Formation Of Metamorphic Core Complexes Research Articles

Structures, flow stresses, and estimated strain rates in metamorphic rocks of the Small Cyclades Islands Iraklia and Schinoussa (Aegean Sea, Greece)

Like the larger Cyclades Islands (e.g. Naxos, Paros, Ios) the Small Cyclades Islands form part of the Attic-Cycladic crystalline basement belt. Subduction in the Paleogene was associated with high-pressure metamorphism, followed by a Barroviantype overprint, local partial melting and granitic plutonism in the Neogene. On the islands of Iraklia and Schinoussa Neogene metamorphism did not exceed mid-greenschist grade, and there are some well preserved older high-pressure/low temperature metamorphic assemblages. The rocks and structural geology of both islands had previously not been analysed in detail. Rocks present are calcite and dolomite marbles, in part with preserved paleosoils (metabauxites and metalaterites), and a suite of metapelitic schists with associated glaucophane schists and piemontite quartzites. There is a ductile deformation history comprising four events. The two older ones (D1, D2) are associated with high strains, and have resulted in a subhorizontal foliation, a N-S trending stretching lineation, and recumbent isoclinal folds with variable axial orientations. At least D1 is bracketed by the growth of glaucophane, with uncertain kinematics. D2, syn-blueschist grade on Iraklia, however, has a clear top-to-north sense of shear. D3 and D4 caused upright folds with N-S and E-W trending fold axes, respectively. This is evidence for late, two-phase horizontal shortening. As the deformed rocks show only a slight static metamorphic overprint, we have estimated flow stresses from dynamically recrystallized grain sizes of quartz and calcite. Coincident flow stress estimates are 28-62 MPa in quartzites, and 31-56 MPa in the marbles. Together with syntectonic temperature extimates, and applying published flow laws, this leads to estimated deformation rates on the order of 10−12 to 10−14 sec−1. Even though deformation history and kinematics is very similar to that on Naxos Island, the clear relation of the earlier part to high-pressure metamorphism on Iraklia and Schinoussa speaks in favour of a subduction-related setting here. This is in contrast to the later formation of metamorphic core complexes in the north (Naxos) and south (Ios).

Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located <30 km from an active mid‐ocean ridge (MOR), a >1 km thickness of blueschist‐derived mylonites formed in a midcrustal shear zone during the Pliocene at ∼400–500°C. This top‐to‐the‐north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCC's lower plate have been exhumed along a detachment as a result of >50 km of slip at rates of >12 mm/yr. The inactive, back‐tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio‐Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling‐hinge style MCC, probably since <0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ∼30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ > 0.8) at depth, and provide a sufficient mechanism for activating low‐angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1–6 m.y. timescale. Deep‐seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB to cause MCC formation, vs. those in which slip is distributed across an imbricate zone of more uniform strength normal faults. The Trobriand fault connects in the eastern Woodlark rift to the Owen Stanley fault in the Papuan Ranges, which is probably moving at nearly the full plate velocity.

Discussion on Mesozoic extensional structures of the Fangshan tectonic dome and their subsequent reworking during collisional accretion of the North China BlockJournal, Vol.163, 2006, 127–142

Yu Wang writes: On the basis of descriptions of the stratigraphic sequences and structural features in the field, and interpretations of published data, Yan et al . (2006 a ) reach the surprising conclusion that early Mesozoic (early–middle Triassic) ESE-oriented extension resulted in formation of the Fangshan metamorphic core complex. If correct, the authors have made an important contribution to understanding of the tectonic evolution of North China. However, the way in which they cite previously published work to support their model, and discrepancies between their findings in the field and those of other workers raise a number of problems that will be discussed below. ### Citing of published work. Yan et al . (2006 a ) cited many references (e.g. Lei et al . 1994; Niu et al . 1994; Yu & Zhang 1996; Chen 1999; Fu 1999; Meng et al . 2003) in support of their conclusions about early Triassic extension and formation of metamorphic core complexes, as well as Late Cretaceous normal faulting. In many cases, however, the cited references do not lend support to their interpretations. For example, the Du Shan granite and surrounding Archaean and Proterozoic metamorphic rocks (Yu & Zhang 1996) contain no evidence for extension or development of metamorphic core complex features, but instead record NE-trending, strike-slip ductile shearing, not top-to-the-SE extension. The Malanyu dome is an old metamorphic sequence (Chen 1999), and no one has previously interpreted it as an extensional dome associated with SE-directed extensional shear. Zhang et al . (1991) did not examine any aspect of the geology in the area of the so-called Yuerya metamorphic core complex. Furthermore, no dome has been identified here by any previous geologists; instead, it is a c . 175–174 Ma (U–Pb sensitive high-resolution ion microprobe ages, Luo et al . 2001) granitic sheet-like …

Cenozoic topographic and climatic response to changing tectonic boundary conditions in Western North America

This study presents an oxygen isotopic record from the Paleocene to the Pliocene based on the analysis of predominantly paleosol carbonate from intermontane basins in southwestern Montana and eastern Idaho. δ 18O values of calcite decrease by 7 to 10‰ between ∼ 50 and 47 Ma in southwestern Montana and Idaho most likely as a result of an increase in elevation of 2.5 to 3.5 km. This rise in elevation is roughly contemporaneous with the emplacement of the nearby Challis Volcanics, and the formation of metamorphic core complexes in the hinterland of the Sevier thrust belt. Moreover, when compared to previous oxygen isotopic studies that show oxygen isotopic shifts of similar magnitude occurring later (in the late Eocene to early Oligocene in northeastern Nevada, and late Oligocene to Miocene in southern Nevada), the results of this study add to a growing body of evidence for a spatial and temporal migration of high surface elevations from north to south in the Great Basin of western United States. This surface uplift history supports tectonic models calling for north to south removal of the Farallon slab or delamination of the mantle lithosphere.

2: Hydrothermal ore deposits related to post-orogenic extensional magmatism and core complex formation: The Rhodope Massif of Bulgaria and Greece

The Rhodope Massif in southern Bulgaria and northern Greece hosts a range of Pb–Zn–Ag, Cu–Mo and Au–Ag deposits in high-grade metamorphic, continental sedimentary and igneous rocks. Following a protracted thrusting history as part of the Alpine–Himalayan collision, major late orogenic extension led to the formation of metamorphic core complexes, block faulting, sedimentary basin formation, acid to basic magmatism and hydrothermal activity within a relatively short period of time during the Early Tertiary. Large vein and carbonate replacement Pb–Zn deposits hosted by high-grade metamorphic rocks in the Central Rhodopean Dome (e.g., the Madan ore field) are spatially associated with low-angle detachment faults as well as local silicic dyke swarms and/or ignimbrites. Ore formation is essentially synchronous with post-extensional dome uplift and magmatism, which has a dominant crustal magma component according to Pb and Sr isotope data. Intermediate- and high-sulphidation Pb–Zn–Ag–Au deposits and minor porphyry Cu–Mo mineralization in the Eastern Rhodopes are predominantly hosted by veins in shoshonitic to high-K calc-alkaline volcanic rocks of closely similar age. Base-metal-poor, high-grade gold deposits of low sulphidation character occurring in continental sedimentary rocks of synextensional basins (e.g., Ada Tepe) show a close spatial and temporal relation to detachment faulting prior and during metamorphic core complex formation. Their formation predates local magmatism but may involve fluids from deep mantle magmas. The change in geochemical signatures of Palaeogene magmatic rocks, from predominantly silicic types in the Central Rhodopes to strongly fractionated shoshonitic (Bulgaria) to calc-alkaline and high-K calc-alkaline (Greece) magmas in the Eastern Rhodopes, coincides with the enrichment in Cu and Au relative to Pb and Zn of the associated ore deposits. This trend also correlates with a decrease in the radiogenic Pb and Sr isotope components of the magmatic rocks from west to east, reflecting a reduced crustal contamination of mantle magmas, which in turn correlates with a decreasing crustal thickness that can be observed today. Hydrogen and oxygen isotopic compositions of the related hydrothermal systems show a concomitant increase of magmatic relative to meteoric fluids, from the Pb–Zn–Ag deposits of the Central Rhodopes to the magmatic rock-hosted polymetallic gold deposits of the Eastern Rhodopes.

Compositional diversity of Eocene–Oligocene basaltic magmatism in the Eastern Rhodopes, SE Bulgaria: implications for genesis and tectonic setting

Basaltic magmatism occurred only rarely within the extensive Eocene–Oligocene volcanic activity in the Eastern Rhodope Mts., SE Bulgaria. The earliest mafic volcanism started at ca. 34 Ma with K-rich trachybasalts strongly enriched in large ion lithophile elements (LILE), particularly Ba, Sr, Pb, Th, and light rare earth elements (REE) relative to the high field strength elements (HFSE). They have high 87Sr/ 86Sr ratios (0.70688–0.70756), low 144Nd/ 144Nd (0.51252–0.51243), and very high 207Pb/ 204Pb (15.74–15.76) and 208Pb/ 204Pb (39.07–39.14) at low 206Pb/ 204Pb (18.72–18.73) ratios, reflecting high degrees of crustal contamination. Shoshonitic basalts and absarokites and calc-alkaline and high-K calc-alkaline magmas, which erupted between 33 and 31 Ma, have decreasing Sr isotope initial ratios from west (0.70825) to east (0.70647) at approximately constant 143Nd/ 144Nd isotopic compositions (0.51252–0.51243) and slightly decreasing 207Pb/ 204Pb (15.66–15.72) and 208Pb/ 204Pb (38.80–38.96) and increasing 206Pb/ 204Pb (18.73–18.90) in comparison with the trachybasalts. All these rocks are characterized by negative Nb–Ti and Eu anomalies. They resulted from different degrees of partial melting of enriched asthenosphere, and the magmas were later contaminated by the Rhodopian crust. The end of the magmatic activity (28–26 Ma) was marked by emplacement of alkaline dykes, spatially associated with metamorphic core complexes. They are characterized by low 87Sr/ 86Sr (0.70323–0.70338), high 144Nd/ 144Nd (0.51290–0.51289), and 206Pb/ 204Pb (18.91–19.02) at lower 207Pb/ 204Pb (15.52–15.64) and 208Pb/ 204Pb (38.59–38.87) ratios, consistent with an origin from a source similar to OIB-like European Asthenospheric reservoir contaminated by depleted mantle lithosphere. The Eastern Rhodope Eo-Oligocene mafic magmatism formed as part of the prolonged extensional tectonics of the whole Rhodope region in Late Cretaceous–Paleogene time, similar to those in the U.S. Cordillera and Menderes Massif (Turkey). Initiation of extension is constrained by the formation of metamorphic core complexes, low-angle detachment faults, and supradetachment Maastrichtian–Paleocene sedimentary basins, intimately associated with 70–42 Ma granitoids and metamorphism which record mantle perturbation. The Eo-Oligocene stage started with block faulting, sedimentary basin formation, and extensive acid-intermediate and basic volcanism over the entire Eastern Rhodope area. The order of emplacement of the basalts from high-Ba trachybasalts through shoshonites, calc-alkaline and high-K calc-alkaline basalts, and finally to purely asthenospheric-derived alkaline basalts, with progressively decreasing amount of crustal component, reflects upwelling asthenospheric mantle. Most of the models proposed in the literature to explain extension and magmagenesis in the Rhodopes and the Mediterranean region cannot be applied directly. Critical evaluation of these models suggest that some form of convective removal of the lithosphere and mantle diapirism provide the most satisfactory explanation for the Paleogene structural, metamorphic, and magmatic evolution of the Rhodopes.

Tectonics of the late Mesozoic wide extensional basin system in the China–Mongolia border region

AbstractThe China–Mongolia border region contains many late Mesozoic extensional basins that together constitute a regionally extensive basin system. Individual basins within the system are internally composed of a family of sub‐basins filled with relatively thin sedimentary piles mostly less than 5 km in thickness. There are two types of sub‐basins within the basins, failed and combined, respectively. The failed sub‐basins are those that failed to continue developing with time. In contrast, the combined ones are those that succeeded in growing by coalescing adjacent previously isolated sub‐basins. Thus, a combined sub‐basin is bounded by a linked through‐going normal fault that usually displays a corrugated trace on map view and a shallower dip on cross‐section. Along‐strike existence of discrete depocenters and alternation of sedimentary wedges of different types validate the linkage origin of combined sub‐basins. Localized high‐strain extension resulted in large‐amount displacement on linked faults, but contemporaneously brought about the cessation of some isolated fault segments and the formation of corresponding failed sub‐basins in intervening areas between active linked faults. Some combined sub‐basins might have evolved into supradetachment basins through time, concurrent with rapid denudation of footwall rocks and formation of metamorphic core complexes in places. A tectonic scenario of the broad basin system can be envisioned as an evolution from early‐stage distributed isolated sub‐basins to late‐stage focused combined or/and supradetachment sub‐basins bounded by linked faults, accompanied by synchronous cessation of some early‐formed sub‐basins.Initiation of the late Mesozoic extension is believed to result from gravitational collapse of the crust that had been overthickened shortly prior to the extension. Compression, arising from collision of Siberia and the amalgamated North China–Mongolia block along the Mongol–Okhotsk suture in the time interval from the Middle to Late Jurassic, led to significant shortening and thickening over a broad area and subsequent extensional collapse. Pre‐ and syn‐extensional voluminous magmatism must have considerably reduced the viscosity of the overthickened crust, thereby not only facilitating the gravitational collapse but enabling the lower‐middle crust to flow as well. Flow of a thicker crustal layer is assumed to have occurred coevally with upper‐crustal stretching so as to diminish the potential contrast of crustal thickness by repositioning materials from less extended to highly extending regions. Lateral middle‐ and lower‐crustal flow and its resultant upward push upon the upper crust provide a satisfying explanation for a number of unusual phenomena, such as supracrustal activity of the extension, absence or negligibleness of postrift subsidence of the basin system, less reduction of crustal thickness after extension, and non‐compression‐induced basin inversion, all of which have been paradoxical in the previous study of the late Mesozoic basin tectonics in the China–Mongolia border region.

Electrical conductivity images of Quaternary faults and Tertiary detachments in the California Basin and Range

Comparison of an electrical resistivity section derived from magnetotelluric (MT) data to a geologic section extending eastward from the Sierra Nevada near latitude 36°20′N shows that the crust is dominated by steeply dipping conductive features that correlate with active strike‐slip faults. While there is a subhorizontal conductor at a depth ∼20 km beneath some of the profile, it is broken by vertical structures associated with the active strike‐slip faults. The continuous subhorizontal anomalies in the lower crust typically observed in extensional regions are therefore absent in the resistivity section. The present‐day strike‐slip tectonic regime as indicated by geodetic data in this part of the Basin and Range is not producing features that could be inferred to indicate subhorizontal shear zones resulting from lateral crustal flow during extension. Because the Miocene tectonic regime resulted in the formation of metamorphic core complexes and thus was accompanied by such flow, the present regime appears to represent a fundamental transition in the mode of crustal deformation in the region. A serendipitous result of our study was the identification on resistivity sections of carbonate aquifers in the upper crust. Comparison of resistivities from the MT section to measured fluid resistivities from springs and boreholes suggests that the aquifers must be heterogeneous, with more saline brines occupying the deepest portions of the carbonates.

Tectonic denudation of a Late Cretaceous–Tertiary collisional belt: regionally symmetric cooling patterns and their relation to extensional faults in the Anatolide belt of western Turkey

Thermochronological data reveal that the Late Cretaceous–Tertiary nappe pile of the Anatolide belt of western Turkey displays a two-stage cooling history. Three crustal segments differing in structure and cooling history have been identified. The Central Menderes metamorphic core complex represents an ‘inner’ axial segment of the Anatolide belt and exposes the lowest structural levels of the nappe pile, whereas the two ‘outer’ submassifs, the Gördes submassif to the north and the Çine submassif to the south, represent higher levels of the nappe pile. A regionally significant phase of cooling in the Late Oligocene and Early Miocene affected the outer two submassifs and the upper structural levels of the Central Menderes metamorphic core complex. In the northern part of the Gördes submassif, cooling was related to top-to-the-NNE movement on the Simav detachment, as the apatite fission-track ages show a northward-younging trend in the direction of movement on this detachment. In the Çine submassif, relatively rapid cooling in Late Oligocene and Early Miocene times may have been related to top-to-the-S extensional reactivation of the basal thrust of the overlying Lycian nappes. The second phase of cooling in the Anatolide belt is related to Pliocene to Recent extension resulting in the formation of the Central Menderes metamorphic core complex in the inner part of the Anatolide belt. Core-complex development caused the formation of supra-detachment graben, which document the ongoing separation of the Central Menderes metamorphic core complex from the outer submassifs.

PET and antipsychotic drugs — future

We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

Issues in the routine measurement of quality of life outcome

We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

Metamorphic core complex formation by density inversion and lower-crust extrusion.

Metamorphic core complexes are domal uplifts of metamorphic and plutonic rocks bounded by shear zones that separate them from unmetamorphosed cover rocks. Interpretations of how these features form are varied and controversial, and include models involving extension on low-angle normal faults, plutonic intrusions and flexural rotation of initially high-angle normal faults. The D'Entrecasteaux islands of Papua New Guinea are actively forming metamorphic core complexes located within a continental rift that laterally evolves to sea-floor spreading. The continental rifting is recent (since approximately 6 Myr ago), seismogenic and occurring at a rapid rate ( approximately 25 mm yr-1). Here we present evidence-based on isostatic modelling, geological data and heat-flow measurements-that the D'Entrecasteaux core complexes accommodate extension through the vertical extrusion of ductile lower-crust material, driven by a crustal density inversion. Although buoyant extrusion is accentuated in this region by the geological structure present-which consists of dense ophiolite overlaying less-dense continental crust-this mechanism may be generally applicable to regions where thermal expansion lowers crustal density with depth.

Late orogenic crustal extension in the northern Menderes massif (western Turkey): evidence for metamorphic core complex formation

The Simav metamorphic core complex of the northern Menderes massif, western Turkey, consists of a plutonic (Tertiary) and metamorphic (Precambrian) core (footwall) separated from an allochthonous cover sequence (hanging wall) by a low-angle, ductile-to-brittle, extensional fault zone (i.e. detachment fault). The core rocks below the detachment fault are converted into mylonites with a thickness of a few hundred metres. Two main deformation events have affected the core rocks. The first deformational event (D1) was developed within the Precambrian metamorphic rocks. The second event (D2), associated with the Tertiary crustal extension, includes two distinct stages. Stage one is the formation of a variably developed ductile (mylonitic) deformation (D2d) in metamorphic and granitic core rocks under greenschist facies conditions. The majority of the mylonites in the study area have foliations that strike NNW to NNE and dip SW to SE. Stretched quartz and feldspar grains define the mineral lineation trending SW-NE direction and plunging gently to SW. The kinematic indicators indicate a top-to-NE sense of shear. Stage two formation of brittle deformation (D2b) that affected all core and cover rocks. D2b involves the development of cataclasites and high-angle normal faults. An overall top towards the north sense of shear for the ductile (mylonitic) fabrics in the core rocks is consistent with the N-S regional extension in western Turkey.

A Tibetan model for the early Tertiary western United States

A comparative study of the geology of the early Tertiary western United States and the modern Tibetan and Turkish–Iranian Plateaux indicates that these three regions, situated in the hinterland of major orogenic belts, share a common taphrogenic evolutionary path. Both the Tibetan and Turkish–Iranian Plateaux have high mean elevations above sea level (>5 km and 2.0–2.5 km, respectively), have experienced fast uplift and widespread unroofing during the last 10–8 Ma, and are undergoing extension nearly perpendicular to the plate convergence direction. Successive accretionary and collisional events accompanied by significant shortening and underplating during the formation of these plateaux resulted in the generation of overthickened continental crust (>50 km) that is currently in the process of extending. A two-stage magmatism, characterized by widespread calc-alkaline volcanism followed and/or overlapped by bimodal (basaltic to rhyolitic) volcanism, occurs synchronously with crustal extension and thinning. The plateaux in general have an internal (closed) drainage system and are drained by antecedent rivers near their edges. The early Tertiary western US Cordillera displayed similar geological and geomorphic features to those observed in the Tibetan and Turkish–Iranian Plateaux in the aftermath of a prolonged Mesozoic orogeny and underwent an orogenic collapse starting around mid-Tertiary time. A N-trending crustal welt with a maximum thickness of 60 km and an elevation around >3 km formed a highland with alpine flora in the hinterland of the Cordilleran orogenic belt and separated internal drainage basins and saline lakes on the east from major flood plains of antecedent river systems on the west. This highland subsequently became the locus of crustal extension, metamorphic core complex formation, and wide-spread calcalkaline volcanism (9ignimbrite flare-up9). This early stage of orogenic collapse was followed by the Basin and Range extension, characterized by block faulting and bimodal volcanism, starting around 18–16 Ma. The Basin and Range extension and associated subsidence resulted in development of the Great Basin with a mean elevation of 1.5 km above sea level and a crustal thickness of 30 km. Similarities in the post-orogenic evolutionary paths of these three regions indicate that continental lithosphere reacts to overthickening in a consistent way through large-scale crustal remobilization and extensional collapse, regardless of the mode and nature of its orogenic build-up.

Structural and geochronological constraints on the role of partial melting during the formation of the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome, British Columbia

At the latitude of the Thor-Odin dome, the Shuswap metamorphic core complex exposes a ~15 km thick structural section composed of an upper unit that preserved Mesozoic metamorphism, structures, and cooling ages, separated from the underlying high-grade rocks by low-angle detachment zones. Below the detachments, the core of the complex consists of an amphibolite-facies middle unit overlying a migmatitic lower unit exposed in the core of the Thor-Odin dome. Combined structural and super high resolution ion microprobe (SHRIMP) U-Pb geochronology studies indicate that the pervasive shallowly dipping foliation and east-west lineation developed in the presence of melt during Paleocene time. SHRIMP analyses of complexly zoned zircon grains suggest that the migmatites of the lower unit crystallized at ~56 Ma, and a syntectonic leucogranite at ~60 Ma. We suggest that leucogranite migrated upward from the migmatites through an array of dikes and sills that permeated the middle unit and ponded to form laccoliths spatially related to the detachment zones. The similarity in ages of inherited zircon cores in the two migmatite and the leucogranite samples suggests a genetic link consistent with the structural analysis. Following the crystallization of migmatites, the terrane cooled rapidly, as indicated by argon thermochronology. We propose that exhumation of the core of the Canadian Cordillera during the formation of the Shuswap metamorphic core complex occurred from ~60 to 56 Ma at a time when the crust was significantly partially molten. These structural and temporal relationships suggest a genetic link between mechanical weakening of the crust by partial melting, late-orogenic collapse, and exhumation of high-grade rocks in the hinterland of a thermally mature orogenic belt.

Tectonic evolution of the Priest River complex, northern Idaho and Washington: A reappraisal of the Newport fault with new insights on metamorphic core complex formation

New geologic mapping, 40Ar/39Ar thermochronometry, and geobarometry in the Middle Eocene Priest River metamorphic core complex provide the basis for unraveling the role of en echelon fault systems in core complex formation and for determining the scale of crustal fragments that form during continental extension. Four faults occur in the Priest River complex. The east verging Purcell Trench fault zone on the eastern side consists of two distinct en echelon fault segments separated by an unfaulted homocline. The U‐shaped Newport fault system on the northwestern side is a conjugate normal fault set. The west verging eastern Newport fault terminates within the Silver Point Wrencoe pluton, which was intruded syntectonically into the fault zone. The east verging western Newport fault merges with the east verging Spokane dome mylonite zone in the underlying infrastructure. New geobarometric data show that this midcrustal shear zone, which evidently forms part of the regional basal décollement of the Cordilleran fold and thrust belt, also records significant Eocene extensional shearing. Rocks that formed beneath the mylonite zone at a depth of 30–35 km are juxtaposed against rocks that formed at a depth of 10 km above the zone. Eocene 40Ar/39Ar chrontours in the southern part of the infrastructure record progressive exhumation and quenching that becomes younger eastward. In the northern fragment of the infrastructure, alternating domains of progressive westward exhumation/quenching and progressive eastward exhumation/quenching occur beneath the eastern Newport fault and the northern Purcell Trench fault, respectively. These relationships form the basis for a new model of the evolution of the Priest River complex. The southern part of the infrastructure was exhumed by a major east verging detachment system comprising the western Newport fault and the reactivated eastern part of the Spokane dome mylonite zone, into which the western Newport fault merges. This master detachment fault, which roots in the southern Purcell Trench on the east side of the core complex, plunges beneath the northern part of the infrastructure (the Selkirk Crest block). Below the detachment, the infrastructure appears to be intact; above the detachment, the crust extended along a set of relaying conjugate detachment faults. These faults are the eastern Newport fault and the north and south Purcell Trench faults. Kinematic analysis shows that the Selkirk Crest block is a crustal‐scale extensional horse that was stranded as continued extension moved the underlying metamorphic infrastructure out from beneath it and toward the west along the master detachment. This study shows that large tracts of midcrustal rocks can be translated and stranded as allochthonous fragments during continental crustal extension.

Model for the development of kyanite during partial convective overturn of Archean granite–greenstone terranes: the Pilbara Craton, Australia

Restricted occurrences of early, syn‐ and late‐kinematic kyanite adjacent to large domal batholiths in the Archean granite–greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo‐mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstone keels, reflecting rapid, near‐isothermal burial. The narrow zones of kyanite‐bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low‐grade greenstone succession, dragged to mid‐crustal depths (6 kbar) during greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low‐strain greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher‐temperature aluminosilicate polymorphs during thermal recovery.Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid‐crustal depths (c. 20 km) before rebound, providing a three‐dimensional glimpse of Archean dome‐and‐keel structures. The general occurrence of large granitoid domes in Archean granite–greenstone terranes, restriction of rare kyanite to the adjacent, high‐strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history.

Structural and geochronological constraints on the role of partial melting during the formation of the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome, British Columbia

Structural and geochronological constraints on the role of partial melting during the formation of the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome, British Columbia

Self-consistent rolling-hinge model for the evolution of large-offset low-angle normal faults

Research Article| December 01, 1999 Self-consistent rolling-hinge model for the evolution of large-offset low-angle normal faults Luc L. Lavier; Luc L. Lavier 1Lamont-Doherty Earth Observatory of Columbia University, and Department of Earth and Environmental Sciences, Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar W. Roger Buck; W. Roger Buck 1Lamont-Doherty Earth Observatory of Columbia University, and Department of Earth and Environmental Sciences, Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Alexei N. B. Poliakov Alexei N. B. Poliakov 2Laboratoire de Géophysique et Tectonique, CNRS UMR 5573, Université de Montpellier Cedex 05, France Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (12): 1127–1130. https://doi.org/10.1130/0091-7613(1999)027<1127:SCRHMF>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Luc L. Lavier, W. Roger Buck, Alexei N. B. Poliakov; Self-consistent rolling-hinge model for the evolution of large-offset low-angle normal faults. Geology 1999;; 27 (12): 1127–1130. doi: https://doi.org/10.1130/0091-7613(1999)027<1127:SCRHMF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated. The controversy focuses primarily on whether the low-angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high-angle orientation after large offsets. We describe a self-consistent numerical model for the extension of a brittle layer that can spontaneously produce normal-fault structures. In our formulation, a fault or faults form because strength is locally reduced with increasing strain. If the reduction in fault strength is <∼10% of the total strength of the layer, then faults lock after an offset smaller than the layer thickness and new faults form. Larger strength reduction leads to single faults that continue to slip no matter how large the fault offset. If the strength reduction occurs by the loss of cohesion, then we see the unlimited offset faults for layers <11–22 km thick for reasonable values of cohesion. The key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications

Strain varies systematically from weakly-developed, outward-dipping, S-tectonites in the ∼3320–3310±10 Ma Mount Edgar Batholith to intensely deformed, subvertical, L-tectonites in greenstones of the Warrawoona syncline. A consistent ‘greenstone-down/batholith-up’ sense of shear is recorded in batholithic domal margins and adjacent high-grade supracrustal rims: lineations converge to a central zone of subvertical extension (zone of sinking) along the synclinal axis. At domal margins, early kinematic granitoid sheets and ‘intrusive diatexites’ are subconcordant to a well-developed, dome-parallel schistosity, but late- to post-kinematic intrusives are discordant, high-level plutons. All granitoids are the same age, within analytical error. These syn-doming features conform with structural tests for diapirs, and differ from those expected during metamorphic core complex formation and cross-folding. Diapirism is part of a larger process involving partial convective overturn of the crust. Based on strain patterns and kinematic criteria, we argue that deformation was initiated by sinking of greenstones, which rapidly subsided; the domes then rose passively as accommodation structures. Ongoing doming ( D 4) partitioned strain to the southern rim of the batholith and uplifted a wedge of the Warrawoona Syncline keel. We consider that crustal overturn occurred in response to mantle plume activity: Eruption of a 5–10 km thick, ∼3325 Ma, mafic–ultramafic greenstone pile onto an older granite–greenstone terrain created a negatively buoyant crust, but convective overturn occurred some 5–20 Ma later, triggered by widespread anatexis (thermal softening). Convective overturn may have been a common Archaean process, occurring in response to a hotter mantle, and represents an end-member deformation mechanism that includes thrust-accretion of lithotectonic assemblages in other Archaean terranes, such as the Superior Province of Canada.