Stratigraphy and ductile structure of the Presidential Range, New Hampshire: Tectonic implications for the Acadian orogeny J. Dykstra Eusden Jr. John M. Garesche Aaron H. Johnson Jenna-Marie Maconochie Department of Geology, Bates College, Lewiston, Maine 04240 Steven P. Peters Jonathan B. O’Brien Beth L. Widmann } ABSTRACT mation is interpreted as an olistostromal fected by the Early Devonian Acadian melange and has been subdivided into three orogeny. These rocks are extremely well ex- The Presidential Range of New Hamp- different members and six submembers on posed, thus offering a rare opportunity to shire with its unique high relief contains ex- the basis of lithologic variations in the accurately and clearly describe the complex ceptionally well exposed rocks of the Aca- gneiss and subordinate units. stratigraphic and structural features in this dian orogenic hinterland. These rocks are The Presidential Range preserves three part of the northern Appalachians. Though within the Central Maine terrane, a belt of phases of ductile folding (F1, F2, and F3) the Acadian orogeny has been comprehen- complexly metamorphosed and deformed and a single phase of thrust faulting (T1). sively studied by numerous geologists, Aca- Silurian and Devonian metasedimentary Well-constrained southeast-, east- or north- dian tectonic models remain controversial rocks. The Presidential Range lies between east-facing directions and consistent asym- due to the lack of distinctive lithotectonic regions to the south dominated by refolded metry of F1 nappe limbs suggest that nappe assemblages. Notably absent from the nappes and synkinematic high-grade re- vergence was easterly. Severe refolding of Acadian orogen is obducted ocean crust, gional metamorphism and regions to the some nappes resulted during F2 collapse accretionary melange, or unequivocal arc northeast dominated by overlapping multi- of the nappe pile. The west-vergent (?) volcanics. Additionally, effects of the Aca- ple deformation and contact metamorphism Greenough Spring thrust fault truncated F1 dian orogeny vary considerably along the produced by late- and post-Acadian plu- and F2 structures and juxtaposed nonmig- length of the orogen. For example, the Pres- tons. To better understand these complex matized and migmatized rocks. F3 east-ver- idential Range lies between regions to the tectonic variations and, in particular, to gent folding dominates the alpine zone and south dominated by refolded nappes and clearly and accurately determine the effects represents the final tightening of Acadian synkinematic high-grade regional metamor- of the Devonian Acadian orogeny in a well- structures, which occurred after early gra- phism and regions to the northeast domi- exposed region, the bedrock geology in the nitic plutonism and prior to the emplace- nated by overlapping multiple deformation alpine zone of the Presidential Range was ment of postkinematic granitic sheets. and contact metamorphism produced by mapped in great detail. The results of the The structural and sedimentological evi- late- and post-Acadian plutons. Our ap- stratigraphic and ductile structural analy- dence supports the existence of a subduc- proach toward a better understanding of the ses are presented here. tion system that was active beginning in the Acadian orogeny is a combined analysis of Five metasedimentary formations have early Silurian on the east flank of the Bron- the stratigraphy and ductile deformation been recognized and constitute a conform- son Hill Anticlinorium with a west-dipping and the relationship of these geologic ele- able stratigraphy, which, from oldest to subducting slab geometry. This system per- ments to possible tectonic models. We have youngest, are the Silurian (?) Rangeley, sisted throughout the Acadian. done this at a detailed scale to reveal with Perry Mountain, Smalls Falls, and Madrid clarity the nature of deformation in the Ap- Formations, and the Devonian (?) Littleton INTRODUCTION palachian hinterland. Formation. The Littleton Formation has The Presidential Range is located near been subdivided into fifteen different mem- This paper details the results of a five-year the axis of the Central Maine Terrane bers and three submembers based on vari- project to map the bedrock geology in the (Fig. 1; Zen et al., 1986; Rankin, 1994; ations in bedding style of the schists and alpine zone of the Presidential Range, New Thompson et al., 1993; Stewart et al., 1993) quartzites and other lithologic peculiarities. Hampshire. The Presidential Range is an and is near the center of the large area of The Madrid, Smalls Falls, and Perry Moun- important region for geologic study as it Acadian high-grade metamorphism. To the tains Formations are extremely thin, con- contains Silurian and Devonian metasedi- south and east, the Central Maine Ter- sistent with their deposition near the mentary cover rocks that postdate the Tac- rane is bounded by the Massabesic Gneiss Silurian tectonic hinge. The Rangeley For- onic orogeny and have been significantly af- Complex, an outlier of late Precambrian GSA Bulletin; April 1996; v. 108; no. 4; p. 417–436; 17 figures; 2 inserts. 417 Figure 1. Simplified structural map of New Hampshire showing the location of the study area. 418 Geological Society of America Bulletin, April 1996 STRATIGRAPHY AND STRUCTURE OF THE PRESIDENTIAL RANGE Avalonian (?) basement, and the Merri- Mount Washington, the highest wind speed These stratigraphic correlations are used on mack-Harpswell terrane, composed of meta- ever recorded in the world gusted at 231 the bedrock geologic map of New Hamp- sedimentary rocks of uncertain age (possibly mph. Gusts of over 100 mph have been re- shire by Lyons et al. (1992). Silurian to Precambrian) (Rankin, 1994; corded every month of the year (AMC, The Silurian metasedimentary rocks of Thompson et al., 1993; Stewart et al., 1993). 1988) and were frequent throughout the the Presidential Range represent the middle To the west and northwest, the Central field seasons. to distal portion (Moench and Pankiwskyj, Maine Terrane abuts the Bronson Hill An- The Presidential Range, with its unique 1988) of a depositional basin, the Kronos ticlinorium, which represents a magmatic natural environment, has always been a ha- Ocean (Berry and Osberg, 1989), that was arc of Ordovician age, with a cover of thin ven for scientific exploration. Many topo- flanked by the Bronson Hill Volcanic Arc on Silurian and somewhat thicker Devonian graphical features have been named after the west and the microcontinent, Avalon, on sediments. The Central Maine Terrane is re- scientific researchers. Jackson, Huntington, the east (Berry and Osberg, 1989; Bradley, garded as an eastward-thickening succession Hitchcock, and Agassiz were among the re- 1983). Within the deepest portions of the of deep-water turbidites, adjacent to the searchers who worked in the Presidential basin, Silurian deposition accounted for up Bronson Hill Anticlinorium, filled with Si- Range during the 1800s. Many geologists to 3.5 km of siliciclastic and lesser carbonate lurian shales and conglomerates (Rangeley have since studied various aspects of the ge- turbidites, which are covered by up to 2.5 km Formation), quartzose turbidites (Perry ology, and research continues today. We dis- of Devonian turbidites (Hatch et al., 1983; Mountain Formation), anoxic shales (Smalls cuss more recent studies done during this Hatch and Moench, 1984; Moench and Falls Formation), and calcareous turbidites century, including the classic works of Bill- Pankiwskyj, 1988). (Madrid Formation), and topped by early ings and Fowler-Billings. The Silurian Rangeley, Perry Mountain, Devonian turbidites and minor volcanics and Smalls Falls Formations are believed to from an eastern source (Littleton Forma- Previous Work have a southeast sediment transport direc- tion) (Moench and Pankiwskyj, 1988; Han- tion, shed from the eroding Bronson Hill arc son and Bradley, 1989). This basin, together Stratigraphy. Billings (1941) originally (Hanson and Bradley, 1989). The sediments with the Bronson Hill Anticlinorium, was correlated the metamorphic rocks of the of the Madrid Formation were transported multiply deformed and metamorphosed Mount Washington to Silurian and Devo- along the axis of the basin, toward the south- during the large-scale crustal shortening and nian formations 40.2 km (25 mi) to the west west (Hanson and Bradley, 1989). The De- thickening of the Acadian orogeny. in the Littleton–Mount Moosilauke area of vonian Littleton sediments had an overall New Hampshire. Billings et al. (1946), Bill- northwesterly transport direction, toward THE PRESIDENTIAL RANGE ings and Fowler-Billings (1975), and Billings the margin of the pre-Acadian North Amer- et al. (1979) subsequently reassigned all of ican continent (Hanson and Bradley, 1993). The Presidential Range of New Hamp- these metamorphic rocks to the Devonian The basin discussed above essentially de- shire is one of the most popular recreational Littleton Formation, by introducing the scribes an extensional or passive tectonic areas in the Northeast. Culminating with Boott member and splitting the Littleton setting throughout the deposition of the Si- Mount Washington, elevation 1916.5 m Formation into upper and lower members. lurian and Devonian sediments. Although (6288 ft; the highest peak in the Northeast), Hatch et al. (1983) correlated the meta- this model has been widely accepted, it is the Range has an extensive alpine zone sedimentary rocks in the Pinkham Notch often difficult to rationalize