and >1.4 Ga rocks of Laurentia. Major and trace-element compositions of these rocks are consistent with volcanic protoliths that range in composition from basalt to dacite. These cordierite–gedrite rocks have low CaO (average 1.2 wt.%) and major element and oxygen-isotope ratios suggestive of hydrothermal alteration before metamorphism. Rare-earth element (REE) compositions also indicate igneous protoliths, although some REE patterns have been modified by local melt extraction. The trace-element compositions of cordierite–gedrite rocks, and neodymium-isotope systematics, are similar to those of metavolcanic rocks in the Composite Arc Belt and are consistent with the extension of the Composite Arc Belt to the base of the boundary thrust zone.

Computation of Present Strain Rate Field of Qinghai-Tibetan Plateau and Its Geodynamic Implications

Many researchers calculated strain rate of significant differences from the same GPS measurement data. In this paper, we use the Kriging method in geostatistics to GPS velocity field. Interpolating the scattered GPS velocity data of Qinghai-Tibetan plateau and its adjacent areas to grid point values by Kriging, we calculate the strain rates from these nodal values in each grid cell similar to derivative of shape functions (essentially Lagrange interpolation function) in finite element algorithm, and obtain the stable distribution of strain rate field in Qinghai-Tibetan plateau. The results show that the main part of Qinghai-Tibetan plateau is in the state of compression in north-south direction, and extension in west-east direction. On the contrary, in the eastern part of Tibet, the strain rate is compressive in west-east and extensional in north-south direction. The orientations of principal strain rates are consistent with those of the P axis and T axis in focal mechanism. The high values of maximum compressive principal strain rates are located in the Himalayan main boundary thrust zone (MBT) and the surrounded regions. The maximum extensive principal strain rates are higher than those of the compressive ones in the main part of the interior of Qinghai-Tibetan plateau. Also, the surface dilation strain rate shows that it is in the state of surface compression in Himalayan and its surrounded areas, and in the state of surface extension in the interior of Qinghai-Tibetan plateau. The distribution of maximum shear strain rate clearly displays the outlines of some main active fault zones. The result of the strain rate in this study suggests that the contemporary tectonic strain of Tibet inherits the long term geological deformation.

Applications of seismic pattern recognition and gravity inversion techniques to obtain enhanced subsurface images of the Earth's crust under the Central Metasedimentary Belt, Grenville Province, Ontario

SUMMARY Project Lithoprobe's Abitibi–Grenville transect seismic reflection lines 32 and 33 traverse the exposed Central Metasedimentary Belt (CMB) located in the Grenville province of the Precambrian Shield of Canada in southern Ontario. These seismic lines image a zone with a protracted deformational history spanning more than 300 Myr. Detailed examination of the commercially processed stacked sections reveals a number of significant deficiencies in some important areas. The image quality in these zones of reduced coherency needs to be enhanced to examine specific features and their relation to the surface geology. Examination of near-vertical seismic data from Lines 32 and 33 revealed that the signal-to-noise ratio was not improved by stacking, due to misalignment of signals even after static, normal moveout corrections and residual static corrections. The presumed reason is that reflected seismic energy following long ray paths in heterogeneous media suffers from relative advances and delays in its propagation, and hence arrives at slightly different times at the receivers, tending to be poorly aligned relative to its theoretical traveltime curves. A pattern recognition (PR) method for signal enhancement followed by energy stacking in moving time windows was used in this study to improve the images in spite of misalignments. Reprocessing has refined the geometry of the reflection profiles. The objective of this paper is to use enhanced images of the seismic reflection data obtained by using a PR approach together with gravity data, using 2.5-D forward and 3-D inversion routines, to give an improved model of subsurface structure in the vicinity of lines 32 and 33. Line 32 is dominated by southeast-dipping reflectors soling into the lower crust. The listric geometry of the strong reflection packages of the CMB boundary thrust zone is interpreted to represent a crustal-scale ramp–flat geometry that accommodated northwest-directed tectonic transport of the CMB. This interpretation is also supported by the gravity data.

A seismic-based cross-section of the Grenville Orogen in southern Ontario and western Quebec

A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction - wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac-Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir-Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12-15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010-980 Ma, 1080-1060 Ma, and 1170-1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25-30 km corresponding to the top of a high-velocity lower crustal layer.

GENESIS OF CORDIERITE - GEDRITE GNEISSES, CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE, ONTARIO, CANADA

Cordierite- and gedrite-bearing lithologies crop out along the base of the Central Metasedimentary Belt boundary thrust zone (CMBbtz) in Ontario, the boundary between the Central Metasedimentary Belt and the Central Gneiss Belt. It has been proposed that these rocks represent metamorphosed and partially melted Al- and Mg-rich sedimentary rocks. Whole-rock oxygen isotope values (δ18O in the range 6–8‰) from these rocks are not consistent with closed-system metamorphism of either soils or pelitic sediments, and cannot be explained solely by extraction of partial melt from pelitic rocks. The whole-rock δ18O and chemical composition of cordierite–gedrite rocks in the CMBbtz indicate that these rocks represent volcanic rocks hydrothermally altered by seawater. After hydrothermal alteration, these rocks were metamorphosed to the upper amphibolite facies, stabilizing the cordierite + gedrite assemblages. Diffusion modeling of quartz–garnet fractionations suggests intermineral exchange of oxygen in a closed system at moderate fugacity of H2O during slow cooling after regional metamorphism. We believe that cordierite–gedrite lithologies in the CMBbtz mark the site of Mid-Proterozoic rifting at the margin of Laurentia.

Time of metamorphism beneath the Central Metasedimentary Belt boundary thrust zone, Grenville Orogen, Ontario: accretion at 1080 Ma?

New U–Pb zircon and titanite data from the Muskoka domain, Grenville Province, Ontario, provide protolith and metamorphic ages for the southwestern Central Gneiss Belt. Discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 ± 6 Ma and a lower intercept age of 1064 ±18 Ma. U–Pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207Pb/206Pb age of 1079 ± 3 Ma. A weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the Central Metasedimentary Belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 ± 13 Ma and a lower intercept age of 1066 ± 8 Ma. The charnockitic vein yielded concordant zircon ages of 1077 ± 2 Ma. The upper intercept ages are interpreted in terms of protolith crystallization, and the concordant and lower intercept ages as Grenviilian high-grade metamorphism and associated anatexis. We have found no evidence for a ca. 1190–1160 Ma metamorphic event in these rocks, as required by some regional tectonic interpretations. We conclude that emplacement of the Central Metasedimentary Belt over the Central Gneiss Belt, which caused high-grade metamorphism in the Muskoka domain, occurred at or shortly before ca. 1080 Ma, and that this marks the time of accretion of the Central Metasedimentary Belt to the southeast margin of Laurentia.

The Parry Sound domain: a far-travelled allochthon? New evidence from U–Pb zicon geochronology

We report U–Pb zircon ages for metaplutonic and metasedimentary rocks from three lithotectonic assemblages within the Parry Sound allochthon of the Central Gneiss Belt, southwestern Grenville Orogen: the basal Parry Sound, interior Parry Sound, and Twelve Mile Bay assemblages. Magmatic crystallization ages for granitic to tonalitic gneisses from the basal Parry Sound assemblage fall in the range 1400–1330 Ma. Younger intrusions include the Parry Island anorthosite dated at 1163 ± 3 Ma and a crosscutting mafic dyke bracketed between 1151 and 1163 Ma. Dated at [Formula: see text] a tonalitic gneiss from the overlying interior Parry Sound assemblage is slightly younger than the older group of rocks from the basal Parry Sound assemblage. 207Pb/206Pb ages for zircons from a quartzite of the basal Parry Sound assemblage range from 1385 Ma to the Neoarchaean. An absolute maximum age for this quartzite is 1436 ± 17 Ma. In contrast, detrital zircons from a quartzite of the Twelve Mile Bay assemblage constrain the age of deposition at post-ca. 1140–1120 Ma. We speculate that Grenvillian-age zircons within this quartzite were derived from rocks in the Adirondack Highlands and Frontenac terrane, implying that part of the Parry Sound domain and these terranes were contiguous during deposition of the quartzite. Our data support previous interpretations that the Parry Sound domain is allochthonous with respect to its surroundings, and suggest that the most likely source region of the basal Parry Sound domain lies southeast of the Central Gneiss Belt, within the Central Metasedimentary Belt boundary thrust zone or the Adirondack Highlands. This implies the possibility of 100–300 km of displacement of the domain. Emplacement of the Parry Sound domain into its present position must have occurred relatively late in the orogen's history, by about 1080 Ma.