Research Article| November 01, 1993 Contrasting thermal histories across the Flying Point fault, southwestern Maine: Evidence for Mesozoic displacement DAVID P. WEST, JR.; DAVID P. WEST, JR. 1Department of Geological Sciences, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar DANIEL R. LUX; DANIEL R. LUX 1Department of Geological Sciences, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar ARTHUR M. HUSSEY, II ARTHUR M. HUSSEY, II 2Department of Geology, Bowdoin College, Brunswick, Maine 04011 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1993) 105 (11): 1478–1490. https://doi.org/10.1130/0016-7606(1993)105<1478:CTHATF>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation DAVID P. WEST, DANIEL R. LUX, ARTHUR M. HUSSEY; Contrasting thermal histories across the Flying Point fault, southwestern Maine: Evidence for Mesozoic displacement. GSA Bulletin 1993;; 105 (11): 1478–1490. doi: https://doi.org/10.1130/0016-7606(1993)105<1478:CTHATF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 40Ar/39Ar analysis of hornblende, muscovite, biotite, and K-feldspar across the Flying Point fault in southwestern Maine reveals important information on the timing of thermal events and subsequent cooling history of the region. The Flying Point fault is the most significant structure associated with the Norumbega fault zone in southwestern Maine. It forms the boundary between two different lithotectonic sequences, offsets metamorphic isograds, and marks a remarkable time-temperature discontinuity. Southeast of the Flying Point fault in the Casco Bay Group, early Carboniferous hornblende ages and late Carboniferous muscovite and biotite ages reflect slow cooling following Middle to late Devonian (Acadian) metamorphism and deformation. No significant (>300 °C) late Paleozoic thermal event affected these rocks. In contrast, rocks northwest of the Flying Point fault were affected by a significant (>500 °C) late Paleozoic (Alleghanian) thermal event as indicated by latest Carboniferous-Permian hornblende cooling ages. This represents the northernmost occurrence of Alleghanian high-grade metamorphism reported in the Appalachian orogen. Cooling below muscovite and biotite closure temperatures northwest of the Flying Point fault did not occur until the Early Triassic. The thermochronologic data indicate that the rocks currently juxtaposed across the Flying Point fault underwent drastically different thermal histories during Permian and Triassic time. Approximately 4 km of post-Paleozoic west-side-up displacement along the Flying Point fault can account for the presently observed time-temperature discontinuity. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.
From studies of structure, metamorphism, and geochronology we have evidence that the Norumbega Fault Zone represents a transition from mid- to shallow crustal levels in a dextral, transcurrent shear zone within the northern Appalachian Orogen. The Norumbega Fault Zone strikes parallel to the orogen (northeast-southwest), is ~5 to 30 km wide, and is characterized by distributed ductile dextral shear fabrics in the southwestern section with a transition to brittle fabrics toward the northeast. Within the zone of distributed shear, deformation is partitioned into local zones of very high strain. Upright, isoclinal folds are common in areas of high and low strain. Metamorphic grade decreases from amphibolite facies in the southwest to sub-green schist facies in the northeast. 40Ar/39Ar mineral ages from recrystallized minerals in a high strain zone, regional cooling ages in areas of lower strain, and metamorphic textures are consistent with a polyphase history of deformation. We interpret a younging trend in 40Ar/39Ar cooling ages toward the northeast, together with the deformational fabrics andmetamorphic features, to represent exhumation of the southwestern section of the Norumbega Fault Zone from mid-crustal levels during the polyphase history of this transcurrent zone. The Norumbega Fault Zone may therefore serve as a model for deformational processes at mid- to shallow crustal levels in active strike-slip systems.
RÉSUMÉ
Des études de la structure, du métamorphisme et de la géochronologic nous fournissent la preuve que la zone faillée de Norumbega représente une transition des niveaux crustaux moyens à des niveaux peu profonds dans une zone de cisaillement transversal dextrale à l'intérieur de la partie septentrionale de l'orogene appalachien. La zone faillée de Norumbega suit une direction paralléle a l'orogéne (nord-est-sud-ouest); die a cinq à 30 km de largeur; et elle est caractérisec par des structures cisaillées dextrales déformables se ramifiant dans la section sud-ouest avec une transition a des structures cassantes vers le nord-est. A l'intérieur de la zone de cisaillement ramifié, la déformation est divisée en zones locales de très forte contrainte. Les plis droits isoclinaux sont courants dans les secteurs de forte et faible contrainte. L'intensité du métamorphisme décroit d'un faciès à amphibolite dans le sud-ouest à un faciès secondaire de schistes verts dans le nord-est. Les périodes minérales 40Ar/39Ar des minéraux recristallises dans une zone de forte contrainte, les périodes de refroidissement régional dans les secteurs de faible contrainte et les textures métamorphiques correspondent à une orogénese polyphasée. Nous interprétons la tendance de rajeunissement dans les périodes de refroidissement 40Ar/39Ar vers le nord-est, de même que les textures de déformation et les caractéristiqucs métamorphiques, comme une exhumation de la section sud-ouest de la zone faillée de Norumbega depuis les niveaux crustaux moyens pendant l'histoire polyphasée de cette zone transversale. La zone faillée de Norumbega pourrait, par consequent, servir de modèle pour les processus de déformation aux niveaux variant de moyens à peu profonds dans les systèmes a décrochement.
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A growing body of evidence demonstrates that Atlantic-style passive margins have experienced episodes of uplift and volcanism in response to changes in mantle circulation long after cessation of rifting. Passive margins are thus an attractive archive from which to retrieve records of mantle circulation and lithospheric alteration. However, this archive remains under-utilized due to difficulty in deciphering the surficial records of passive margin tectonism and linking them to seismic velocity structure. Here we present a new approach to unraveling the tectonic history of passive margins using U-Pb dating of calcite in faults and fractures along the eastern North American margin. These ages show a 40 Myr long period of continuous fracturing and faulting from ~115 to 75 Ma followed by another episode in Mio-Pliocene time. We argue that the former event represents a response to Cretaceous lithospheric alteration whereas the latter records development of modern relief in the northern Appalachians.
