Cordilleran Metamorphic Core Complexes Research Articles

Constrictional strain in a non-coaxial shear zone: implications for fold and rock fabric development, central mojave metamorphic core complex, california

Finite strain and fold analyses of footwall mylonites in the central Mojave metamorphic core complex (CMMCC) reveal two distinct deformation paths that generated non-coaxial constrictional strain. The first path, recorded in the Waterman Hills, accomplishes constrictional strain through the formation of L-tectonites at the grain scale. The second path, recorded in the Mitchel Range, involves a combination of plane strain at the grain scale and Y-axis shortening through syn-mylonitic folding. The ductile shear zone in the Waterman Hills is approximately 500 m thick and is developed entirely within a Miocene syn-kinematic granodiorite. Strain magnitude decreases structurally downward. The mylonites contain abundant NE-directed non-coaxial microstructures yet are L-tectonites. R f/ø analysis of quartz-ribbon grain shapes indicate prolate strains ( K=6) and bulk chemical data suggest that mylonitization was isochemical or involved volume loss. Therefore, ductile shearing in the Waterman Hills involved true constrictional strain at the grain scale. Immediately to the southeast in the Mitchel Range, the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre-Tertiary basement. The mylonitic fabric comprises a well-developed foliation and NE-trending stretching lineation. R f/ø strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite. In contrast to the Waterman Hills, finite strains in these L- S-tectonites approximate plane strain ( K = 1.1). However, the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation. The coaxial folds range from open to isoclinal. We interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished Y-axis shortening in the shear zone. Macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone, suggesting that Y-axis shortening continued after the fault zone had reached the brittle regime. Corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in Cordilleran metamorphic core complexes. Therefore, constrictional strain may be common in core complexes. Strain compatibility with upper plate rocks in core complexes may be maintained by Y-axis shortening along conjugate strike slip faults. A nearly uniaxial stress field ( σ 3 < σ 2 ≈ σ 1) is consistent with most of the structures formed in Basin and Range extension and could generate the observed constrictional strain.

Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia

The recognition of large areas of gneiss within granitic complexes of the northeastern part of the Eastern Goldfields Province of Western Australia, based on analysis of regional aeromagnetic data, has resulted in a re-evaluation of the structural data for that region. Five magnetic domains have been established in the granitic complexes, which are marginal to the main greenstone belts. At least two of these domains have a distinctive domal form, and also have internal domeshaped regions. The Ballard and Laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes. Boundary relationships at the Laverton and Ballard domains show that the domes are concordant sheets, and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level. Strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower-grade greenstones was initially along relatively gently-dipping shear zones which were active during uplift. Structural relationships around both the domal granitic complexes and the later granite batholiths, however, show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks. The tectonic hypothesis presented for the Eastern Goldfields is similar to that proposed for the formation of Cordilleran metamorphic core complexes. The uplift event responsible for the crustal extension required to allow the emplacement of the gneiss, may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts.

Western Old Woman Mountains shear zone: Evidence for late ductile extension in the Cordilleran orogenic belt

Research Article| September 01, 1991 Western Old Woman Mountains shear zone: Evidence for late ductile extension in the Cordilleran orogenic belt Brian S. Carl; Brian S. Carl 1Vanderbilt University, Nashville, Tennessee 37235 Search for other works by this author on: GSW Google Scholar Calvin F. Miller Calvin F. Miller 1Vanderbilt University, Nashville, Tennessee 37235 Search for other works by this author on: GSW Google Scholar Author and Article Information Brian S. Carl 1Vanderbilt University, Nashville, Tennessee 37235 Calvin F. Miller 1Vanderbilt University, Nashville, Tennessee 37235 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (9): 893–896. https://doi.org/10.1130/0091-7613(1991)019<0893:WOWMSZ>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 Brian S. Carl, Calvin F. Miller; Western Old Woman Mountains shear zone: Evidence for late ductile extension in the Cordilleran orogenic belt. Geology 1991;; 19 (9): 893–896. doi: https://doi.org/10.1130/0091-7613(1991)019<0893:WOWMSZ>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 Rocks within the 1-km-thick Western Old Woman Mountains shear zone (WSZ) contain ductilely deformed quartz and ductilely and brittlely deformed feldspars, indicating greenschist to lower amphibolite facies mylonitization. Foliation measured in 73 Ma mid-crustal granitoids within the zone generally dips west-southwest, and sense-of-shear indicators demonstrate top-to-the-west sense of movement parallel to gently southwest plunging lineation. The top of the shear zone is not exposed, but 10 km to the west unmetamorphosed Late Cretaceous-age upper crust is present. The WSZ is thus most simply interpreted as a normal-sense low-angle shear zone. Timing of deformation is constrained to the interval 73 to ca. 65 Ma by the age of deformed granitoids and 40Ar/39Ar chronology. This interval coincides with a period of rapid cooling of rocks now exposed in the Old Woman Mountains that probably resulted from 4 to 8 km of unroofing. We suggest that movement along the WSZ is responsible for at least some of this unroofing The proposed history involves tectonic denudation along a low-angle ductile shear zone and is similar to that demonstrated for Tertiary Cordilleran metamorphic core complexes. 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.

Volume loss, fluid flow and state of strain in extensional mylonites from the central Mojave Desert, California

Compositional changes in mylonitic rocks from the central Mojave metamorphic core complex, California, indicate that large volume loss (20–70%) attended formation of mylonite along the Waterman Hills detachment fault. Strong silica depletion and significant mobility of normally immobile elements imply large fluid/rock ratios during mylonitization, possibly as a result of multiple-pass hydrothermal convection connecting near-surface and mid-crustal alteration regimes. Dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica, and therefore was probably not magmatic or connate. Large volume loss in a shear zone should result in apparent flattening strains, yet mylonites throughout the complex, including the mylonites that record large volume loss, exhibit constrictional strains. This inconsistency raises questions about the applicability of the shear-zone model to Cordilleran metamorphic core complexes.

Crustal structure of the Valhalla complex, British Columbia, from Lithoprobe seismic-reflection and potential-field data

The Valhalla complex, situated in the Omineca crystalline belt in southeastern British Columbia, is a Cordilleran metamorphic core complex bordering the suture zone between Quesnellia and North American rocks. The region is tectonically interposed between a convergent plate margin along Canada's west coast and the stable North American craton, and is characterized by a crustal thickness of ~ 35 km, high surface heat flux, and elevated lower crustal electrical conductivity. In this study, Lithoprobe deep-crustal seismic-reflection data, potential-field data, and geological constraints have been used to gain a better understanding of crustal structure in the vicinity of the Valhalla complex. Analysis of Bouguer gravity and total-field aeromagnetic data indicates that mafic oceanic rocks and various syn- and post-accretionary granitoid plutonic rocks are not major constituents of the upper crust underlying the complex. The seismic data reveal a moderately reflective upper crust and image several fault zones, including a very high amplitude, west-dipping reflection that is interpreted as a significant Late Cretaceous or Paleocene thrust fault. The fault-zone reflectivity may be related to compositional heterogeneity and (or) seismic anisotropy associated with mylonites. The lower crust appears to be nonreflective, in contrast with other areas of high surface heat flux and elevated lower crustal conductivity. Taken together, the various data show that the Valhalla complex is likely cored by North American metasedimentary rocks and reveal features related to the Jurassic to Paleocene compressional fabric, which has been largely overprinted at the surface by subsequent Eocene extension.

Folding of mylonitic zones in Cordilleran metamorphic core complexes: Evidence from near the mylonitic front

Metamorphic core complexes of North America are asymmetric when viewed perpendicular to the direction of tectonic transport. On one side, mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper-plate transport, and the mylonitic rocks have a down-dip sense of shear. On the opposite side, the mylonitic zone rolls over to a dip opposed to the direction of upper-plate transport, diverges downward from the detachment fault, and dies out upward across a mylonitic front. New kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone. Folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization.

Black Mountains crustal section, Death Valley extended terrain, California

Recent reconstructions of Cenozoic extension in the southern Great Basin juxtapose the Panamint Range above the Black Mountains and indicate major tectonic denudation of the Black Mountains during Tertiary extension. Structural and geologic characteristics of the Black Mountains and consideration of the regional geology of surrounding ranges suggest that the Black Mountains block may represent a 10 to 30 km preextensional crustal section that has been uplifted along a major detachment zone. Geobarometry and ductile deformation features in a synrift, intermediate to mafic batholith and metamorphism of Eocambrian sedimentary rock on the western side of the range appear to support this hypothesis. These data suggest that the Black Mountains block is one of the youngest and perhaps among the deepest exposed examples of Cordilleran metamorphic core complexes.

Detachment zones of Cordilleran metamorphic core complexes: thermal, fluid and metasomatic regimes

Metamorphic core complexes in Arizona are characterized by extensional tectonics, in which listrically faulted and unmetamorphosed volcanic rocks of an upper plate are tectonically superimposed on high-grade rocks of a lower plate along low angle detachment faults. The detachment faults are marked by microbreccias and chloritic breccias which developed by hydraulic fracturing and metasomatism that overprinted subjacent amphibolite facies mylonites. Oxygen and hydrogen isotope data indicate that the mylonites formed at ∼500 °C under conditions of low water/rock ratio. Chloritic breccias developed at 300–350 °C during incursion of hydrothermal fluids along the detachment fault. Hydrothermal alteration involved massive additions of Fe, Mn, and Mg, with concomitant depletions of K and Na, reflecting the hydrolysis of K-feldspar and plagioclase to chlorite.

Mid-crustal Cretaceous roots of Cordilleran metamorphic core complexes

Research Article| April 01, 1988 Mid-crustal Cretaceous roots of Cordilleran metamorphic core complexes J. Lawford Anderson; J. Lawford Anderson 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 Search for other works by this author on: GSW Google Scholar Andrew P. Barth; Andrew P. Barth 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 Search for other works by this author on: GSW Google Scholar Edward D. Young Edward D. Young 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 Search for other works by this author on: GSW Google Scholar Author and Article Information J. Lawford Anderson 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 Andrew P. Barth 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 Edward D. Young 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (4): 366–369. https://doi.org/10.1130/0091-7613(1988)016<0366:MCCROC>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 J. Lawford Anderson, Andrew P. Barth, Edward D. Young; Mid-crustal Cretaceous roots of Cordilleran metamorphic core complexes. Geology 1988;; 16 (4): 366–369. doi: https://doi.org/10.1130/0091-7613(1988)016<0366:MCCROC>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 Thermobarometry for Cretaceous to mid-Tertiary plutonism and deformation in the lower plate of Whipple and Santa Catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust. Moreover, they are characterized by a striking acceleration of tectonic decompression coincident with middle Tertiary, low-angle detachment faulting leading to erosional and tectonic unroofing by mid-Miocene time.Depth estimates for emplacement of five intrusive suites within the Whipple complex include (1) 33 ±4 km for the peraluminous, 89 Ma Whipple Wash plutonic suite; (2) 29 ±1 km for the 73 Ma Axtel quartz diorite; (3) 16 ±5 km for mylonitization and synkinematic plutonism at 26 Ma; (4) 6.2 ±1.9 km for the postkinematic, 19 Ma War Eagle gabbro-quartz diorite complex; and (5) 5.2 ±2.3 km for a 17 Ma postkinematic granodiorite. Decompression initially occurred at a low rate of 0.3 mm/yr from 89 to 26 Ma and increased to approximately 2 mm/yr during the late Oligocene to middle Miocene. Estimated depths for four pluton emplacement or deformational events in the Santa Catalina Mountains include (1) 21 ±1 km for the magmatic epidote-bearing, 68 Ma Leatherwood quartz diorite; (2) 15 ±3 km for the garnet, two-mica, 47 Ma Wilderness granite; (3) 9.3 ±1.9 km for post-Wilderness mylonitzation; and (4) 6.3 ±2.6 km for the 27 Ma Catalina monzogranite. Post-Laramide decompression, estimated at 0.3 mm/yr, accelerated to 1.3 mm/yr prior to the cessation of detachment faulting.Whereas most batholithic terranes of the North American Cordillera are representative of an upper crustal setting, core complexes provide, as a consequence of their tectonic evolution, a petrological and structural view into middle crustal processes. 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.

Geochemistry of Late Cretaceous granitoids from northeastern, Washington: Implication for genesis of two-mica, Cordilleran granites

Mesozoic two-mica granites and I-type granodiorites from northeastern Washington have initial /sup 87/Sr//sup 86/Sr ratios around 0.7100, similar to many other Cordilleran granitoids. Metapelite and calcsilicate country rocks, equivalent to the Belt Supergroup, have measured /sup 87/Sr//sup 86/Sr ratios in the range of 0.91 to 0.98. Unlike many of their Cordilleran counterparts, the northeastern Washington two-mica granites are enriched in U, Rb, and other incompatible elements, and therefore cannot have been derived from depleted lower crust sedimentary sources. Both the two-mica granites and the I-type granodiorites have similar rare-earth-element and trace-element enrichment patterns. Trace-elements modeling shows that it is feasible to derive the two-mica granites by 60% partial melting of a granodiorite-like source. The close association of Cordilleran metamorphic core complexes and older I-type granodiorites with two-mica granites supports this model.

Extensional shearing deformation and kilometer‐scale “a”‐ type folds in a Cordilleran Metamorphic Core Complex (Raft River Mountains, northwestern Utah)

Cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations. In the Raft River Mountains a mylonitic zone and a late superimposed detachment fault separate Precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous Paleozoic rocks which suffered a brittle extensive deformation. We suggest that the mylonitic ductile deformation is the result of Tertiary extensional tectonics. Strain analysis shows that (1) strain results from large‐scale progressive shearing deformation in a non‐coaxial regime, (2) E–W regional stretching lineations represent the direction of shearing and transport, (3) shear criteria indicate an east directed movement, (4) strain magnitude is increasing eastward, (5) kilometer‐scale folds parallel to the lineations are interpreted as “a”‐type folds formed by eastward shearing of a previously northward dipping layering. Measurement of striated faults in the upper plate gives an E–W direction of extension parallel to the lineations. Thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics. A model is proposed to explain the observed geometry and kinematics of the structures. The Mesozoic to Paleocene Cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes. Important uplift, abnormal heating, and granite emplacement occurred later during the collapse of the former belt. The combined effects of thermal and gravitational instabilities during Tertiary extensional tectonics enabled the development of a ductile shear zone localized at the Precambrian basement‐Paleozoic cover interface. The brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust.

Eocene structural development of the Valhalla Complex, southeastern British Columbia

The Valhalla complex, a Cordilleran metamorphic core complex, is a 100 km by 30 km structural culmination within the Omineca belt of southeastern British Columbia. It comprises sheets of granitic orthogneiss ranging in age from 100 to 59 Ma with intervening paragneiss of uncertain age and stratigraphic correlation. The complex is roofed by the ductile Valkyr shear zone and the ductile/brittle Slocan Lake fault zone; the upper plate comprises lower grade metasedimentary rocks intruded by middle Jurassic plutons. The Valkyr shear zone and the Slocan Lake fault zone deform 62 and 59 Ma granitic sheets in their footwalls. The easterly directed Valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern, western, and southern margins. The shear zone was active between 59 and 54 Ma under amphibolite facies conditions. The juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the Valkyr shear zone is a significant structure with large displacement. There is evidence to support an easterly rooting direction consistent with an extensional origin; its surface breakaway is suggested to be west of the Valhalla complex. The Slocan Lake fault zone on the eastern side of the complex is a gently (30°), easterly dipping ductile/brittle normal fault which roots to the east. It was active between 54 and approximately 45 Ma and truncates the Valkyr shear zone. Timing and structural relationships indicate that the Valkyr shear zone and the Slocan Lake fault zone are genetically related. Movement on the ductile Valkyr shear zone, arching of the complex, and displacement on the Slocan Lake fault zone occurred as a continuum in Early to Middle Eocene time. This paper documents the presence of significant Eocene ductile strain in the Valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized.

The Mesozoic‐Cenozoic tectonothermal evolution of the Ruby Mountains, East Humboldt Range, Nevada: A Cordilleran Metamorphic Core Complex

A combined K‐Ar and 40Ar/39Ar geochronological study of the Ruby Mountains‐East Humboldt Range, Nevada, a Cordilleran metamorphic core complex, has revealed a complex tectonothermal history. The three structural subdivisions of the core complex (migmatitic core, mylonitic zone, and cover) share magmatic and deformational histories in part, but also display important contrasts in structural style, metamorphic paragenesis, and thermal history. During the Late Jurassic (ca 160 Ma), miogeoclinal rocks were polyphase‐deformed, metamorphosed under amphibolite facies conditions, and pervasively intruded by peraluminous granitic magmas, thereby forming an igneous and metamorphic complex. Late Jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks. Late Cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved Cretaceous incremental‐release 40Ar/39Ar hornblende spectra and by U‐Pb monazite ages from pelitic schist exposed in the northern Ruby Mountains. Between ca 45 Ma and 20 Ma, the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low‐angle, normal‐sense, simple shear zones. As a result, rocks metamorphosed at depth in the Mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals. Near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation. However, it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 Ma metamorphism or if distinct, locally superposed late Mesozoic and/or middle Tertiary thermal overprints had affected the cooling history. A major extensional simple shear zone is marked by an approximately 1‐kilometer‐thick mylonite zone that experienced late kinematic, superposed brittle deformation. A geometric consequence of the development of low‐angle extensional fault zones is the eventual translation of colder rocks over hotter rocks. Such a history is supported by the 40Ar/39Ar data. For example, mylonitic mafic orthogneisses from a major allochthon record 40Ar/39Ar hornblende plateau ages between 44 and 48 Ma and biotite plateau ages between 32 and 33 Ma, and are structurally positioned above mylonitic rocks of the northern Ruby Mountains which record biotite cooling ages of ca 22–24 Ma. Such relations suggest either multiple episodes of Tertiary mylonitization or a protracted, multiphase history of mylonitization. However, after ca 20 Ma, all of the presently exposed mylonitic rocks were maintained below 100°C and have only experienced upper crustal brittle deformation. This article contains supplementary material.

Fluid transport in lineaments

Fluid transport in lineaments

Regional correlation of extension directions in Cordilleran metamorphic core complexes

Cordilleran metamorphic core complexes exhibit Tertiary extensional deformation, expressed either as detachment faults or detachment faults plus ductile mylonitic shear zones. Extension directions, as indicated by stretching lineations within mylonite and striations along detachment faults, fall into regional groups in which the directions are similar in trend throughout each group. Asymmetric fabrics on both small and large scale give a sense of shear and indicate that tectonic vergence within groups is directed outward from a central axis. The regional consistency of extension directions implies a regional control of extension in metamorphic core complexes.

Fluid infiltration into fault zones: Chemical, isotopic, and mechanical effects

Fluid infiltration into fault zones and their deeper-level counterparts, brittle-ductile shear zones, is examined in diverse tectonic environments. In the 2.7 Ga Abitibi greenstone belt, major tectonic discontinuities, with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism. During reverse motion on the structures, accommodating shortening of the belt, these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids. Such fluids were characterised by δ18O=+6±2, δD=−50±20, δ13C=−4±4, and temperatures of 270 to 450°C, probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism. Hydrothermal fluids were more radiogenic (87Sr/86Sr=0.7010 to 0.7040) and possessed higher μ than did contemporaneous mantle, komatiites or tholeiites, and thus carried a contribution from older sialic basement. A provinciality of87Sr/86Sr and δ13C is evident, signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres. Mineralised faults possess enrichments of large ion lithophile (LIL), LIL elements, including K, Rb, Ba, Cs, B, and CO2, and rare elements, such as Au, Ag, As, Sb, Se, Te, Bi, and W. Fluids were characterised by XCO 2≈0.1, neutral to slightly acidic pH, low salinity ≤3 wt-%, K/Na=0.1, they carried minor CH4, CO, and N2, and they underwent transient effervescence of CO2 during decompression. Clastic sediments occupy graben developed at fault flexures. The40Ar/39Ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years. At the Grenville front, translation was accommodated along two mylonite zones and an intervening boundary fault. The high-temperature (580°C) and low-temperature (430 to 490°C) mylonite zones, formed in the presence of deep-level crust-equilibrated fluids of metamorphic origin. Late brittle faults contain quartz veins precipitated from fluids with extemely negative δ18O (−14 per mil) at 200 to 300°C. The water may have been derived from downward penetration into fault zones of precipitation of low18O on a mountain range induced by continental collision, with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. Archean gneisses overlie Proterozoic sediments along thrust surfaces at Lagoa Real, Brazil; the gneisses are transected by brittle-ductile shear zones locally occupied by uranium deposits. Following deformation at 500 to 540°C, in the presence of metamorphic fluids and under conditions of low water-to-rock ratio, shear zones underwent local intense oxidation and desilication. All minerals undergo a shift of −10 per mil, indicating discharge of meteoric-water-recharged formation brines in the underlying Proterozoic sediments up through the Archean gneisses, during overthrusting; ≈1000 km3 of solutions passed through these structures. The shear zones and Proterozoic sediments are less radiogenic (87Sr/86Sr=0.720) than contemporaneous Archean gneisses (0.900), corroborating the transport of fluids and solutes through the structure from a large external reservoir. Major crustal detachment faults of Tertiary age in the Picacho Cordilleran metamorphic core complex of Arizona show an upward transition from undeformed granitic basement through mylonitic to brecciated and hydrothermally altered counterparts. The highest tectonic levels are allochthonous, oxidatively altered Miocene volcanics. This transition is accompanied by an increase of 12 per mil in δ18O, from +7 to +19, and a 400°C decrease in temperature. Lower tectonic levels acted as aquifers for the expulsion of large volumes of higher-temperature reduced metamorphic fluids and/or evolved formation brines. The Miocene allochthon was influenced by a lower-temperature reservoir inducing oxidative potassic alteration; mixing occurred between cool downward-penetrating thermal waters and the hot, deeper aqueous reservoir. In general, flow regimes in these fault and shear zones follow a sequence, from conditions of high temperature and pressure with locally derived fluids at low water-to-rock ratios, during initiation of the structures, to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution and at shallower crustal levels there was incursion of oxidising fluids from near-surface reservoirs into the faults. In general, magmatism, tectonics, and fluid motion are intimately related.

Cordilleran metamorphic core complexes in Arizona: A contribution from geomorphology

Cordilleran metamorphic core complexes consist of granitic cores surrounded by, in an outward direction, mylonite, chloritic breccia, and microbreccia. These rocks are separated by a low-angle detachment fault from typically unmetamorphosed but normally faulted rocks. Rocks above the detachment fault have been transported, relative to the core rocks, in one consistent direction, giving a distinct lineation to rocks below the detachment fault. Dome-shaped metamorphic core-complex mountains show considerable structural control of landforms. Mylonitic rocks develop triangular dome facets around the margins and low relief areas in the central parts of the complexes. Chloritic breccia is much less resistant to erosion and has been almost completely removed from many complexes. Drainage patterns in metamorphic core complexes are mainly radial, but an older set of drainage lines parallel to mylonitic lineation, especially channels that drain down the crests of antiformal ridges, suggests that drainage developed on a ramp on the detachment fault and the underlying breccia prior to lineation-parallel arching.

The tiger formation: Paleogene fluvial sediments deposited adjacent to a deforming Cordilleran metamorphic core complex, northeastern Washington

The Eocene to Oligocene(?) Tiger Formation and associated upper-plate rocks of northeastern Washington are juxtaposed along low-angle fault zones with structurally lower, or “lower-plate”, rocks of the Spokane dome, a metamorphic core complex. Strata of the Tiger Formation that were derived solely from upper-plate rock are termed intraplate strata and those that include clasts of lower-plate rock are termed extraplate strata. The Cliff Ridge and Ione regions are situated on opposite sides of an arch of typical lower-plate rock. The Tiger Formation here consists of predominantly conglomeratic intraplate strata that were deposited by braided streams on both valley fills and alluvial fans. These strata were derived from a tectonically active source terrane where lower-plate rock now exists. Intraplate strata of the Cusick region consist of conglomeratic, sandstone, and siltstone facies. Alluvial fans and a sandy braided-alluvial plain graded downstream to a floodplain-dominated environment. Intraplate strata exposed in the Cliff Ridge, Ione and Cusick regions record deposition in response to differential uplift of the core complex in the subsurface. Extraplate conglomeratic strata deposited in alluvial-fan and braided alluvial-plain environments conformably and unconformably overlie the intraplate sandstone and siltstone facies of the Cusick region. These extraplate conglomerates were derived from the area now occupied solely by the core complex and record the erosion of intraplate Tiger strata and other upper-plate rocks, exposure of the fault zone at the paleosurface, and erosion of the core complex. The Tiger Formation provides valuable insight to the spatial and temporal distribution of deformation that accompanies the development of a Cordilleran metamorphic core complex.

Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression

Research Article| September 01, 1984 Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression Peter J. Coney; Peter J. Coney 1Department of Geosciences, Laboratory of Geotectonics, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Tekla A. Harms Tekla A. Harms 1Department of Geosciences, Laboratory of Geotectonics, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information Peter J. Coney 1Department of Geosciences, Laboratory of Geotectonics, University of Arizona, Tucson, Arizona 85721 Tekla A. Harms 1Department of Geosciences, Laboratory of Geotectonics, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1984) 12 (9): 550–554. https://doi.org/10.1130/0091-7613(1984)12<550:CMCCCE>2.0.CO;2 Article history First Online: 01 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 Peter J. Coney, Tekla A. Harms; Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression. Geology 1984;; 12 (9): 550–554. doi: https://doi.org/10.1130/0091-7613(1984)12<550:CMCCCE>2.0.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 Cordilleran metamorphic core complexes form a belt of uplifted metamorphic rock that extends from southern Canada to northwestern Mexico just west of, or astride, the foreland thrust belt of the North American Cordillera. During the past several years the age and tectonic significance of the core complexes have been a topic of considerable controversy. Some geologists view the complexes as an uplifted erogenic core zone that formed behind the thrust belts mainly during Mesozoic regional compression. An opposing view is that they are mainly Tertiary in age and of extensional origin. We support a model that unifies these seemingly inconsistent views by suggesting that Mesozoic crustal telescoping resulted in an overthickened plateaulike crustal welt along the Cordilleran hinterland. During Cenozoic time this gravitationally unstable mass spread laterally, resulting in deep-seated crustal extension. The extension was aided by a thermal pulse of Cenozoic magmatism that reduced crustal viscosity and by a lowering of intraplate convergent stress fields due to changing plate kinematics. The chief advantage of the model is that it reconciles opposing views as to the age and tectonic significance of the complexes and places them in a more comprehensible setting amid Mesozoic-Cenozoic Cordilleran thermotectonic history. 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.

Geology and origin of Meatiq Dome, Egypt: A Precambrian metamorphic core complex?

Meatiq Dome, a metamorphic complex in the Precambrian basement of the Eastern Desert of Egypt, exhibits many of the essential features of Cordilleran metamorphic core complexes. It is an antiformal structure with low-dipping foliation and unidirectional mineral-slickenside lineation. The core consists of granite gneiss and is conformably overlain by a heterogeneous, isoclinally folded, mylonitic carapace. The carapace grades upward into a nonmylonitic cover of low-grade ophiolitic rocks. The mylonitic rocks are interpreted to have formed in a low-angle ductile shear zone during a compressional tectonic event accompanied by syntectonic intrusion of tonalite. Doming postdated mylonitization. The absence of a brittle detachment surface at Meatiq Dome is its primary distinction from the otherwise similar Cordilleran complexes.