Robert H. Moench

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England ☆

Abstract The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U–Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, N.H., the type area, northeast to Milan, N.H., and Oquossoc, Me.; it also includes black slate of the Partidge Formation (C. bicornis zone graptolites, ∼457 Ma). Related metamorphosed intrusive are the tonalitic Joslin Turn pluton (469±2 Ma), the Cambridge Black granitic pluton (468±3 Ma), and gabbro, tonalite (467±4 Ma), and sheeted diabase of the Chickwolnepy instructions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465±6 and 461±8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484±5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443±4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441–452 Ma) and Oliverian (435–456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoosuc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotropic data, the Ammonoosuc sequence and its correlative and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain–Jim Pond–Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ∼475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte–Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ∼470 Ma. Ammonoosuc eruptions probably ended at ∼460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ∼3–5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (∼456–435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack–Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley–Gaspe basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

Carboniferous U-Pb age of the Sebago batholith, southwestern Maine: metamorphic and tectonic implications.

Two phases (pink and white granite) of the Sebago batholith of southwestern Maine have been dated by the U-Pb zircon method. Identical upper concordia intercepts of both rocks indicate an intrusive age of 325 ± 3 m.y. for the batholith. The lower intercept of the pink-phase sample, 114 ± 13 m.y., is inferred to represent episodic lead loss due to the intrusion of the nearby Cretaceous Pleasant Mountain stock. The lower intercept of the white-phase sample, 18 ± 21 m.y., suggests only modern dilatancy lead loss. Monazites have ages of 272 m.y. (pink) and 282 m.y. (white) which are thought to be cooling ages. Rb-Sr whole-rock data have low initial 87 Sr/ 86 Sr ratios of 0.7031 (pink) and 0.7053 (white). These data, in conjunction with published 40 Ar/ 39 Ar, Rb-Sr, K-Ar, and fission-track ages, suggest that little or no uplift occurred in this part of New England until the Permian and that the uplift rate from 275 m.y. to 225 m.y. was ∼3 times as rapid as was the rate for 225 m.y. to the present. The Carboniferous age of the Sebago batholith suggests that currently accepted metamorphic and tectonic interpretations for southwestern Maine and for east-central New Hampshire require revision.

Premetamorphic Down-to-Basin Faulting, Folding, and Tectonic Dewatering, Rangeley Area, Western Maine

The Rangeley area of western Maine is underlain by a thick sequence of dominantly eugeosynclinal metasedimentary rocks of Ordovician, Silurian, and Devonian age. The dominant structural pattern of these rocks is defined by tight, upright, northeast-trending passive flow folds and by three major normal faults along which younger rocks on the southeast are down-faulted against older rocks on the northwest. Each normal fault, together with a major syncline and a complementary anticline farther southeast, defines a geometrically related fault-fold unit. In best-exposed units, displacement along the faults increases in the direction of plunge of the synclines and of increasing structural relief in the syncline-anticline pairs. A genetic relation between normal faulting and folding is inferred. The dominant fault-fold pattern represents the oldest recognized deformation in the area. Slaty or phyllitic cleavage of this deformation is typically subparallel to the axial surfaces of folds, but locally crosses the faults and the axial surfaces of tight folds at low angles. Metamorphosed clastic dikes along the cleavage suggest that cleavage formation was in part a diagenetic dewatering process. This process probably graded, however, into low-grade metamorphism at depth. It was quickly followed by emplacement of large plutons, local superposed passive slip and flexural slip folding, and by two recognized events of greenschist and amphibolite facies metamorphism. Porphyroblasts of these events have grown across slip cleavages as well as older phyllitic cleavage, and metamorphic zones cross the dominant fault-fold pattern. Deformation, as well as sedimentation, is considered to have been controlled by the ancestral Merrimack synclinorium—a strongly linear two-sided trough that persisted at least from Late Ordovician through Early Devonian time. The fault-fold pattern is inferred to have evolved over a long period of time, as follows: (1) Rapid deposition of 15,000 to 20,000 ft of nearly-impermeable clastic sediments in Late Ordovician and Early Silurian time on the southeast-dipping slope of the sedimentary trough; mass weakened in depth by excess fluid pressure. (2) Continuing sedimentation, down-to-basin creep with associated slump faulting and folding, probably beginning in Middle Silurian time; faults flattened basinward in depth along lower boundary of zone of excess fluid pressure. (3) Horizontal compression developed parallel to slide direction as mass piled against material in the trough; incipient slaty cleavage developed normal to compression, improving vertical permeability. (4) Pore fluids expelled vertically, permitting the slumping mass to compact horizontally, and fold with at least 25 percent shortening. The process culminated in Early Devonian time, during and after deposition of the youngest exposed rocks in the area.

Relation of S2 Schistosity to Metamorphosed Clastic Dikes, Rangeley-Phillips Area, Maine

In the Rangeley-Phillips area of Maine, a section of lower Paleozoic clastic sedimentary rocks that may be more than 35,000 feet thick has been tightly folded along northeast-trending axes, intruded by large plutons, and variably metamorphosed. Two generations of schistosity are recognized. The older schistosity, S 2 , pervades pelitic strata in most of the area and grades from slaty through phyllitic cleavage to schistosity, roughly with increasing metamorphic grade. It is typically nearly parallel to axial surfaces of the major folds of the area. At several localities thinly tabular metasandstone and metashale dikes are about parallel to S 2 . They are best developed near the axial surfaces of folds, but are present along fold limbs as well, and have been found in all the major pelitic units of the area. Metasandstone dikes, the largest of which are about half an inch thick, at least 6 feet high, and of unknown horizontal extent, characteristically extend downward from graded beds of metasandstone. Some are paired with metashale dikes that extend upward from metashale beds along refracted cleavage in metasandstone. At places folded beds of meta-sandstone are penetrated by many closely spaced paper-thin to half- inch-thick metashale dikes, producing a compositional cleavage lamination. The discovery of similar clastic dikes approximately parallel to slaty cleavage in the slates of the Delaware Water Gap area of Pennsylvania and New Jersey led J. C. Maxwell to the conclusion that slate exposed there is an end product of compression, folding, compaction, and dewatering of a thick mass of rapidly subsiding, nearly impermeable pelitic sediments. This process, which may be called tectonic compaction, best explains S 2 schistosity in the Rangeley-Phillips area. It was followed here, however, by metamorphism, which was accompanied by growth of micas and other minerals parallel to the old S 2 fabric, producing phyllites and schists. Later the rocks were sheared and metamorphosed again, which caused younger slip cleavage and schistosity, S 3 , in the manner proposed by W. S. White in Vermont.

Basalts of the Mount Taylor Volcanic Field, New Mexico

Basalt flows of the upper Cenozoic Mount Taylor volcanic field in central New Mexico are divisible into four stratigraphic sequences that define the general petrologic progression: early undersaturated alkalic basalt, two sequences of relatively silicic alkalic basalt, and late olivine tholeiite. This progression is characterized by increasing SiO 2 and decreasing K 2 O, TiO 2 , P 2 O 5 , Sr, U, and Th. Recent experimental data permit interpretation of such a progression in terms of fractionation of magma at progressively shallower levels in the upper mantle. In the western United States, Cenozoic basalts of such compositional diversity occur mainly along boundaries between major tectonic provinces, especially around the margins of the Basin and Range province and the Colorado Plateau.

Precambrian Folding in the Idaho Springs-Central City Area, Front Range, Colorado

Metasedimentary gneisses in the Idaho Springs-Central City area of the Front Range have been deformed twice in Precambrian time. The older and major deformation was plastic folding; it was accompanied by intrusion of a series of plutons and sheets, recrystallization of the meta-sedimentary rocks, and metamorphism of the earlier members of the igneous series. It produced a major fold system consisting mainly of open, but disharmonic asymmetric and upright anticlines and synclines whose axes trend sinuously north-northeast and are spaced 1-2 miles apart. Over most of the area the fold axes are nearly horizontal or plunge at low angles, but in the southern part the axes plunge steeply northeastward. Small folds and a well-developed mineral alignment characteristically parallel the major fold axes ( B o ); small-scale folds, boudinage and sparse mineral alignment are present in the A o direction. The younger deformation was dominantly cataclastic and restricted chiefly to a 2-mile-wide zone in the southeast part of the area. Within this zone small folds were developed locally in the relatively incompetent rock masses, and intense granulation was developed locally in the more competent units. Cataclastic products are pervasively distributed, however, through all the rocks in the zone. The younger folds are mainly terrace, monoclinal, and chevron types; the largest has a breadth of about 400 feet. These folds trend N. 55° E., are remarkably straight, and plunge at various angles, largely depending upon their position on the older, larger folds. They consistently are strongly asymmetric and show their northwest limbs raised structurally. Associated with these folds are two lineations, one (B y ) parallel to the fold axes and one (A y ) oriented at about 80° to the fold axes. The younger deformation is a manifestation of extensive Precambrian shearing defined by a zone of intense cataclasis that extends both northeast and southwest of this region.

The Piermont allochthon revisited and redefined at its type locality: Discussion

[Timms (2004)][1] has presented a very restricted interpretation of an important structural feature in the New England Appalachians that I originally termed the Piermont allochthon and interpreted as a far-traveled Acadian thrust sheet ([Moench et al., 1987][2]; [Moench, 1990][3]). As described