Development Of Metamorphic Core Complexes Research Articles

Constraints on the thermal evolution of metamorphic core complexes from the timing of high-pressure metamorphism on Naxos, Greece

Metamorphic core complexes are classically interpreted to have formed during crustal extension, although many also occur in compressional environments. New U−(Th)−Pb allanite and xenotime geochronologic data from the structurally highest Zas Unit (Cycladic Blueschist Unit) of the Naxos metamorphic core complex, Greece, integrated with pressure−temperature−time (P−T−t) histories, are incorporated into a thermal model to test the role of crustal thickening and extension in forming metamorphic core complexes. Metamorphism on Naxos is diachronous, with peak metamorphic conditions propagating down structural section over a ∼30−35 m.y. period, from ca. 50 Ma to 15 Ma. At the highest structural level, the Zas Unit records blueschist-facies metamorphism (∼14.5−19 kbar, 470−570 °C) at ca. 50 Ma, during northeast-directed subduction of the Adriatic continental margin. The Zas Unit was subsequently extruded toward the SW and thrust over more proximal continental margin and basement rocks (Koronos and Core units). This contractional episode resulted in crustal thickening and Barrovian metamorphism from ca. 40 Ma and reached peak kyanite-sillimanite−grade conditions of ∼10−5 kbar and 600−730 °C at 20−15 Ma. Model P−T−t paths, assuming conductive relaxation of isotherms following overthrusting, are consistent with the clockwise P−T−t evolution. In contrast, extension results in exhumation and cooling of the crust, which is inconsistent with key components of the thermal evolution. Barrovian metamorphism on Naxos is therefore interpreted to have resulted from crustal thickening over a ∼30−35 m.y. time period prior to extension, normal faulting, and rapid exhumation after a thermal climax at ca. 15 Ma.

The role of gravitational body forces in the development of metamorphic core complexes

Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands.

Formation and Evolution of Supradetachment Basins During Continental Extension: Insights From the Fuxin Basin in NE China

Supradetachment basins can record the stratigraphy and development of metamorphic core complexes (MCCs). The Fuxin supradetachment basin, which lies immediately to the west of the Yiwulvshan MCC in NE China, provides an excellent opportunity to establish the relationship between supradetachment basins and MCCs. In this study, we conducted field investigations, sedimentary facies analysis, and seismic profile interpretation to decipher the sedimentary processes and structural evolution of the basin and link them to the development of the Yiwulvshan MCC. The Fuxin Basin is filled predominantly by syn-rift volcanic sedimentary and post-rift clastic rocks, which developed in four stages, namely, proto-rift, fault subsidence, transition, and compression. The Fuxin Basin developed simultaneously with the two stages (earlier faulting-dominated and later exhumation stages) of development of the MCC. Erosion of the core of the Yiwulvshan MCC provided clastic material to the Fuxin Basin. Based on multi-stage reconstruction of the formation and evolution of the Fuxin Basin and Yiwulvshan MCC, we propose that rollback of the Paleo-Pacific Plate and retreat of the subduction trench provided the geodynamic setting for the crustal extension that formed the basin and MCC.

Middle-late Miocene normal faulting in the intermontane Tarom basin during the collisional deformation of the Arabia-Eurasia collision zone, NW Iran: A regional process or a local feature?

• Magnetic fabric in the central Tarom Basin records NE-SW layer parallel shortening. • Magnetic fabric in the SW margin of the basin is a primary sedimentary fabric. • NE-SW orogen-perpendicular syndepositional extension occurs in the SW margin. • Normal faults in the SW margin were induced by gravity instabilities. • Middle-late Miocene extension is not regionally pervasive. The upper plate of the Arabia-Eurasia collision zone experienced orogen-perpendicular to orogen-parallel extension from 25–22 to 10–9 Ma. Although such an extension occurred during widespread collisional deformation, it is not clear if it is a local feature or if represents a major phase of upper plate extension. In this study we combine anisotropy of magnetic susceptibility (AMS) with fault kinematic analysis and sedimentologic data from 16.2- to 7.6-My-old deposits of the Upper Red Formation of the intermontane Tarom Basin (NW Iran). These strata present syndepositional, normal faults and offer the possibility to gain new insights into the spatial extent of such a Miocene extension. AMS data from the central and northern sectors of the basin document a tectonic fabric with a magnetic lineation parallel to the strike of the orogen, suggesting a compressional tectonic overprint. Conversely, the southern margin of the basin presents a purely sedimentary magnetic fabric despite a ~NE–SW orogen-perpendicular extension. This suggests that basin formation was not driven by extensional tectonics. Rather, the normal faults are gravity instabilities induced as also documented by coeval landslide deposits. This allows concluding that the orogen-perpendicular extension observed in few sectors of the collision zone is not regionally pervasive and hence it is not controlled by large-scale processes. Combined, our results indicate that if orogen-parallel extension associated with tectonic denudation and metamorphic core complex development occurred in certain sectors of the collision zone (Takab complex), it must have ended before 19–16 Ma, when widespread upper plate contractional deformation started.

Characterizing syn-convergent extension along the Neybaz-Chatak detachment shear zone, Central Iran: Insights from microstructures, quartz petrofabrics and flow vorticity analysis

Integrated microstructural, quartz petrofabric and flow vorticity analyses on mylonitic rocks in the lower plate of the Neybaz Chatak Detachment Shear Zone provide significant information regarding the kinematic characteristics coeval with the Chapedony metamorphic core complex development within the Central East Iranian Microcontinent. Several kinematic shear sense indicators including quartz c-axis fabrics, S–C fabrics, rotated feldspar and mica fish reveal that the detachment shear zone has experienced a consistent top-to-the NE-directed progressive shear deformation. Detailed investigations on recrystallization types of quartz and feldspar in the mylonites demonstrate that the deformation occurred under the amphibolite facies condition corresponding to mid-crustal levels. A combination of investigations on mineral assemblages, microstructures of quartz and feldspar, and quartz lattice preferred orientations (LPOs) show that deformation temperatures ranged from 400 to 645 °C that were varied across a distance of ~21 km in the extension direction (NE-SW). The western and eastern parts of the study area experienced lower temperatures compared to the central parts. The gneissic mylonite record sub simple shear deformation (Wm = 0.56–0.85, 64–36%) associated with an increasing of pure shear component. These data highlight that the metamorphic rocks of the study underwent a non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime responsible for formation of the core complex.

Late Triassic uplift, magmatism and extension of the northern North China block: Mantle signatures in the surface

The Late Triassic saw a period of broad uplift, vigorous magmatism and supracrustal stretching in the northern North China block, but the driving mechanism has been uncertain. Previous investigations have mostly dealt with individual tectonic processes, such as petrogenesis of igneous complexes, extensional deformations, or basin development, and the proposed models were aimed to account for some specific processes. This study makes a holistic treatment of Late Triassic denudation, magmatism, and rift basins in an attempt to unravel the driver for these coeval tectonic processes. Late Triassic uplift is registered by a regional disconformity separating Jurassic from Lower–Middle Triassic or older units, and Upper Triassic oligomict conglomerates are suggestive of early-stage peneplanation. The extensive uplift of the northern North China block was also accompanied by the development of the Great Ordos basin in the southern North China block, which exhibits striking southward variation from alluvial/fluvial through deltaic to lacustrine depositional settings. Vigorous magmatism is typified by mantle-sourced alkaline and mafic complexes distributed in an east–west narrow belt. The development of metamorphic core complex and some isolated rift basins is clearly indicative of crustal extension. Spatial variation from coeval island-arc calc-alkaline to backarc bimodal magmatism in the Amurian superterrane hints at southward subduction of the Mongol-Okhotsk plate. All the geologic records point to the plausibility that these synchronous tectonic processes must have been genetically linked and were likely induced by southward subduction of the Mongol-Okhotsk plate. Here, we argue that the Mongol-Okhotsk plate subduction not only triggered arc and backarc magmatism in the Amurian superterrane, but also led to decompression melting and focused upwelling of the upper mantle ahead of the subducting slab beneath the northern North China block. The Late Triassic intraplate extensive uplift, magmatism, and extension in the northern North China block are accordingly interpreted as the signatures of the focused mantle upwelling.

Phase equilibria modelling and LASS monazite petrochronology: P–T–t constraints on the evolution of the Priest River core complex, northern Idaho

AbstractPhase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.

Geology, mineralization, and geochronology of the Qianhe gold deposit, Xiong’ershan area, southern North China Craton

The Qianhe gold deposit in the Xiong’ershan area is located along the southern margin of the Archean-Paleoproterozoic North China Craton. The deposit consists of six orebodies that are hosted in Paleoproterozoic andesites to basaltic andesites and structurally controlled by roughly EW-trending faults. Individual orebodies comprise auriferous quartz veins and disseminated Au-bearing pyrite within hydrothermally altered rocks on both sides of, or close to, the veins. Ore-related hydrothermal alteration has produced various mixtures of K-feldspar, quartz, sericite, chlorite, epidote, carbonate, and sulfides. Pyrite is the most important ore mineral, associated with minor amounts of galena, sphalerite, and chalcopyrite. Other trace minerals include molybdenite, arsenopyrite, scheelite, rutile, xenotime, and parisite. Gold occurs mostly as native gold and electrum enclosed in pyrite or along microfractures of sulfides and quartz. Microthermometric measurements of primary inclusions in auriferous quartz suggest that gold and associated minerals were precipitated in the range of 160–305 °C from aqueous or carbonic-aqueous fluids with salinities of 6–22 wt% NaCl equiv. Samples of molybdenite coexisting with Au-bearing pyrite have Re–Os model ages of 134–135 Ma, whereas ore-related hydrothermal sericite separates yield 40Ar/39Ar plateau ages between 127 and 124 Ma. The Re–Os and 40Ar/39Ar ages are remarkably consistent with zircon U–Pb ages (134.5 ± 1.5 and 127.2 ± 1.4 Ma; 1σ) of the biotite monzogranite from the Heyu-intrusive complex and granitic dikes in and close to the Qianhe gold mine, indicating a close temporal and thus possibly genetic relationship between gold mineralization and granitic magmatism in the area. Fluid inclusion waters extracted from auriferous quartz have δD values of −80 to −72 ‰, whereas the calculated δ 18OH2O values range from 3.1 to 3.8 ‰. The hydrogen and oxygen isotopes from this study and previous work indicate that ore fluids were likely derived from degassing of magmas, with addition of minor amounts of meteoric water. Gold mineralization at Qianhe is temporarily coincident with pervasive bimodal magmatism, widespread fault-basin formation, and well development of metamorphic core complexes in the whole eastern North China Craton that have been interpreted as reflecting reactivation of the craton in the late Mesozoic after prolonged stabilization since its formation in the late Paleoproterozoic. It is therefore concluded that the Qianhe gold deposit formed as a result of this craton reactivation event.

Multistage Cenozoic extension of the Albion–Raft River–Grouse Creek metamorphic core complex: Geochronologic and stratigraphic constraints

Despite numerous studies on the structural evolution of metamorphic core complexes, there is still little consensus on the set and sequence of processes that bring deep levels of the crust to the surface during extension. This problem is partially related to the fact that core complexes expose polydeformed rocks, the history of which has been challenging to decipher. New geochronological and structural data combined with existing data provide improved insight into the Cenozoic extensional evolution of the Albion–Raft River–Grouse Creek (ARG) metamorphic core complex. The Cenozoic extensional history of the core complex can be divided into several distinct stages based on the geochronology and structure of igneous and metamorphic rocks in the lower plate of the complex combined with the geochronology and regional geologic context of sedimentary and volcanic rocks flanking the complex. Initial volcanism and plutonism was Eocene age (42–34 Ma), related to a regional southward-younging magmatic event. The development of high-temperature (sillimanite grade) metamorphic fabrics and mineral assemblages in footwall rocks was mostly Oligocene (ca. 32–25 Ma), synchronous with the diapiric rise and intrusion of evolved plutons to mid-crustal depths (∼10–15 km), formed by partial melting and remobilization of the deeper crust. There is no evidence for associated volcanism or basin development at the surface during this time span. The metamorphic and plutonic rocks of the core complex apparently remained at depth for ∼10–12 m.y. until the Middle Miocene (ca. 14 Ma), when they were exhumed by Basin and Range faulting.

Metamorphic Core Complex dynamics and structural development: Field evidences from the Liaodong Peninsula (China, East Asia)

Metamorphic Core Complexes (MCC) constitute remarkable features within wide rifts. Based on analogue and numerical modelling, MCC dynamics and structural development are mainly controlled by first geothermal gradient, second the compositional layering, and after the strain rate and partial melting. In the Late Mesozoic, continental extension occurred in East Asia leading to the development of MCC, magmatism and extensional sedimentary basins. Based on an integrated study (i.e. structural and finite strain analysis, petrofabrics, Anisotropy of Magnetic Susceptibility (AMS) and U/Pb on zircon dating), this paper aims at constraining the tectonic evolution and deformation mechanisms in the South Liaodong Peninsula (NE China). The Gudaoling massif is identified as a “migmatitic” MCC developed from Upper Jurassic (ca. 157–154Ma) to Early Cretaceous (ca. 128–113Ma). Intrusion of the Yinmawanshan synkinematic pluton (Early Cretaceous) to the south of the dome marks out the final stages of shearing along the Gudaoling detachment zone and of exhumation of the MCC into the upper crust. The Gudaoling MCC and the Yinmawanshan pluton stay in line with the coeval South Liaodong MCC, to the south, all making a ~140×30km wide extensional band formed during a regional E–W to NW–SE crustal stretching. The area shows a bi-phased development with a “slow” and a “fast” stage which corresponds to (1) crustal necking and (2) dome amplification/exhumation stages according to published thermo-mechanical modelling results. Finally, the Gudaoling MCC lower unit almost exclusively displays migmatites and anatectic granitoids of Late Jurassic and Early Cretaceous, respectively. Occurrence of partial melting during earlier stages of extension seems controlling the initiation of MCC (as a soft anomaly within the lower crust). In East Asia, a regional-scale thermal event, during Jurassic–Cretaceous times, may have significantly reduced the bulk lithosphere strength and locally induced partial melting, emphasizing pervasive deformations and development of a wide rift system.

Mylonitization in the lower plate of the Buckskin-Rawhide detachment fault, west-central Arizona: Implications for the geometric evolution of metamorphic core complexes

In metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down-dip continuation of a detachment fault, or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault. Detailed microstructural, fabric, and strain data from mylonites in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, constrain the structural development of the lower plate shear zone. Widespread exposures of ∼22–21 Ma granitoids of the Swansea Plutonic Suite enable us to separate Miocene strain coeval with core complex extension from older deformation. Mylonites across the lower plate consistently record top-to-the-NE-directed shear. Miocene quartz and feldspar deformation/recrystallization mechanisms indicate ∼450–500 °C mylonitization temperatures that were relatively uniform across a distance of ∼35 km in the extension direction. Quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction. Strain recorded in the Swansea Plutonic Suite is also relatively uniform in the extension direction, which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault. Altogether these data suggest the mylonitic shear zone initiated with a ≤4° dip and was unroofed by a more steeply dipping detachment fault system. Lower plate mylonites in the Buckskin-Rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down-dip continuation of a detachment fault.

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

The present paper demonstrates that exposed semi-brittle–brittle detachment fault zones, in addition to footwall mylonites and syn-extensional granitoids, can be used to date the timing of faulting and constrain the history and evolution of metamorphic core complexes. We employed Rb–Sr geochronology on micas, sampled directly from a part of the Simav detachment fault (SDF) zone in the northern Menderes Massif (western Turkey). The exposed part of the fault zone is marked by ∼3-m thick zone of low-grade mylonites/foliated cataclasites, in which mylonitic fabrics in orthogneisses are overprinted by fabrics of semi-brittle deformation. The low-grade mylonites/foliated cataclasites are characterized by coexistence of brown and green biotites. Rb–Sr ages on muscovite and brown and green biotite from the low-grade mylonites/foliated cataclasites are ca. 30 Ma, 17–13 Ma and 12–10 Ma, respectively; green biotite ages are interpreted as dating fluid-assisted deformation-induced dynamic recrystallization and suggest that a part of the SDF was active during a 12–10 Ma interval. The ca. 30 Ma muscovite ages date dynamic crystallization and probably beginning of extensional exhumation of the northern Menderes Massif. The coexistence of brown and green biotites in the same sample indicates retrogressive processes associated within a detachment faulting during which green biotites have recrystallized from primary brown biotites with an age of 19 ± 2 Ma in this area. This further means that the isotopic system became opened during faulting and that the green biotite ages therefore record the activity of the SDF. We have also dated an orthogneiss sample exposed well away from the detachment fault zone (devoid of any retrogressive processes); muscovites and biotites from this sample yield Rb–Sr ages of 45.7 ± 0.6 and 18.17 ± 0.18 Ma, respectively. The biotite age is in accord with regional biotite ages (19 ± 2 Ma) and record cooling of the footwall rocks of the detachment fault. We argue that the ca. 46 Ma muscovite age record synkinematic recrystallization during, and lend credibility to interpretation of, the prograde Barrovian-type regional Main Menderes Metamorphism as an Eocene event and that, prior to the onset of extensional detachment formation, the northern Menderes Massif (or at least the rocks we measured) experienced Eocene temperatures in excess of 500 °C to get this age. New brown biotite and secondary green biotite ages together with previously published age data argues that the footwall rocks reached to biotite cooling temperature conditions by ca. 18 Ma and stayed at similar conditions for a longer period until ca. 12 Ma. The ca. 18–12 Ma time interval corresponds to a period of tectonic quiescence during the Neogene and is attributed to the locking of SDF activity by the intrusion and rapid cooling of I-type syn-extensional granites. Secondary green biotite Rb–Sr ages indicate that the fault then become active during 12–10 Ma. Published low-temperature apatite–zircon fission track and apatite (U–Th)/He ages further suggest that displacement along the entire fault zone has continued until 8.0 ± 0.5 Ma. Combined with previous radiometric and low-temperature thermochronologic ages, the new ages argue for episodic core-complex formation and the Simav detachment fault activity between ca. 30 and 8 Ma. The data therefore argues for a complex history of core-complex formation in the northern Menderes Massif; it occurs in pulses and involves periods of alternating rapid and slow cooling/denudation rates.

Granite intrusion in a metamorphic core complex: The example of the Mykonos laccolith (Cyclades, Greece)

The Aegean domain is a well-suited place to study the formation of metamorphic core complex (MCC) and to investigate the role of syn-tectonic granites on their development. In the northern Cyclades, the Mykonos–Delos–Rhenia MCC is characterized by the intrusion of a kilometer-scale Late Miocene pluton of I-type granitoids within a migmatitic gneiss dome. New combined AMS (anisotropy of magnetic susceptibility) and microstructural studies on the Mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers. The Mykonos granitoids form a laccolith-like intrusion with a N70°E long axis. The laccolith is strongly asymmetric with an outlying root zone to the SW and a major body mainly developed to the NE. The laccolith construction is due to successive pulses of more or less differentiated magma that intruded the Cycladic Blueschist Unit. The attitude of stretching markers suggests an important (about 60°) vertical-axis local rotation phenomenon in the cycladic upper crust during the exhumation of the Mykonos MCC. Structural data suggest a four-stage evolution of the Mykonos MCC: (i) a first stage characterized by flat shearing toward the N–NE and by the formation of a domal structure in migmatitic paragneisses with multi-scale generation of folds with axes either perpendicular or parallel to the regional stretching, as a result of the interplay between regional N20°E-directed extension and EW shortening; (ii) a second stage marked by the emplacement of the Mykonos laccolith at 13.5 ± 0.3 Ma at the top of the migmatitic paragneisses; (iii) the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions; and (iv) the late stage marked by an acceleration of the exhumation of the Mykonos MCC. This exhumation was accommodated by important rotations of upper crustal blocks. During the end of the exhumation processes, around 10 Ma, deformation localized at the top of the laccolith in semi-ductile conditions and then in brittle conditions in the major detachment plane. Our study shows that the Cycladic plutonism event had no role on the initiation of the MCC. However, the geometry of the Mykonos intrusion supports that the magmas are “sucked” into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the MCC development. This acceleration was marked by a very fast exhumation of the laccolith after its emplacement.

Post-orogenic extension and metamorphic core complexes in a heterogeneous crust: the role of crustal layering inherited from collision. Application to the Cyclades (Aegean domain)

SUMMARY The development of metamorphic core complexes (MCC) corresponds to a mode of lithospheric continental stretching that follows collision. In most of the models that explain the formation of the MCC, high thermal gradients are necessary to weaken the lower crust and to induce its ascent. Such models fail to explain the exhumation of high pressure–low temperature metamorphic rocks in metamorphic core complex structures as observed in the Cycladic Blueschists in the Aegean domain. Besides, account for the lithological crustal stratification induced from collision has never been tested. In this paper, we use fully coupled thermomechanical modelling to investigate the impact of structural heritage and initial thermal gradient on the behaviour of the post-orogenic continental lithosphere. The models are designed and validated by petrological, structural and time data from the Cyclades. As a result, high thermal gradients (Moho temperature higher than 800 ◦ C) are neither necessary nor always sufficient to induce the development of a metamorphic core complex. At the contrary, the rheological layering of the crust inherited from collision is a first-order parameter controlling the development of extensional structures in post-orogenic settings. ‘Cold’ MCC can develop if the crust is made of a strong nappe thrust on top of weaker metamorphic cover and basement units, as observed in the Cyclades.

Cenozoic crustal extension in southeastern Arizona and implications for models of core-complex development

In conventional models of Cordilleran-style metamorphic core-complex development, initial extension occurs along a breakaway fault, which subsequently is deformed into a synform and abandoned in response to isostatic rebound and new faults breaking forward in the dominant transport direction. The Catalina core complex and associated geology in southeastern Arizona have been pointed to as a type example of this model. From southwest to northeast, the region is characterized by the NW–SE trending Tucson basin, the Catalina core complex, the San Pedro trough and the Galiuro Mountains. The Catalina core complex is bounded by the top-to-the-southwest Catalina detachment fault along its southwestern flank and the low-angle, northeast-dipping San Pedro fault along its northeastern flank. The Galiuro Mountains expose non-mylonitic rocks and are separated from the San Pedro trough to the southwest by a system of low- to moderate-angle southwest-dipping normal faults. This Galiuro fault system is widely interpreted to be the breakaway zone for the Catalina core complex. It is inferred to be folded into a synform beneath the San Pedro trough, to resurface to the southwest as the San Pedro fault, and to have been abandoned during slip along the younger Catalina detachment. This study aimed to test this model through analysis of field relations and geochronological age constraints, and reprocessing and interpretation of 2-D seismic reflection data from the Catalina core complex and San Pedro trough. In contrast to predictions of the conventional breakaway zone model, we raise the possibility of a moderate-angle, southwest-dipping detachment fault beneath the San Pedro trough that could extend to mid-crustal depths beneath the eastern flank of the Catalina Mountains. We present an alternative kinematic model in which extension was accommodated by a pair of top-to-the-southwest normal-fault systems (the Catalina and Galiuro detachment faults), with the only major difference between the two being the magnitude of displacement, which was greater for the Catalina detachment.

Tectonic and structural control of fluvial channel morphology in metamorphic core complexes: The example of the Catalina-Rincon core complex, Arizona

Fluvial channels in metamorphic core complexes are preferentially oriented parallel and perpendicular to the direction of tectonic extension. This pattern has been variably attributed to such causes as tectonic tilting during extension, channel elongation by slip along the range-bounding detachment fault, and the exploitation of extension-related joint sets during channel incision. In this paper we use field measurements, digital elevation model analyses, and numerical modeling to test hypotheses for the tectonic and structural control of fluvial channels in metamorphic core complexes, using the Catalina-Rincon core complex in southern Arizona, USA, as a type example. Field measurements and aerial photographic analyses indicate that channels of all sizes exploit steeply dipping joint sets during fluvial incision. As a consequence, channels become preferentially aligned along those joint sets. First and second Strahler-order channels preferentially exploit a joint set oriented perpendicular to the extension direction, while higher-order channels preferentially exploit a joint set oriented parallel to the extension direction. While these observations support the joint-exploitation hypothesis for structural control of drainage architecture, numerical modeling indicates that the spatial distribution of rock uplift during the initial phase of extension plays a crucial role by determining which joint set is preferentially exploited by channels of which Strahler orders. Numerical models indicate that higher-order channels exploit the joint set that is most closely aligned with the direction of initial tectonic tilting, even if that tilting is active for only a short period of time following the initiation of uplift. We conclude that the drainage architecture in the Catalina-Rincon core complex is the result of a combination of joint exploitation and tectonic tilting mechanisms. Structure also plays an important role in controlling the longitudinal profiles of channels in metamorphic core complexes. Channels in the Catalina-Rincon core complex are characterized by structurally controlled knickpoints with a wide distribution of heights and spacings. Field observations indicate that the occurrence of structurally controlled knickpoints and the resulting variability in longitudinal profile form is related to spatial variations in joint density. Numerical models that incorporate spatial variations in joint density using a stochastic bedrock erodibility coefficient are capable of reproducing the statistical properties of longitudinal profiles in the Catalina-Rincon core complex, including the power spectrum of longitudinal profiles and the frequency size distribution of structurally controlled knickpoints. The results of this study illustrate the important roles played by both jointing and the spatial distribution of rock uplift on the geomorphic evolution of metamorphic core complexes. More broadly, the study provides a recipe for how to incorporate joint-related structural controls into landscape evolution models.

The role of partial melting and extensional strain rates in the development of metamorphic core complexes

Orogenic collapse involves extension and thinning of thick and hot (partially molten) crust, leading to the formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. Two-dimensional thermo-mechanical Ellipsis models evaluate the parameters that likely control the formation and evolution of MCC: the nature and geometry of the heterogeneity that localizes MCC, the presence/absence of a partially molten layer in the lower crust, and the rate of extension. When the localizing heterogeneity is a normal fault in the upper crust, the migmatite core remains in the footwall of the fault, resulting in an asymmetric MCC; if the localizing heterogeneity is point like region within the upper crust, the MCC remains symmetric throughout its development. Therefore, asymmetrically located migmatite domes likely reflect the dip of the original normal fault system that generated the MCC. Modeling of a severe viscosity drop owing to the presence of a partially molten layer, compared to a crust with no melt, demonstrates that the presence of melt slightly enhances upward advection of material and heat. Our experiments show that, when associated with boundary-driven extension, far-field horizontal extension provides space for the domes. Therefore, the buoyancy of migmatite cores contributes little to the outer envelope of metamorphic core complexes, although it may play a significant role in the internal dynamics of the partially molten layer. The presence of melt also favors heterogeneous bulk pure shear of the dome as opposed to the bulk simple shear, which dominates in melt-absent experiments. Melt presence affects the shape of P-T-t paths only slightly for material located near the top of the low-viscosity layer but leads to more complex flow paths for material inside the layer. The effect of extension rate is significant: at high extension rate (cm yr − 1 in the core complex region), partially molten crust crystallizes and cools along a high geothermal gradient (35 to 65 °C km − 1 ); material remains partially molten in the dome during ascent. At low strain rate (mm yr − 1 in the core complex region), the partially molten crust crystallizes at high pressure; this material is subsequently deformed in the solid-state along a cooler geothermal gradient (20 to 35 °C km − 1 ) during ascent. Therefore, the models predict distinct crystallization versus exhumation histories of migmatite cores as a function of extensional strain rates. The Shuswap metamorphic core complex (British Columbia, Canada) exemplifies a metamorphic core complex in which an asymmetric, detachment-controlled migmatite dome records rapid exhumation and cooling likely related to faster rates of extension. In contrast the Ruby Mountain-East Humboldt Ranges (Nevada, U.S.A.) exhibits characteristics associated with slower metamorphic core complexes.

Exhumation of a Cretaceous rift complex within a Late Cenozoic restraining bend, southern Mongolia: implications for the crustal evolution of the Gobi Altai region

Abstract: The Gobi Altai region of southern Mongolia contains intramontane basins dominated by Jurassic–Cretaceous clastic and volcanic deposits. The origin of the basins is poorly documented because Late Cenozoic transpressional mountain building and associated alluvial sedimentation have overprinted and obscured the older Mesozoic history. In this study, we report the discovery of Cretaceous high-angle normal faults and a major low-angle extensional detachment fault bordering the Altan Uul range in southern Mongolia, which indicates that NW–SE crustal stretching was responsible for creating the accommodation space for thick Jurassic–Cretaceous sedimentary accumulations in southern Mongolia. The detachment fault separates chlorite-grade metasedimentary rocks in the footwall from intensely sheared and flattened unmetamorphosed conglomerates and breccias in the hanging wall. The significant metamorphic break across the detachment fault, associated extensional structures in the hanging wall, and domal form of the footwall block indicate that NW Altan Uul has some structural and topographic characteristics typically associated with metamorphic core complexes. However, NW Altan Uul lacks ductile extensional fabrics along the detachment fault and in the adjacent footwall, and limited stretching in the upper plate suggests less total extensional slip than is reported from typical metamorphic core complexes. Unconformable relations in hanging-wall strata and palynological data indicate that extensional detachment faulting at NW Altan Uul occurred in the Aptian (120–112 Ma) similar in time to metamorphic core complex development previously reported in Mongolia–China border areas and the Daqing Shan of northern China. Post-detachment fault Cretaceous sedimentation buried NW Altan Uul, which is now exposed only as a result of erosional denudation associated with uplift of the recent Nemegt–Altan Uul restraining bend. The discovery of significant Aptian crustal extension in the southern Gobi Altai suggests that Early Cretaceous diffuse rifting encompassed an even wider region than was previously recognized, including areas of the eastern Altai, central, southern and eastern Mongolia, and adjacent areas of northern and northeastern China and Transbaikalia, constituting one of Earth's largest continental interior extensional provinces. Workers investigating the neotectonic development of the Gobi Altai should consider the extent to which the pre-existing rift basin architecture may have influenced the recent range development and network of seismically active faults in the region.

Dynamics and structural development of metamorphic core complexes

The development of metamorphic core complexes (MCCs) in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code, accounting for elastic‐brittle‐ductile properties of constituent rocks. For MCCs to develop, the middle lower crust and the sub‐Moho mantle are required to be weak enough to flow laterally, so as to simultaneously feed the exhuming dome and enable the Moho to keep a flat geometry. The conditions are satisfied with initial Moho temperatures of 800°C or higher, with crustal thicknesses of 45 km or greater, and with initial effective viscosities lower than 1020 Pa s and 1022 Pa s in the lower crust and the underlying mantle, respectively. A compositional (mainly density) anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation. During a first stage of “upper crust necking,” the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust. When the first ductile layers reach the surface, a second stage of “dome amplification and widening” occurs. Dome amplification is accommodated by horizontal flow in the ductile crust, giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone. The system then rapidly becomes asymmetric, with the localization of a detachment zone along one dome limb, further accommodating dome widening. Thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone. Depending on initial Moho temperature, the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip.

Cenozoic Crustal Evolution and Mantle Dynamics of Post-Collisional Magmatism in Western Anatolia

Post-collisional magmatism in western Anatolia followed a continental collision event in the Early Eocene, and occurred in discrete pulses that appear to have propagated from north to south over time. The first episode occurred during the Eocene and Oligo-Miocene and was subalkaline in nature, producing medium-to high-K calc-alkaline granitoids and mafic to felsic volcanic rocks. Partial melting and assimilation-fractional crystallization of enriched subcontinental lithospheric mantle-derived magma(s) were important processes in the genesis and evolution of the parental magmas, which experienced decreasing subduction influence and increasing crustal contamination through the Early Eocene-Early Miocene. This magmatic episode coincided with continued regional compression and development of a thick orogenic crust, and was influenced by an influx of asthenospheric heat and melts provided by lithospheric slab break-off. Extensional tectonics replaced the regional compression by the Middle Miocene, following the initial collapse of the western Anatolian orogenic welt, and resulted in the development of metamorphic core complexes and horst-graben structures. The second main episode of magmatism occurred during the Middle Miocene (16-14 Ma) and produced mildly alkaline rocks that show a decreasing amount of crustal contamination and subduction influence through time. Although melting of a subduction-modified lithospheric mantle continued, an asthenospheric mantle-derived melt contribution played a major role in the generation of these mildly alkaline magmas. The inferred asthenospheric melt contribution was a result of delamination of the lowermost part of the lithospheric mantle and/or partial convective removal of the sub-continental lithospheric mantle (SCLM). The third episode of post-collisional magmatism started around ~12 Ma and continued through the Late Quaternary. The main melt source for this phase carried no subduction component and was generated by the decompressional melting of asthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in the Aegean extensional province. Lithospheric-scale extensional fault systems acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface. Post-collisional magmatism in western Anatolia thus displays compositionally distinct episodes controlled by slab break-off, lithospheric delamination, and asthenospheric upwelling and decompressional melting, reflecting the geodynamic evolution of the eastern Mediterranean region throughout the Cenozoic. These events and the associated processes in the mantle took place primarily in response to the plate tectonic evolution of the region and collectively constitute a time-progressive template for the mode and nature of the post-collisional magmatism common to most alpine-style orogenic belts.