Deeper Structural Levels Research Articles

Thrust tectonics in the Picentini Mountains, Southern Apennines, Italy

Detailed geological mapping carried out in the Picentini Mountains, Southern Apennines, Italy, allowed to reconstruct the geometry of the fold and thrust belt in this region. Contractional structures were formed during multiple episodes of ENE to NNE shortening and were cut during later extension by low- and high-angle normal faults. Based on timing of emplacement and geometrical relationships between thrust units, we worked out a kinematic model of thrust tectonics. Basinal (Sicilid and Lagonegro) units were thrust eastward onto a carbonate-platform-basin system (CPBS) starting in the Serravallian-Tortonian, and were in turn overridden by the CPBS units by means of deeper decollement thrusts. Later contraction, starting from late Tortonian-Messinian times, built up a 15-km-thick antiformal stack during SSW-NNE shortening. We applied the forward kinematic model of thrust imbrication to perform a qualitative palinspastic restoration of a regional cross-section through this area, based on published interpreted seismics and other subsurface data. The thrust tectonics of the Picentini Mountains and more northward regions was controlled in the internal sectors mostly by envelopment thrusting of carbonate platform thrust sheets, which formerly were the floor complex of the Tortonian thrust belt, while multiple progressive decollement of the basinal roof complex occurred in the external part of the belt. Shallow crustal extension on low-angle faults with transport direction oblique to orthogonal to contractional transport was responsible for contemporaneous thinning during the accretion of the antiformal stack at deeper structural levels.

The Malaguide-Alpujarride boundary: a major extensional contact in the Internal Zone of the eastern Betic Cordillera, SE Spain

The Internal Zone of the Betic Cordillera in southern Spain was formed by thrusting and horizontal ductile shortening in the Late Cretaceous (?)-Palaeogene, but was subjected to a major phase of vertical thinning during the Neogene. The contact between the Malaguide (the structurally highest thrust complex) and the underlying Alpujarride Complex, is an example of several major tectonic contacts across which the stratigraphy has been repeated by thrusting, but the metamorphic grade increases abruptly downwards. Detailed work in the eastern Betic Cordillera documents the relationships across this contact. In the Sierra Espuña, petrographic and illite crystallinity studies show that the highest imbricate thrusts of Malaguide rocks have only been diagenetically altered, whereas the deeper parts of the thrust stack exhibit anchizone facies metamorphism. In the northern Sierra de las Estancias, apatite fission track studies confirm that there, too, Malaguide rocks have never been above 200 °C. In both areas, the underlying Alpujarride rocks exhibit epizone or greenschist facies metamorphism and a major fault zone with both brittle and ductile structures separates the two complexes. The observed rapid increase in metamorphic grade over very small structural thicknesses suggests that part of the original metamorphic zonation has been excised across the contact. Thus, in its present form, we interpret the Malaguide/ Alpujarride boundary as an extensional contact. The extensional motion took place in Early-Middle Miocene times, as constrained by fission-track cooling ages. The fault zone at the contact is sub-horizontal in the Sierra Espuña, but in the Sierra de las Estancias it is mainly sub-vertical. Kinematic data from a 5–20 m thick calcmylonite layer within the fault zone in the Sierra de las Estancias, corrected for later tilting, indicates that the extensional movement was originally directed with the top to the east-northeast. The interpretation of the Malaguide/Alpujarride boundary as an extensional contact is similar to recent interpretations of the Nevado-Filabride/Alpujarride contact at deeper structural levels in the central and eastern Betics. The identification of another major extension fault system confirms that Miocene extension was widespread at all structural levels. During the early-middle Miocene, the Sierra Espuña and the northern margin of the Sierra de las Estancias lay on the boundary between the region undergoing horizontal extension in the Internal Zones and the Alboran Sea to the south, and the region undergoing horizontal shortening in the External Zones to the north. As a result, the detailed history of the Alpujarride/Malaguide contact is complicated: extensional structures cross-cut structures related to earlier Alpine shortening and were then affected by further shortening as convergence with the External Zones continued.

Late-orogenic extensional tectonics at the NW margin of the Caledonides in Scotland

Abstract A major ductile extensional structure throwing down several kilometres to the ESE probably marks the southern portion of the NW Caledonian orogenic margin in Scotland. This structure, the Sound of Iona Fault, is probably related to the intensely mylonitized zones that cut the Lewisian and adjacent Iona Group (?Torridonian) metasedimentary cover on Iona. The fault-related mylonites predated the 414±3 Ma Ross of Mull Granite and are inferred to have postdated the c. 425 Ma Moine Thrust. It is concluded that the Sound of Iona Fault is one of the many late Caledonian faults in Scotland, but is exposed at a deeper structural level than normal. Its position at the orogenic margin of the Caledonides is similar to that of the Loch Gruinart Fault on Islay, hinting that (a) major steep fault(s) may be a significant feature of the largely concealed Caledonian margin south of Skye

The reflection Moho beneath the southern Canadian Cordillera

The transition from the crust to the mantle beneath the Canadian portion of the North American Cordillera varies in depth, geometry, and tectonic age across the orogen. These variations are rarely spatially related to the positions of morphologic or tectonic belts based on surface geology, nor to nearly 25 km of structural relief identified in outcrop and on seismic reflection data. The Moho in this region is thus interpreted to be a long-lived feature, perhaps as old as Proterozoic in the eastern part of the Cordillera, that probably has been active as a structural boundary during periods of crustal contraction and subsequent crustal stretching. Recognition of the Moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural levels beneath the interior of the orogen: they likely project to the base of the crust, where they flatten and cause imbrication of crustal rocks.

Charnockitic and monzonitic Pan-African series from north-central Nigeria: Trace-element and Nd, Sr, Pb isotope constraints on their petrogenesis

Major- and trace-element studies indicate that the evolution of the calc-alkaline (charnockitic) and sub-alkaline (monzonitic) Pan-African rocks in north-central Nigeria cannot be explained in terms of a unique or classical process of fractional crystallisation. At Toro, the crystallisation history of the more differentiated members appears more complex, with the participation of accessory phases. The quartz-fayalite monzonites show evidence of a cumulative phase during their differentiation. The fayalite-bearing quartzmonzonites are not common rocks and their occurrence in the Nigerian basement remains the only reported exposure in the Pan-African belt of West Africa. Like the charnockitic suite at Toro, they were emplaced during the Pan-African and at deep structural levels unlike the adjacent high-level Jurassic peralkaline granites of Jos Plateau. The application of multiple radiogenic tracers (Sr, Nd and Pb) indicates the involvement of pre-existing crust in the formation of these rocks ( ϵ Sr(i) = + 30 to + 86, ϵ Nd(i) = − 15.5 to − 5.5, 206 Pb 204 Pb i = 16.34–17.71 and 207 Pb 204 Pb i = 15.48–15.58 for Toro Charnockitic Complex; and ϵ Sr(i) = + 19 to + 51 and ϵ Nd(i) = − 8.0 to − 4.0 for the quartz-fayalite monzonites of Bauchi). Long crustal residence ages ( T Nd) of between 2.0 and 1.5 Ga preclude their complete derivation from the mantle. The bulk of evidence is consistent with the partial melting of a U-poor source typical of the lower crust during the Pan-African orogeny and in particular the involvement of older Precambrian crust.

Timescales of orogeny: Jurassic construction of the Klamath Mountains

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU’s FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 94YCJ2454, Timescales of orogeny: Jurassic construction of the Klamath Mountains, B.R. Hacker, M.M. Donato, C.G. Barnes, M.O. McWilliams, and W.G. Ernst). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.Classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous. Isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts. Forty‐five new 40Ar/39Ar ages from the Klamath Mountains illuminate the deformation, metamorphism, magmatism, and sedimentation involved in the Jurassic construction of that orogen, leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes. The new geochronologic relationships show that many Jurassic units of the Klamath Mountains had 200 Ma or older volcanoplutonic basement. Subsequent formation of a large ∼170 Ma arc was followed by contractional collapse of the arc. Collision with a spreading ridge may have led to large‐scale NW–SE extension in the central and northern Klamaths from 167 to ∼155 Ma, coincident with the crystallization of voluminous plutonic and volcanic suites. Marked cooling of a large region of the central Klamath Mountains to below ∼350°C at ∼150 Ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels. These data demonstrate that the Klamath Mountains—and perhaps other similar orogens—were constructed during areally and temporally variant episodes of contraction, extension, and magmatism that do not fit classical definitions of orogeny.

Structural and metamorphic relations in the southwest Seward Peninsula, Alaska: Crustal extension and the unroofing of blueschists

An oblique crustal section on the southwestern Seward Peninsula, Alaska, exposes polydeformed, polymetamorphosed rocks thatrecordextensionalthinningofthecrust in the Late Cretaceous. The rocks experienced an early (M1a) high-pressure/lowtemperature metamorphism (pumpellyiteactinolite facies in the upper part of the section; blueschist facies at deeper structurallevels)followedbyagreenschist-facies overprint(M1b)andaccompaniedbyD1deformation. A second deformational event, D2, is responsible for the prominent, gently dipping foliation, northwest-southeast‐ trending stretching lineations, and abundant recumbent isoclinal folds throughout the section. Metamorphism during D2(M2) variedfromextremelylow-gradeattheshallowest structural levels to upper amphibolite- to granulite-facies within the Kigluaik gneissdome.Apatitefission-trackagesfrom the shallowest structural levels indicate thatcoolingbelow ;120 &Ctookplaceatca. 70‐100 Ma, during the same time span as high-grade metamorphism and pluton intrusion at depth. Significant vertical attenuationofthecrustalsectionapparentlytook place during D2 deformation, resulting in close spacing of both M1aand M2isograds and overall thinning of the crust. This extensional deformation played an important role in the exhumation of the blueschist-facies rocks but may have followed an earlier period of partial erosional unroofing of these rocks.

What brought them up? Exhumation of the Dabie Shan ultrahigh-pressure rocks

Metamorphic coesite and diamond in the Dabie Shan, eastern China, testify to subduction of continental crust to >100 km depth. Exhumation of these ultrahigh-pressure rocks through the crust encompassed two stages. (1) South-dipping foliation, southeast-plunging stretching lineation, lineation-parallel isoclinal folds, and boudins indicate extreme subhorizontal shortening and subvertical extension during top-to-northwest shearing at 200–180 Ma. Syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability. (2) Northwest-southeast subhorizontal extension from 133 to 122 Ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the Dabie Shan. Pluton cores at deep structural levels have weak hypersolidus fabrics, and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions. Deformation is concentrated in greenschist facies mylonites and ultramylonites along the Xiaotian-Mozitang detachment fault at the northern topographic limit of the Dabie Shan. Our preferred exhumation model involves two stages: Triassic indentation—vertical extrusion and erosion—followed by Cretaceous plate margin transtension.

Development of structurally inverted basins: a case study from the West Coast, South Island, New Zealand

Abstract Cretaceous-Palaeogene basins in the West Coast of South Island, New Zealand, underwent inversion during the Neogene. Initial extension was oriented WNW-ESE and had a typical magnitude of c. 5%, whilst subsequent shortening in the same direction was typically c. 3–4%. This deformation produced a NNE-SSW trending basin and range province. Shortening was commonly partitioned into narrow zones along basin and range margins, leaving unreactivated extensional structures inbetween. Such margins are characterized by subparallel and en echelon Cretaceous — Palaeogene faults, which have been reactivated as predominantly reverse faults during the Neogene. Reverse faults propagated into the syn-inversion sedimentary sequence with increasingly shallow dip. Inversion caused local erosion of the original rift deposits. New back-thrusts, low-angle thrusts and folds developed during the late Neogene. Locally, incompetent Eocene mudstones deformed in a quasi-plastic manner, so that reverse faults at deeper structural levels became decoupled from thrust faults at shallower levels. Computer-aided section restoration has allowed validation of the interpretations, provided estimates of the timing and magnitude of deformation, and given a better understanding of the sequential development of inversion structures. Natural inversion structures show both contrasts and similarities to those produced in sand-box experiments.

40Ar/ 39Ar thermochronologic constraints on the tectonothermal evolution of the Northern East Humboldt range metamorphic core complex, Nevada

The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40Ar/ 39Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63–49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550°C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270°–350°C), but time gaps of > 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of 40Ar 39Ar mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and adjacent areas during latest Oligocene to earliest Miocene time. Such thermal gradients could be readily explained if the RM-EHR detachment fault dipped > 30° between the 300°C and 100°C isotherms. Finally, 40Ar 39Ar biotite cooling ages increase southward through the East Humboldt Range, compatible with northward extrapolation of a previously recognized pattern of WNW-younging biotite cooling ages from the Ruby Mountains. A simple model involving the propagation of footwall uplift in the direction of tectonic transport beneath an initially listric normal fault can explain the principle features of the Oligocène to Miocene thermochronologic data set for the RM-EHR.

Constraints from 40Ar/39Ar geochronology on the tectonothermal history of the Kapuskasing uplift in the Canadian Superior Province

The Kapuskasing uplift (KU) in the Superior Province of the Canadian Shield has been interpreted as an oblique cross section through the Archean mid-crust. However, the time of juxtaposition of the granulites of the KU against the lower grade rocks of the Abitibi greenstone belt (AGB) along the Ivanhoe Lake fault zone is problematic. To constrain the postmetamorphic tectonothermal history of the KU, we have conducted 57 40Ar/39Ar step-heating analyses on mineral and rock samples collected in a transect across the southern KU and adjacent AGB. The age spectra record a complex thermal history. Amphiboles from the AGB in the footwall of the Ivanhoe Lake fault zone have ca. 2.66 Ga dates, similar to closure ages for amphiboles from farther east in the AGB. Amphibole dates of 2.46–2.52 Ga from the deepest structural levels of the KU place an upper limit on the time of major uplift of the granulites and their juxtaposition with the AGB. Biotite and muscovite dates from the transect cluster into three age groups. The presence in the deepest structural levels of the KU of biotite with 2.40–2.45 Ga dates indicates that significant uplift (15–20 km or more) of the granulites had occurred by this time. Micas with dates in the 2.25–2.30 Ga range are close to fault zones; these dates may indicate a ca. 2.30 Ga episode of fault reactivation. Feldspar, fault-related whole rocks, and some micas record events post 2.1 Ga. These correspond to the emplacement of mafic and lamprophyric dykes and fault reactivation.

Thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada Batholith, Tehachapi Mountains, California

We present thermodynamic estimates of pressures, temperatures, and volatile activities in variably deformed, gabbroic to granitic, Cretaceous (115–100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. Al contents of hornblende in granitoids imply igneous emplacement at ∼8 kbar in the southernmost Tehachapi Mountains, with lower pressures (3–7 kbar) to the north. Metamorphic pressures and temperatures for garnet‐bearing paragneisses and metaigneous rocks were estimated on the basis of garnet‐hornblende‐plagioclase‐quartz and garnet‐biotite‐plagioclase‐quartz thermobarometers. Disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite‐based thermobarometers to rocks of contrasting composition and mineralogy. Preferred pressures cluster at 7.1–9.4 and 3.6–4.3 kbar. Incomplete knowledge of reaction histories, however, limits our interpretation of the lower pressures because they are minimum estimates. The ∼4‐kbar samples are all from a small area and, if our interpretation is correct, they imply a local, more shallow event superimposed on crust once residing at deeper structural levels. Garnet‐hornblende and garnet‐biotite temperatures are less coherent, likely owing to retrograde Fe‐Mg exchange, and range from 570° to 790°C, The majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling; they are not granulites. Country rock paragneisses are typically migmatized at “peak” metamorphic conditions near that of the wet granite solidus (>690°C). Veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase. Thermodynamic estimates of volatile activities in these garnet‐bearing assemblages suggest variable, mostly CO2‐rich fluid compositions, in the absence of any pervasive fluid flux. The igneous rocks of the Tehachapi Mountains were thus intruded at depths of ∼30 km, making them the deepest known exposed components of the Cretaceous Sierra Nevada batholith. Metamorphism occurred at these great depths and, perhaps, locally after ∼15 km of uplift before ∼87 Ma, implying an uplift rate of 1.2 mm/yr. (A minimum uplift rate is 0.6 mm/yr.) This original uplift and possible subsequent uplift events may have been related to underthrusting of a block of Rand Schist from what is now the southeast, with concomitant widespread ductile deformation. The deduced pressure‐temperature and uplift history is similar to those of high‐pressure/high‐temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not wan‐anted. When compared with higher‐level intrusive rocks from analogous portions of the Sierra Nevada batholith to the north, the Tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average, but displaying a similar level of isotopic hybridization involving mantle and crustal sources. The batholith is quartz‐rich at these levels, suggestive of a weak, ductile middle crust susceptible to prolonged deformation and possible delamination.

Identification of magmatic and meteoric fluid sources and upward- and downward-moving infiltration fronts in a metamorphic core complex

A combined study of O, H, and C isotope variations in the Ruby-East Humboldt Range metamorphic core complex has identified infiltration of aqueous fluids originating from both the surface of the earth, and from deep crustal or mantle levels. At deep structural levels, metamorphic fluids flowed upward and were rich in water, had mantle-like isotopic compositions, and were probably derived from crystallizing magmas. Conversely, at higher structural levels, a major meteoric-hydrothermal system developed in response to rapid uplift, causing meteoric fluids to penetrate downward into plastically deforming metamorphic rocks at depths of 10 to 15 km below the surface. Infiltration fronts associated with both processes have been recognized, allowing us to determine fluid fluxes and flow geometries at mid-crustal levels.

Homogenization and lowering of 18O/ 16O in mid-crustal rocks during extension-related magmatism in eastern Nevada

The oxygen isotope systematics of Tertiary volcanic rocks of east-central Nevada and of plutonic and metamorphic rocks of the Ruby Mountains-East Humboldt Range core complex provide complementary evidence for major 18O-depletion and 18O/ 16O homogenization of mid-crustal rocks during metamorphism and magmatism. The δ 18O value of crustal source material for silicic volcanic rocks decreased from between +9 and +11‰ to between +7 and +8‰ over ∼ 5 Ma. Mid-crustal metasedimentary and granitic rocks in the East Humboldt Range have δ 18O values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels. Exchange with deep-seated mantle-derived igneous rocks, or fluids derived therefrom, is the most plausible 18O-depletion mechanism. Intrusion of these mafic magmas promoted crustal melting and fluid migration. Homogenization of 18O/ 16O resulted from migration of high-temperature fluids and melts at mid-crustal levels, and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks.

Structural evolution of the Lewis plate in Glacier National Park, Montana: Implications for regional tectonic development

Detailed geologic mapping in southern Glacier National Park, Montana, reveals four episodes of deformation in the hanging wall of the Lewis thrust. (1) Pre-Lewis thrust structures include west- and east-dipping imbricate thrusts, conjugate contraction faults, and west- and east-directed bedding-parallel faults. Although these structures are truncated from below by the Lewis thrust, their development was kinematically compatible with the emplacement of the Lewis plate. Thus, they may have formed during early stages of the emplacement of the Lewis plate. (2) Syn-Lewis thrust structures include the Late Cretaceous-early Tertiary Lewis thrust, west-dipping duplexes, east-dipping normal faults, and the Akamina syncline, a broad fold that lies directly west of the Lewis thrust and extends northwestward for about 120 km from southern Glacier Park, western Montana, to southeastern British Columbia and southwestern Alberta, Canada. The development of the duplexes and the normal faults may have been related to east-verging simple-shear deformation during emplacement of the Lewis plate. The formation of the segment of the Akamina syncline in the study area was the consequence of development of the duplexes in the Lewis plate, because strata above the duplexes are concordant with the syncline. The syncline is, however, disconcordant with the Lewis thrust. This observation contrasts strongly with the well-established concordant relationship between the Lewis thrust and the Akamina syncline in its hanging wall in Canada, about 100 km north of the study area. We propose that the formation of the Akamina syncline on a regional scale was related to the development of duplexes and imbricate thrusts at two structural levels, one above and one below the Lewis thrust. During the development of these duplexes, the Lewis thrust transferred horizontal shortening laterally along the strike of regional compressional structures from its footwall in the Paleozoic-Mesozoic strata to its hanging wall in the Proterozoic strata. We speculate that development of the broad-fold belt, a major structure in the fold-and-thrust belt in the southern Canadian Rocky Mountains and western Montana, was related to duplex formation at deep structural levels below the folds. (3) Post-Lewis thrust contractional structures include a high-angle reverse fault that cuts the Lewis thrust and strikes N70°W, which is about 30°-40° more to the west than the average strike of the syn- Lewis thrust structures. The development of this fault represents a change in compressional direction after emplacement of the Lewis plate. (4) Post-Lewis thrust extensional structures include southwest-dipping normal faults. These faults truncate the post-Lewis thrust reverse fault and are part of the Eocene-Oligocene Rocky Mountain trench normal fault system.

The nature of sialic basement to the Dunnage Zone, northern Newfoundland: evidence from crustal xenoliths

Quartzofeldspathic xenoliths representing the probable structural basement to the northern Dunnage Zone occur in lamprophyre dykes on Alcock Island and vicinity, Notre Dame Bay. The dykes occur close to the boundary between seismically defined lower crustal blocks (LCB) below the Dunnage Zone, so the assignment of the xenoliths to a specific subsurface source is equivocal. The xenoliths include (i) clinopyroxene-bearing meta-(quartz) diorite to tonalite; (ii) plagioclase + quartz-rich rocks; and (iii) garnetiferous gneiss, including granulite. Paragneiss is absent, so the gneissic xenoliths do not appear to be direct analogues of Gander Zone metasediments thought to underlie parts of the Dunnage Zone farther to the southeast. The xenoliths also differ from Grenvillian crust exposed in western Newfoundland: the granulites are mineralogically distinct (contain clinopyroxene + garnet rather than orthopyroxene + garnet) and yield higher metamorphic P (approx. 8–10 kbar versus approx. 3–7 kbar) (1 kbar = 100 MPa), and the granitoid xenoliths are dioritic–tonalitic rather than granitic. Consequently, we propose that the xenoliths are derived from a non-Gander, non-Grenville source that may include Central LCB and (or) exotic basement material. We cannot, however, exclude the possibility that the xenoliths represent deeper structural levels of Grenvillian or Gander Zone crust than are presently exposed in outcrop.

Relative timing of emplacement of an Archean lode-gold deposit in an amphibolite terrane: the Eastmain River deposit, northern Quebec

The Eastmain River deposit is located within the Archean Upper Eastmain River greenstone belt, approximately 320 km north of Chibougamau. The regional stratigraphy is dominated by ultramafic to felsic volcano-sedimentary units and aluminous sedimentary strata that have undergone amphibolite-grade regional metamorphism.The deposit is hosted by a narrow brittle–ductile shear zone striking northwest and dipping 45 °to the northeast. This shear zone affects a lithologically varied unit composed of MgO-rich rocks and mafic and felsic volcanics. Kinematic indicators within this deformation zone indicate thrust-sense displacement.Within this shear zone, surface drilling has outlined three ore zones characterized by deformed quartz–sulphide veins that form elongated lenses whose long axes parallel the steeply pitching stretching lineation, which plunges to the northeast.Hydrothermal alteration is restricted to within a few metres of the ore veins. It consists of two different mineral assemblages that are both associated with ore veins: a prograde garnet-bearing assemblage, and an epidote-bearing assemblage that locally retrogrades the garnet-bearing assemblage. The main pulse of Au mineralization appears to be relatively late, as suggested by grains of native gold occurring in fractures within quartz and sulphide.The general absence of carbonate alteration around the lodes in rocks generally susceptible to such a process contrasts strongly with Archean lode-gold deposits developed in greenschist-facies metamorphic terranes. Furthermore, the presence of garnet within ore veins raises a number of questions regarding the timing of mineralization of this deposit. On the other hand, the main pulse of Au mineralization appears relatively late, as is the case for the majority of Archean lode-gold deposits.Structural relations and alteration assemblages suggest a deeper structural level of emplacement, at least for the quartz-sulphide veins and the hydrothermal alteration. It is suggested that the Eastmain River deposit formed from a syntectonic hydrothermal system during peak metamorphic conditions.

The Self-Referentiability of Thorstein Veblen’s Theory of the Preconceptions of Economic Science

(1990). The Self-Referentiability of Thorstein Veblen’s Theory of the Preconceptions of Economic Science. Journal of Economic Issues: Vol. 24, No. 3, pp. 695-718.

Kinematics of wrench and divergent‐wrench deformation along a central part of the Border Ranges Fault System, Northern Chugach Mountains, Alaska

The Border Ranges fault system (BRFS) bounds the inboard edge of the subduction‐accretion complex of southern Alaska. In Eocene time a central segment of this fault system was reactivated as a zone of dextral wrench‐ and oblique‐slip faulting having a cumulative strike‐slip offset of at least several tens of kilometers, but probably less than 100 km. Early wrench folds are upright, trend at less than 45° to the strike of adjacent faults and developed with fold axes oriented subparallel to the axis of maximum incremental stretch λ1. These en echelon folds rotated and tightened with progressive deformation and then were overprinted by younger wrench folds that trend at about 60° to adjacent throughgoing faults. The latter folds are interpreted as forming during a late increment of distributed wrench deformation within the BRFS that included a component of extension (divergence) orthogonal to the mean strike of the fault system. A sharp releasing bend in exposures of a strike‐slip fault originally at >4 km depth today coincides with a narrow pull‐apart graben bounded by oblique‐normal faults that dip toward the basin. Widening of this pull‐apart graben by brittle faulting and dike intrusion accommodated less than 2 km of strike‐slip and was a late‐stage phenomenon, possibly occurring at supracrustal levels. Prior to formation of this graben during a period of predominantly ductile deformation at deeper structural levels, wrench‐folded rocks on one side of the nonplanar fault were translated around the releasing bend without significant faulting or loss of coherence. Kinematically, the earlier deformation was accomplished by fault‐bend folding and rotation of a relatively deformable block as it passed through a system of upright megakinks. Such a ductile mechanism of fault block translation around a strike‐slip bend may be typical of intermediate levels of the crust beneath pull‐apart grabens and may be transitional downward into heterogeneous laminar flow occuring along curved segments of ductile shear zones. Some degree of fault‐bend folding of strike‐slip fault blocks around releasing bends may be one reason why the amount of extension measured across natural pull‐apart basins is commonly observed to be less than the amount of strike‐slip along their master faults.

An oxygen isotope discontinuity in high-grade rocks of the East Humboldt Range, Nevada

RECENT studies of metamorphic terranes document sharply contrasting 18O/16O systematics1. Some regions are isotopically homogeneous, but with decreased 18O/16O ratios (see, for example, refs 2–4), whereas others preserve steep gradients in oxygen isotope composition5–7. These results indicate that some terranes experience very large, pervasive fluid fluxes, whereas others may, at least locally, remain virtually fluid-absent. Here we report a very detailed oxygen isotope study of a high-grade metamorphic terrane, revealing striking systematic variations that are closely correlated with structural depth. In particular, metamorphic rocks that are low in 18O, first detected at deep structural levels in a preliminary study of the area8, are now shown to be restricted to a clearly defined zone several hundred metres thick and extending for at least 3 km laterally. The most striking feature is the existence of a very sharp oxygen isotope discontinuity marking the upper boundary of the low-18O zone and separating it from an isotopically heavier and more heterogeneous (although still high-grade) overlying region.