Early Paleozoic orogenesis in the Maine-Quebec Appalachians (Maine, Quebec).

摘要

Accretionary orogens, such as the Appalachian orogen, form by episodic docking of oceanic and continental fragments. Two factors that exert significant control on the development of an accretionary orogen are: (1) the nature and source of the accreting fragments, and (2) the thermal and deformational structure of the crust. This study addresses aspects of both of these controls.; In the Northern Appalachians, a long-lived but untested hypothesis suggested that Early Paleozoic accretion in western Maine, which marked the initiation of Appalachian development, involved the docking of an island arc. My goal was to test this hypothesis for the Maine-Quebec segment of the orogen, where the Boundary Mountains terrane had been identified as a possible collider. Combining the techniques of mapping, structural analysis, petrography, U-Pb zircon and monazite geochronology, geochemistry, and geochemical modeling, I present the following interpretations related to the geologic history of the region. (1) the Chain Lakes massif, which cores the Boundary Mountains, was an Ordovician arc-marginal basin receiving sediments eroded from a Laurentian source. (2) Anatexis of the Chain Lakes massif disrupted the original sedimentary-volcanic sequence. (3) The Boil Mountain Complex and Jim Pond Formation, which lie along the southern margin of the Chain Lakes massif, do not represent an ophiolite, as previously thought. (4) The Boundary Mountains represent a Laurentian-derived microcontinent that served as the nucleus for part of a regional arc system that collided with Laurentia in the Ordovician.; The thermal and deformational processes described herein relate, respectively, to anatexis and pluton emplacement. Review and numerical modeling of the causes of low-pressure anatexis, which affected the Chain Lakes massif, indicate that appropriate pressure-temperature conditions are possible in regions of crustal-scale detachment faulting, percolative magma flow, or where thin lithosphere is accompanied by plutonic activity. Analytical kinematic modeling of the consequences of dike-fed pluton emplacement suggests that if published physical properties of granitic magmas are correct, host rocks surrounding an in-situ expanding pluton must deform at rates several orders of magnitude faster than typical tectonic strain rates. Such strain rates almost certainly must be accommodated by processes other than dislocation creep.