Metamorphic Core Complex Research Articles

Tectono‐magmatic and Stratigraphic Evolution of the Cycladic Basement, Ios Island, Greece

AbstractThe Cycladic Basement (CB) and the overlying Cycladic Blueschist Unit (CBU) are part of the Paleogene Cycladic subduction complex exposed in Miocene metamorphic core complexes in the distended back‐arc of the retreating Hellenic subduction zone of the southern Aegean. While the Cenozoic tectono‐metamorphic evolutions of the CB and the CBU have been the foci of numerous studies, this study presents new laser ablation inductively coupled plasma mass spectrometry bedrock and detrital zircon (DZ) U‐Pb ages that place robust constraints on the presubduction tectonic, magmatic, and paleogeographic evolution of the CB. Zircon U‐Pb ages of crystalline CB are ~306‐330 Ma, demonstrating local plutonism associated with regional voluminous, protracted Carboniferous magmatism related to Paleo‐Tethys subduction. The plutons intruded the CB metasedimentary host‐rock sequence, characterized by distinct Gondwanan DZ U‐Pb provenance, Neoproterozoic to early Paleozoic maximum depositional ages, and synmagmatic, contact metamorphic zircon rims (~300‐330 Ma). DZ U‐Pb dating revealed postmagmatic Permian metasedimentary rocks (~270‐295 Ma) that unconformably overlie the CB and have unimodal DZ spectra that indicate exhumation of the CB prior to Permian deposition within extensional basins, as well as mark the onset of CBU deposition prior to formation of the Pindos rift domain. These U‐Pb results clarify the late Paleozoic‐early Mesozoic evolution of the CB as a peri‐Gondwanan terrane composed of Neoproterozoic and early Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and exhumed in the Permian, prior to Triassic rifting and CBU deposition. Additionally, these data provide a chronostratigraphic framework and illuminate subduction‐related juxtaposition within the CB metasedimentary sequence.

Reconstruction of Mid‐Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications

AbstractThe Rincon Mountains metamorphic core complex, located east of Tucson, Arizona, consists of an arched footwall of foliated crystalline rocks bounded above by the generally outward dipping, Oligocene‐Miocene San Pedro extensional detachment fault. The southwest trending axes of corrugations in the detachment fault, and in footwall foliation and lithologic layering, parallel mylonitic lineation, and inferred top‐southwest displacement on the fault. An upper plate fault block within a synformal fault groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced 34‐38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro detachment fault. Much of the footwall of the detachment fault in the eastern Rincon Mountains consists of metasedimentary tectonites derived largely from Paleozoic carbonates that were buried beneath Proterozoic crystalline rocks forming the hanging wall of the Laramide Wildhorse Mountain thrust. These tectonites were later exhumed by displacement on the San Pedro detachment fault. Structural reconstruction supports the interpretation that the carbonate tectonites localized extensional faulting along the San Pedro detachment fault at crustal depths where carbonates would be weak and deform by crystal plasticity while quartzo‐feldspathic rocks would be strong and brittle. This weak zone is located adjacent to the greatest width of exposed extension‐parallel mylonitic fabrics in southeastern Arizona and may have been associated with the earliest initiation of extension in the region. Domains of low‐strength carbonates may be an underappreciated influence on extensional tectonics in cratonic southwestern North America.

THERMOCHRONOLOGY OF GRANITOID BATHOLITHS AND THEIR TRANSFORMATION INTO METAMORPHIC CORE COMPLEXES (EXAMPLE OF SONG‐CHAI MASSIF, NORTHERN VIETNAM)

Based on the reconstruction of the thermal evolution of granitoid batholith, represented by the Song‐Chai gneiss‐granite massif (Northern Vietnam), the long‐term existence of granitoid magma at deep levels of the Earth's crust (H≥25 km, Δt~20–50 Ma) is established. The geodynamic analysis of the granitoid batholith and mathematical modeling of its thermal history shows that the magmatic chamber should be considered as a thermal trap at the lower level of the crust, which preserved residual granite melts for a long time. Activation of the magmatic chamber occurs in post‐collisional strike‐slip fault zones and is accompanied by tectonic exhumation of large crustal segments. As a result, the batholith is transformed into a Cordilleran‐type metamorphic core complex, residual rare‐metal melts are emplaced, and, commercial deposits are thus formed.

Compressional origin of the Naxos metamorphic core complex, Greece: Structure, petrography, and thermobarometry

AbstractThe island of Naxos, Greece, has been previously considered to represent a Cordilleran-style metamorphic core complex that formed during Cenozoic extension of the Aegean Sea. Although lithospheric extension has undoubtedly occurred in the region since 10 Ma, the geodynamic history of older, regional-scale, kyanite- and sillimanite-grade metamorphic rocks exposed within the core of the Naxos dome is controversial. Specifically, little is known about the pre-extensional prograde evolution and the relative timing of peak metamorphism in relation to the onset of extension. In this work, new structural mapping is presented and integrated with petrographic analyses and phase equilibrium modeling of blueschists, kyanite gneisses, and anatectic sillimanite migmatites. The kyanite-sillimanite–grade rocks within the core complex record a complex history of burial and compression and did not form under crustal extension. Deformation and metamorphism were diachronous and advanced down the structural section, resulting in the juxtaposition of several distinct tectono-stratigraphic nappes that experienced contrasting metamorphic histories. The Cycladic Blueschists attained ∼14.5 kbar and 470 °C during attempted northeast-directed subduction of the continental margin. These were subsequently thrusted onto the more proximal continental margin, resulting in crustal thickening and regional metamorphism associated with kyanite-grade conditions of ∼10 kbar and 600–670 °C. With continued shortening, the deepest structural levels underwent kyanite-grade hydrous melting at ∼8–10 kbar and 680–750 °C, followed by isothermal decompression through the muscovite dehydration melting reaction to sillimanite-grade conditions of ∼5–6 kbar and 730 °C. This decompression process was associated with top-to-the-NNE shearing along passive-roof faults that formed because of SW-directed extrusion. These shear zones predated crustal extension, because they are folded around the migmatite dome and are crosscut by leucogranites and low-angle normal faults. The migmatite dome formed at lower-pressure conditions under horizontal constriction that caused vertical boudinage and upright isoclinal folds. The switch from compression to extension occurred immediately following doming and was associated with NNE-SSW horizontal boudinage and top-to-the-NNE brittle-ductile normal faults that truncate the internal shear zones and earlier collisional features. The Naxos metamorphic core complex is interpreted to have formed via crustal thickening, regional metamorphism, and partial melting in a compressional setting, here termed the Aegean orogeny, and it was exhumed from the midcrust due to the switch from compression to extension at ca. 15 Ma.

Protracted late- to post-Ottawan cooling and exhumation of the mid crust in a Grenvillian inlier, central Appalachians, USA – New data and a new hypothesis

New 40Ar/39Ar hornblende and biotite data from granulite-facies gneisses across the Grenvillian New Jersey Highlands inlier in the central Appalachians permit estimates of the rates of regional cooling and exhumation following the Ottawan phase of the Grenvillian Orogeny. 40Ar/39Ar hornblende and biotite results yielded plateau ages of ca. 947–914 Ma and ca. 887–819 Ma, respectively, constraining post-Ottawan cooling from the peak metamorphic temperature of ∼770 °C to ∼300 °C at a slow rate of 1.9–3.5 °C/Myr. Following assumptions about the physical properties of these mid-crustal rocks, we calculate they were exhumed from a peak burial depth of ∼22 km, and passed through the 500 °C and 300 °C isotherms at depths of ∼14 km and ∼7.5 km, respectively, indicating a slow exhumation rate of 0.06–0.11 km/Myr.The dual signal of slow cooling and exhumation is interpreted to be due to an extended period of elevated mantle heat flow triggered by delamination or convective removal of the lower mantle lithosphere beneath the orogen. Together with lower crustal intrusion and (or) crustal underplating by asthenospheric magmas, this is interpreted to have resulted in extensional collapse of the overthickened crust, juxtaposition of crustal levels, and the formation of large metamorphic core complexes. Mafic intrusions and (or) underplating not only provided an important heat source in the lower crust, but also increased the average crustal density, thereby slowing the rates of uplift, erosion and exhumation. We interpret the dual signal as a cryptic signature of pervasive extensional orogenic collapse after the Ottawan phase of the Grenvillian Orogeny.

Structural doming tectonics: intracontinental deformation of the SW margin of the Yangtze Block, China

The southwest margin of the Yangtze Block underwent multiple tectonic events including the collision between the Yangtze and the South China Plate, Caledonian folding and uplift of South China, subduction of the western Pacific Plate, and Cenozoic continental escape during the Indo–Asia collision. The structural framework and tectonic evolution of the area are currently unclear. Through field petrologic and structural studies, microstructures within the deformed units, and estimates of the temperature of deformation, we constrain the conditions of deformation and further classify at least four-stage deformation. The first stage involved northeastward thrusting that produced folds, mylonites, and dynamically recrystallized quartz and calcite. Electron backscatter diffraction data indicate that deformation during this event occurred at < 400 °C, corresponding to a depth of ~ 10 km. The second stage of deformation involved uplift, extension, and the generation of a domal structure. During the third event, high-angle normal faults formed within the cover sequence that surrounds the dome. The fourth stage of deformation occurred in the Cenozoic and involved motion along sinistral and dextral strike–slip faults, which cut structures that formed during the previous stages of deformation. The first deformation event was related to Triassic N–S collision with the Indochina block; the second and third stages resulted from far-field compression related to westward subduction of the paleo-Pacific plate; and the fourth event was related to displacement along the Ailaoshan Red-River strike–slip fault. The dome in the study area represents a transitional structure between a metamorphic core complex and a gneissic dome, which we refer to as structural doming.

Accelerating exhumation in the Eocene North American Cordilleran hinterland: Implications from detrital zircon (U-Th)/(He-Pb) double dating

AbstractBasins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve records of deformation that are unavailable in footwall rocks. Following prolonged late Mesozoic–early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth’s best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermochronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105–740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (&amp;lt;4–6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to &amp;lt;100 m.y. in 45–43 Ma strata and to &amp;lt;10 m.y. in 43–41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1–20 m.y. lag times. These data reflect acceleration of basement exhumation rates by &amp;gt;1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere.

Mesozoic tectonic evolution of eastern China: Insights from the Changshan Islands, NE China

The Changshan islands are scattered among the Yellow Sea east of the Liaodong Peninsula, west of North Korea and north of the Sulu Orogenic Belt. They consist of three lithotectonic units: i) Archean gneiss, ii) Paleoproterozoic marble and micaschists, and iii) Neoproterozoic weakly metamorphic sedimentary rocks. Three stages of deformation (D1-D3) documented in this study indicate that Changshan islands are a key region to decipher the Mesozoic tectonic evolution of eastern China. The earliest deformation D1 corresponds to a ductile deformation with NE-SW trending mineral and stretching lineations and top-to-the-NE sense of shear. The deformation D2, observed in the micaschists and sedimentary rocks, is characterized by the SW-verging folds with NE-dipping axial planar cleavage S2 and SW-directed thrusting. D2 could be considered as the back-folding and back-thrusting of the D1. D3 is expressed by the NE-SW trending low-angle brittle and ductile detachment faults representing a NW-SE extensional event.To the east of Guanglu and Guapi islands, two samples from a granite pluton that only record D3 fabrics yield SIMS zircon ages of 165.4 ± 1.6 Ma and 164.7 ± 1.5 Ma, which indicate that D1 and D2 occurred earlier than 165 Ma, while D3 is younger than this age. Similar geometric and kinematic features with those of the South Liaodong Peninsula suggest that D1 and D2 occurred in the Middle-Late Triassic as the structural response of overriding plate to the continental subduction of the South China Block (SCB) beneath the North China Craton (NCC), while the D3 occurred in the Early Cretaceous during the development of the Liaonan Metamorphic Core Complex (MCC). The ductile shear zone and brittle normal faults of the Changshan islands represent the early extensional structures of the Liaonan MCC. They may develop further southeastward and controlled the early development of the North Yellow Sea Basin.

Structural and Geomorphic Evidence for Rolling‐Hinge Style Deformation of an Active Continental Low‐Angle Normal Fault, SE Papua New Guinea

AbstractTo what degree low‐angle normal faults (LANFs) deform by a “rolling‐hinge” mechanism is still debated for continental metamorphic core complexes (MCCs). The Mai'iu fault in SE Papua New Guinea is one of the best preserved and fastest slipping active continental LANFs on Earth, providing an ideal setting in which to evaluate footwall deformation and doming in MCCs. We analyzed structural field data from the exhumed slip surface and subjacent footwall of the Mai'iu fault, together with geomorphic data interpreted from aerial photographs and GeoSAR‐derived digital terrain models. The exhumed part of the Mai'iu fault forms a smooth, continuous surface, traced at least 28 km in the slip direction. The fault emerges from the ground near sea level with a northward dip of ≤22°N and flattens southward over the crest of the Suckling‐Dayman Dome. Its most southern mapped portion dips ~12°S. Geomorphic and structural evidence indicates updip tectonic transport of the footwall and progressive back‐tilting of the exposed part of the fault and the underlying foliation through &gt;26°. We infer that antithetic (northside‐up) dip slip on an array of steep‐dipping faults striking parallel to the Mai'iu fault accommodated some of the exhumation‐related inelastic bending of the footwall. The exhuming footwall was subject to late‐stage slip‐parallel contractional strain as recorded by a postmetamorphic crenulation foliation that strikes parallel to the curved Mai'iu fault trace, by folds of bedding in a large rider block that is stranded on the current footwall and by strike‐parallel warps in the exhumed fault surface. Geodynamic modeling predicts the observed footwall strain.

Extensional Structures in the Ophiolites from Cuba and China: New Constraints to Slowly‐Spreading Ancient Oceanic Lithosphere

AbstractA group of low‐angle normal faults developed in banded gabbro of Moa Ophiolite, Cuba. The dark gabbro was cut into puddings by several normal faults, while light gabbro was just swelling in layer thickness. In Hongliuhe ophiolite at eastern segment of South Tien Shan Suture Zone in China, the extensional deformation concentrates on fine cumulus gabbro which is typically mylonitized. Abundant structural features were discovered in HLH ophiolite such as S‐C foliation, C’ foliation, extensional crenulation cleavage, small toughness normal fault, low‐angle normal faults and high‐angle normal faults. According to the above tectonic phenomenon from the ophiolite belts in Cuba and China, we will get the conclusion: the maximum principal compressive stress (b1) is vertical to cumulus bedding, and the maximum tensile stress (b3) is paralleling to cumulus bedding. Considering of the above evidence, the extensional tectonic event should developed at mid‐ocean ridge. Due to seafloor spreading, the maximum tensile stress is paralleling to cumulus layer, and extensional tectonic is kept in cumulus gabbro. In this way, normal faults developed in dark gabbro, while brittle‐ductile extensional developed in light gabbro. A large number of domes, folds paralleling to ocean ridge and detachment faults represented by low angle normal fault were discovered near ocean ridge in Indian Ocean and Atlantic Ocean. In this way, materials from deep oceanic lithosphere (e.g. gabbro, mantle peridotite) outcrop at the crust surface of ocean basin. The above evidences from China and Cuba are consistent with extensional tectonic and metamorphic core complex from slowly and super‐slowly spreading Indian Ocean and Atlantic Oceanic lithosphere based on ODP. Therefore, extensional deformation in the ophiolite belt is of significant meaning for clarifying the formation process and mechanism of ancient oceanic basin.

Tectonic Inheritance Following Failed Continental Subduction: A Model for Core Complex Formation in Cold, Strong Lithosphere

AbstractInherited structural, compositional, thermal, and mechanical properties from previous tectonic phases can affect the deformation style of lithosphere entering a new stage of the Wilson cycle. When continental crust jams a subduction zone, the transition from subduction to extension can occur rapidly, as is the case following slab breakoff of the leading subducted oceanic slab. This study explores the extent to which geometric and physical properties of the subduction phase affect the subsequent deformation style and surface morphology of post subduction extensional systems. We focus on regions that transition rapidly from subduction to extension, retaining lithospheric heterogeneities and cold thermal structure inherited from subduction. We present numerical models suggesting that following failed subduction of continental crust (with or without slab breakoff), the extensional deformation style depends on the strength and dip of the preexisting subduction thrust. Our models predict three distinct extensional modes based on these inherited properties: (1) reactivation of the subduction thrust and development of a rolling‐hinge detachment that exhumes deep crustal material in a domal structure prior to onset of an asymmetric rift; (2) partial reactivation of a low‐angle subduction thrust, which is eventually abandoned as high‐angle, “domino”‐style normal faults cut and extend the crust above the inherited thrust; and (3) no reactivation of the subduction fault but instead localized rifting above the previous subduction margin as new rift‐bounding, high‐angle normal faults form. We propose that the first mode is well exemplified by the young, rapidly exhumed Dayman‐Suckling metamorphic core complex that is exhuming today in Papua New Guinea.

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.

Generation of high-Mg diorites and associated iron mineralization within an intracontinental setting: Insights from ore-barren and ore-bearing intrusions in the eastern North China Craton

High-Mg andesitic rocks (HMAs) are commonly generated through slab-mantle interactions in arc settings. In this study, however, we report the high-Mg diorites in the Zibo and Laiwu areas of the eastern North China Craton (NCC), which were particularly produced during lithospheric thinning within an intracontinental setting. Some of the rocks generated large-scale Fe skarn deposits (e.g., the Zibo quartz monzonite and the Laiwu Kuangshan monzonite), whereas the others are ore-barren (e.g., the Zibo gabbroic diorite and the Laiwu Jinniushan and Jiaoyu monzodiorites), providing an excellent case to reveal how the non-arc HMAs were formed and the crucial factors controlling the Fe mineralization. All the rocks are characterized by high Mg# values (51–84) at andesitic compositions (52.5–65.0 wt%) and show arc-like geochemistry (e.g., enriched in LREEs and LILEs and depleted in HREEs and HFSEs), of which the strongly mineralized intrusions are more felsic with higher Mg# values (69–84) and stronger fractionation between LREEs and HREEs. Zircon U-Pb dating indicates that these rocks were emplaced at the early Cretaceous (130–132 Ma), coeval with the peak thinning of the lithosphere in the eastern NCC. All the rocks have transitional Sr-Nd-Pb isotopic compositions (0.704726 < (87Sr/86Sr)i < 0.707507, −16.0 < εNd(t) < −5.4, 16.334 < (206Pb/204Pb)i < 17.629, 15.205 < (207Pb/204Pb)i < 15.447, 36.306 < (208Pb/204Pb)i < 37.754) between the local lithospheric mantle and the ancient crust. Combined with their high Mg# and arc-like geochemistry, they were most likely derived from an anciently metasomatized and recently hydrated subcontinental lithospheric mantle (SCLM) with various crustal contamination. Magma mixing modelling suggests that in the Zibo area little crustal materials were involved into the gabbroic diorite, but ~20% middle-upper crustal materials were assimilated into the monzodiorite and quartz monzonite during magma ascent. In the Laiwu area, lower crustal materials (at least ~30%) were significantly involved into the Jinniushan, Jiaoyu and Kuangshan intrusions through magma mixing at the base of the lower crust. Sedimentary rocks (mainly carbonates interbedded with coal-bearing shales, sandstones and evaporitic rocks) were particularly assimilated into the strongly mineralized intrusions in both the Zibo and Laiwu areas. Such assimilation probably contributed to promote fluid exsolution and metal extraction from the melts, since the strongly mineralized intrusions have the lowest H2O contents (2.4–3.2 wt% H2O), the highest Mg# values and the lowest oxygen fugacities (2.5 < ∆FMQ < 3.1). It is thus considered that the assimilation of sedimentary rocks is crucial for generating ore-forming fluids. P-T calculations indicate that all the high-Mg diorites were emplaced shallowly (mainly T < 760 °C and P < 140 MPa). In combination with the co-development of bimodal volcanics, metamorphic core complexes and rift basins in the eastern NCC during the early Cretaceous, extreme lithospheric extension should have occurred in the study region. Such intracontinental extension was likely caused by the asthenospheric upwelling induced by the stagnant Paleo-Pacific Plate in the transition zone beneath the eastern Asian continent. The geodynamic regime is different from those of the typical arc settings.

Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings

AbstractMetamorphic core complexes (MCCs) are interpreted as domal structures exposing ductile deformed high-grade metamorphic rocks in the core underlying a ductile-to-brittle high-strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension. Extension is supposedly the driving force that has governed exhumation. However, numerous core complexes, notably Himalayan, Karakoram and Pamir domes, occur in wholly compressional environments and are not related to lithospheric extension. We suggest that many MCCs previously thought to form during extension are instead related to compressional tectonics. Pressures of kyanite-and sillimanite-grade rocks in the cores of many of these domes are c. 10–14 kbar, approximating to exhumation from depths of c. 35–45 km, too great to be accounted for solely by isostatic uplift. The evolution of high-grade metamorphic rocks is driven by crustal thickening, shortening, regional Barrovian metamorphism, isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension. Extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel (channel flow) following peak metamorphism beneath a passive roof stretching fault. In Naxos, low-angle normal faults associated with regional Aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening. The fact that low-angle normal faults exist in both extensional and compressional tectonic settings, and can actively slip at low angles (&lt; 30°), suggests that a re-evaluation of the Andersonian mechanical theory that requires normal faults to form and slip only at high angles (c. 60°) is needed.

The Gediz Supradetachment System (SW Turkey): Magmatism, Tectonics, and Sedimentation During Crustal Extension

AbstractUnraveling the evolution of supradetachment basins developed in the hanging wall of low‐angle detachment faults may be an invaluable tool in reconstructing the tectonic evolution of highly extended terrains. These basins may record major regional tectonic events related to the exhumation of metamorphic core complexes, and the reconstruction of their evolution helps to quantify the amount of extension accommodated by such processes. Here we present stratigraphic and structural field evidence and micropaleontological constraints to the Neogene‐to‐Quaternary evolution of the supradetachment Gediz Graben that developed on top of the exhuming Central Menderes Massif (SW Turkey). This basin displays three different structural styles during its evolution: (i) it initiated as a ramp basin following the activation of the Gediz Detachment in the Middle Miocene, (ii) evolved as a half graben during the late Miocene following the activation of high‐angle brittle faults at its southern margin, and (iii) reached its final symmetric graben configuration in Late Pliocene (?)‐Quaternary times following the activation of its northern margin. New micropaleontological data document a short‐lived upper Tortonian marine episode in the basin, and major along‐strike variations in exhumation are documented on its southern margin. Our reconstruction shows how sedimentary basins originally formed in the hanging wall of detachment faults may eventually end up in tectonic contact with the mylonitic footwall. Finally, we highlight the importance of magmatism in localizing the deformation in highly extended terrains and in controlling the evolution of supradetachment systems.

Structural characteristics and geochronology of thrust faulting in the Jiulianshan anticlinorium, the Lower Yangtze region, East China

The Jiulianshan anticlinorium is located at the southern part of the Mount Maoshan–Jiulianshan in the Lower Yangtze region, where a number of new‐found thrust faults were identified. Movement of these thrust faults had resulted in Palaeozoic strata lying unconformably above Cretaceous strata. However, the Lower Yangtze region was mostly under an extensional tectonic regime during the Late Mesozoic, which is inconsistent with existence of the nappe in the Mount Maoshan–Jiulianshan. Therefore, what the structural characteristics are and when these thrust faults developed are important for understanding the evolution of the South China Block. Based on detailed field observations, these new‐found thrust faults outcropped in the Liqiao, Yangxian, and Miaopu areas. These faults, SE‐dipping deeply, developed a meters‐wide fault‐and‐fracture zone of width in meters. The footwall was composed of the Early Cretaceous pyroclastic rock or tuff, which indicates that these faults developed since the Early Cretaceous. Two undeformed dykes formed in the fault‐and‐fracture zone of Miaopu area and a footless intrusion in the hanging wall, which give emplacement ages of ca. 132 Ma by using zircon U–Pb LA‐ICP‐MS dating and thus implies that thrust fault motion occurred prior to or around this time. The volcanic tuff and pyroclastic rocks, located in the footwall of the Yangxian and Miaopu thrust fault, are dated at ca. 137 and 134 Ma, respectively, indicating that fault motion must have occurred after this time. The Qiaotouyang pluton emplaced in the core of one of the folds in the fold‐and‐thrust belt at ca. 141 Ma, which indicates that the fold‐and‐thrust belt of Tongling Uplift formed prior to 141 Ma. Integrating other evidences, such as magmatic quiescence, large‐scale and short‐duration volcanic eruptions, and beginning time of metamorphic core complex and rift basin, it is inferred that Early Yanshanian thrust faults were reactivated during the Late Mesozoic, resulting in thrusting within the Jiulianshan anticlinorium.

Evolution of a rapidly slipping, active low-angle normal fault, Suckling-Dayman metamorphic core complex, SE Papua New Guinea

Evolution of a rapidly slipping, active low-angle normal fault, Suckling-Dayman metamorphic core complex, SE Papua New Guinea

Comprehensive Study of Metamorphic Core Complex in Southern Liaodong Peninsula, NE China

Through this work and comprehensive research on a large amount of previous data, four metamorphic core complexes were identified in the south of Liaodong peninsula, which were respectively Jinzhou metamorphic core complex, Wanfu metamorphic core complex, Xinfang metamorphic core complex and Lizifang metamorphic core complex. Xinfang metamorphic core complex and Lizifang metamorphic core complex were discovered and proposed for the first time. They have kinematic unity, geometric asymmetry, tectonic magma joint activity and the difference in formation time.

Tectonic Mode Switches Recorded at the Northern Edge of the Australian Plate During the Pliocene and Pleistocene

AbstractWe report new data from medium‐high grade metamorphic rocks found at the northern margin of the Lengguru Fold Belt in West Papua. The study involved a systematic analysis of cross‐cutting structures to establish the relative timing of deformation, together with isotopic dating to define when these tectono‐thermal events occurred. These data show that the region underwent multiple episodes of deformation within the last six million years. Metamorphic mineral growth was associated with the development of ductile shear zones. This episode occurred during a phase of crustal stretching associated with the formation of a metamorphic core complex. Metamorphic zircon growth at 4.9 to 5.3 Ma was documented in two of the dated samples. These data are interpreted to postdate the peak pressure and temperature conditions of the phase of regional crustal stretching. The shear fabrics associated with the metamorphic core complex were later overprinted by at least two generations of folds. The change in mode from crustal extension to shortening reflects a tectonic mode switch. A subsequent mode switch is documented by numerous brittle extensional faults that cross‐cut the earlier formed ductile fabrics. We interpret ca. 0.75–1.51 Ma (U–Th)/He age data to reflect cooling associated with the later stages of crustal shortening (marked by folds) or the later extensional unroofing of the peninsula. This work demonstrates that an orogen can record multiple tectonic mode switches within several million years. These outcomes should be considered in studies of ancient orogens where analytical uncertainties associated with isotopic dating may mask short‐lived mode switches.

Geodynamic Complexes and Structures of Transbaikalia: Record in Gravity Data

Abstract —The Transbaikalian region comprises several known geologic structures: the Mesozoic Mongolia-Okhotsk orogen, the Cenozoic Baikal rift system, and the world largest Angara–Vitim granitic batholith. They all formed upon heterogeneous Neoproterozoic– Early Paleozoic continental-margin complexes of the Siberian craton. The region is subject to the influence of mantle plumes, which induced Mesozoic–Cenozoic volcanism and controlled structural and lithological changes in the crust in the early history. Transbaikalia, which has been a scene of multiple tectonic events, is a model area for geophysical (in particular, gravity) surveys for various geological and geodynamic applications. As a novel approach, we interpret geological and geodynamic data from the region with reference to the pattern of free-air and Bouguer gravity anomalies revealed by satellite altimetry. Bouguer anomalies highlight large structures in the lithospheric mantle which were produced in the Cenozoic mainly by the activity of mantle plumes. Basaltic lava fields were confirmed to be almost coeval with mantle anomalies and to record the presence of the plume head at the crustal base. However, the origin of the Late Paleozoic Angara–Vitim granitic batholith was only tentatively attributed to the plume activity, based on gravity data. Mesozoic metamorphic core complexes (MCC) and basins that formed during the evolution of the Mongolia–Okhotsk orogen show up clearly on the map of free-air anomalies. Most of the MCC revealed in Transbaikalia coincide with oval gravity highs and border negative elongate features corresponding to Mesozoic basins. The zone of Cenozoic tectonism stands out in the pattern of free-air anomalies as maximum gravity contrasts, with the values changing from –110 to –120 mGal in basins to + 90 or +100 mGal in ranges. This zone encompasses rift basins filled with Cenozoic or, locally, Mesozoic sediments, which jointly form a domino-like system of rhomb-shaped structures typical of the Baikal rift system and, in general, of Cenozoic Central Asia resulted from the far-field effect of the India–Eurasia collision.