Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington

Wernicke, Brian P.; Getty, Stephen R.

doi:10.1130/0016-7606(1997)109<1149:isagci>2.3.co;2

Cited by 48 publications

(61 citation statements)

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“…Our rock starts out at a depth of ∼ 25 km, where high temperatures (> 500 • C) mean that flow stresses are low, and pressure differences related to density or topographic gradients may result in lateral flow of material (Block and Royden, 1990;McKenzie and Jackson, 2002;McKenzie et al, 2000;Rey et al, 2011;Wernicke, 1992;Wernicke and Getty, 1997). The resulting deformation is characterized by hightemperature, coarse-grained microstructures that may have Figure 3.…”

Section: Distributed Deformation Zonementioning

confidence: 99%

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Cooper

Platt

2017

Solid Earth

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Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains-East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized-distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

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Section: Distributed Deformation Zonementioning

confidence: 99%

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Cooper

Platt

2017

Solid Earth

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“…The Chelan block is composed by a mixture of Palaeozoic supracrustal rocks (Cascade Holden River assemblage, Napeequa Twisp Valley assemblage, Swakane gneiss, Skagit gneiss) and numerous plutons of different ages ranging from Cretaceous to early Miocene (Tabor et al 1987). Thermobarometric data from the Swakane Gneiss and Napeequa complex indicate that the Chelan block was metamorphosed at upper amphibolite facies conditions at pressures of 1.0-1.2 GPa and a temperatures of 640-750°C (Brown and Walker 1993;Valley et al 2003;Gordon et al 2010) before 96 Ma (Wernicke and Getty 1997;Valley et al 2003;Gordon et al 2010). The emplacement depths of the plutons vary between 0.7 GPa and shallower levels.…”

Section: Geological Settingmentioning

confidence: 99%

“…Sparse sphene and hornblende cooling ages of the Chelan Complex suggest cooling below 500°C occurred around 60 Ma (Mattinson 1972;Tabor et al 1987) and have a common cooling path with the Napeequa unit (Matzel 2004). At the same time, the Skagit gneiss to the North was buried and underwent a migmatitic event (Wernicke and Getty 1997;Whitney et al 1999;Matzel et al 2008). The Chelan Complex and the Napeequa unit do not record late Cretaceous/early Tertiary burial, but were already exhumed at this time (Paterson et al 2004).…”

Section: Tectonic History Of Chelan Complex In the Framework Of Northmentioning

confidence: 99%

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

Dessimoz

Müntener

Ulmer

2011

Contrib Mineral Petrol

153

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Amphibole fractionation in the deep roots of subduction-related magmatic arcs is a fundamental process for the generation of the continental crust. Field relations and geochemical data of exposed lower crustal igneous rocks can be used to better constrain these processes. The Chelan Complex in the western U.S. forms the lowest level of a 40-km thick exposed crustal section of the North Cascades and is composed of olivine websterite, pyroxenite, hornblendite, and dominantly by hornblende gabbro and tonalite. Magmatic breccias, comb layers and intrusive contacts suggest that the Chelan Complex was build by igneous processes. Phase equilibria, textural observations and mineral chemistry yield emplacement pressures of *1.0 GPa followed by isobaric cooling to 700°C. The widespread occurrence of idiomorphic hornblende and interstitial plagioclase together with the lack of Eu anomalies in bulk rock compositions indicate that the differentiation is largely dominated by amphibole. Major and trace element modeling constrained by field observations and bulk chemistry demonstrate that peraluminous tonalite could be derived by removing successively 3% of olivine websterite, 12% of pyroxene hornblendite, 33% of pyroxene hornblendite, 19% of gabbros, 15% of diorite and 2% tonalite. Peraluminous tonalite with high Sr/Y that are worldwide associated with active margin settings can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process. Crustal assimilation during fractionation is thus not required to generate peraluminous tonalite.

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“…Some gneisses were exhumed from >40 km, with crystallization and closure ages from the late Cretaceous to ca. 55 Ma (Miller and Bowring, 1990;Haugerud et al, 1991;Wernicke and Getty, 1997;Valley et al, 2003;Paterson et al, 2004;McLean et al, 2006;Whitney et al, 2008). Of particular interest, then, is how the sedimentary basins behaved during mid-Eocene time.…”

Section: Regional Setting and Tectonic Historymentioning

confidence: 99%

“…These terranes were subsequently translated along a system of strike-slip faults along the northern Cordilleran margin. Strike-slip faulting continued through the late Cretaceous and early Tertiary, along with intermittent magmatism, volcanism, and exhumation of mid-crustal rocks (Miller and Bowring, 1990;Haugerud et al, 1991;Umhoefer and Miller, 1996;Wernicke and Getty, 1997;Valley et al, 2003;Paterson et al, 2004;Whitney et al, 2008). All of the cited geologic studies emphasized crustal shortening of pre-Tertiary rocks but concluded (with variable certainty) that the once deep-seated rocks were exhumed by extension or transtension in the Eocene.…”

Section: Introductionmentioning

confidence: 99%

The Chiwaukum Structural Low: Cenozoic shortening of the central Cascade Range, Washington State, USA

Cheney

Hayman

2009

GSA Bulletin

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The central Cascade Range of Washington State has become a testing ground for theories surrounding the exhumation of deep-seated arcs, generation of both arc and fl ood-basalt volcanism, strike-slip faulting that translated crustal blocks along the Cordilleran margin, and development of fault-bounded basins. A central existing hypothesis is that the region underwent either regional extension or transtension during the Eocene. Both the extension and transtension models derive from the interpretation that clastic Eocene formations were deposited syntectonically in local basins. Geologic mapping and structural analyses presented here support an alternative hypothesis: that these formations are preserved in regional synclines, not in separate depositional basins. The type area for the Eocene history is the so-called Chiwaukum graben or Chumstick basin, here renamed the Chiwaukum Structural Low. The southwestern boundary of the Chiwaukum Structural Low includes post-depositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive and arkosic, early Eocene Swauk Formation over arkosic, mid-Eocene strata that have previously been called the Chumstick Formation. Cataclastic structures provide independent evidence for the reverse faulting. Elsewhere, 39-42 Ma rocks unconformably overlie the folds. The reverse faults and fold hinges are cut by northerly striking strike-slip faults, which likely are of late Eocene age. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group, the gentler folding of which largely defi nes the regional map pattern around the Chiwaukum Structural Low. Instead of an extensional or transtensional history, the Eocene-to-Recent history of the Central Cascade region is characterized by multiple periods of folding and reverse faulting alternating with periods of strike-slip faulting.

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Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington

Cited by 48 publications

References 0 publications

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

The Chiwaukum Structural Low: Cenozoic shortening of the central Cascade Range, Washington State, USA

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