Exhumation Of Metamorphic Core Complexes Research Articles

Deformation conditions and kinematic vorticity within the footwall shear zone of the Wildhorse detachment system, Pioneer metamorphic core complex, Idaho

Detailed analysis of numerous sequential fabrics and microstructures preserved in the footwall shear zones of detachment fault systems can be used to clarify the spatial relationships and timing of deformation related to emplacement and exhumation of metamorphic core complexes. To quantify the kinematics during deformation, we conducted microstructural analyses along two ∼300 m transects across mylonitic sections of the footwall of the Eocene Wildhorse Detachment system within the Pioneer metamorphic core complex. These results enable comparison of deformation at different structural levels during exhumation and between monomineralic and polymineralic lithologies.In both transects, mylonitic rocks preserve normal-sense asymmetric structures such as recrystallized quartz oblique foliation, S/C–C′ structures, and asymmetric feldspar porphyroclasts. Quartz microstructures, CPO patterns, and kinematic vorticity analyses consistently indicate simple shear-dominated deformation, which is predicted for late stage, shallow-level deformation within a rolling hinge detachment system exhuming a metamorphic core complex. Variations in c-axis patterns and active slip systems are associated with temperature, influenced by structural level, and grain boundary processes related to lithology. To determine the degree of non-coaxiality and minimize limitations and uncertainties with kinematic vorticity methods, we used two methods to evaluate flow vorticity. Kinematic vorticity ranges from 0.55 to 0.95 for the oblique quartz foliation (δ/β) and C′-type shear bands (C′-c) methods. Quartz recrystallized grain size ranges from 10 to 37 μm, indicating maximum differential stresses of 80–110 MPa, which is consistent with the footwall shear zones of numerous metamorphic core complex detachment systems.

Crustal flow driving twin domes exhumation and low-angle normal faulting in the Menderes Massif of western Anatolia

Lower crustal flow in regions of post-orogenic extension has been inferred to explain the exhumation of metamorphic core complexes and associated low-angle normal (detachment) fault systems. However, the origin of detachment faults, whether initially formed as high-angle or low-angle shear zones, and the extension is symmetric or asymmetric remains enigmatic. Here, we use numerical modeling constrained by geophysical and geological data to show that symmetric extension in the central Menderes Massif of western Anatolia is accommodated by the crustal flow. Our geodynamic model explains how opposite dipping Gediz and Büyük Menderes detachment faults are formed by ∼40° footwall rotation. Model predictions agree with seismic tomography data that suggests updoming of lower crust beneath the exhumed massifs, represented as “twin domes” and a flat Moho. Our work helps to account for the genetic relation between the exhumation of metamorphic core complexes and low-angle normal faulting in both Cordillera and Aegean orogenic regions and has important implications on crustal dynamics in extensional provinces.

Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse

The (ultra)high-pressure Western Gneiss Region of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continent–continent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite U–Pb zircon samples date melt crystallization at c. 405 Ma in the Øygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Øygarden and Gulen domes yield 40Ar/39Ar ages mostly between 405 and 398 Ma, recording the exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the Western Gneiss Region show abrupt breaks (10–30 myr) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain the segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge v. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments.Supplementary material: Complete analytical results are available at https://doi.org/10.6084/m9.figshare.c.5241710

Exhumation of Metamorphic Core Complexes through Progressive Subhorizontal Shearing, Doming, and Detachment Faulting: Insights from the Cretaceous Liaonan Metamorphic Core Complex, Eastern North China Craton

Abstract The Early Cretaceous Liaonan metamorphic core complex (MCC), eastern North China craton, provides a field setting to evaluate progressive middle-upper crustal subhorizontal shearing, doming, and detachment faulting. The MCC is bounded by a western Jinzhou detachment fault zone (JDFZ) and a southern Dongjiagou shear zone (DSZ) that were primarily suggested to be two segments of the master detachment fault zone. Integrated structural, microstructural, quartz c-axis fabrics, and fluid inclusion analysis and zircon U-Pb dating on mylonites and syn-kinematic granites along the DSZ and JDFZ reveal that the DSZ possesses deformation characteristics that are obviously different from those along the JDFZ. The DSZ is composed of a Lower Unit of sheared Archean gneisses and an Upper Unit of sheared Neoproterozoic metasedimentary rocks, between which there is an obvious tectonic discontinuity contact (TDC). Rocks from below and above the TDC possess structures and fabrics with consistent geometries and kinematics with those along the JDFZ. A metamorphic break exists between the two units that were sheared at contrasting deformation conditions. Dating of zircons from syn-kinematic granitic dikes from DSZ yields an age of ca. 134 Ma, which is similar to the ages of early shearing along the JDFZ. It is concluded that the Jinzhou and Dongjiagou faults formed parts of a detachment faulting with top-to-the WNW kinematics. Exhumation of the Liaonan MCC shearing initiation along both the JDFZ and DSZ at an early stage (ca. 133~134 Ma), subsequent progressive shearing, and doming during slow cooling and exhumation before ca. 120 Ma, followed by fast cooling and rapid exhumation of the MCC by detachment faulting along the JDFZ until ca. 107 Ma.

Extent and Cessation of the Mid‐Cretaceous Hikurangi Plateau Underthrusting: Impact on Global Plate Tectonics and the Submarine Chatham Rise

AbstractSubduction of oceanic plateaux are events that have occurred infrequently in Earth's history. Although rare, these events are thought to considerably influence regional tectonics and global plate motions. In the mid‐Cretaceous the oceanic Hikurangi Plateau collided with the formerly active East Gondwana margin: An event which falls into the same of a sudden change from subduction to extensional processes, including graben formation, development rift basin development, and exhumation of metamorphic core complexes in the Zealandia continent as well as a global‐scale plate reorganization event. In this study, we use recently acquired seismic refraction and gravity data along one profile across the submarine Chatham Rise, which represent a former accretionary wedge of the East Gondwana subduction zone. We demonstrate that the southward extent of the subducted Hikurangi Plateau in the lower crust beneath the submarine Chatham Rise along our profile is only ~150. This is ~150 km less than the previously suggested extent. Furthermore, we interpret that a slice of the subducted Phoenix Plate remains attached to the southern edge of the Hikurangi Plateau. We suggest that cessation of subduction in response to the Hikurangi Plateau jamming as well as the rollback and detachment of the Phoenix Plate slab played an important role in the changing tectonic forces across Zealandia in mid‐Cretaceous. Moreover, we suggest that the subduction cessation along the Hikurangi Plateau segment led to the fragmentation of the Gondwana subduction zone and Phoenix Plate, which significantly influenced and potentially prolonged the global mid‐Cretaceous plate reorganization event.

Miocene geochronology and stratigraphy of western Anatolia: Insights from new Ar/Ar dataset

Understanding the dynamic evolution of orogenic belts and intra-continental basins depend on field-based (tectono-) stratigraphic observations paired with geochronologic data such as 40Ar/39Ar analyses. Independent dating of tectono-stratigraphic units is a crucial tool to place them in a broader framework. In this study, we focus on the geodynamic development of western Anatolia, with an emphasis on the timing and progression of volcanism along the İzmir-Balıkesir Transfer Zone (İBTZ). We present 36 new 40Ar/39Ar ages of both volcanic complexes/domes and volcanic rocks coevally emplaced within Miocene sediments from western Anatolian extensional basins. In combination with existing ages from the literature and paleontological records from Miocene basin in-fills, we build an improved and integrated stratigraphic framework for the region. Our results show a remarkable break in volcanism from this area during the Langhian (15.97–13.82 Ma), encompassing a major unconformity in the İBTZ, and a pulse in the exhumation of metamorphic core complexes in western Anatolia. Hence we attribute this magmatic pause to the tectonic reorganization and change in the partitioning of extensional deformation between Cycladic and Menderes core complexes, facilitated by the acceleration of roll-back of African oceanic lithosphere below western Anatolia.

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.

Synconvergent exhumation of metamorphic core complexes in the northern North American Cordillera: COMMENT

Synconvergent exhumation of metamorphic core complexes in the northern North American Cordillera: COMMENT

Synconvergent exhumation of metamorphic core complexes in the northern North American Cordillera

Continental metamorphic core complexes of the northern North American Cordillera were exhumed during the early Paleogene while the Farallon–North American plate convergence rate remained high. Such convergent boundary conditions can excite localized mid-crustal exhumation in numerical simulations of collisions of softer accreted terranes with a rigid, irregular craton margin. Resulting simulated temperature-time ( T - t ) paths match T - t paths interpreted from observed footwall exposures in several core complexes of the northern North American Cordillera, including variability of maximum temperatures, duration of exhumation, and exhumation velocities among individual complexes.

Rheology of the middle crust under tectonic extension: Insights from the Jinzhou detachment fault zone of the Liaonan metamorphic core complex, eastern North China Craton

Although exhumation of metamorphic core complexes (MCCs) have been traditionally attributed to collapse of orogenically overthickened crust, recent studies reveal that they can also result from extension of lithosphere with normal crustal thickness. For example, there is an ongoing debate about the mechanisms responsible for exhumation of the Liaonan MCC, which occurred during late Mesozoic lithospheric thinning of the North China Craton. This paper attempts to present rheological constraints on middle to upper crustal detachment faulting during the exhumation of the Liaonan MCC and, therefore, on the genesis of the MCC based on a comprehensive study of the microstructural and fabric characteristics of tectonites from the Jinzhou master detachment fault of the Liaonan MCC.The Jinzhou detachment fault zone comprises a thick sequence of fault rocks of middle to shallow crustal depths. Three types of mylonitic rocks characterize middle crustal deformation along the detachment fault during early Cretaceous lithospheric extension. Microstructural and fabric studies reveal that these fault rocks were formed via different mechanisms of crystal plastic deformation and dynamic recrystallization at temperatures from ca. 300 to 650°C. The flow stresses at different crustal depths were calculated using classical paleopiezometers. This study suggests that a pre-heated crust was responsible for the low flow stresses during the Jinzhou detachment faulting, which is somehow different from, e.g., the Whipple Mountain detachment faulting in the North American Cordillera. Based on inferences about the properties of middle to upper crustal flow associated with the Jinzhou detachment faulting, it is suggested that the Liaonan metamorphic core complex was formed by tectonic extension of cratonic lithosphere with a normal crustal thickness, instead of being a typical Cordilleran-type core complex that occurred in a setting with overthickened crust.

The relationship between gold mineralization, exhumation of metamorphic core complex and magma cooling: Formation of the Anjiayingzi Au deposit, northern North China Craton

The Anjiayingzi gold deposit is hosted by the Jiguanzi Quartz Monzonite in the footwall of the Kalaqin Metamorphic Core Complex (MCC) at Chifeng in the northern part of the North China Craton (NCC). An earlier (ca. 135–125Ma) top-to-the-NE ductile–brittle shear and a later (ca. 121Ma) top-to-the-NW and SE brittle detachment normal faulting are recognized around the Kalaqin MCC. The Jiguanzi Quartz Monzonite intruded the footwall of the Kalaqin MCC at ca. 133±1Ma with crystallization temperatures of 640–691°C, pressures of 167–271MPa and an average depth of 8.6km. Gold veins in the Anjiayingzi gold deposit are hosted by the monzonite and controlled by the NE-trending brittle–ductile and brittle fault zones. These ore-bearing faults are products of a regional sinistral shearing event coinciding with the earlier top-to-the-NE ductile–brittle shear at the Kalaqin MCC. The ore of the Anjiayingzi deposit is characterised by its low Au/Ag ratio (<1) and high sulphide content (4–30vol.%). The assemblage associated with gold mineralization consists of: (1) quartz–coarse-grained pyrite; (2) quartz–fine-grained pyrite; (3) quartz–polymetallic sulphide; and (4) quartz–carbonate. The ore formed at 245–358°C from a H2O–CO2–NaCl fluid system with salinities of 1.3–15.6wt.% NaCl equiv. Trapping pressures of ore fluids were 50–110MPa, corresponding to an average depth of ~2.5km under a lithostatic load and ~7.5km under a hydrostatic load. Fluid mixing and immiscibility are potential causes for the rapid precipitation of sulphides carrying Au. The stable isotope δD (−80.3 to −96.5‰) and δ18OH2O (3.7 to 5.5‰) of the ore fluids, the consistent Pb isotope compositions between the ore-bearing sulphides and K-feldspar from the Jiguanzi Quartz Monzonite, and a >2billionyear time gap between the gold mineralization and metamorphism of the basement all indicate that the mineralizing fluids and metals are derived from a concealed intrusion near the major eastern Louzidian Fault in the Kalaqin MCC. This proposal is supported by the presence of mafic magmatic enclaves and rhyolite dykes in the Jiguanzi Quartz Monzonite, and higher average Thtotal (322°C) and δ34S (2.4‰) of Eastern veins than those of Western veins (with a Thtotal of 287°C and δ34S value of 0.3‰). The ca. 133–126Ma gold mineralization at Anjiayingzi was formed during rapid cooling of ca. 133–126Ma Jiguanzi Quartz Monzonite at a cooling rate of 42–71°C/Ma, and fast exhumation and cooling of the Kalaqin MCC between ca. 135 and 121Ma at a cooling rate of 30°C/Ma. This process was accompanied by fluid immiscibility and mixing within the structural transition from ductile–brittle to brittle deformation with resultant fluid-pressure fluctuations between the supra-hydrostatic and hydrostatic regimes.

Tectonic inversion of an asymmetric graben: Insights from a combined field and gravity survey in the Sorbas basin

The formation of sedimentary basins in the Alboran domain is associated with the exhumation of metamorphic core complexes over a ca. 15 Ma period through a transition from regional late-orogenic extension to compression. An integrated study coupling field analysis and gravity data acquisition was performed in the Sorbas basin in the southeastern Betic Cordillera. Detailed field observations revealed for the first time that extensional tectonics occurred before shortening in this basin. Two extensional events were recorded with NW-SE to N-S and NE-SW kinematics respectively; the first of which being likely responsible for the basin initiation. Tectonic inversion of the basin then occurred around 8 Ma in an overall ca. N-S shortening context. 2D-gravity sections reveal that the basin acted as an active depocenter as the basin floor locally exceeding 2 km-depth is characterized by a marked asymmetric architecture. Based on this integrated study, we explore a new evolutionary scenario which can be used as a basis for interpretations of the Neogene tectonic history of the southeastern Betics.

Thermomechanics of an extensional shear zone, Raft River metamorphic core complex, NW Utah

A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10−12 and 10−14 s−1. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone.

Giant Mesozoic gold provinces related to the destruction of the North China craton

Lode gold deposits in Precambrian cratons represent the world's major gold source and were mostly generated during formation and stabilization of the cratons. However, there is an extraordinary exception in the North China craton (NCC), where lode gold deposits formed after prolonged stabilization of the craton. Molybdenite Re–Os and hydrothermal sericite and biotite 40Ar/39Ar dating of major gold deposits from the Xiaoqinling district, southern NCC, bracket their emplacement in the range of 154.1±1.1 to 118.9±1.2Ma (n=23), postdating formation of the craton by more than 1.7 billion years. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (1.52–0.22Ra) and mantle-like Ne isotopes (20Ne/22Ne=10.02−9.22 and 21Ne/22Ne=0.033−0.027), indicating presence of mantle-derived fluids in the ore system. Measured δ34S of pyrite and δD and δ18O of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic/mantle source for sulfur and ore fluids. Gold deposits of similar ages also widely occur in the eastern and northern margins of the NCC, which, together with those in the Xiaoqinling district, have a total reserve of ∼2500t gold, forming the only known giant late Mesozoic gold province in the world's Precambrian cratons. These deposits formed coevally with extensive felsic to mafic magmatism, development of intracontinental rift basins, and exhumation of metamorphic core complexes across the eastern NCC, events interpreted as indicating thinning and destruction of the lithosphere beneath the craton. Rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids, sulfur and, by inference, other ore components to form the giant gold provinces.

Metamorphic Core Complexes vs. synkinematic plutons in continental extension setting: Insights from key structures (Shandong Province, eastern China)

Continental extension is an important geodynamical process mostly diagnostic of a peculiar behaviour of the crust accommodated by geological structures that highlight these specific conditions in the crust. Though a Metamorphic Core Complex (MCC) reveals a much stronger crustal extension than a synkinematic pluton, the nature of those two structures implies different implications in terms of crustal extension mechanisms and geodynamic significations. In eastern Asia, a major continental extensional event occurred during Mesozoic and early Cenozoic times. The resulting various extensional events described in previous studies consist in large intracontinental basins, important volcanism, emplacement of plutons in the upper crust and exhumation of MCCs. An efficient description of MCCs and plutons in eastern Asia is essential to discriminate important differences of those two structures in terms of strain amount undergone by continental crust and geodynamic significations. In that way, an integrated structural and geophysical study has been realized to precisely discriminate the structure in the Jiaodong Peninsula (Shandong Province, eastern China), previously regarded as simple sheared plutons. A succession of three main stages have been identified all pertaining to a NW–SE extensional setting: (1) the exhumation of the Late Jurassic–Early Cretaceous Linglong MCC below the SE-dipping Linglong detachment fault, (2) the emplacement of the Guojialing syntectonic pluton below the N-dipping extensional Guojialing intracrustal shear zone (130–124 Ma) and (3) a penetrative brittle normal faulting associated with gold mineralizations (∼120 Ma). As a result, the maximum amount of extension in Jiaodong Peninsula, characterized by partially-melted lower to middle crust upward into the Linglong MCC should be revised to Late Jurassic–Early Cretaceous period.

Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula, eastern North China craton, and consists of the Jinzhou detachment fault, Proterozoic–Paleozoic sedimentary rocks in its upper plate, and exhumed high-grade Archean metamorphic rocks and Early Cretaceous granitic plutons in the lower plate. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. The footwall mylonite zone is 2.5–3.5 km thick and includes Early Cretaceous (128–118 Ma) granitic rocks, together with older metamorphic rocks. The 40Ar/39Ar ages of muscovite, hornblende, biotite, and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120 and 107 Ma, from the time of initial crystallization of zircons (122–118 Ma) at 700–800 °C in syntectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at ∼500 to ∼200 °C. Throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity, reflects regional extensional tectonics in the Early Cretaceous. This accompanied lithospheric thinning, possibly resulting from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous.

Lithospheric structure of the Aegean obtained from P and S receiver functions

Combined P and S receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the Aegean region are used to map the geometry of the subducted African and the overriding Aegean plates. We image the Moho of the subducting African plate at depths ranging from 40 km beneath southern Crete and the western Peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern Greece. However, the dip of the Moho of the subducting African plate is shallower beneath the Peloponnesus than beneath Crete and Rhodes and flattens out beneath the northern Aegean. Observed P‐to‐S conversions at stations located in the forearc indicate a reversed velocity contrast at the Moho boundary of the Aegean plate, whereas this boundary is observed as a normal velocity contrast by the S‐to‐P conversions. Our modeling suggests that the presence of a large amount of serpentinite (more than 30%) in the forearc mantle wedge, which generally occurs in the subduction zones, may be the reason for the reverse sign of the P‐to‐S conversion coefficient. Moho depths for the Aegean plate show that the southern part of the Aegean (crustal thickness of 20–22 km) has been strongly influenced by extension, while the northern Aegean Sea, which at present undergoes the highest crustal deformation, shows a relatively thicker crust (25–28 km). This may imply a recent initiation of the present kinematics in the Aegean. Western Greece (crustal thickness of 32–40 km) is unaffected by the recent extension but underwent crustal thickening during the Hellenides Mountains building event. The depths of the Aegean Moho beneath the margin of the Peloponnesus and Crete (25–28 and 25–33 km, respectively) show that these areas are also likely to be affected by the Aegean extension, even though the Cyclades (crustal thickness of 26–30 km) were not significantly involved in this episode. The Aegean lithosphere‐asthenosphere boundary (LAB) mapped with S receiver functions is about 150 km deep beneath mainland Greece, whereas the LAB of the subducted African plate dips from 100 km beneath Crete and the southern Aegean Sea to about 225 km under the volcanic arc. This implies a thickness of 60–65 km for the subducted African lithosphere, suggesting that the Aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the Cyclades.

Crustal flow beneath Iceland

Theoretical and experimental studies indicate that when oceanic crust is hotter than about 800°C, variations in crustal thickness drive lower crustal flow. We investigate the nature of crustal flow beneath Iceland, where zero‐age crust varies in thickness from under 20 km to over 40 km over a distance of 100 km and temperatures exceed 1000°C below depths of a few kilometers. We model the regional characteristics of crustal flow using the two‐dimensional channel flow equation with depth‐dependent viscosity. The model predicts the observed decay in crustal thickness contrasts between zero‐age and off‐axis crust on shore Iceland and development of a sharp Moho step as the plate moves away from the rift axis. These features become locked in place as plate cooling increases the viscosity. Observations are best matched by model predictions when the solidus viscosity is of order 1018 Pa s, in agreement with viscosity estimates from deformation rates associated with deglaciation and plate boundary processes. Crustal flow acts to erase the crustal thickness memory of ridge‐plume interaction, so that maps of Icelandic crustal thickness can be used neither to derive a detailed plume flux history nor to test in detail whether the Iceland Plume center is fixed relative to other hot spots. Crustal flow beneath Iceland is an unusually clear example of the kind of flow postulated to explain continental phenomena including exhumation of metamorphic core complexes and associated development of low‐angle detachments within extensional terrains and sharp‐edged plateau topography in major collisional belts.

Episodic, two-stage Neogene extension and short-term intervening compression in Western Turkey: field evidence from the Kiraz Basin and Bozdağ Horst

Western Anatolia (Turkey) is a region of widespread active N-S continental extension that forms the eastern part of the Aegean extensional province. The extension in the region is expressed by two distinct/different structural styles, separated by a short-term gap: (1) rapid exhumation of metamorphic core complexes along presently low-angle ductile-brittle normal faults commenced by the latest Oligocene-Early Miocene period, and; (2) late stretching of crust and, consequent graben evolution along Plio-Quaternary high-angle normal faults, cross-cutting the pre-existing low-angle normal faults. However, current understanding of the processes (tectonic quiescence vs N-S continental compression) operating during the short-time interval is incomplete. This paper therefore reports the results of recent field mapping and structural analysis from the NE of Küçük Menderes Graben—Kiraz Basin—that shed lights on the processes operating during this short-time interval. The data includes the thrusting of metamorphic rocks of the Menderes Massif over the Mio-Pliocene sediments along WNW-ESE-trending high-angle reverse fault and the development of compressional fabrics in the metamorphic rocks of the Menderes Massif. There, the metamorphic rocks display evidence for four distinct phases of deformation: (1) southfacing top-N ductile fabrics developed at relatively high-grade metamorphic conditions, possibly during the Eocene main Menderes metamorphism (amphibolite facies) associated with top-N thrust tectonics (D1); (2) top-S and top-N ductile gentle-moderatley south-dipping extensional fabrics formed at relatively lower-grade metamorphic (possibly greenschist facies) conditions associated with the exhumation of Menderes Massif along presently low-angle normal fault plane that accompanied the first phase of extension (D2); (3) moderately north-dipping top-S ductile-brittle fabrics, present configuration of which suggest a thrust-related compression (D3); and (4) south-facing approximately E-W-trending brittle high-angle normal faults (D4) that form the youngest structures in the region. It is interpreted that D4 faults are time equivalent of graben-bounding major high-angle normal faults and they correspond to the second phase of extension in western Anatolia. The presence of thrust-related D3 compressional fabrics suggests N-S compression during the time interval between the two phases of extension (D2 and D4). The results of the present study therefore support the episodic, two-stage extension model in western Anatolia and confirm that a short-time, intervening N-S compression separated the two distinct phases.

Contrasting Late Cretaceous with Neogene ore provinces in the Alpine-Balkan-Carpathian-Dinaride collision belt

AbstractInternal sectors of the Alpine-Balkan-Carpathian-Dinaride (ABCD) orogen comprise fundamentally different ore deposits along strike in three temporally and spatially distinct belts. These were formed by several short-lived, late-stage collisional processes (including slab break-off) during the Late Cretaceous and Oligocene to Neogene times. Reconstruction of Late Cretaceous (c. 92–65 Ma) collisional structures, magmatic features and mineralization reveals contrasting variations along strike, including the following.(1a)The Late Cretaceous ‘banatite’ magmatic belt, which extends from the Apuseni mountains to the Balkans, associated mainly with porphyry Cu-Au, massive sulphide and Fe-Cu skarn mineral deposits. In respect to their country rocks and geodynamic setting, the magmatism is interpreted to represent either post-collisional or Andean-type calc-alkaline due to continuous subduction or break-off of the subducted lithosphere.(1b)The Alpine-West Carpathian sector, which is characterized by strong Late Cretaceous metamorphic/deformational overprint, lack of magmatism and both syn- and late-orogenic formation of metasomatic and metamorphogenic talc, magnesite, siderite and vein- and shear zone-type Cu and As-Au due to the exhumation of metamorphic core complexes.(2a)The Oligocene-Miocene Serbomacedonian-Rhodope metallogenic zone extends across several structural units from the Bosnian Dinarides to the Rhodopes and to Thrace. It includes both a belt with volcanic-hosted and vein-type Pb-Zn deposits and a belt of porphyry Cu-Au-Mo and epithermal Au mineralization, which is more common in the south. Both belts appear to relate to microcontinent collision and associated subsequent magmatism, again possibly due to slab break-off.(2b)Different types of mineralization were also formed along the internal Inner Carpathian and Alpine sectors during Late Oligocene to Miocene collision. In the Alps, mineralization formed due to eastward extrusion of fault-bounded blocks into the Carpathian arc. Associated mineral deposits are always related to exhumation of metamorphic core complexes and include: sub-vertical mesothermal Au-quartz veins and replacement As-Ag-Cu ore bodies within the metamorphic core complex, fault-bounded mineralization (Pb-W-Au) along low-angle ductile normal faults along the upper margins of the metamorphic core complex, mineralized (Sb-Au) strike-slip faults and sub-vertical Au-Ag-Sb-bearing tension veins.(2c)In contrast, nearly all Miocene ore deposits within the Carpathians are related to volcanic activity contemporaneous with the invasion of fault-bounded blocks into the Carpathian arc. These have been related to slab break-off and cessation of subduction. Mineral deposits include structurally controlled Au-Sb-Cu-Pb-Zn ore bodies within shallow volcanic edifices, with a preference for steep tension veins parallel to the motion direction of laterally escaping crustal blocks.