Cees R. van Staal

Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians

Abstract During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma). The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderians lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395–c. 375 Ma is tentatively attributed to slab-breakoff. Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.

Brunswick subduction complex in the Canadian Appalachians: Record of the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon

The Brunswick subduction complex in the New Brunswick part of the Canadian Appalachians records the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon. The Brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the Quebec reentrant of the Laurentian margin. This part of the margin experienced less underthrusting and exhumation and overprinting by orogen-parallel faulting than the adjacent promontories where collision started earlier. The early, southeast to east vergent thrust-related structures represent a progressive D1 deformation that formed in response to northwestward subduction of the previously extended Gander margin and subsequent tectonic unroofing of the subduction complex. The original, shallow northwestward dipping envelope to S1 was deformed in the Late Silurian by D2 upright folds and associated shear zones into a steep belt during terminal collision. The D2 structures probably formed in response to sinistral transpression. Together, D1 and D2 indicate that convergence was oblique and sinistral. Most rocks incorporated in the subduction complex formed in the Tetagouche back arc basin that evolved from rifting of an Arenig magmatic arc built on the Gander margin into a wide marginal basin. Subduction was initiated in the Late Ordovician (≈455 Ma) in the back arc basin following collision of the Middle Ordovician Popelogan arc with Laurentia in the Caradoc, shortly after its opening in the late Arenig (≈473 Ma). The closure history of the Iapetus Ocean involved more than one subduction zone and arc-continent collision and rivals the southwestern Pacific Ocean in its complexities.

Taconian orogeny and the accretion of the Dashwoods block: A peri-Laurentian microcontinent in the Iapetus Ocean

The stratigraphy of the external Humber zone of the western Newfoundland Appalachians records protracted Neoproterozoic–Early Cambrian rifting, followed by development of a passive margin that persisted until late Early Ordovician (Arenigian) time (ca. 475 Ma). However, adjacent metamorphic rocks, derived from the Laurentian margin and preserved in the Dashwoods subzone, were deformed, overthrust by ophiolites, and intruded by arc plutons by 488 Ma. The adjacent Notre Dame subzone also records isotopic evidence of interaction with the margin by 488 Ma. We propose that a microcontinent (Dashwoods microcontinent) was rifted from Laurentia during the Early Cambrian after an earlier, Neoproterozoic opening of the Iapetus Ocean, and was separated from the margin by a narrow oceanic tract (Humber seaway). Attempted subduction of the Dashwoods microcontinent prior to 488 Ma was followed by closure of the Humber seaway in the Taconian orogeny.

A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic ’building blocks’ in order to test the validity of the implication of independent lithotectonic histories for the two segments. Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia. Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri-Gonwanan Ganderia and Carolinia. This deduction implies that any first-order differences between northern and southern segments post-date Late Ordovician consolidation of a large portion of the orogen.

Middle to late Paleozoic Acadian orogeny in the northern Appalachians: A Laramide-style plume-modified orogeny?

The Laramide orogeny of the western United States is proposed as a modern analogue for the Silurian-Devonian Acadian orogeny and subsequent diachronous, voluminous, short-lived magmatism and basin formation in the northern Appalachians. Shallowing of the Benioff zone accounts for several enigmatic features associated with plate convergence in the northern Appalachians, including (1) Wenlockian-Ludlovian termination of arc-related magmatism in the Avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 Ma, and (2) diachronous migration of the Acadian deformation front from ca. 415 Ma in the southeast to ca. 370 Ma in the northwest, extending more than 600 km into the continental interior. The flattening of the subduction zone is attributed to overriding of a plume by the convergent margin, which may explain (1) the abrupt termination of magmatic quiescence by 380–370 Ma, and voluminous felsic magmatism and production of plume-related lamprophyres in the southeast (Meguma terrane) as the plume thermally eroded the oceanic lithosphere, causing melting of the lower crust; (2) Late Devonian regional high-temperature, low-pressure metamorphism in the Meguma terrane related to the thermal anomalies above a plume; (3) synchronous Devonian emplacement of Meguma gold deposits and associated siderophile elements, possibly derived from fluid circulation above an ascending plume; (4) rapid Late Devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume; (5) migration of magmatism to the north (Avalon terrane, Cobequid highlands) so that plume-related Carboniferous magmatism occurred in and around the Carboniferous-Permian Maritimes basin; (6) the high-density lens at the base of the crust beneath the Maritimes basin, the product of plume-derived underplated mafic rocks; and (7) a subsidence mechanism for formation of the Maritimes basin by cooling of a decapitated plume head.

Age Constraints on the Tectonic Evolution and Provenance of the Pie de Palo Complex, Cuyania Composite Terrane, and the Famatinian Orogeny in the Sierra de Pie de Palo, San Juan, Argentina

Abstract New U-Pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the Pie de Palo Complex represent a Mesoproterozoic fragment of suprasubduction zone oceanic crust. A gabbroic pegmatite, interpreted to have formed during arc rifting or subsequent back-arc spreading, yielded a U-Pb age of 1204 +5.3/–4.7 Ma. Highly tectonized ultramafic-mafic cumulates, occurring at the structural base of the Pie de Palo Complex and previously interpreted to represent remnants of a primitive arc phase, prior to rifting and back-arc spreading, could not be dated, but should be older than 1204 Ma if these inferences are correct. Tabular, sill-like bodies of leucogabbro/diorite and calc-alkaline tonalite/granodiorite sills yielded ages of 1174±43 and 1169 +8/–7 Ma respectively. They may represent a younger, more evolved arc phase established after arc rifting or a younger, tectonically unrelated Mesoproterozoic arc. SHRIMP-analysis of metamorphic zircon rims with low Th/U ratios in VVL 110 gave a 206Pb/238U age of 455±10 Ma, similar to lower intercept dates determined by discordia lines. Combined, these data indicate that the bulk of the amphibolite facies metamorphism present in the Pie de Palo Complex was generated during the Famatinian Orogeny. Analysis of six single detrital zircon grains in a metasedimentary, quartzofeldspathic garnet-mica schist, tectonically interleaved with the igneous rocks of the Pie de Palo Complex, and tentatively correlated with the Difunta Correa metasedimentary sequence of other workers, yielded three age populations: 1150–1160 Ma; 1050–1080 Ma and 665 Ma, indicating that these sedimentary rocks were deposited during the late Neoproterozoic or Early Paleozoic. In addition, they confirm structural evidence that intercalation of rocks of the Pie de Palo Complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications. These ages are also consistent with a Laurentian provenance, and earlier interpretations that these rocks once represented a sedimentary cover to the Pie de Palo Complex. The zircon population of 1050–1080 Ma could be derived from Grenville-age felsic plutons identified elsewhere in the Pie de Palo Complex by other workers. However, no evidence has been found in our samples for a Grenville-age orogenic event, invoked previously to explain accretion of the oceanic Pie de Palo Complex to Laurentia prior to the late Neoproterozoic/Early Cambrian rifting and drift of Cuyania.

Provenance and tectonic evolution of Ganderia: Constraints on the evolution of the Iapetus and Rheic oceans

We provide estimates for the width, timing, and rates of opening and closing of the Iapetus and Rheic oceans, the evolution of which profoundly infl uenced Paleozoic global paleogeography. These estimates are primarily derived from the transfer of Ganderia and Avalonia from Gondwana to Laurentia, which led to closure of the Iapetus Ocean and opening of the Rheic Ocean. Ganderia, a long-lived arc terrane, separated from the paleo-Caribbean margin of Amazonia at 505 Ma with a latitudinal speed of ~9 cm/a northward, initiating the Rheic Ocean as a backarc basin. Ganderia’s trailing edge was reduced to ~5 cm/a following opening of a 600‐800-km-wide backarc basin within Ganderia at 475 Ma. Opening and closing of the Iapetus Ocean was largely driven by far-fi eld stresses, slab pull in some places and slab rollback in others.

Promontory-promontory collision in the Canadian Appalachians

In Cape Breton Island and southwestern Newfoundland, the Appalachian orogen is extremely narrow; the rocks were strongly deformed and underwent high-grade Barrovian-type metamorphism in Silurian time, and the orogen-parallel Silurian thrusts are dominantly west-vergent, antithetic to the westward subduction. This area is bounded by two transverse dextral wrench faults, the Canso fault to the southwest and the Gunflap Hills fault to the northeast. These tectono-stratigraphic, structural, and metamorphic features are explained by Silurian collision between the St. Lawrence promontory on the Laurentian margin and the Cabot promontory on the Avalon margin during the final closure of the Iapetus ocean. The west-vergent antithetic thrusting is interpreted to be related to tectonic wedging, while the two transverse wrench faults accommodated differential movement from promontories to reentrants during the collision. Promontory-promontory collision should be a common process and probably played a significant role in shaping the tectonic framework of many orogenic belts.

Crustal structure, fossil subduction, and the tectonic evolution of the Newfoundland Appalachians: Evidence from a reprocessed seismic reflection survey

Reprocessed Lithoprobe seismic reflection data across the Appalachian orogen in Newfoundland provide images of an Ordovician–Devonian collision zone that separates Laurentia from Ganderia, an accreted peri-Gondwanan microcontinent. Prominent reflectivity within Ganderian basement tapers westward and merges with reflections that project beneath the Moho, outlining a probable Ordovician to Devonian subduction zone. Reflectivity within Ganderian basement likely originates from transposed compositional layering within Cambrian–Neoproterozoic arc basement. Migmatites and other high-grade rocks of the Meelpaeg allochthon were likely extruded in the Devonian toward the northeast. The reflection Moho may have been established in the Devonian in parts of Newfoundland by partial melting of the lower crust. Reflection truncations outline a near-vertical Carboniferous strike-slip fault zone that cuts the entire crust.

Post-Taconic blueschist suture in the northern Appalachians of northern New Brunswick, Canada

A narrow belt of Late Ordovician-Early Silurian blueschist, at least 70 km long, separates an allochthonous fragment of back-arc oceanic crust of the Middle Ordovician Fournier Group from underlying, rift-related volcanic rocks of the Middle Ordovician Tetagouche Group in northern New Brunswick, Canada. The basalts on both sides of the blueschist belt are predominantly metamorphosed to greenschist facies conditions. The blueschist belt is interpreted to be an out-of-sequence thrust zone that accommodated tectonic transport of higher pressure rocks on top of lower pressure rocks during post-peak blueschist facies metamorphism. The blueschists have higher Fe{sub 2}O{sub 3}/FeO ratios and total iron contents in comparison to otherwise chemically equivalent basalts of the Fournier and Tetagouche Groups that have been metamorphosed into greenschists. The blueschist belt was probably the site of channelized flow of oxidizing fluids during active deformation ina subduction complex formed during the closure of a wide Taconic back-arac basin in Late Ordovician-Silurian time.