Normal Ductile Shear Zones Research Articles

Neogene polyphase deformation related to the Alboran Basin evolution: new insights for the Beni Bousera massif (Internal Rif, Morocco)

Located in the Internal domain of the Rif belt, the Beni Bousera massif is characterized by a stack of peridotites and crustal metamorphic units. The massif is intruded by granitic dykes and affected by several normal ductile shear zones. Structural, petrological and 40Ar–39Ar dating analyses performed on these two elements highlight that (1) the granitic dykes are emplaced within major N70° to N140° trending normal faults and shear zones, resulted from an NNE-SSW extension (2) the Aaraben fault in its NE part is characterized by N70° to N150° trending ductile normal shear zones, resulted from a nearly N-S extension and (3) the age of this extensional event is comprised between 22 and 20 Ma. Available paleomagnetic data allow a restoration of the initial orientation of extension, which was nearly E-W contemporary with the Alboran Basin opening in back-arc context, during the Early Miocene. At the onset of the extension, the peridotites were somehow lying upon a partially melted continental crust, and exhumed during this event by the Aaraben Normal Shear Zone. Afterward, the Alboran Domain suffered several compressional events.

The characteristics of the structure, strain and kinematic vorticity of the Wulian detachment fault zone, Shandong Peninsula, China

As one of the vital tectonic units of the Wulian Metamorphic Core Complex (MCC), the Wulian detachment fault zone (WDFZ), which developed in the Jiaodong peninsula, separates the lower plate of the ultrahigh pressure (UHP) metamorphic rocks in the Sulu orogenic belt from the upper plate of the early Cretaceous Zhucheng basin and the basin basement. The fault zone generally strikes NNE with a dip in the west along the southern portion of the MCC and strikes NE with a dip in the WNW along the northern portion. The fault zone displays a wavy-tile-shaped extension on the plane, principally composed of the fault breccias and mylonite and transits downward to the mylonitic gneiss As a whole, the detachment fault zone shows a top-to-west or a WNW extension. By calculating the harmonic mean, we obtain a Flynn index K of 0.98–2.0, and the mean value is approximately 1.35 in the fault zone. According to the polar Mohr construction, the extensional crenulation cleavage, the RS/θ, and the quartz C-axial fabric methods, we acquire mean kinematic vorticity values of 0.64–0.97, 0.76–0.93, 0.6–0.92, and 0.63–0.98 with mean values of 0.83, 0.80, 0.78 and 0.86, respectively, for mylonite and promylonite. The strain measurement results and the kinematic vorticity values indicate that the WDFZ is a normal ductile shear zone developed in the extensional setting. The kinematic track shows that the kinematic vorticity value decreases gradually from the NW to the SE as a whole. A simple shear dominates in the middle and upper parts of the shear zone, which is reflected by a higher vorticity value (>0.75, up to 0.98), a low thinning rate and a lower K value. In contrast, toward the footwall, the pure shear is increased significantly, showing a lower vorticity value (<0.70, low to 0.64), a relatively high thinning rate and a higher K value. Combined with the geotectonic background, the development and evolution of the WDFZ should respond to lithospheric thinning and the destruction of the North China Craton (NCC). As a result, the WDFZ can be defined as a thinning normal shear zone developed in the extension tectonic setting and the combined result of the simple shear caused by the crust extension and pure shear led by the rapid uplift of the footwall and magmatic upwelling.

Miocene crustal extension following thrust tectonic in the Lower Sebtides units (internal Rif, Ceuta Peninsula, Spain): Implication for the geodynamic evolution of the Alboran domain

In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10kbar and 780–820°C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550°C and 1–3kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.

Paleoarchean partial convective overturn in the Singhbhum Craton, Eastern India

Analyses of fold superposition structures and kinematics of shear zones along a 120km long E–W corridor across the Singhbhum Craton (Eastern India) are combined with U–Pb–Th chemical ages of texturally constrained monazites in supracrustals, gneisses and granitoids to reconstruct the Paleoarchean tectonic history of the western part of the Craton. Metamorphic sub-greenschist facies shallow marine to platformal sediments of the Iron Ore Group (IOG) and the accordantly deformed Jagnnathpur meta-lavas were accreted onto a basement composed of >3.4Ga amphibolite facies para-amphibolites/quartzites/muscovite schists (Older Metamorphic Group, OMG) and anatectic Older Metamorphic Tonalite Gneisses (OMTG). Crustal shortening was followed by emplacement-ascent of the Singhbhum granitoid pluton, SGP (3.2–3.3Ga) synchronous with sinking of supracrustal screens along N/NNE-trending steep-dipping sinistral–normal ductile shear zones. Deformation microstructures indicate the bulk of the pluton responded to exhumation-strain by low-T (≤500°C) ductile flow of quartz and K-feldspar, with plagioclase being largely unaffected. But granitoids along the western margin of pluton emplaced concurrent with westward-retreating exhumation of the thickened crust was deformed at T≥650°C. Since all granitoids in the pluton share the same N/NNE-trending tectonic fabric, microstructural features imply phased emplacement of the pluton during exhumation strain. The transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3.2 and 3.4Ga is explained by a two-stage partial convective overturn model involving structures typical of supracrustal-down, granitoid-up sense of movement. The tectonic restoration in the western part of the Singhbhum Craton is new and complements growing evidence that accretion and convective overturn may have operated during the Paleoarchean.

The Sha’it–Nugrus shear zone separating Central and South Eastern Deserts, Egypt: A post-arc collision low-angle normal ductile shear zone

The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (∼680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ∼600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ∼600 Ma, and followed arc collision and NW-ward ejection of nappes.

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex, Central Eastern Desert, Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.

El Sibai gneissic complex, Central Eastern Desert, Egypt: Folded nappes and syn-kinematic gneissic granitoid sheets – not a core complex

The El Sibai area of the Central Eastern Desert (CED) of Egypt consists of an ophiolitic association of arc metavolcanics, ophiolitic rocks, mélange, metasediments and minor mafic intrusions; and a gneissic association of amphibolite, gneissic diorite, tonalite, granodiorite and granite. Previous studies of the El Sibai area have identified the gneissic association as a lower crustal infrastructure in sheared contact with upper crustal ophiolitic association suprastructure, and have presented it as an example of a metamorphic or magmatic core complex. Detailed structural remapping of the El Sibai area reveals that the gneissic association rocks are not infrastructural but form a unit within the ophiolitic association nappes. Furthermore, the El Sibai structure is not domal in shape, and is not antiformal. The main gneissic association rocks are tabular intrusions roughly concordant with the shears dividing the ophiolitic association into nappes, and are syn-kinematic with the nappe stacking event (∼700–650 Ma). The gneissic granite tabular intrusions and their ophiolitic host were later folded about upright NW–SE trending mainly open folds during a NE–SW directed shortening event (∼625–590 Ma). Subsequently, NW–SE regional extension effects became evident including low angle normal ductile shear zones and mylonites. The latest gneissic red granites are syn-kinematic with respect to these shear zones. Probably continuing from the low-angle shearing event were steep normal faults, and sinistral WNW and N–S trending transcurrent faults (∼590–570 Ma). The normal faults mark the southeastern and maybe also the northwestern limits of the El Sibai gneissic association rocks. The El Sibai complex is not a core complex, but exemplifies the overlap of NW–SE folding and NW–SE extensional which is a significant theme of CED regional structure.

Formation of new continental crust in Western British Columbia during transpression and transtension

Crustal growth in the Coast Mountains, along the leading edge of the Canadian Cordillera, was the result of processes associated with horizontal flow of material during transpression and subsequent transtension, and the vertical accretion of mantle derived melts. From 85 to 58 Ma, as exotic terranes were translated northward during transpression, the crust was thickened to about 55 km, and melt that originated from a mix of mantle-derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones. When the orogen extended, between 58 and 50 Ma, there was large-scale decompression melting in the mantle and dehydration melting in the lower crust. Voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones. By 50 Ma, the final crustal thickness of the new continental crust was about 34 km. Comparison of seismic data with other crustal sections suggests that the crust-forming processes identified in western British Columbia have general applicability to models for the formation of continental crust.

Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western Mediterranean

The Ligurian basin, western Mediterranean Sea, has opened from late Oligocene to early Miocene times, behind the Apulian subduction zone and partly within the western Alpine belt. We analyze the deep structures of the basin and its conjugate margins in order to describe the tectonic styles of opening and to investigate the possible contributions of forces responsible for the basin formation, especially the pulling force induced by the retreating subduction hinge and the gravitational body force from the Alpine wedge. To undertake this analysis, we combine new multichannel seismic reflection data (Malis cruise, 1995) with other geophysical data (previous multichannel and monochannel seismic sections, magnetic anomalies) and constrain them by geological sampling from two recent cruises (dredges from Marco cruise, 1995, and submersible dives from Cylice cruise, 1997). From an analysis of basement morphology and seismic facies, we refine the extent of the different domains in the Ligurian Sea: (1) the continental thinned margins, with strong changes in width and structure along strike and on both sides of the ocean; (2) the transitional domain to the basin; and (3) a narrow, atypical oceanic domain. Margin structures are characterized by few tilted blocks along the narrow margins, where inherited structures seem to control synrift sedimentation and margin segmentation. On the NW Corsican margin, extension is distributed over more than 120 km, including offshore Alpine Corsica, and several oceanward faults sole on a relatively flat reflector. We interpret them as previous Alpine thrusts reactivated during rifting as normal faults soling on a normal ductile shear zone. Using correlations between magnetic data, seismic facies, and sampling, we propose a new map of the distribution of magmatism. The oceanic domain depicts narrow, isolated magnetic anomalies and is interpreted as tholeitic volcanics settled within an unroofed upper mantle, whereas calcalkaline volcanism appears to be discontinuous but massive and has jumped in space and time, from the beginning of rifting on the Ligurian margin (∼30 Ma), toward the Corsican margin at the end of the Corsica‐Sardinia block rotation (∼16 Ma). This space and time shift reveals the importance of the rollback of the Apulian slab and of the migration of the Alpine‐Apennines belt front toward the E‐SE for driving basin formation. We also state that initial rheological conditions and inherited crustal fabric induce important changes in the styles of deformation observed along margins and between conjugate margins. In the NE Ligurian basin the prerift Alpine crustal thickening together with slow rollback velocity likely contribute to distribute strain across the whole NW Corsican margin, whereas farther south the inherited Hercynian structural pattern combined with a faster rollback of the subducting plate tend to focus the extension at the foot of the margin, up to the Sardinian rift which ends within the SW Corsican margin. Therefore the mode of opening and the margin structures mainly depend on the balance between intrinsic, inherited crustal heterogeneity (fabric and rheological changes) and external conditions imposed by rollback of the subducting lithosphere.

Folding in high-grade rocks due to back-rotation between shear zones

Folds with a sense of asymmetry opposite to the bulk shear sense may form in high-grade rocks due to back-rotation between ductile shear zones. Layers back-rotated into the shortening field can undergo additional buckle folding. With progressive deformation, shear zones and domains of back-rotated foliation between shears rotate towards the X– Y (flattening) plane of the finite strain ellipsoid. In high-grade gneiss, the migration of melt into shear zones may facilitate displacement along them and accentuate back-rotation of material between shear zones. Folding during back-rotation occurs when the spacing between shear zones does not increase sufficiently to accommodate the length of back-rotated layers or between non-parallel shear zones. Folds formed by back-rotation between shear zones commonly have pegmatites sub-parallel to their axial planes, thickened overturned limbs and occur in localised packages. Associated shear zones have the same sense of shear on both attenuated limbs of folds formed by back-rotation. Examples are given for folds between transcurrent and normal ductile shear zones in the Albany Mobile Belt and Leeuwin Complex (Western Australia) and the Dharwar Craton (India).

Metamorphic evolution, partial melting and Miocene exhumation of lower crust in the Edough metamorphic core complex, west Mediterranean orogen, eastern Algeria

The Edough massif is the easternmost crystalline massif of the Maghrebides which represent the southeastern part of the west Mediterranean orogen. It emerges from below the greenschist–facies Tellian units that represent the thrusted Mesozoic to Eocene passive paleomargin of northern Africa deposited on the thinned continental crust. The top of the crystalline pile of the Edough massif is characterised by metasedimentary rocks affected by medium- to high-pressure (12–14 kbar) and medium-temperature metamorphic conditions of ca. 600 °C. The northern part of the massif as well as the overlying allochthonous amphibole–peridotite unit in the north is characterised by high-pressure granulite facies ( T∼800 °C and P≥12 kbar). Nearly isothermal decompression is documented in metapelites by late- to post-kinematic crystallisation of sillimanite and andalusite overprinting kyanite. The core of the dome comprises essentially diatexite and granite gneisses. A late low-pressure evolution and overprinting (2–3 kbar) took place in Paleozoic rocks in proximity to granite–gneisses. The predominant igneous mineralogy of granite–gneisses suggests that they represent synmetamorphic and syn-kinematic sheeted intrusions generated through a high-degree melting of metasediments. Geochronology of the metamorphic rocks (U–Pb on monazite, Sm–Nd on garnet–WR pairs, 40Ar/ 39Ar on amphiboles and micas) yields ages bracketed between 24 and 16 Ma, suggesting that regional metamorphism of the Edough massif took place during the Late Tertiary, synchronous with the greenschist–facies metamorphism recorded in the overlying Mesozoic Tellian units. The Edough massif represents a metamorphic core complex exhumed through tectonic denudation assisted by syn- to late-metamorphic low-angle, N-dipping normal ductile shear zones. An exhumation rate of 6–8 km/Ma and associated cooling rate of 200–300 °C/Ma is implied. We conclude that the Edough massif represents part of the former thinned continental crust of Africa involved in a Late Eocene northward subduction beneath the suboceanic domain that separated Paleo-Europe and Africa in Mesozoic time. The upward syn-metamorphic tectonic emplacement at ∼40–45 km depth of slices of spinel peridotites detached from the lower mantle may be responsible for the local granulite facies occurrences. Their emplacement is a consequence of the long-lived transtensional (sinistral, then dextral) kinematics having taken place between the African and European plates.

Probable anticlockwise P,–T evolution in extending crust: Hlinsko region, Bohemian Massif

ABSTRACT In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low‐grade metasedimentary series (Hlinsko schists) and high‐grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late‐Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used.The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P–T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P‐T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A low aH2O is required to explain the presence of andalusite‐biotite‐bearing assemblages, and could be related to the presence of abundant graphite.In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P–T histories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P–T path recorded in the hangingwall pelites is related to the rapid, extension‐controlled sedimentation of the overlying flysch series.

Late orogenic extension in the Bohemian Massif: petrostructural evidence in the Hlinsko region

The tectonic contact between low-grade metase-dimentary series and high-grade rocks in the Hlinsko region (Bohemian Massif) is commonly interpreted as a thrust of the Barrandian sediments over the upper Moldanubian nappe.The sediments occur in an E-facing synform that contains a tonalitic laccolith on its eastern boundary with the Moldanubian, and is truncated by a granodiorite pluton to the west. The synform represents a late deformational folding event related to the granodiorite intrusion. NW-oriented normal shear in the tonalite is indicated by S-C microstructures. Kinematic criteria associated with the major foliation and lineation development in the metasediments also indicate a north-westward, normal shear. In addition, Moldanubian gneiss display late shear bands due to north-westward, normal shear. Consequently, the presumed thrust is a low-angle, normal shear zone.Low-pressure type metamorphism (3 < P < 4 x 102 MPa) coeval with the major deformational phase in pelites of the Hlinsko synform is attributed to both the tonalite aureole and the extensive HT metamorphism (under P > 6 x 102 MPa) that has affected the underlying Moldanubian.The possibly polyphase normal fault is consistent with the meta-morphic pressure jump between the metasediments and the Moldanubian.We suggest that the tonalite intruded syntectonically within the normal ductile shear zone active during waning stages of the Variscan orogeny.

Tatla Lake Metamorphic Complex: An Eocene Metamorphic Core Complex on the southwestern edge of the Intermontane Belt of British Columbia

The Tatla Lake Metamorphic Complex (TLMC), which lies on the southwest side of the Intermontane Belt (IMB) in British Columbia, has characteristics typical of a metamorphic core complex: anticlinorial amphibolite‐grade gneissic and migmatitic core underlying a 1− to 2.5+‐km‐thick zone of mylonite and ductilely sheared metamorphic rocks which is in fault contact beneath an upper plate of low‐metamorphic‐grade cover rocks of the 1MB. Ductile shearing in the TLMC involved tonalitic to granodioritic orthogneiss and structurally overlying amphibolite‐grade metasedimentary rocks and greenschist‐grade chlorite‐actinolite‐albite schist. Structures observed throughout the ductilely sheared rocks include a gently dipping mylonitic foliation (Ss), containing a mineral lineation (Ls) which trends toward 280° (100°) ± 20°. Minor folds of variable trend (Fs), almost exclusively confined to metasedimentary rocks, are interpreted as synductile shear. Vergence of these folds defines a movement sense and direction of top toward 290° ± 20°. Kinematic indicators from rocks not deformed by synductile shear folds indicate a tops‐to‐the‐west sense of shear, while those within metasedimentary rocks (deformed by Fs folds) yield conflicting results, with a tops‐to‐the‐west sense predominating. Calculated a directions from Fs folds which deform Ls lineations indicate nearly horizontal Ds movement, subparallel to 290°–110°. The entire metamorphic core of the TLMC has been deformed by upright, west to west‐northwest trending, shallowly plunging map‐scale folds (F3). The steeply dipping, northwest trending Yalakom fault cuts all units and forms the southwestern margin of the TLMC. U‐Pb zircon geochronology has documented the existence of Cretaceous (107–79 Ma, in the core) and Eocene 55–47 Ma, (in the mylonitic zone) deformation and metamorphism in the TLMC. K‐Ar dates for biotite and hornblende of 53.4–45.6 Ma record the uplift and cooling of the TLMC. During early and middle Eocene time (55–47 Ma) metamorphic rocks of the TLMC were carried to higher crustal levels along the footwall of the TLMC normal ductile shear zone. Final uplift and development of F3 folds (post‐47 Ma) are possibly related to dextral motion along the Yalakom fault The TLMC has structural style and timing of deformation similar to other metamorphic core complexes in southeastern British Columbia. Local and regional evidence is consistent with the formation of the TLMC in a regional extensional setting within a vigorous magmatic arc. Similar interlayered gneisses with near‐horizontal layering and foliation may underlie the entire southern IMB.