Metamorphic Break Research Articles

Tethyan sutures of northern Turkey

Abstract The two main Tethyan sutures of Turkey, the İzmir-Ankara-Erzincan and the Intra-Pontide sutures, are reviewed through several well-studied transects crossing the suture regions. Both sutures have formed during the Early Tertiary continental collisions following northward subduction of Tethyan oceanic lithosphere. The İzmir-Ankara-Erzincan suture is represented along most of its c. 2000 km length by Paleocene and younger thrust, which emplace the upper crustal rocks of the northern continent over that of the southern continent with an intervening tectonic layer of Cretaceous subduction-accretion complexes. These thrusts constitutes a profound stratigraphic, structural, magmatic and metamorphic break, of at least Carboniferous to Palaeocene age and form the main boundary between Laurasia and Gondwana in the Turkish transect. Voluminous subduction-accretion complexes of Triassic and Cretaceous ages occur respectively to the north and south of the suture giving the antithetic subduction polarities during these two periods. This, and evidence for a major accretionary orogeny of Late Triassic age north of the İzmir-Ankara-Erzincan suture suggest that two separate oceanic lithospheres, of Carboniferous to Triassic (Palaeo-Tethys) and of Triassic to Cretaceous ages (Neo-Tethys) respectively have been consumed along the suture. The final continental collision along the İzmir-Ankara-Erzincan suture was slightly diachronous and occurred in the earliest Palaeocene to the west and in the Late Palaeocene to the east. The c. 800 km long Intra-Pontide suture is younger in age and have formed during the Early Eocene and younger continental collisions linked to the opening of the Western Black Sea Basin as an oceanic back-arc basin. At present the North Anatolian Fault, which came into existence in the Late Miocene, follows the course of the older Intra-Pontide suture.

New evidence for coesite in eclogite and gneisses: Defining an ultrahigh-pressure province in the Western Gneiss region of Norway

Research Article| October 01, 1997 New evidence for coesite in eclogite and gneisses: Defining an ultrahigh-pressure province in the Western Gneiss region of Norway Alice Wain Alice Wain 1Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information Alice Wain 1Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, United Kingdom Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (10): 927–930. https://doi.org/10.1130/0091-7613(1997)025<0927:NEFCIE>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 Alice Wain; New evidence for coesite in eclogite and gneisses: Defining an ultrahigh-pressure province in the Western Gneiss region of Norway. Geology 1997;; 25 (10): 927–930. doi: https://doi.org/10.1130/0091-7613(1997)025<0927:NEFCIE>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 Widespread evidence of ultrahigh-pressure metamorphism is reported from the Nordfjord and Stadtlandet area of the Western Gneiss region in Norway, outlining a distinct ultrahigh-pressure province. Coesite has been found as inclusions in garnet, omphacite, or kyanite in eclogite from 5 new localities, and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities. Quartz pseudomorphs after coesite, within garnet and clinozoisite, are reported for the first time from associated gneisses, which is evidence for in situ ultrahigh-pressure metamorphism, at least at a local scale. The boundary to an ultrahigh-pressure province is defined by a metamorphic break from lower pressure (<2.5 GPa) eclogites to coesite or quartz pseudomorph after coesite eclogites (>2.8 GPa) within a 10-km-wide mixed zone in the Nordfjord area. This zone may mark the disrupted tectonic boundary to a small ultrahigh-pressure unit and casts doubt on the assumption that the Western Gneiss region behaved as a coherent terrane during burial and exhumation. 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.

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian orogen, Baltic Shield, southwestern Sweden

The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vänern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north. Structural studies in three areas north of lake Vänern (Värmlandsnäs, Kil and Torsby-Charlottenberg) suggest that the MZ displays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist-facies conditions. In the northernmost Torsby-Charlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defines the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within this thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorwegian Frontal Deformation Zone (SFDZ) in the frontal part of the orogen. The MZ is inferred to have formed in an extrusive (dispersive) tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and shortening was absorbed, at this stage, by crustal thickening. Late, possibly out-of-sequence thrusting with a right-lateral, strike-slip sense of shear along the SFDZ marked the waning stages of crustal shortening in the Sveconorwegian orogen.

The evolution of the Hornelen Basin detachment system, western Norway: Implications for the style of late orogenic extension in the southern Scandinavian Caledonides

The geology of the Devonian Hornelen Basin and the surrounding Western Gneiss Region (WGR), western Norway has been examined in detail. Structural, clast provenance and geothermobarometric data have been combined to reconstruct the motion history of part of a major late-orogenic extensional shear zone in the southern Scandinavian Caledonides, the Nordfjord-Sogn Detachment (NSD). The NSD in the Hornelen Basin area is a zone of mylonites 4–6 km thick, tectonostratigraphically constrained within the Lower and Middle Allochthons (LMA) and the uppermost part of the WGR. It separates eclogite-bearing gneisses of the WGR from greenschist and amphibolite facies rocks of the Middle and Upper Allochthons, marking a profound metamorphic break and indicating major excision of crustal section. Kinematic indicators give a consistent “top to the west” shear sense, indicating a reversal of the movement sense of the Scandian nappes, with the ductile strain being distributed through the entire LMA section. The WGR was exhumed from depths of up to 100 km in the footwall of the shear zone, partly aided by pure-shear vertical flattening below the NSD and extension and erosion of the hanging wall. The majority of the movements on the NSD in the Hornelen Basin area occurred under amphibolite facies conditions prior to 410 Ma. Subsequent shear caused limited greenschist retrogression along extensional crenulation cleavages. Depending on interpretation of mica 40Ar- 39Ar ages, this lower-grade shearing may be due to a continuation of movements, or renewal of motion associated with opening of the Hornelen Basin at about 385 Ma. The lack of clear evidence of penetrative greenschist facies mylonites in the Hornelen area may be due to the faulted omission of tectonites of this grade. Following Scandian A-subduction and eclogite facies metamorphism at 425 Ma, the P-T-t path of the WGR shows very rapid decompression associated with early slip on the extensional detachment horizons. Unroofing was initially near isothermal, followed by a phase of rapid near-isobaric cooling. After 410 Ma decompression and cooling slowed, as slip on the NSD slowed or even ceased. By the Middle Devonian, the WGR lay at about 10–15 km depth and the crust had almost returned to isostatic equilibrium. At this time, a brittle fault, splaying from the NSD, ramped upwards through the Upper Allochthon, opening a scoop-shaped, west-facing half-graben, the Hornelen Basin. A further 50 km of heave on the NSD occurred during alluvial infilling of the basin. Subsequently, the basin fill was folded by (Late Devonian?) N-S compression and the basin margins were modified by Mesozoic faulting. These features are consistent with an orogenic collapse model for southern Scandinavia that involves large-scale backsliding of the Scandinavian Caledonides within tectonostratigraphic units. By this mechanism the Baltic Shield could have been efficiently unloaded, with tectonic translations producing significant metamorphic breaks within a seemingly preserved tectonostratigraphic sequence. The model favours orogenic collapse driven by an external pull, rather than internal body forces.

Caledonian compressional and late-orogenic extensional deformation in the Staveneset area, Sunnfjord, Western Norway

The studied area in Western Norway constitutes part of the hangingwall of the extensional Kvamshesten Detachment Zone (KDZ). The KDZ separates eclogite-bearing lower crust from a hangingwall consisting of a Precambrian basement-cover pair, Silurian continental margin sediments, a Silurian obduction mélange, a Late-Ordovician ophiolite and the Devonian Kvamshesten Basin, deposited unconformably on the Pre-Devonian rocks. Contractional deformation related to ophiolite obduction and to the Caledonian Scandian Orogeny produced a suite of SE-verging structures developed under greenschist facies conditions. As orogenic collapse commenced, extensional shear zones were formed in the upper and middle crust, re-activating contractional shear zones and fabrics. The direction of transport on the inherited fabrics was reversed from top-to-southeast to top-to-northwest, and structures related to the regional extension were superimposed on the contractional structures. Back-folding of the main contractional foliation by asymmetric W-vergent folds, together with NW and W-directed shearing along weak lithologies and semi-ductile faulting in the mélange, are the main structural expressions of early stages of the orogenic collapse in the Staveneset area. There is apparently no metamorphic break between the contractional fabrics and the earliest extensional structures. The extensional structures developed, however, under progressively more brittle conditions. The Devonian sediments were deposited upon a sequence of rocks in the upper plate of the Kvamshesten Detachment that had already undergone significant extensional deformation and tectonic exhumation in the Late-Silurian-Early Devonian. The extensional deformation in the upper plate of the Western Norwegian extensional detachments has up to recently been largely ignored in the discussion of the structural control of the formation of the Devonian basins.

Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains

The northern part of the Saih Hatat window, Oman, shows high-pressure metamorphic rocks derived from shelf sediments and pre-Premian continental basement, and is atypical of sub-ophiolite metamorphism elsewhere. The high-pressure rocks are divided into structural units originally bounded by foreland-propagating thrusts formed during ophiolite obduction, although now many contacts are backthrusts, normal faults or extensional shear zones. Metamorphic breaks exist across many unit boundaries. The deepest unit (As Sifah) has eclogite-facies assemblages in metabasites and metapelites which record evolution of P – T conditions along a clockwise path culminating al 23±2.5kbar, 540±75°C, contrasting markedly with overlying units (5–10 kbar, 200–500°C), although separated by &lt;l0km on the ground. The dominant penetrative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-facies extensional fabric. Phengites from eclogite-facies schist show discordant 40 Ar/ 39 Ar apparent ages. Our tectonic model relates all the high- P units to a single convergent event in the Late Cretaceous. The As Sifah eclogites were exhumed in two stages: (i) tectonic emplacement against other units at c . 20–25 km depth, and (ii) exhumation of the entire high- P zone by culmination collapse after obduction.

Uplifted ophiolitic rocks on Isla Gordon, southernmost Chile: implications for the closure history of the Rocas Verdes marginal basin and the tectonic evolution of the Beagle Channel region

A succession of mafic rocks that includes gabbro, sheeted dikes and deformed pillow basalts has been mapped in detail on Isla Gordon, southernmost Chile and is identified as an upper ophiolitic complex representing the uplifted floor of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. The complex was uplifted, deformed, and regionally metamorphosed prior to the intrusion of an undeformed 90 Ma granodiorite that cuts the complex. The complex appears para-autochthonous, is gently tilted to the northeast and is internally sheared by near-vertical foliation zones. No evidence for obduction was observed although the base of the complex is not exposed. The ophiolitic rocks have been regionally metamorphosed to mid-upper greenschist levels. Isla Gordon is bounded by the northwest and southwest arms of the Beagle Channel, two important structural boundaries in the southernmost Andes that are interpreted to have accommodated north-side-up and left-lateral displacements. Directly north of Isla Gordon is the Cordillera Darwin metamorphic complex that exposes the highest grade metamorphic rocks in the Andes south of Peru. On the north coast of Isla Gordon a volcaniclastic turbidite sequence that is interpreted to have been deposited above the mafic floor is metamorphosed to lower greenschist levels in strong metamorphic contrast to amphibolite-grade othogneisses exposed in Cordillera Darwin only 2 km away across the northwest arm of the Beagle Channel. The profound metamorphic break across the northwest arm of the Beagle Channel and the regional northeast tilt of the ophiolitic complex are consistent with the previously proposed hypothesis that Isla Gordon represents the upper plate to an extensional fault that accommodated tectonic unroofing of Cordillera Darwin. However, limited structural evidence for extension was identified in this study to support the model and further work is needed to determine the relative importance of contractional, extensional and strike-slip displacements during the closure of the Rocas Verdes marginal basin and uplift of Cordillera Darwin. The Isla Gordon ophiolitic complex is correlative with other regional occurrences of ophiolitic rocks including the previously studied Tortuga, Sarmiento and Larsen Harbour complexes. The existence of the Isla Gordon ophiolitic complex helps link the known occurrences of the marginal basin floor into a semi-continuous belt that sheds light on the original continuity of the basin.

Metamorphic zones, terranes, and Cenozoic faults in the Marlborough Schist, New Zealand

Abstract The northeast‐striking Picton Fault Zone is a major structural and metamorphic break that divides the Marlborough Schist into two separate blocks. The northwestern (Kaituna) block shows a regular increase in textural and metamorphic grade to the southeast from prehnite‐pumpellyite to greenschist facies and textural zone I to IV. The southeastern (Arapawa) block shows an irregular increase in grade to the southeast from prehnite‐pumpellyite to pumpellyite‐actinolite facies and textural zone I to IIB. The Kaituna block is correlated with the Caples and Torlesse Terranes and the Otago Schist of the southern South Island. The Arapawa block is correlated with the Waipapa Terrane (possibly also the Torlesse Terrane) and Kaimanawa Schist of the North Island. Isotects and the Picton Fault Zone are dextrally offset by 10–15 km across the ENE‐striking Queen Charlotte Fault Zone. The latter should be included with the late Cenozoic Marlborough faults.

Tectonics of the lower crust: a view from the Usambara Mountains, NE Tanzania

In the Pare and Usambara Mountains, NE Tanzania, approximately 4000 km 2 of granulites display regular moderate northeast dips of layering and foliation and down-dip plunge of pervasive stretching lineation. Folds, other than scarce tight microfolds, appear to be absent; thrusts, only mapped in the North Pare Mountains, are probably numerous elsewhere. The granulites pass up, without any evidence of stratigraphic, structural or metamorphic break, into slightly lower-grade metasediments, including conspicuous marbles. These rocks show major tight folds. Comparison with the Turoka Fold in south Kenya, here re-interpreted as a sheath fold, supports interpretation of the stretching lineation in the Usambara granulites and overlying metasediments as the transport direction, transverse to the N-S Mozambique Belt. The regional lineation pattern indicates correlation of the transverse Usambara and Turoka lineations with those transverse to the Mozambique Belt farther north in Kenya. These are dated at ∼830 Ma; it is concluded that the Usambara stretching lineation, foliation and associated structures were also formed at that time. The fabrics show that this intense deformation took place in granulite facies conditions. The gradual change from their intense planar and linear deformation to the slightly more inclined fold and thrust deformation in the overlying metasediments is interpreted as implying an initially nearly flat shear zone in the lower crust during the Mozambique Belt orogeny; continuing movement imbricated and refolded the initially flat foliation, giving the northeastward dip across the granulites. The curved trend of the stretching lineation through the Usambara Mountains is attributed to strains during the late Mozambiquian deformations.

Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from 40Ar/39Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore‐continent–Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjåkkå Nappe is composed of c. 600‐Ma‐old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjåkkå Nappe is situated between eclogite‐bearing nappes (Mikka and Tsäkkok nappes) which underwent high‐P metamorphism at c. 500 Ma during westward subduction of the Baltoscandian margin. 40Ar/39Ar mineral ages of c. 520–500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjåkkå Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510–490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470–460 Ma are recorded from massive dyke protoliths within the Sarektjåkkå Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island‐arc sequences represented by structurally lower horizons of the Köli Nappes).Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520–500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian–Early Ordovician.

Thermal and kinetic constraints on the tectonic applications of thermobarometry

AbstractMetamorphic petrology, and in particular quantitative thermobarometry, offer the possibility of identifying faults in metamorphic terrains by the metamorphic and/or thermobarometric breaks that occur across them. Furthermore, the sense of the thermobarometric break (e.g. warmer, deeper rocks on top of colder, shallower ones) and its magnitude could be useful tools for determining the sense and magnitude of the fault. The sensitivity of thermobarometry to tectonic variables such as fault throw and uplift rate, has been tested by a series of one-dimensional numerical re-equilibration models for both thrust and normal faults. In each of these models, the post-tectonic re-equilibration of a model garnet-biotite geothermometer is simulated by coupling one-dimensional numerical thermal and garnet diffusion models. After cooling and uplift, a thermobarometric temperature was calculated by using a near-rim garnet composition (5–10 µm from the rim) calculated by the re-equilibration model and biotite from the model matrix. The models varied the thermal structure of the model orogen prior to faulting, the depth of the fault, the structural throw of the fault, and the uplift rate following faulting.All models produced zoned garnet porphyroblasts that recorded P-T conditions that were different from those at the time of fault motion. Most of the models produced thermobarometric breaks that were in the same sense as the temperature break at the time of fault motion. Normal faults produced a normal thermobarometric gradient with higher temperatures recorded below the fault than above it. Many, but not all of the thrust models produced a thermobarometric inversion near the fault, with higher temperatures locally recorded above the fault than below it. However, the magnitude of thermobarometric break correlated poorly with the fault throw. Most models developed thermobarometric breaks that were much smaller than the breaks that existed at the time of fault motion. The size of the thermobarometric break was commonly of the same magnitude as would be generated from microprobe analytical error. The models suggest that for metamorphic rocks whose thermal peak does not exceed a narrowly defined closure temperature, thermobarometry faithfully recorded the P-T conditions of the metamorphic peak. The closure temperature increases slightly with increasing uplift rate, but overall is rather insensitive to the uplift rate or other tectonic variables. For rocks whose thermal peaks is above the closure temperature, however, the model thermobarometer recorded a temperature that was very close to the closure temperature.

Metamorphism in allochthonous and autochthonous terranes of the western United States

The haphazard accretion of exotic terranes during continental reassembly results in a crustal college typified by genetically unrelated lithotectonic belts. Profound chronologic, lithologic, geochemical, and metamorphic breaks characterize such suture zones. However, post-metamorphic differential vertical uplift and erosion can generate a marked discontinuity in grade within a single lithotectonic entity, and in contrast, post-amalgamation recrystallization of an exotic terrane assembly can produce an isofacial metamorphic overprint. Thus the tectonic context of metamorphic mineral parageneses must be interpreted with caution. In spite of the presence of allochthonous terranes, the western U.S. Cordillera in general is characterized by gradual sectorial enlargements towards the modern edge of the continent, by coherent, broadly continuous isotopic or geochemical provinces, and by systematic oceanward decreases in the metamorphic intensities of the constituent lithic assemblages, both within a belt, and across a series of belts. These relationships hold over a wide range of scales, from that of a physiographic province to that of a quadrangle-sized area. Examples described include chronologic, isotopic, igneous and metamorphic belts of (1) the entire western conterminous U.S. Cordillera, (2) the Phanerozoic Sierran-Klamath basement terrane assembly, and (3) the Great Valley and Franciscan sedimentary couplet derived from the late Mesozoic Sierra Nevada-TClamath arc. For these cases, systematic recrystallization-deformation trends and nearly in situ growth of sialic crust are evident. Mapped metamorphic and structural discontinuities reflect dislocations involving spatially associated, co-evolving continental lithotectonic units, and, except for fartravelled oceanic fragments, do not imply wholesale juxtaposition of exotic, genetically unrelated terranes.

Metamorphism and crustal stacking in the North Indian Plate, North Pakistan

The northern part of the Indian Plate in North Pakistan is composed of a number of large-scale crustal nappes, each of which are stratigraphically distinct, and which were stacked late in the main phase of southeasterly directed thrusting associated with the Himalayan event. The major nappes recognised in the Swat to Kaghan area of North Pakistan are the Besham, Swat, Hazara, Banna, Lower Kaghan and Upper Kaghan nappes. Metamorphism was synchronous with early stages of deformation. Within each nappe the metamorphic grade increases upwards, an overall inversion that represents post-metamorphic imbrication within individual nappes, synchronous with the main phase of nappe stacking, rather than a “hot iron” type inversion as described under the MCT in Nepal and India. As a result of this “within-nappe” imbrication each thrust slice within any particular nappe contains rocks of a higher metamorphic grade than those in the slice below, with sharp metamorphic breaks across the imbricating thrusts as well as across the major shears that bound the crustal-scale nappes. Uplift along these imbricating thrusts initiated cooling of the stack. K-Ar mica and hornblende cooling ages imply that much of this uplift was completed by 30 Ma, or within 20 Ma of the collision.

Metamorphic break across postmetamorphic thrust faults: An unreliable indicator of structural throw

The recent proliferation of quantitative geobarometers for common metamorphic rocks has produced a potentially powerful tectonic tool for metamorphic terranes. The geobarometric break across postmetamorphic faults could be useful for determining the structural throw of the fault. However, the metamorphic break will correspond to the structural throw on the fault only if the metamorphic conditions preserved in rocks near the fault are the P-T conditions of the rocks just prior to fault movement. In general, one-dimensional thermal models for tectonically thickened orogens do not produce metamorphic breaks across thrust faults, suggesting that anomalous conditions might be required to preserve postmetamorphic thrust faults (PMTF). This study examined three classes of one-dimensional numerical thermal models in order to simulate the evolution of a PMTF. Class I models try to preserve the PMTF by rapid uplift following thrusting. Class II models allow for cooling and uplift between the metamorphic peak and thrusting. Class III models consider the effect of the underthrusting of a cool sheet beneath the thrust of interest. The relation between the geobarometric break across the fault and its structural throw is examined. All three classes of models manage to produce and preserve metamorphic inversions, given the proper starting conditions. However, in all class I and class III models, as well as most class II models, rocks below the thrust heat and reach their metamorphic peak subsequent to thrusting. This implies that the metamorphic conditions preserved across a PMTF do not represent the P-T conditions at the time of thrusting. The models investigated here all produced barometric breaks less than or equal to the throw of the fault. This suggests that the geobarometric break could place a lower limit on fault throw. However, the class II models demonstrated the importance of upper plate cooling and uplift to the preservation of a metamorphic inversion. Preferential uplift and cooling of the warmer upper plate rocks prior to thrusting could produce a barometric break that exceeds the fault throw. The geobarometric break across a PMTF may be either greater or less than that which would correspond to the structural throw of the fault and is therefore likely to be a rather unreliable indicator of its structural throw.

An allochthonous cover in northern Vestranden, Western Gneiss Region, central Norway

Abstract The present studies in the northwesternmost part of the Western Gneiss Region, Norway, have demonstrated the existence of previously unknown metasediments and metavolcanics in the Caledonian nappe units. Tectonic and metamorphic breaks as well as lithological differences separate these units from the underlying migmatized gneissic granites and granodiorites. Structural concordance between basement and cover developed as a result of high strain induced by nappe translation. An intense interfolding of basement and cover units postdated the thrusting event. Metamorphic and lithological contrasts in the area are generally preserved. Garnet-biotite thermometry data suggest that the temperatures during post-thrusting metamorphism did not exceed 600°C. Later movements were cataclastic in character and often occurred parallel to older thrust planes.

Structure and stratigraphy of the metamorphic rocks in part of Northwest County Mayo, Ireland

Summary A new geological map covering over 1000 square km of n.w. Co. Mayo is presented. Metasediments, probably Moine and Dalradian in age, have been deformed and metamorphosed during the Caledonian orogeny together with the pre-Moine Erris Complex (Laxfordian? rocks). The gneissose Erris Complex is commonly preserved except where significant Caledonian reconstitution to schists has occurred. Rocks of the Erris Complex were tectonically emplaced within the metasediments as major anticlinal cores, and as transgressive thrust wedges during the first Caledonian deformation. A significant structural and metamorphic break occurs across the Corraun Fault which is probably the south-westward continuation of the Leannan Fault of County Donegal.

Stratigraphy and Metamorphism of Late Precambrian Rocks in Central Northeastern Nevada and Adjacent Utah

Stratigraphy and Metamorphism of Late Precambrian Rocks in Central Northeastern Nevada and Adjacent Utah