Scott M. Johnston

Exhuming Norwegian ultrahigh-pressure rocks: Overprinting extensional structures and the role of the Nordfjord-Sogn Detachment Zone

[1] The Nordfjord-Sogn Detachment Zone (NSDZ) is widely cited as one of the primary structures responsible for the exhumation of Norwegian (ultra)high-pressure (UHP) rocks. Here we review data from the considerable volume of research describing this shear zone, and compile a strikeparallel cross section along the NSDZ from the Solund Basin in the south to the Soroyane UHP domain in the north. This cross section highlights several previously unrecognized patterns, revealing a shear zone with top-to-the-west asymmetric fabrics that (1) initiated at amphibolite facies, (2) overprints metamorphic breaks and tectonostratigraphic contacts, and (3) has a gradational continuum of muscovite cooling ages. These patterns constrain the kinematic evolution of the NSDZ and suggest a new three-step model for the exhumation of Norwegian (U)HP rocks. The initial stages of exhumation were characterized by the rise of crustal rocks from (U)HP depths to the base of the crust by buoyancydriven mechanisms not specified in this paper. Mantle exhumation was followed by top-to-thewest, normal-sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation. In the final stages of crustal exhumation, top-W brittleductile detachments soled into and partially excised this ductile shear zone, dropping the Devonian basins into contact with rocks of varying tectonostratigraphic levels. This new interpretation of the NSDZ is significant as it accounts for the extreme crustal excision observed in western Norway using three sequentially overprinting structures active at different stages of UHP rock exhumation. Citation: Johnston, S. M., B. R. Hacker, and T. B. Andersen (2007), Exhuming Norwegian ultrahigh-pressure rocks: Overprinting extensional structures and the role of the Nordfjord-Sogn Detachment Zone, Tectonics, 26, TC5001,

Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway

The Nordfjord-Sogn Detachment Zone of western Norway represents an archetype for crustal-scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure (UHP) terranes. In this paper, we investigate the role of normal-sense shear zones with respect to UHP exhumation using structural geology, thermobarometry, and geochronology of the Hornelen segment of the Nordfjord-Sogn Detachment Zone. The Hornelen segment of the zone is a 2–6 km thick, top-W shear zone, primarily developed within amphibolite-grade allochthonous rocks, that juxtaposes the UHP rocks of the Western Gneiss Complex in its footwall with lower-grade allochthons and Carboniferous-Devonian Basins in its hanging wall. New thermobarometry and Sm/Nd garnet geochronology show that these top-W fabrics were initiated at lower crustal depths of 30–40 km between 410 Ma and 400 Ma. Structural geology and quartz petrofabrics indicate that top-W shear was initially relatively evenly distributed across the shear zone, and then overprinted by discrete ductile-brittle detachment faults at slower strain rates during progressive deformation and exhumation. These results require a three-stage model for UHP exhumation in which normal-sense shear zones exhumed UHP rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile-brittle structures.

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

Abstract Electron back-scatter diffraction (EBSD) was used to measure the crystal preferred orientations (CPOs) from 101 samples across the ultrahigh-pressure Western Gneiss region of Norway to assess slip systems, sense of shear, CPO strength, and strain geometry. The CPOs suggest a dominance of prism ⟨a⟩ slip, with lesser amounts of prism [c] slip and basal ⟨a⟩ slip; there are few Type I and Type II girdles. The major structural feature in the study area – the high-strain, top-W, normal-sense Nordfjord–Sogn Detachment Zone – is characterized by asymmetric and strong CPOs; an eastern domain with strong asymmetric CPOs shows top-E shear. Strain throughout the study area was characterized by a mix of plane strain and constriction with no evidence of flattening. Adjacent gneiss and quartzite/vein samples have similar CPOs.

Reply to comment by R. Bousquet et al. on ''Subduction factory: 1. Theoretical mineralogy, densities, seismic wave speeds and H 2 O contents''

[1] Steady increases in the mineral physical property database, new methods of calculating the physical properties of rocks, and the growing understanding of ultrahighpressure rocks [e.g., Liou et al., 2000] has stimulated considerable effort in recent years to calculate rock physical properties at high to ultrahigh pressure. One approach [e.g., Hacker et al., 2003] is to use the mineral abundances and mineral compositions of natural rocks determined in the field and in experiments. Another approach [e.g., Kerrick and Connolly, 2001] uses the thermodynamic properties of minerals to calculate mineral abundances and mineral compositions. Both approaches have strengths and weaknesses [Hacker et al., 2003]: the former is grounded in the reality that the mineral abundances and mineral compositions actually exist in natural rocks, the latter has the internal consistency and elegance that comes from a purely theoretical approach. Between these two end-member methodologies lie a range of approaches, including that presented by Bousquet et al. [1997]. Hacker et al. [2003] and Bousquet et al. [1997] both used metamorphic facies determined from a combination of natural and experimental studies; the boundaries of the facies fields are relatively similar but differ in key areas where temperatures are low (and thus equilibrium is often not attained) or pressures are unusually high (an area of active research). Hacker et al. [2003] used natural mineral abundances and compositions to calculate rock physical properties from a mineral equation of state that uses a third-order finite strain approximation with full consideration of the effects of changes in pressure and temperature. Bousquet et al. [1997] calculated mineral abundances from assumed mineral compositions and calculated rock densities at 1 bar and 25 C [Goffe et al., 2003] using an unspecified procedure. Because the thermal expansivities and compressibilities of key minerals such as feldspar translate into density changes of 15% over the 800 C to 3 GPa pressure range of interest [Hacker and Abers, 2004], the differences in these approaches will necessarily lead to different calculated densities. [2] Our calculated densities do not violate thermodynamics as implied by Bousquet et al. [2005]. With increasing temperature, thermodynamics requires the entropy of a system to increase whereas the density of the system may increase or decrease. With increasing pressure, thermodynamics requires the density of a system to increase, but we and Bousquet et al. [1997, 2005] present calculated densities for the solid minerals, not the whole system which includes H2O in addition to the solid minerals. The specific example cited, of a temperature-dependent increase in density at 800 C and 1.0–1.5 GPa, is caused by the dehydration of amphibole. Our new results ‘‘contradict’’ our previous results [Hacker, 1996] because our new calculations explicitly consider the effect of Fe. [3] We are perplexed by Bousquet et al.’s [2005] suggestion that our density calculations contradict densities of natural rocks measured by Austrheim [1987]. Austrheim [1987] measured granulite facies anorthositic rocks with densities of 2.79–3.21 g/cm and mafic eclogites of 3.5– 3.6 g/cm but did not specify mineral abundances or compositions, precluding a direct comparison. If, however, we use mineral compositions from Austrheim and Griffin [1985] and assume mineral abundances typical for granulites (50% plagioclase, 25% garnet, and 25% pyroxene) and eclogites (50% garnet and 50% pyroxene), we calculate STP densities of 3.20 and 3.61 g/cm, in agreement with those measured by Austrheim, who noted that the presence of microcracks and alteration in his rocks would lower the density measurements. It is even more difficult to make comparisons between our calculated densities and the other papers cited by Bousquet et al. [2005]. Green and Ringwood [1967] did not measure the densities of their experimental rocks. Ito and Kennedy [1971] did not measure garnet compositions and reported only a single pyroxene composition. Bousquet et al. [2005] are correct to note that within individual facies fields our calculated densities depend only on the formula weight, molar volume, expansivity a, @a/@T, isothermal bulk modulus KT, @KT/@P, shear modulus m, @m/@P, G = (@lnm/@lnr)P, Gruneisen parameter gth, and the second Gruneisen parameter dT = (@lnKT/@lnr)P; we do not model the changes in mineral composition and abundance within those individual fields. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B02207, doi:10.1029/2004JB003490, 2005

Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada

Studies of pre-Grenvillian (1.4–1.3 Ga) plutons offer insight into the dynamics of arc amalgamation and backarc rifting prior to continental collision during the Ottawan orogeny. The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a northeast-southwest–trending thrust zone consisting of metaplutonic thrust sheets enveloped in gneissic tectonites and calcitic-dolomitic marble. Tonalitic CMBbtz thrust sheets (Dysart and Redstone), located in the southern Ontario Grenville Province (Canada) are made up of upper amphibolite facies, foliation-concordant metatonalite (+ amphibole ± biotite ± accessory zircon and titanite) and amphibolite (± biotite ± clinopyroxene). These thrust sheets are thought to have formed and amalgamated onto the Laurentian margin prior to Ottawan orogeny. Major and trace element analyses show that the metatonalite rocks have calc-alkaline affinity and amphibolite rocks have both calc-alkaline and tholeiitic affinities, suggesting an arc environment. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) geochronology of zircon from the two thrust sheets yield igneous ages of ca. 1350–1300 Ma for diorite and granodiorite, ca. 1150–1100 Ma ages from Dysart tonalite interpreted to record metamorphic zircon growth, and a ca. 1086 Ma Ottawan metamorphic age from a Dysart amphibolite. The ca. 1150–1100 Ma metamorphic event has not been previously documented within these thrust sheets of the CMBbtz, but correlates well with thermal events in the allochthonous Parry Sound domain to the west, as do ca. 1350 Ma igneous ages of tonalite in both areas. These data support the hypothesis that the CMBbtz and Parry Sound domain may have been initially linked. Widespread ca. 1350 Ma crust along with distinct 1460–1400 Ma depleted mantle model ages (TDM) are also consistent with a shared genesis with the Dysart–Mount Holly suite in New York and Vermont, and support the correlation between the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt as a rifted arc. Alternatively, the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt may represent contemporaneous arc development at different parts of the convergent margin; however, we support the correlation between the CMBbtz thrust sheets and the Adirondack-Highlands–Mount Holly belt.

The role of calcite-rich metasedimentary mylonites in localizing detachment fault strain and influencing the structural evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Although crystalline rocks dominate the footwall of the Buckskin-Rawhide detachment fault in west-central Arizona (USA), we estimate that thin (<1 to 100 m thick) calcite-rich metasedimentary mylonites were present along 25%–35% of the detachment fault, and in parts of the footwall they were continuous for ~30 km in the slip direction. New field observations, geochronology, and detailed microstructural data provide insight into the origin of these metasedimentary rocks and their role in the structural evolution of the detachment fault system. We propose that calc-mylonites along the Ives Peak footwall corrugation were derived from locally overturned Pennsylvanian–Permian strata that were buried to mid-crustal depths beneath a southeast-vergent Cretaceous thrust fault, which was reactivated in the Miocene by the parallel Buckskin detachment fault shear zone. In some areas these laterally persistent calc-mylonites were smeared out along the detachment fault during incisement into the crystalline footwall, forming a thin carapace of rheologically weak rocks structurally below the original weak zone. Metasedimentary mylonites consistently record top-to-the-northeast simple shear parallel to the detachment fault slip direction. Strain, synmylonitic veins, and paleostress recorded in these mylonites increase toward the detachment fault. Marble mylonites <1 m below the detachment fault preserve strong calcite crystallographic preferred orientations and lack cataclastic deformation that characterizes quartz-rich rocks along the detachment fault. In addition, unlike quartzofeldspathic mylonites, calc-mylonites typically lack extension via postmylonitic normal faults and associated horizontal axis rotation. Paleopiezometry and rheological modeling of the metasedimentary mylonites suggest that when quartzite layers were being sheared at ~100 MPa and 10−13 to 10−14 s−1 near the brittle-plastic transition, marble layers could have been strained ~100× faster at ~20 MPa. Detachment fault strain localized within the metasedimentary rocks, and the calcite marbles exerted significant control on the rheology of the footwall shear zone. This study highlights the important role that inherited weak zones may play in influencing the location, geometry, rheology, and style of deformation associated with detachment fault systems. LITHOSPHERE; v. 10; no. 2; p. 172–193; GSA Data Repository Item 2018080 | Published online 30 January 2018 https://doi.org/10.1130/L699.1

Mechanical Mixing of Garnet Peridotite and Pyroxenite in the Orogenic Peridotite Lenses of the Tvaerdal Complex, Liverpool Land, Greenland Caledonides

The Tvaerdal Complex is an eclogite-bearing metamorphic terrane in Liverpool Land at the southern tip of the Greenland Caledonides. It is a Baltic terrane that was transferred to Laurentia during the Scandian orogeny. It exposes a few small garnet dunite and harzburgite lenses, some containing parallel layers of garnet pyroxenite and peridotite (including lherzolite). Sm–Nd mineral ages from the pyroxenites indicate recrystallization occurred at the same time ( 405Ma) as eclogite recrystallization in the enclosing gneiss. Geothermobarometry indicates these eclogites and pyroxenites shared a similar pressure-temperature history. This congruent evolution suggests pyroxenite-bearing peridotite lenses were introduced from a mantle wedge into subducted Baltic continental crust and subsequently shared a common history with this crust and its eclogites during the Scandian orogeny. Some garnet peridotite samples contain two garnet populations: one Cr-rich (3 5–6 2wt % Cr2O3) and the other Cr-poor (0 2–1 4wt %). Sm–Nd analyses of two such garnet peridotites define two sets of apparent ages: one older (>800Ma) for Cr-rich garnets and the other younger (<650Ma) for Cr-poor garnets. We propose that the younger Cr-poor garnets were derived from fractured and disaggregated garnet pyroxenite layers (i.e. are M2) and were mixed mechanically with older (i.e. M1) garnets of the host peridotite during intense Scandian shearing. Mechanical mixing may be an important mantle process.

Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages

Metamorphic core complexes within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, magmatism, and exhumation within the footwalls of high-angle Basin and Range normal faults. In models proposed for the formation of metamorphic core complexes there is a close temporal and spatial link between upper crustal normal faulting, lower crustal ductile extension and flow, and detachment faulting. To provide constraints on the timing of ductile extension in the northern Snake Range metamorphic core complex (Nevada) and thereby test these models, we present new 238 U- 206 Pb dates on zircons from both deformed and undeformed rhyolite dikes intruded into this core complex. The older age bracket is from the northern dike swarm, which was emplaced in the northwestern part of the range pretectonic to syntectonic with ductile extension. The younger age bracket is from the Silver Creek dike swarm, which was emplaced in the southern part of the range after ductile extensional deformation. The 238 U- 206 Pb zircon ages from these dikes provide tight bounds on the timing of ductile extension, between 37.806 ± 0.051 Ma and 22.49 ± 0.36 Ma. Our field observations, petrography, and 238 U- 206 Pb zircon ages on these dikes combined with published data on the geology and kinematics of extension, moderate- and low-temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins, are interpreted as recording an episode of localized upper crustal brittle extension during the late Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the northern Snake Range detachment soon after, or simultaneously with, emplacement of the older dike swarm. Exhumation of the lower plate continued in a rolling hinge–isostatic rebound style; the western part of the lower plate was exhumed first and the eastern part extended ductilely either episodically or continuously until the latest Oligocene–earliest Miocene, when the post-tectonic younger dike swarm was emplaced. Major brittle slip along the eastern part of the northern Snake Range detachment and along high-angle normal faults exhumed the lower plate during middle Miocene.

An Orphaned Baltic Terrane in the Greenland Caledonides: A Sm-Nd and Detrital Zircon Study of a High-Pressure/Ultrahigh-Pressure Complex in Liverpool Land

Liverpool Land, at the southern tip of the Greenland Caledonides, exposes a composite metamorphic terrane: the midcrustal granulite-facies Jaettedal Complex tectonically juxtaposed against the eclogite-facies, peridotite-bearing Tvaerdal Complex. The Jaettedal Complex is a Laurentian terrane, whereas the Tvaerdal Complex was proposed by earlier investigators to be a Baltic terrane. PT estimates (880°–920°C at 35–40 kbar) and Sm-Nd mineral isochrons from Tvaerdal eclogites indicate that recrystallization occurred under ultrahigh-pressure (UHP) metamorphic conditions ≈400 m.yr. ago, the same time and under similar conditions as the Western Gneiss Complex of the Norwegian Caledonides. Detrital zircons from the Tvaerdal Complex, analyzed for U-Pb, Lu-Hf, and trace elements by laser ablation inductively coupled plasma mass spectrometry, give concordant Mesoproterozoic ages but not the Archean and ≈1.8 Ga Proterozoic ages characteristic of Laurentian terranes. Most remaining concordant U-Pb ages are 411–375 Ma (i.e., Scandian), which contrast with older (≈460–410 Ma) zircon ages from the Jaettedal Complex as well as other Laurentian terranes. Both the Precambrian and the Scandian age sets confirm the Tvaerdal Complex as an orphaned Baltic terrane. The Jaettedal Complex underwent a lengthy Caledonian history as part of a continental arc system during the closure of Iapetus, whereas the Tvaerdal Complex was a fragment of the approaching Baltic passive margin. UHP metamorphism occurred when this margin subducted into the mantle beneath Laurentia. We propose that the Tvaerdal Complex separated from Baltica and rose through the hot mantle wedge to the base of the overriding Laurentian crust by diapirism, a process that may explain its abundant anatectic granitoid intrusions.