Mylonitic Shear Zones Research Articles

Mid‐Cretaceous extensional tectonics of the Yukon‐Tanana Terrane, Trans‐Alaska Crustal Transect (TACT), east‐central Alaska

Mid‐Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon‐Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end‐member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid‐Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity spreading.

Microstructural and crystal fabric evolution during shear zone formation

The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Regional geological setting of the Limpopo Belt

The ∼ 2.7 Ga Limpopo Belt granulite terrane is situated between two lower-grade granite-greenstone cratons, the Kaapvaal Craton in the south and the Zimbabwe Craton in the north. This granulite-facies terrane is subdivided into three zones, each with a distinctive geological signature and separated from each other, and from the adjacent cratons, by well defined shear zones: (1) the Southern Marginal Zone, composed of granite-greenstone material very similar to that of the Kaapvaal Craton, has retrogressed granulites on its southern margin in the hanging wall of the northward-dipping terrane boundary (Hout River Shear Zone) separating the granulite terrane from the adjacent Kaapvaal Craton; (2) the Northern Marginal Zone, a granite-greenstone terrane at granulite grade, which is separated from the Zimbabwe Craton by a southward-dipping terrane boundary; and (3) the Central Zone, a unique crustal element, with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at ∼ 3250 Ma. This zone cannot be related to the Kaapvaal Craton or to the Zimbabwe Craton. It is separated from the two Marginal Zones by mylonitic shear zones with horizontal stretching lineations. The entire Limpopo Belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite-greenstone terranes of the Kaapvaal and Zimbabwe Cratons. This distinctive pattern developed during the Limpopo Orogeny, probably as a consequence of continental collision during the late Archaean.

Stress magnitude, strain rate, and rheology of extended Middle Continental Crust inferred from quartz grain sizes in the Whipple Mountains, California

Knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension. We analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the Whipple Mountains, California, using grain‐size piezometers and kinetic laws for grain growth. Mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 µm. Analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures ≤500°C, compatible with estimates from two‐feldspar thermometry. Quartz grain‐size piezometers suggest that the mylonitization occurred under differential stresses (σ1–σ3) of ∼40–150 MPa, or maximum shear stresses of 20–75 MPa. Extrapolation of quartzite flow laws to 500°C indicates that the mylonitization occurred at strain rates faster than 10−14 s−1. These estimates suggest that the mylonitic zone within the Whipple Mountains had an effective viscosity of the order of 1018±4−1020±4 Pa s. These low viscosities and rapid strain rates, combined with seismic reflection data showing that continental crust is layered, suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high‐viscosity regions separated by low‐viscosity zones.

Shear localisation in upper mantle peridotites

Upper mantle peridotite bodies at the earth's surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle. Deformation which occurred at high temperatures (T>950±50°C) is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres. Below 950±50°C strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites. The microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes. The most important strain softening processes are inferred to be structural and reaction induced softening. Structural softening processes include dynamic recrystallisation and strain-induced transitions from dislocation creep to some form of grain-size-sensitive (GSS) creep. Reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by GSS creep and the formation of weak sheet silicates such as phlogopite, chlorite, talc and antigorite. From experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910±160°C. This temperature range is inferred to be a significant rheological transition in the upper mantle. Below 910±160°C deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct, perhaps weak, rheology. At higher temperatures strain is accommodated in much wider deformation zones.

The geology and tectonic evolution of Waigeo Island, NE Indonesia

Waigeo occupies a critical position between the Halmahera-Philippine arcs to the northwest and Australia-New Guinea to the southeast. The island consists of a deformed ophiolitic basement of supra-subduction zone type overlain by probable Paleogene forearc sedimentary rocks. The forerarc basement is cut by mylonite shear zones and the basement and sedimentary cover rocks were deformed by southward-directed thrusts and associated folds during the Oligocene. The deformed sequences are intruded by basic dykes, thought to be coeval with basalts and andesites of Late Oligocene age. The entire Paleogene sequence is overlain unconformably by a thick (up to 2000 m) sequence of Miocene limestones which accumulated during a tectonically quiet period. A final phase of deformation occurred during the Pliocene which caused the development of two very large anticlines and an intervening syncline, associated with left-lateral wrench faults. Waigeo is interpreted to have been situated in a forearc position in an intra-oceanic island arc during the early Paleogene, forming part of the East Halmahera-Waigeo forearc terrane. The Waigeo arc terrane collided with a continental block in about the Middle Oligocene, contemporaneous with similar arc-continent collision in northern New Guinea. A period of tectonic quiescence during the Miocene was followed by Pliocene deformation in Waigeo related to a left-lateral wrench faulting on splays of the Sorong Fault in northern New Guinea. The Pliocene deformation is interpreted as resulting from compression on a right-stepping restraining bend in this wrench fault system.

Cooling and inferred uplift/erosion history of the Grenville Orogen, Ontario: Constraints from 40Ar/39Ar thermochronology

Stepwise 40Ar/39Ar degassing experiments of 57 mineral separates of hornblende, muscovite, biotite, and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the Grenville Orogen in Ontario. The 40Ar/39Ar data support an interpretation of slow, nearly uniform cooling (1°–4°C/m.y.) from temperatures of ˜500°C to below ˜150°C. In the Central Gneiss Belt (CGB) hornblendes cooled through Ar closure between 930 and 1025 Ma, whereas in the Central Metasedimentary Belt (CMB) hornblendes record the following range in 40Ar/39Ar cooling ages: 1104 Ma in the Frontenac terrane, 1007–1067 Ma in the Sharbot Lake terrane, 919–1026 Ma in the Elzevir terrane, and 972 Ma in the Central Metasedimentary Belt Boundary Zone. Regional uplift/erosion rates of 0.03–0.14 km/m.y. have been estimated for the Grenville Orogen in Ontario based on the 40Ar/39Ar data, a model retrograde P‐T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambrian and Ordovician sediments. These erosion rates are consistent with rates estimated for other Proterozoic or Archean granulite terranes but are an order of magnitude slower than active orogens such as the Alps and Himalayas. A regular variation in hornblende 40Ar/39Ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the CMB. The pattern of 40Ar/39Ar ages is interpreted to reflect late‐tectonic extension, consistent with field observations in the Central Metasedimentary Belt Boundary Zone and elsewhere in the CMB. Up to 13 km of vertical displacement is inferred for some rocks in the CMB between the time they cooled below closure to argon diffusion in hornblende (˜500°C) and their exposure at the surface (˜25°C).

Stretching of the deep crust at the slow-spreading Southwest Indian Ridge

The 500 metres of gabbro drilled in the Southwest Indian Ocean during ODP Leg 118 allow direct observation of early, high-temperature extensional deformation in the deep crust of a slow-spreading rift valley. About 30% of the recovered gabbros have been plastically stretched at progressively decreasing temperatures (granulite- to lower amphibolite-facies metamorphic conditions), increasing water/rock ratio, and probably increasing deviatoric stresses. The early solid-state strain in granulite facies metamorphic conditions produced low-dipping foliations and low strain gradients. At this stage, mylonitic shear zones only formed in oxide-rich gabbros. Strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy. In upper to lower amphibolite-facies metamorphic conditions, the ductile strength of the gabbro was probably higher, and deformation concentrated in mylonitic shear bands, which may have provided access to seawater-derived fluids. Strain localization at this stage was accompanied by brittle failure and boudinage, in the mylonitic foliation, of individual porphyroclasts, or of thick gabbro intervals. The influence of such brittle failure on hydrothermal circulations was critical: it allowed for extensive crust/seawater interactions at high temperatures (upper amphibolite facies conditions). In a spreading context where continuous magma supply provides heat and new crustal material to the axial region, the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite-facies conditions. This may lead to the steady-state “necking” configuration described by Tapponnier and Francheteau (1978). Alternatively, a discontinuity of the magma supply, allowing the lower crust of the axial region to cool below granulite-facies temperatures, should cause mylonitic shear zones characteristic of amphibolite-facies deformation to extend deeper into the axial domain, eventually leading to the detachment fault configuration proposed by Karson (in press). As magma supply to slow-spreading oceanic ridges is likely to be discontinuous, necking and faulting periods should alternate through time.

The tectonic and rheological evolution of an attenuated cross section of the continental crust: Ivrea crustal section, southern Alps, northwestern Italy and southern Switzerland

The tectonic and rheological evolution of the southern Alpine continental crust is reconstructed from structural, petrological, and radiometric studies in the Ivrea and Strona-Ceneri basement units. The deep crust of the southern Alps acquired its present compositional and metamorphic zonation during Paleozoic magmatism and amphibolite-to granulite-facies regional metamorphism. Inferred strength contrasts between lower crustal and upper mantle rocks in the Ivrea zone are low at the high temperatures of regional metamorphism. Late Paleozoic transtension and basic to intermediate magmatism in all crustal levels preceded extensional faulting associated with the formation of a passive continental margin during early Mesozoic time. Extensional uplift and cooling of the basement section is coupled with crustal-scale trends of increasing rheological stratification, grain size reduction, and strain localization. Mylonitic shear zones in the lower crust and a broad zone of noncoaxial shear at the base of the intermediate crust in the Ivrea zone are inferred to have controlled the high-strain rheology of the attenuating southern Alpine crust. The deep crust in the thinned Ivrea crustal cross section transferred noncoaxial strain from the upper mantle to the intermediate and upper crust. Final uplift and exposure of the deep southern Alpine crust occurred during Late Cretaceous to Tertiary Alpine thrusting.

Geochronology of a rhyolite dyke from Nassarawa-Egon in Central Nigeria

A 50-m thick rhyolite dyke which cuts the Pan-African crystalline basement complex at Nassarawa-Egon, in central Nigeria, yielded a 10-point Rb-Sr whole-rock isochron age of 171±3 Ma and an initial87Sr/86Sr ratio of 0.71137±0.00020. The petrography and major oxide geochemistry of the rock indicate that it has a calc-alkalic affinity. The dyke occupies the axis of an ancient mylonitic shear zone situated at the edge of the Benue Rift, and both strike at a high angle with respect to the long axis of the rift. We therefore consider that the emplacement of the dyke is related to the doming which predated the formation of the Benue Rift (Olade, 1975). The calculated age of the dyke is much older than those of the Jurassic Younger Granites in the closest neighbourhood (e.g. Mada complex; 147 Ma). Furthermore, the Jurassic Granites and their associated volcanics and subvolcanics are all alkalic. This work provides further evidence that acid, pre-rift volcanism began by mid-Jurassic, although the oldest exposed sedimentary fill is mid-Cretaceous in age.

U-Pb geochronologic constraints on the age of thrusting, crustal extension, and peraluminous plutonism in the Little Rincon Mountains, southern Arizona

The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age 66{plus minus}10 Ma for zircon and a concordant age 51{plus minus}2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age 24{plus minus}12 Ma for zircon and an age of 30{plus minus}6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension.

Temperature and pressure of mylonitization in a Tertiary extensional shear zone, Ruby Mountains-East Humboldt Range, Nevada: Tectonic implications

In the northern Ruby Mountains-East Humboldt Range metamorphic core complex, a kinematically coupled mylonitic shear zone and brittle-plastic detachment fault compose a low-angle extensional fault system that is mid-Tertiary in age. Geothermobarometry of pelitic schists from the mylonitic shear zone yields a temperature and pressure estimate of 820-900 K (580-620 °C) and 310-370 MPa (3.1-3.7 kbar) for mylonite petrogenesis. The shear-zone-detachment-fault system excised 11-12 km of crustal section and accommodated at least 15-17 km of normal-sense dip slip.

Opposite thrust systems in northern Victoria Land, Antarctica: Imprints of Gondwana's Paleozoic accretion

Two major thrust systems with contrasting senses of displacement transect the Wilson terrane crust of northern Victoria Land, Antarctica. Along both mylonitic shear zones the central high-grade metamorphic basement is detached and thrust divergently toward the west and east over synorogenic, lower grade fore-arc and back-arc basin sedimentary rocks, respectively. Deformation was preceded by pervasive high-temperature-low-pressure metamorphism. Granites intruded the basement prekinematically and postkinematically. The structures are interpreted as results of early Paleozoic subduction of the paleo-Pacific oceanic crust under the Antarctic craton.

Geothermobarometry in albite-garnet orthogneisses: A case study from the Gran Paradiso nappe (Western Alps)

The aim of this paper is to estimate syntectonic P-T conditions within albite- and garnet-bearing orthogneisses. These rocks are generally characterized by the assemblage quartz + albite + biotite + phengite + CaFe-garnet + epidote + titanite. Garnet contains up to 55 mole per cent of grossular. K-feldspar is a relict magmatic phase.P-T conditions are estimated using several independent methods. First, it is shown that exchange reactions based on the Fe-Mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks, due to the high grossular content of garnet. Second, maximum and minimum pressures are constrained, respectively, by the occurrence of albite instead of jadeite + quartz and by the assemblage phengite + biotite + quartz. Third, phase equilibria in albite- and garnet-bearing metagranites are modelled in the system K2O-CaO-FeO-Al2O3-SiO2-H2O. Equilibrium curves are calculated for the observed phase compositions. Uncertainties in P-T estimates mainly result from the choice of appropriate non-ideal solution models for the garnet.An application is developed for granites from the Gran Paradiso nappe (Western Alps). These granites show an heterogeneous deformation of Alpine age expressed by mylonitic shear zones cutting across weakly deformed domains. Estimated P-T conditions for the synkinematic assemblages are 10–16 kbar at 550±50°C.

Structural analysis of a ductile-brittle Precambrian shear zone in the Sierra Madre, Wyoming: western extension of the Cheyenne belt?

Abstract Existing plate tectonic models for the Cheyenne belt, a Proterozoic suture at the southern margin of the Archean Wyoming craton, are based largely on data from the Medicine Bow Mountains, southern Wyoming. This paper tests the hypothesis that a major structural and geochronological discontinuity in the Sierra Madre of southern Wyoming is the western extension of the Cheyenne belt. The Sierra Madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones. The eastern segment records early ductile thrusting (ca. 1750 Ma) and later dextral strike-slip faulting as revealed by macrostructural, microstructural, and quartz fabric analysis. Structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the Medicine Bow Mountains. The correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting 〈 c 〉 slip in synkinematic granites in both ranges. Isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive. The western segment of the Sierra Madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone. Kinematic analysis within the cataclastic zone suggests that it formed during north-directed thrusting. East-striking, south-dipping thrust faults and associated overturned-to-the-north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone. This spatial association of north-vergent structures and their apparent kinematic compatibility suggests that they are genetically related. Two major northwest-striking, cataclastic, dextral strike-slip in the eastern Sierra Madre appear to merge with the cataclastic thrust fault. These cataclastic faults are interpreted as parts of a north-vergent, thrust-tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north. It is not known whether this deformational event occurred in response to continued, post-suture convergence to the south or if it represents a separate and distinct Precambrian event. The prominent bend in the Sierra Madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system.

Differential stress, strain rate, and temperatures of mylonitization in the Ruby Mountains, Nevada: Implications for the rate and duration of uplift

Knowledge of the magnitude of the differential stress during the formation of mylonitic rocks provides constraints on mechanical and thermal models for the exhumation of the metamorphosed foot walls of major low‐angle detachment faults. We have analyzed the differential flow stress during the mylonitization of quartzose rocks in the Ruby Mountains, Nevada, using grain‐size piezometers and kinetic laws for grain growth. Quartzites from mylonitic shear zones in Lamoille Canyon and Secret Creek gorge have grain sizes of 91–151 μm and 42–64 μm, respectively. The peak temperature during mylonitization was 630°±50°C, and analysis of grain‐growth kinetics indicates that mylonitization continued during cooling to temperatures ≤450°C. Quartz grain‐size piezometers suggest that the mylonitization occurred under differential stresses (σ1‐σ3) of 38–64 MPa, or maximum shear stresses of 19–32 MPa. Extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10−10 and 10−13 s−1; arguments presented in the paper suggest that the likely range of strain rates is 10−11 to 10−12 s−1. These strain rates are compatible with displacement rates of the order of 23 mm yr−1 along a 1.5‐km‐thick simple shear zone. Such a shear zone dipping 15° would produce an uplift rate of 5.8 km m.y.−1 and a horizontal extension rate of 22 km m.y.−1. This uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1.5‐km‐thick simple shear zone dipping 15° in 2.6 m.y.

Nature and origin of seismic reflection fabric, Ruby‐East Humboldt Metamorphic Core Complex, Nevada

Seismic reflection profiling across exposed upper and middle crustal rocks of the Ruby‐East Humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during Tertiary extensional deformation. The goals of this study were to trace a Tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex. Reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks. Constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections. East and southwest dipping reflectors in the seismic section appear to correlate with exposed, opposing dipping mylonitic foliation domains. The opposing dips of the mylonitic layering may reflect warping of the normal‐sense shear zone during tectonic exhumation coupled with an overall anastomosing character. Deeper in the section, a heterogeneous reflection character correlates with increases in velocity shown from wide‐angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad‐scale pure shear and localized simple shear. Lower crustal rocks apparently achieved granulite facies metamorphism during extension. A maximum of 6–8 km of mafic material could have been added to the crust during Cenozoic extension. The Moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite. Reflections from the Moho show no significant upwarping of the base of the crust beneath the core complex. This suggests that the lower‐crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 Ma which was responsible for the exhumation of the metamorphic core complex.

Nd isotopes and the origin of 1.9-1.7 Ga Penokean continental crust of the Lake Superior region

Nd isotopic data on 26 samples demonstrate the origin of the Early Proterozoic crust of the Penokean orogen. The three major components are (1) the Marquette Range Supergroup, a predominantly miogeoclinal continental-margin sequence deposited before 1.85 Ga on Archean basement; (2) ca. 1.88 Ga felsic metavolcanic rocks of the Wisconsin magmatic terrane to the south, of evolved island-arc affinity; and (3) 1.87-1.76 Ga granitoids that intrude the metavolcanic rocks. Initial ϵ Nd values for the metavolcanic rocks of the magmatic terrane range between 0.0 and +2.4 and T DM ages between 1.9 and 2.2 b.y. The volcanic rocks primarily represent new crustal material that had only a limited Archean input, probably through mixing of subducted sediments into the magma source area. ϵ Nd (T) values for sedimentary rocks in the lower part of the Marquette Range Supergroup indicate an Archean source, most likely the 2.7 Ga Superior province to the north. On the other hand, the sedimentary rocks in the upper part of the Marquette Range Supergroup (graywackes of the upper Michigamme Formation) have initial ϵ Nd values between -0.8 and +1.5, indicative of an Early Proterozoic source. These graywackes probably are foredeep deposits derived from the volcanic rocks of the Wisconsin magmatic terrane to the south. Thus, the time of deposition of the upper Michigamme Formation dates the final convergence of the Wisconsin magmatic terrane with the continental margin. The granitoids of the Wisconsin magmatic terrane have a wide range of ϵ Nd (T) values, from -4.5 to +4.0. They represent mixtures of variable amounts of new crustal material and recycled Archean detritus. The more negative ϵ Nd (T) values occur in samples close to the northern boundary of the Wisconsin magmatic terrane, which is marked by a mylonitic shear zone (the Niagara fault zone). We suggest that the miogeoclinal lower Marquette Range Supergroup rocks having an Archean Nd signature became involved in magma genesis close to the collisional boundary, whereas increasingly lesser amounts of this older material were available for mixing farther away from the suture. The 1.9-1.7 Ga Penokean events involved major growth of new crust from the mantle.

Teleseismic travel-time anomalies and deep crustral structure in central Australia

SUMMARY Teleseismic travel times have been recorded along two lines of instruments traversing the Amadeus Basin and southern Arunta Block of central Australia for the purpose of examining the deep crustal and upper-mantle structure underlying these features. Pronounced differential travel times, approaching 1.5 s, and with major azimuthal variation, have been recorded. Most of the useful observations originated from events to the north (Japan-Marianas) and to the east (Fiji-Tonga) at epicentral distances of 40-65d. A few observations of southerly azimuth events (Macquarie Ridge and South Sandwich Islands) were also recorded. The inferred structure beneath the basin and block exhibits major lateral variations, the essential aspects of which are: (1) a zone of relatively high-velocity material dipping steeply northwards such that the continuation of its southern edge to the surface coincides approximately with a mylonitic shear zone known as the Redbank Deformed Zone; (2) a steeply dipping zone of relatively low-velocity material immediately to the south of this shear zone and partly underlying the high-velocity zone. If the travel times are interpreted in terms of an undulating Moho, variations in Moho depth of 20 km occur over horizontal distances of less than 50 km with relatively shallow Moho depths occurring immediately to the north of the Redbank Deformed Zone. Such an interpretation is consistent with the gravity anomalies observed across the geological structures. The inferred structure is out of hydrostatic equilibrium and departures from local isostasy approach 150 MPa. These stress-differences must have persisted during the 300 Ma following the Alice Springs Orogeny that led to the present crustal structure. Evidence, in the form of the surface topography and seismicity, suggests that some stress relaxation has occurred and still is occurring but not at the shear zone itself. At the time of the orogeny the structure may have been close to isostatic equilibrium, but as erosion took place complete local rebound did not occur, either because horizontal compressive forces still operated or because the strength of the lithosphere increased with time.

Deformational and strain patterns of an intracontinental collision ductile shear zone—an example from the Higher Garhwal Himalaya

The Higher Himalayan metamorphic belt of Garhwal appears to have evolved in a major 15–20 km wide, NE-dipping ductile shear zone of the overthrust-type due to intracontinental crustal shortening during Cenozoic continental collision within the Indian Plate. Out of the four major distinct deformation phases, the most widespread D 2 deformation phase is marked by a prominent S 2 foliation axial-planar to the reclined and recumbent F 2 folds, a coaxial NE-plunging L 2 stretching lineation and syntectonic growth of index metamorphic minerals. These deformational structures are developed during the SW-directed D 2 ductile shear deformation, irrespective of the orientation of the S 2 foliation and thrust zones. Regional strain patterns in quartzite of the Lesser Himalayan Garhwal Group beneath the Main Central Thrust (MCT) and the Central Crystalline Zone using R f/ φ data on quartz clasts and aggregates reveal that the maximum values of finite strain ε s are attained within narrow mylonitic shear zones within the broad ductile shear zone. During its progressive translation towards higher structural levels in late D 2 and D 3 deformation phases, the shortening was accommodated by development of thrust sheets with the MCT and Jutogh Thrust surfaces coinciding with zones of mylonite and maximum strain. At higher levels, ductile shear zones became compressional brittle-ductile zones which resulted in the development of SW-verging F 3a and F 3b folds during the southward migration of the thrust sheets in the Lesser Himalaya.