Metamorphic bathozones and bathograds; a measure of the depth of post-metamorphic uplift and erosion on the regional scale

Carmichael, D. M.

doi:10.2475/ajs.278.6.769

Cited by 169 publications

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“…Without additional independent evidence, his calculated pressure conditions of 6 kbar cannot be considered reliable. We propose a minimum pressure of approximately 3 kbar, equivalent to bathozone 3 of Carmichael (1978). M 2 is characterized by the successive overprinting of rotated staurolite and garnet porphyroblasts by andalusite and cordierite (Fig.…”

Section: P-t Pathmentioning

confidence: 99%

Polymetamorphism and ductile deformation of staurolite–cordierite schist of the Bossòst dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

Mezger

Passchier

2003

Geol. Mag.

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-The Bossòst dome is an E-W-trending elongated structural and metamorphic dome developed in Cambro-Ordovician metasedimentary rocks in the Variscan Axial Zone of the central Pyrenees. A steep fault separates a northern half-dome, cored by massif granite, from an E-W-trending doubly plunging antiform with granitic sills and dykes in the core to the south. The main foliation is a flat-lying S 1/2 schistosity that grades into a steeper-dipping slaty cleavage at the dome margins. Three major deformational and two metamorphic phases can be differentiated. S 1/2 schistosity is an axial planar cleavage to W-vergent recumbent folding that probably occurred in mid-Westphalian time. Peak regional metamorphism M 1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust. Strong non-coaxial deformation recording uniform topto-the-SE extension during D 2a is preserved in staurolite-garnet schists in a 1.5 km thick, shallowly SE-dipping zone in the southeastern dome. A 500 m thick contact aureole (M 2 ) was imprinted on the regionally metamorphosed rocks following the intrusion the Bossòst granite during D 2b . More coaxial deformation prevailed during synkinematic growth of M 2 phases in the inner part of the contact aureole around the northern part of the dome, where it obliterated D 2a fabrics. Progressive non-coaxial deformation continued in the southeastern antiform and is recorded by late-synkinematic growth of cordierite. Successive overprinting of the M 1 staurolite-garnet assemblage by andalusite and cordierite of M 2 is preserved in the southern part. The assemblage muscovite + cordierite + staurolite + biotite is considered metastable, given the low Mn and Zn contents of staurolite and cordierite, and interpreted as the result of prograde metamorphism during decompression. P-T conditions during M 2 were approximately 3 kbar and 600• C. Pervasive crenulations and mesoscopic to regional southerly verging folds are the result of D 3 NNE-SSW compression post-dating ductile deformation and contact metamorphism. Polymetamorphic assemblages of the Bossòst dome preserve a regionally confined zone of ESE-directed extensional shearing within an overall N-S compressional setting. Exact timing of extensional shearing is not known, but can be constrained by recumbent folding during the midWestphalian and granitic intrusions, which confine it to Late Carboniferous time (c. 305 Ma). Crustalscale flat-lying extensional shear zones with similar orientation and time frame are observed in the Hospitalêt massif of the eastern Axial Zone. This suggests that crustal extension, though probably restricted by regional strain partitioning over orthogneiss or intruding granitic bodies within an overall compressive setting, was not uncommon in Late Carboniferous time in the Axial Zone of the Pyrenees.

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Section: P-t Pathmentioning

confidence: 99%

Polymetamorphism and ductile deformation of staurolite–cordierite schist of the Bossòst dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

Mezger

Passchier

2003

Geol. Mag.

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“…La carte des bathogrades ainsi dressée (CARMICHAEL, 1978) pour les séries pélitiques métamorphosées au Dévonien, dans le nord de la Nouvelle-Angleterre implique l'ablation d'une tranche de 17 km par l'érosion. Telle est, en effet, la pression critique correspondant à l'équilibre: andalousite = kyanite + sillimanite.…”

Section: B Rôle Respectif Des Soulèvements Verti-caux Des Aplanisseunclassified

L’érosion différentielle dans les reliefs de l’Estrie et de la Nouvelle-Angleterre, entre Montréal et le piedmont sud-est des White Mountains

Birot¹,

Godard²,

Pelletier³

2007

gpq

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Entre Montréal et Granby, dans une véritable plaine à inselbergs, ces derniers correspondent à des intrusions tantôt syénitiques, tantôt basiques : ils sont plus résistants que les calcaro-schistes cambro-ordoviciens. L'intrusion la plus orientale est celle du mont Mégantic, qui comporte un croissant syénitique enveloppant une bosse de granite. Les intrusions de granite dévonien de l'Estrie et du nord du Vermont sont souvent en creux, ce qui peut s'expliquer soit par leur composition granodioritique (alvéole d'Island Pond), soit par une intense microfissuration (alvéole de Scotstown). La position culminante des White Mountains paraît due (en grande partie) à l'érosion différentielle. Les séries de Littleton, où les unités schisteuses sont aussi résistantes que les éléments quartzitiques et gneissiques. dominent, au nord, les gneiss olivériens, dont la position déprimée est attribuable à leur fracturation, non suivie de recristallisation, et ailleurs, les granites de Bickford, pourtant relativement acides. En position culminante se placent également les syenites et les granites à hastingsite mésozoïques. Quant aux granites dits de Conway, tantôt ils sont troués par d'amples cuvettes, tantôt ils constituent des fiers monolithes limités par des diaclases espacées. C'est que ces roches sont très hétérogènes, tantôt blanches et acides, tantôt grises et riches en biotite et en hornblende, ainsi qu'en plagioclases. Au total, les roches occupant le sommet de l'échelle de dureté sont celles où les perthites et les microperthites constituent une trame continue. Pour les autres variétés de roches, le degré de microfissuration et la composition minéralogique jouent alternativement le rôle principal.Between Montréal and Granby, the studied profile cuts across a true erosion plain where inselbergs coincide with syenitic and basic intrusions more resistant than the Cambro-Ordovician calcareous shales and schists. Mount Mégantic, the most eastern intrusion consists of a syenitic crescent and a granitic boss. In the Eastern Townships and northern Vermont, Devonian granitic intrusions are often hollowed out into basins. This derives either from their granodioritic composition (Island Pond) or from severe microfissuration (Scotstown). The prominent position of the White Mountains seems to be due to selective erosion. The Littleton Series, where schistous units are as resistant as the quartzitic and gneissic elements, rise on their northern boundary above the oliverian gneisses. These were easily eroded since no recrystallization occurred after fracturing. Elsewhere, the Littleton Series rise above Bickford granites despite the rather acid composition of the latter. Mesozoic hastingsite syenites and granites also have a prominent position. As for the granites classified as Conway granites, they range from white and acid varieties to grey varieties with a high content of plagioclases, biotites and hornblende. This heterogeneity may explain the topographic contrast between huge basins and monoliths, the latter being limited by spaced ...

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“…B&W inferred that this assemblage(?) required pressures of at least 5 kbar by assuming temperatures and pressures equivalent to, or above, the breakdown of staurolite to kyanite + Fe-rich garnet [Carmichael, 1978]. This assumption is unwarranted because the kyanite-bearing rocks are above staurolitebearing schists, and in fact, the upward appearance of kyanite coincides with the upward disappearance of staurolite [Rowles, 1982].…”

Section: Constraintsmentioning

confidence: 99%

Comment on “The Snake Range Decollement interpreted as a major extensional shear zone” by John. M. Bartley and Brian P. Wernicke

Gans¹,

Miller²

1985

Tectonics

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Using data from Miller et al. [1983] and Gans and Miller [1983], Bartley and Wernicke [1984] (B&W)haveproposed an alternate interpretation of the northern Snake Range decollement (NSRD) in east-central Nevada. As some key geological relations do not support their model, we feel obliged to comment on their interpretation. We welcome Bartley and Wernicke's interest in our work and feel that this exchange serves to illustrate how differently we view extensional processes.We maintain that the NSRD originated as a transition or decoupling zone between imbricate, high-angle normal faulting above and ductile flow plus pluton emplacement below. Although the decollement presently juxtaposes rocks that contrast strongly in terms of their style of deformation, we contend that the upper plate is closely related to the lower plate in that no stratigraphic section is missing between the two. This, together with the fact that across the entire width of the northern Snake Range the NSRD maintains approximately the same stratigraphic position, suggests that it cannot represent a surface that originally cut downsection to the east and on which there had been a large amount of translation. We have not ruled out all bedding-parallel slip, as B&W implied, and in fact have argued that there undoubtedly had to be some translation on the NSRD. Nevertheless, we pointed out that the comparable magnitudes of lower and upper plate strains, together with the apparently limited regional extent of the NSRD, are compatible with minor translation. In contrast, Bartley and Figure I and are sufficient to evaluate these differences.

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Metamorphic bathozones and bathograds; a measure of the depth of post-metamorphic uplift and erosion on the regional scale

Cited by 169 publications

References 0 publications

Polymetamorphism and ductile deformation of staurolite–cordierite schist of the Bossòst dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

Polymetamorphism and ductile deformation of staurolite–cordierite schist of the Bossòst dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

L’érosion différentielle dans les reliefs de l’Estrie et de la Nouvelle-Angleterre, entre Montréal et le piedmont sud-est des White Mountains

Comment on “The Snake Range Decollement interpreted as a major extensional shear zone” by John. M. Bartley and Brian P. Wernicke

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