The Nappe Theory in the Connecticut

American Mineralogist, Volume 76, pages 689-712, 1991

The nappe theory in the ConnecticutValley region: Thirty-five years since Jim Thompson'sfirst proposal Pprn,nRonrNsor.l Department of Geology and Geography,University of Massachusetts,Amherst, Massachusetts01003, U.S.A. Prrnn J. TnonrpsoN Department of Geology, Cornell College,Mount Vernon, Iowa 52314, U.S.A. D,q,vroC. Er-nnnr Department of Geology, Middlebury College,Middlebury, Vermont 05753, U.S.A.

AssrRAcr The nappe theory in the Bronson Hill anticlinorium of the Connecticut Valley region began with Jim Thompson's analysis of the structure of SkitchewaugMountain in 1954. By 1968 the theory included the recognition of three giant fold nappesformed early in the Devonian Acadian orogeny, with tens of kilometers of east to west overfolding. These were, from lowest to highest,the Cornish, the Skitchewaug,and the Fall Mountain nappes. Recognition ofthe nappeswas basedon careful tracing ofa previously recognizedsequence of distinctive Ordovician, Silurian, and Lower Devonian stratigraphic units that were metamorphosed to the chlorite zone through sillimanite-orthoclase zone. Some of these units proved to be fossiliferous even in the sillimanite zone. Early on, a relationship was recognizedbetween high metamorphic grade and high structural level, implying overfold- ing ofhotter rocks onto cooler ones. In the 1970sdetailed remapping suggestedthat the system included not three but four fold nappes. Some of the rocks previously assignedto the Skitchewauglevel proved to belong to the Bernardston nappe beneath the Skitchewaug.Although shearedand atten- uated fold limbs were part of the nappe theory before 1980, it was only after this that major thrust faults were considered to be important. The theory of major thrusts came about through refinements in stratigraphy, particularly in the Merrimack synclinorium. New mapping near Mount Monadnock, based on the new stratigraphic sequence,has shown that the axial surfacesof the early west-directed fold nappes are truncated above by a major west-directed thrust, the Chesham Pond thrust, which carries higher grade metamorphic and plutonic rocks. To the west in the Hinsdale area,folds in Monadnock sequencestratigraphy are truncated below by a lower thrust, the Brennan Hill thrust, which in turn cuts into the Bernardston fold nappe below. Stratigraphic and structural evidence from the Monadnock and Hinsdale areas provides the key for reevaluation of the inter- action of fold nappes, thrusts, backfolds, and overturned gneiss domes in west-central Massachusetts. Metamorphic studies show different P-Tpaths for rocks on opposite sidesof the thrusts, thus indicating that the thermal structure was as much related to thrusts as to folds, but the final pattern ofisograds appearsto have been superimposedon both. The relationship betweenkinematics and thermal history in the nappeswill be studied for years to come.

INrnonucltotl a paraphrase from the book, The Nappe Theory in the We discussan area in the central New England Appa- l/ps (Heritsch, 1929). Our objective is to explain the de- lachiansalong a tectonic belt extendingfrom the Con- velopmentofatheorythatformationofthegneissdomes preceded west-direct- necticut Valley eastward for about 70 [m Gig. l). The was by formation of regional-scale most striking feature of this region is a row of mantled ed fold and thrust nappes akin to those of the Pennine gneissdomes, the Bronson Hill anticlinorium, in which Zone in the European Alps. This theory began with tonalitic gneissesand related rocks are exposedbeneath Thompson's first proposal published in the Guidebook a well-definedcover sequenceof Ordovician, Silurian and of the New England Intercollegiate Geological Confer- Devonian rocks.The domesare believedto have devel- ence in October 1954 (Thompson, 1954; see also oped during the Devonian Acadian Orogeny. Our title is Thompson, 1956). 0003-o04x/91/0506{689$02.00 689 690 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

4 CONNECTICUT VALLEY BELT MERRIMACKBELT

3rLEMTN. I Lower ____l ERVING ffAITSR. I Devonian LITTLETON LITTLETON

FITCH WARNER MADRID FRANCESTOWNSMALLSFALLS Silurian CLOUGH PERRYMTN. RANGELEY

PARTRIDGE Late AMMONOOSUC Ordovician PLAGIOCLASE GNEISSES

Late MICHOCLINEGNEISS Protgrozoic QUARTZITES

Fig.2. Columnof stratigraphicunits of the ConnecticutVal- ley belt (BronsonHill anticlinorium)and of the Merrimackbelt (synclinorium).

The oldest strata exposedin the Bronson Hill anticli- norium, in the core of the Pelham gneiss dome, are gneisses,quartzites, and subordinate schistsand amphib- olites, believed to be a supracrustalsequence of Late Pro- terozoic age. The stratigraphically deepestunit, the Dry Hill Gneiss (Ashenden, 1973), consisting of microcline- biotite-hastingsite gneiss and interpreted as metamor- phosed alkali rhyolite (Hodgkins, 1985), has recently yielded a high-precisionzircon ageof 613 + 3 Ma (Tuck- er and Robinson, 1990). The Dry Hill Gneiss is grada- tionally overlain by metamorphosed sedimentary units: the Poplar Mountain Gneiss, consisting of metamor- phosed arkose and quartzite, and its more southerly fa- cies,the Mount Mineral Formation, dominated by pelitic schists,quartzites, and amphibolites. In the latter, relics of an earlier granulite-facies metamorphism have been Fig. l. Geologicindex map of west-centralNew England detected beneath the predominant overprint of kyanite- showingthe gneissdomes (black) of the BronsonHill anticli- staurolite zone metamorphism of Acadian age(Robinson norium.From Zenet al. (1968).Owls Head dome mentioned in et al., 1975', Roll, 1986,1987). text lieson latitudeline southwestof Berlin. The next major stratigraphic subdivision consists of the rocks that overlie the Proterozoic rocks in the Pelham dome and form the coresof all of the other domes. These SrnArrcnlpHy oF THE BnoNsou Hrr,r, rockswere originally assignedby Billings (1937, 1956)to ANTICLINORIUM the Oliverian plutonic series,which he supposedto be a The story is dependenton a superbstratigraphic frame- group of Devonian intrusions. They have been consid- work established over decades.A 1949 quotation from ered to be Ordovician or older since the radiometric dat- Billings (1950) is "And lo! stratigraphy led all the rest," ing of zircons by Naylor (1969) from the core of the Mas- a sentiment fully confirmed by our story here. The key coma dome. Although most of these rocks resemble single paper for this stratigraphic framework is Billings's deformed intrusive igneousrocks, they include extensive report on the Littleton-Mooselauke area (Billings, 1937). tracts of strongly layered gneissesand amphibolites re- The essentialcolumn of stratigraphic units for the Bron- semblingmetamorphosed volcanics. Recent work on such son Hill anticlinorium illustrated in Figure 2 comesfrom rocks in large exposuresin the Quabbin Reservoir area this paper with only a few modifications. It was J. B. by Hollocher and Robinson (Robinsonet al., 1989)sug- Thompson's imaginative and creative application of Bill- geststhat the strongly layered gneissesare probably high- ings's stratigraphy that made the nappe theory possible. ly deformed intrusive igneous rocks, originally domi- ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 691 nantly tonalite with a profusion of xenoliths and cross- SELECTED cutting dikes of more mafic rock types. The most recent SYSTEM SERIES CONODONT FOSSIL-BASEDAGE RANGES, OR STAGE ZONES enottsot'lnill nrurtcltruontuir,t agesof these rocks in Massachusettsand southern New l-Littt"ton Fofmation, Litlleton Hampshire obtained from U-Pb in zircon (Tucker and E msian and Whi(efield quadrangles. z. I I Robinson, 1990) suggestthat they fall in the narrow age l-Lilll€lon Formation, range from 454 to 442 + 3/ -2 Ma (Caradocto Ashgill), z; Pragian Beaver Brook, 9= N H2 despiterepeated attempts to identifu older rocks. Contro- , elta Fitch Formalion, o- Mass 3 versy still exists as to the nature ofthe contact between ,u reRaensis l-Bglnardslon, these rocks, Ordovician cover se- I and the overlying uosch midti I quence.It has beenproposed as an intrusive contact (Bill- ings, 1937;Leo et al., 1984;Zartman and ko, 1985),as P rido lian rcmsch e iden sis l-r,,.n Fo.mation, Liltleton,N. H. a (Naylor, I a normal stratigraphic contact 1969), as an un- z L u d lovian conformity (Robinson, 1979, 1981) based on several tr We n lockian quartzite localities at the base of the Ammonoosuc, and J Clouoh Ouartzil€. most recently as a possibledetachment fault (Tucker and a l-Crov6on Mountain. I H.tis Robinson, I 990). Fortunately for our presentstory it can L Iandovo ria be treated simply as another contact within an ordered sequenceof rock layers. Fig. 3. Correlationchart showing fossil-based age estimates The basal unit ofthe Ordovician cover sequenceis the for five setsof Silurian-Devonianstrata from Massachusettsand Ammonoosuc Volcanics, long thought to be Ordovician New Hampshire(from Elbertet al., 1988).References: I : bra- on the basis of regional correlation (Billings, 1937; Rob- chiopods(Boucot and Arndt, 1960\,2: brachiopods(Boucot : : inson, 19 6 3, 197 9 Z,arrmanand ko, 198 5) and now con- and Rumble,1980), 3 conodonts(Elbert et al., 1988),4 ; : firmed as Caradoc by a U-Pb zircon age of 453 + 2 Ma conodonts(Harris et al., 1983),5 brachiopodsand corals (Boucot 1963;Boucot et al., 1958). on a metamorphosed rhyolite from the upper member andThompson, (Tucker and Robinson, 1990). In Massachusettsand southwesternNew Hampshire, where the formation ranges macher and Robinson, 1987; Schumacher,1988; in thickness from less than 30 m to more than 1000 m, Hollocher, 1985). Such assemblagesare nearly unknown the Ammonoosuc Volcanics contain a detailed and com- in the underlying plagioclasegneisses (Hollocher and Lent, plex stratigraphic sequence(Robinson, 1963; Schumach- l987) and in other members of the cover sequence. er, 1988). A lower member is dominated by metamor- The age of the younger part of the stratigraphic column phosed tholeiitic basalts, andesites,and low-K dacites, is more firmly controlled by fossils (Fig. 3), the story of and their hydrothermally altered equivalents, showing which was punctuated on September28, 1870, by C. H. numerous primary features including pillows, conglom- Hitchcock's telegram from the Littleton railroad station, erates,agglomerates, and tuffs. The discontinuous middle "New Hampshire no longer Azoic, Silurian fossils found member is an unusual layer of garnet-amphibole quartz- today" (Hitchcock, I87l). Hitchcock's Silurian designa- ite believed to represent metamorphosed femrginous tion was assignedto rocks later named the Fitch For- chert. The upper member consistsof biotite-muscovite- mation by Billings (1937), and it still holds today (Harris garnet gneiss representing metamorphosed rhyolitic to et al., 1983).However, fossilswere found yearsearlier by dacitic tuff. Hitchcock's father, Edward (1833), in rocks also now as- The upper part ofthe Ordovician cover sequenceis the signedto the Fitch Formation at Bernardston,Massachu- Partridge Formation, dominated by metamorphosed black setts as recently pointed out by Elbert et al. (1988). shales,now graphite-pyrrhotite schists,with subordinate More prophetic was Hitchcock's statement in l9l2 layersand lensesof metamorphosedvolcanics chemically concerning the Clough Quartzite at Moose Mountain, similar to those of the underlying Ammonoosuc Volcan- "This is the key to the geology of this region." It was ics (Hollocher, 1985). The Partridge has also long been essentially Jim Thompson's persistent belief that rocks correlated on the basis of rock types with Caradocian resembling the Clough Quartzite should be assigned to graptolite-bearing slates in northwestern Maine and that unit, despite structural difficulties, that produced the southern Quebec, and this age has now been confirmed nappe theory as we understand it. The Clough is distinc- by a U-Pb zircon age of 449 + 3/ -2 Ma from a bed of tive for three reasons.It is an easily recognizableunit of quartz-phyric rhyolite tuf near the baseof the formation buff-to-white qr:prrtzite and quartz-pebble and quartz- at Bernardston, Massachusetts(Tucker and Robinson, cobble conglomerate(Robinson, 1963; Hatch et al., 1988) 1990). Striking in the metamorphosed volcanics of the that commonly makes resistantridges and peaks(Fig. 4). Ammonoosuc and Partridge are the abundant rocks with Of greater importance is that it rests on a region-wide compositions appropriate for the formation of highly var- unconformity, that can be recognizedeven in highly de- ied assemblagesof amphiboles including hornblende, formed locations, and has been found resting on the pla- cummingtonite, anthophyllite, and gedrite in combina- gioclase gneissesof the domes, on various members of tions with cordierite and other aluminous minerals (Rob- the Ammonoosuc Volcanics, and on the Partridge For- insonet al., 1982b,1986; Spear and Rumble, 1986;Schu- mation, where it is hundreds of meters thick' Such a sig- 692 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

the same area (Fig. 3) have indicated a latest Silurian Pridoli age(Harris et al., 1983),whereas conodonts from Bernardston suggestthe rocks are Lochkovian in the ear- liest Devonian (Elbert et al., 1988). Together, the Clough Quartzite and Fitch Formation represent a period of slow shelf-type deposition on the future site of the Bronson Hill anticlinorium, beginning with a coarse but chemically very mature basal unit of stream or beach origin, followed by increasingly more distal and presumably slowly deposited shallow marine deposits. Locally the Clough and Fitch are up to several hundred meters thick, but the Fitch is commonly absent over long distances and the Clough extends over wide areas with a thickness of no more than l0 m. The dis- continuous nature of the Fitch as well as the Clough sug- geststhere was probably a period of erosion after depo- sition of the earliest Devonian part of the Fitch and prior to deposition of the overlying Littleton Formation (Hatch et al., 1988;Elbert et al., 1988). The Littleton Formation is a key unit of widespread metamorphosedgraphitic shale and interbedded quartz- ose sandstone.It is the youngestunit in the classicBron- son Hill sequenceat Littleton, New Hampshire and is thought to represent the "flysch" derived from tectonic lands to the east in the early stagesof the Acadian Orog- eny (Hatch et al., 1988). The age of the Litrleton at Lit- tleton, New Hampshiro is firmly tied to the Lower Em- sian of the late Lower Devonian (Fig. 3) on the basis of brachiopods originally found by Billings (Billings and I Fig.4. Jim Thompsonon an outcropofcoarse cobble con- Cleaves, 934; Boucot and Arndt, I 960). The Erving For- mation, the Waits River Formation, and the Gile Moun- glomerateof the CloughQuartzite near the summit of Surry Mountain,July 10,1964. tain Formation occur adjacent to the Littleton in the westem part of the Bronson Hill anticlinorium and in the Connecticut Valley synclinorium (Hatch et al., 1988; nificant unconformity cannot be ignored or passedoffas Robinson et al., 1988a).The Erving consistsof well bed- a faciesrelation, and thus servedas a firm mental concept ded biotite-quartz-plagioclase granulite and laminated in the grueling task of regional mapping. In addition, the epidote amphibolite with subordinate interbeds of gray uppor part ofthe Clough is fossiliferous,as first reported nongraphitic schist, calc-silicategranulite, and rare mar- by B. K. Emerson in 1877 at Bernardston,Massachusetts ble. The Waits River Formation consists of gray schist (Dana, 1877). Persistentchasing by Jim Thompson of interbedded with quartzose calcitic marble. The Gile the so-called pit rock, even into rocks of the sillimanite Mountain Formation consistsof gray schist and quartzo- zone, coupled with ample collections and diligent pale- feldspathic granulite with subordinate quartzosemarble. ontologic study by Arthur Boucot, have led to our present Waits River and Gile Mountain both contain local zones firm understanding of the age of the Clough (Fig. 3) as of amphibolite or greenschistrepresenting mafic to inter- late Llandovery (Boucot et al., 1958;Boucot and Thomp- mediate tuffs. The stratigraphic relations of these rocks son, 1963). to the Littleton is uncertain and controversial (Robinson The Clough is locally and discontinuously overlain by et al., 1988a;Hatch, 1987).In the view favoredby Rob- calcareousrocks of the Fitch Formation including calc- inson, based on field relations in Massachusetts(Robin- silicate granulites, biotite-plagioclase granulites and son et al., 1988a),the Erving is consideredto be younger schists, and rare marbles (Billings, 1937; Hatch et al., than and unconformably above the Littleton, and the 1988). (Granulite is used in this paper to denote a meta- Waits River and overlying Gile Mountain are considered morphic rock dominated by granular mineral constitu- to be westward facies of the Erving. In this model, the ents, without any connotation for metamorphic facies.) line along which the upward-facing Gile Mountain is The Fitch Formation at Littleton, New Hampshire, yield- overlain to the east by the upward-facing Littleton, known ed the fossils first recorded by Hitchcock and the bra- locally as the Chicken Yard line (J. B. Thompson and J. chiopods studied more extensively by Biltings and Cleaves L. Rosenfeld,personal communication, 1961),must nec- (1934)and by Berry and Boucot (1970)to determinean essarilybe a thrust (seebelow). early Ludlow age. More recent work on conodonts from In addition to the sequenceof strata, two major sheet- ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 693

Fig. 6. Cartoon sketchof fold interferencepattern at Skit- chewaugMountain. From a field sketchby D.U. Wise.Shows CloughQuartzite wrapped to the north by stratigraphicallyhigh- er Fitch Formation(brick pattern).East-trending anticlinal hinge of the nappeis refoldedby a gentlynorth-plunging syncline.

the 1954 NEIGC Guidebook. Within the heart, a small area of Partridge Formation is surrounded by a contin- uous layer of Clough Quartzite and an incomplete rim of Fitch Formation, all surrounded by Littleton Formation. Thompson's interpretation of this feature is that of a lo- BELLOWSFALLS PLUTON cally east-trending recumbent anticline refolded by a north-trending open syncline (Fig. 6). In seekingthe roots of this isolated structural feature, Thompson ranged to the east side of the Unity dome and north end of the Alstead dome, where he found the appropriate right-side- up autochthon and inverted allochthon, though not yet any synclinal hinges. Thompson's map was drawn at the last minute, and he felt that it was self-explanatory. In editing the guidebook, John Lyons felt that the average Fig. 5. Map and crosssection ofthe regionaround Skitche- reader might need the assistanceof a cross section and waugMountain, Vermont, from Thompson's1954 guidebook added his best interpretation at the bottom. Note in this article.For key to patternssee Figure I I exceptbrick pattern version that the Bethlehem Gneiss of the Mount usedto distinguishSilurian Fitch Formation. early Clough pluton lies in the core of the Skitchewaugnappe, rather than on top ofit, as was found out shortly later. like intrusions played a quasi-stratigraphicrole in the in- During the next several years, the nappe theory flour- terpretation of the nappesof the Bronson Hill anticlinor- ished and expandedbecause of Thompson's enthusiasm, ium (Thompson et al., 1968). These are the Mount the enlistment of Tom N. Clifford (a visiting Common- Clough-Bellows Falls pluton of Bethlehem Gneiss, and wealth Fellow from keds) and the continuing interest of the Cardigan-Ashuelot pluton of Kinsman Granite (Quartz Arthur Boucot in the newer and better fossil localities. Monzonite in the obsoleteterminology). The subsequent- This was but the beginning of the first of three major ly defined stratigraphy of the Merrimack synclinorium phasesin the development ofthe nappe theory described plays a role only in the later part of our story and is below. describedlater. Tnnonv oF THREE FoLD NAPPES Bn'crNNrNc oF THE NAPPETHEoRY In the 1950s and early 1960s the nappe theory grew In 1954 enters our hero for this volume, Jim Thomp- and expanded in its territory. In 1959 Robinson was son, a thorough and diligent worker in easternVermont, tempted by Thompson to seeif it could be applied in the but so far repelled from New Hampshire by stern warn- Mount Grace area in northern Massachusettspreviously ings from Marland Billings. The scene is Skitchewaug mappedby Hadley (1949).In 1961 N. J. Trask begana Mountain near Springfield, Vermont, long before recog- reinterpretation from the Bernardston area into south- nized by Richardson(1931) as possibly a piece of New western New Hampshire. The retirement of M. P. Bill- Hampshire geology thrust into Vermont and more re- ings in 1968 provided the impetus for the first and only cently known for its traces ofcoral and other calcareous general account of the nappe theory in the region fossils in the Clough Quartzite. This shows as a heart- (Thompsonet al., 1968). shapedmap pattern bisectedby the Connecticut River in In the 1968 version there are some changesofdetail in the northwestern part of Figure 5, Thompson's map for the region around Skitchewaug Mountain and the Unity 694 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

Fig. 7. Northern part of Plate 15-la of Thompson et al. ( 1968) with cross sectionsA-A', B-B', and C-C' from their Plate 15- lb. Map area patterns oforiginal colored version have been modified here for black and white printing and are explained in Figure I l. Key to numbered localities and credit for map coverageare also given in Figure I l. ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 695

dome (Fig. 7, rnap and section B-B'), including the iden- 0'l 2 Mesozoic horst defined by the Connecticut tification of a Miles Valley fault on the west and the Grantham fault on the east separatingthe anticlinal nappe hinge at Skitchewaug MASCOMA Mountain from its root zone to the east.Another notable change was the identification of Clough Quartzite and 4'.lt Fitch Formation on the right-side-up upper limb of the {/ Skitchewauganticlinal fold east of the Unity dome, thus ra. 2l rl placing the Bethlehem Gneiss in the "stratigraphic posi- ttt -t: tion" of the Littleton Formation above the nappe. From , > at' this region northeastward, the three levels of Clough LtrL Quartzite could be identified, on the autochthon of the domes, on the inverted limb of the Skitchewaugnappe, | and on the right-side-up limb of the nappe. In everyday ///, ll ,, ,t conversation,Thompson termed thesethe "lower deck," '// ll *t\ "middle deck," and "upper deck." On the upthrown west // IT t fault, and Croy- lll.t side of the Grantham betweenthe Unity .ir'. =1s nappe I don domes, only a tiny remnant of the Skitchewaug rr \rr1\7-. preserved next ,_y'r, is from erosion,but in the axial depression \\ \\ \\!/ \tLlz///- 7 to the northeast, between the Croydon and Mascoma .ir " \\^ -/-/ r =--z domes, the Skitchewaugnappe is shown with an abun- \\ (Fig. Fig. MOUNT- CLOUGH dance of complexities 7, section A-A'; 8). i"' pluiorrr The exposures in the axial depression north of the \+ Croydon dome (Fig. 8) are important to the nappe theory for six reasons.(l) They show extensiveareas ofall three structural levels ("lower, middle, and upper decks"). (2) Fig. 8. Thompson'sdetailed map of the axialdepression be- give They evidenceofa northeast trend ofthe anticlinal tweenthe Croydonand Mascomadomes, modified from Rob- hinge rather than an abnormal east trend as at Skitche- insonet al. (1979;see also Thompsonet al., 1986;or J.B. waug Mountain. (3) They show some evidencefor a com- Thompson,1988). Location is southof MascomaLake shown plementary synclinal hinge, suggestingthat the amplitude at top ofFigure 7. of the nappe at this latitude is about 5 km contrasted with a probable amplitude of at least 20 km farther south (seesection B-B). (4) They demonstratedramatically the limb, however, the place of the Littleton Formation is position of the Mount Clough pluton of Bethlehem Gneiss taken by the Gile Mountain Formation of the Connecti- riding above the Fitch Formation on the upper limb of cut Valley synclinorium sequence. the nappe in the position of the Littleton Formation. (5) The work of Thompson and Clifford in the Bellows They show the variability in thickness of stratigraphic Falls region south of SkitchewaugMountain led to further units with respect to the nappe, with thick regions close discoveries. Here the Skitchewaugnappe was shown to to the anticlinal hinges, and severeattenuation closer to pass under the Bellows Falls pluton and into a large re- the root zones, particularly in the middle deck, which gion of gently dipping foliation northwest of the Keene locally has nearly the character of a thrust fault zone. dome. Above the Bellows Falls pluton at Fall Mountain They also show an interesting northward "forking" ofthe on the west side of the Connecticut Valley fault (Fig. 7, nappe shown by the pattern of the Partridge Formation locality 8), a still higher inverted sequencewas identified south of Mascoma Lake. (6) The regions of Clough (Fig. 7, section C-C). At first assigrredto a mythical Quartzite close to anticlinal hinges have provided the most mother nappeabove the Skitchewaug,this was eventually crucial fossils (Fig. 3), and they come from rocks in the named for Fall Mountain. Thus for the first time the sys- sillimanite zone (Boucot and Thompson, 1963; see also tem of nappes was seento consist of three: the Skitche- Robinson et al., 19791'Thompson et al., 1986; or J. B. waug in the middle, the Cornish below, and the Fall Thompson, 1988)where the calcite fossils are commonly Mountain above (Fig. 9). surrounded by such minerals as diopside and grossular. The root zones ofthe Skitchewaugnappe were vigor- In the region north of SkitchewaugMountain and west ously pursued by Thompson and Clifford in the region of the Croydon dome between Claremont and Lebanon, around the Alstead dome and eventually by Thompson a large fold nappe at a lower level was identified and in the axial depressionbetween the Keene and Alstead named for the town of Cornish (Fig. 7, sectionA-A'). The domes (Fig. 10). Here at last could be seen three expo- strata on the upper limb are of the Bronson Hill sequence. sures of the same synclinal hinge of the nappe highly The oldest rock unit in the nappe is the Post Pond Vol- deformed by later structural features. On the northeast canics,interpreted by Thompson et al. (1968) to be iden- flank of the Keene dome, an odd sock-shaped hinge tical with the Ammonoosuc Volcanics. In the inverted plungessteeply into the Surry Mountain syncline,is tight- 696 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

ALSTEAD DOME

Fig.9. Schematiceast-west cross section illustrating the the- ory ofthree fold nappesas developedup to 1968.patterns for Ordovicianstrata at threestructural levels and foliated Acadian intrusionsare the sameas usedin Figures5,7,8, lO, ll, 12, I and13. KEENE

DOME 'l ly bent in the bottom of that syncline, and emergesagain on the southwest flank of the Alstead dome. The hinge then passesover the southeastcorner of the Alstead dome and plungesnortheast into the southeastlimb of the dome in a very tight attenuated hinge. Taking the three hinge ^L locations together, the original overall l_' trend of the syn- o l/z { : ^^.^^iDKEENE clinal hinge appears to have been toward the northeast, L_l___J ^ rllt 7 Mile l:i:1, DOME and the amplitude of the nappe from this location to the \ anticlinal hinge near Bellows Falls would have been ap- proximately 20 km. An odd feature of the sock-shaped Fig. 10. Thompson'sdetailed map of the axial depression hinge at Surry Mountain is that the cobble conglomerate betweenthe Keeneand Alsteaddomes near Surry Mountain of the Clough has nearly spherical cobbles (see Fig. 4), showingthree synclinal hinges originally interpreted as belong- whereasonly a few hundred meters away on the limbs, ing to the Skitchewaugnappe. Adapted from Traskand Thomp- the same conglomerate has cobbles shaped like sword son (1967).Patterns same as in key to FigureI l. For location blades.The nature and sequenceofdevelopment ofstrain seenumber 10 near bottom ofFigure 7. fabrics of this type still awaits a definitive study. Moore (1949) had mapped the strata in a small graben on the roof of the west lobe of the Keene dome at Hyland northerly when proper account is made of the horizontal Hill (Figs. 7 and ll, locality I l). This included the nor- component of the dip-slip displacementon the Connect- mal Bronson Hill sequence,and then above the normal icut Valley fault. Trask also tentatively identified rem- Littleton, a second quartzite he labeled "Dlq" overlain nants of the Fitch Formation on the upper limb of the by more schist. One afternoon in the summer of 1959, nappe, immediately overlain by the Ashuelot pluton of Robinson showed that the DIq is an inverted limb of Kinsman Granite, apparently in the same structural po- Clough Quartzite in turn overlain by typical sulfidic schists sition as the Mount Clough pluton farther north (Fig. 12, and amphibolites of the Partridge Formation. section D-Dl. The early work of Robinson in the Mount Grace area Work in the Mount Grace quadrangle reached a climax (Fig. I l, localities 12 and I 3) showedthat there was early in the fall of 1966 when Jim Thompson viewed results recumbent folding, but connections with the synclinal root of Robinson's remapping of his 1963 thesis area. Stand- zone at Surry Mountain were not clear. West of the Con- ing upon an outcrop of an extensive unit previously necticut Valley border fault, on the other hand, Trask mapped as the Gray Member of the Partridge Formation, (1964) could show that an inverted limb, tentatively Thompson ofered this quiet prayer, "Oh rock, tell us equated with the Skitchewaug nappe, extends from the your ageand origin!" Within two hours it was answered. Bernardston fossil locality around the east side of the Yet another narrow belt of conglomerateassigned to the Vernon dome and into the axial depression between Clough Quartzite had been located, suggestingthat the Vernon dome and the western lobe of the Keene dome. "gray Partridge" was in fact Littleton, and setting the Thus was shown the identity of the Bernardstoninverted groundwork for the interpretation shown in Figure 13. In limb with the inverted limbs on Hyland Hill and on Surry this, the belt of inverted Clough in the Rum Brook syn- Mountain. Furthermore, Trask identified a synclinal hinge cline along the east flank of the Warwick dome is inter- in the narrow belt east ofSpofford Lake betweenthe west preted as continuous with the inverted belt at Bernards- lobe of the Keene dome and the Ashuelot pluton. When ton, offset vertically about 5 km by the Connecticut Valley this hinge is tied to that at Surry Mountain, the hinge fault. From the Rum Brook synclinethis inverted contact trend appearsto be about N45E but is considerablymore is traced eastwardaround the ends of the Camp Warwick ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 697

Fig. I l. Southernpart ofPlate l5-la ofThompson et al. (1968). Map area patterns oforiginal color version have been modified here for black and white printing. "Triassic" strata are now known to be Late Triassic and Early Jurassic(Zen et al., 1983; Robinson and Luttrell. 1985). 698 ROBINSON ET AL.:NAPPES IN THE CONNECTICUT VALLEY REGION

Foe+

S€o Levsl

-5000

SECTION ALONG LINE D_D'

..\l\ 'i'',, r!is li' l"l r E llrll Km Feel ll t'r: -2 5000 lll,l;

Seo Level :/\/\ /\ / \ /\/\ /\ | r- WARWICK 5000

t- ro,ooo /, \Y=i=r..*=---_-=-:-=: \/ {S0UTH LOBE): a :l-_--- :_-l -t5,ooo

SECTIONALONG LINE

Feet 5000

Seo Level > >(v; ._) -5000 ) MA|N EooY oF .-)/ PELHAM DOME -lo,ooo -r5.oo o '1,/' ',t SECTIONALONG LINE G-G' lrt

Fig. 12. Cross sectionsD-D', E-E', and G-G'from Plate l5-lb of Thompson et al. (1968). Map patterns of color original have been modified here for black and white printing and are explained in Figure I 1. and Keene domes to a synclinal hinge identified at Bliss supported by the evidence for limb shearing already re- Hill. At this time, the core of the nappe was considered ported from north of the Croydon dome. These consid- to be highly attenuated by shearing.Thus, in the central erations were the basis for constructing the cross section part of the Mount Grace quadrangle,it consistedonly of from Bernardstonthrough Mount Grace (Fig. 12, section some 30 m of true Partridge Formation, with younger E-E') and for making senseout of the Amherst inliers in rocks on both sides. At Bliss Hill the core of the nappe central Massachusetts(Fig. 12, section G-G') as part of a was consideredto be only Clough Quartzite with all other far-traveled nappe rooted to the east. units missing as a result of shearing. These ideas were At the end of this early phase,an attempt was made to ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 699

BELLOWS FALLS PLUTON

t,lt!

Z.==-====:-1"1 //: - .\\\ , \\\\ asHUELor 7n/" i 7,2 eLUtoN,z--..., :;

VERNON DOME Fig. 14. Map of the areanorth of the Vernongneiss dome and southofthe BellowsFalls pluton (southwestcorner ofFig. Fig. 13. Map of part of the Mount Gracequadrangle, Mas- 7 and northwestcorner of Fig. I l), showingnew recognition of sachusetts-NewHampshire (east-central part of Fig. I l), adapt- the Bernardstonnappe as distinctfrom the Skitchewaugnappe. ed from Robinson(1967). Patterns same as in key to Figure1 I Adaptedfrom Thompsonand Rosenfeld(1979). Patterns same exceptFitch Formation shown in brick pattern.For locationsee as in key for Figure I I exceptfor brick pattern for Fitch For- alsoinset in Figure17. mation and different patternsfor PartridgeFormation in Ber- nardstonand Skitchewaugnappes that arekeyed in Figure16. rationalize the obvious geometricalrelationships between structural development and regional metamorphism. Theseare obvious in Figuresl5-2 and l5-3 of Thompson was a metamorphic overhang with rocks of the silliman- et al. (1968) where metamorphic zonesare compared di- ite zone above,rocks ofthe kyanite zone below, and rocks rectly with structural levels shown on a map of traces of of the chlorite zone in the middle, in the right location the axial surfacesof nappes.What is especiallystriking is to be sliced offand preservedin the fault zone. The pat- the nearly direct relationship between high metamorphic tern of isogradscannot be simply a product of recumbent grade and high structural level near Fall Mountain at Bel- folding of rocks that were previously metamorphosed, lows Falls and surrounding the Ashuelot pluton. In ad- becausethe dominant metamorphic fabric in most of the dition, in central Massachusetts,the highest sillimanite- rocks appears to have been imposed in later stagesof grade rocks are found on the western, downthrown side metamorphism during formation of the gneissdomes. For of the Connecticut Valley fault adjacent to lower grade this reason it was conceived that the rocks were heated, rocks to the east. Furthermore, Robinson had identified in part by sheetlikeintrusions, at the time they were mov- several slices of rocks of chlorite grade along the fault, ing and that the final imprint of metamorphism occurred juxtaposed to the west by rocks of sillimanite grade and well after emplacement of the nappes. These concepts on the east by rocks of kyanite grade. The most rational have provided fuel for controversy concerningthe length explanation, along this purely dip-slip fault with about 5 of time that such overhanging metamorphic thermal km of vertical displacement, is that before faulting there anomaliescould endure(Sleep, 1979). 700 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

^',',\,J'rlli'vy,;i, \ coRNIS NAPPEr-r .yl W

BERNARDSTON

Fig. 16. Schematiceast-west cross section illustrating the theoryof fourfoldnappes as developedin the 1970s.Partems for Ordovician strata at four structural levels and for foliated Acadianintrusions are the sameas in Figures14 and 15.

SKITCHEWAUG and at Surry Mountain all appear to belong to the Ber- nardston rather than the Skitchewaugnappe. From near Bellows Falls, Thompson then projected the Bernardston nappe northward to the vicinity ofSkitche-

\f, waug Mountain (Fig. l5), where it comes in at a level to between the Cornish and Skitchewaugnappes. The map IJJ pattern in the easternpart of Figure I 5 is somewhat puz- t = zling as to the position ofthe synclinal hinge ofthe Ber- o nardston nappe, but it appearsto be located in the sub- surfacewest of the Unity, Croydon, and Mascoma domes [\t'\ where tho Skitchewaug nappe is directly above the au- tochthon. Thus, this phaseofresearch resulted in the the- '.Y2"i ory that there are four major fold nappesas illustrated in ',r/--\r\=\s Figure 16. FALL MTNJ I.,BELLOWS FALLS $ -- zz \\-\." PLUToN _/ =,,/ For-o N.q.ppBSAND THRUSTNAppES That some of the early west-directedfold nappeshave >- - \\ //-/ limbs locally thinned by faulting has long beenrecognized (Thompson et al., 1968, p. 2ll, 214), but the idea that there are major thrusts is relatively new. The initial in- Fig. 15. Revised map of the vicinity of SkitchewaugMoun- vocation of major thrusting, in addition to folding, to tain (central part of Fig. 7) showing the northward extension of explain the map pattern in the Connecticut Valley region, the Bernardstonnappe. Adapted from manuscript maps provid- involved a tentative resolution ofthe stratigraphic prob- ed by J.B. Thompson. Patterns same as in key for Figure l1 except for brick pattern for Fitch Formation and different pat- lem of the Erving Formation (Robinson et al., 1984, terns for Partridge Formation in Bernardston and Skitchewaug 1988a) in Massachusetts.In this, the true stratigraphic nappesthat are keyed in Figure 16. sequencefrom oldest to youngestwas proposedto be Lit- tleton Formation, Erving Formation, and Waits River and Gile Mountain Formations. Based on complex geo- metric and stratigraphicrelations, the Whately thrust was THnony oF FouR FoLD NAppES proposed to lie along the contact known as the Chicken In the 1970s Thompson reworked the area northwest Yard line betweenthe Littleton Formation to the eastand of the Keene gneissdome in cooperation with J. L. Ro- the Gile Mountain Formation to the west. The strati- senfeld(Thompson and Rosenfeld,1979; Fig. l4). This graphically older Littleton from a deeperpart ofthe basin work showed that earlier investigations had'Jumped the to the east was consideredto have been thrust westward tracks," and that the recumbent fold, identified by Trask over the younger (in this interpretation) Waits River and at Bernardstonand extendedto the southwestside ofthe Gile Mountain Formations. To the east the proposed Keene dome, in fact slips in beneath the Skitchewaug thrust disappearsinto bedding within the Littleton. Lo- nappe southwest of Eellows Falls. This newly identified cated along the axis of the Connecticut Valley low grade nappe was named for Bernardston,using Trask's original metamorphic belt, it is considered to have been a pre- 1964 terminology when he had rightly been doubtful of metamorphic or early metamorphic thrust, but its time the structural correlation. On this basis, the synclinal relations to the fold nappes covered here is uncertain. hingesat Bliss Hill, at the location east of Spofford Lake, Becausethe Chicken Yard line apparently runs north into ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 70r the Monroe line that bounds pre-Silurian rocks of the Cornish nappe from the Gile Mountain to the west, the Whately thrust may run along the lower side of the Cor- PLUTON KEENE nish nappe. Fig.21 Stratigraphy of the Merrimack synclinorium The main idea of the thrust nappesdeveloped through a new concept of the stratigaphy of the Merrimack syn- clinorium. Specifically the sequence of Silurian units identified by Moench and Boudette (1970) in northwest- Fig.1Q Fig.26 ern Maine was extended into central New Hampshire (Hatch et al., 1983) and Massachusetts(Field, 1975 _ry.tll__ Tucker, 1977; Robinson et al., 1982a), and eventually MASS. appliedby P. J. Thompson(1985) to the Mount Monad- nock area,New Hampshire (Fig. l7). As compared to the thin Silurian shelf sequencecharacteristic of the Bronson v\ uz Hill anticlinorium, the Merrimack synclinorium contains 10 a much thicker immature clastic sequence(Fig. 2), and L o, J a the pre-Silurian section is scarceto unknown. The Lower '-@o r OYS HILL I -ll 4 r Silurian consistsmainly of the thick RangeleyFormation, f\l PLUTON dominated by gray- to rusty-weathering feldspathic schists uJo >-1 ='_ T with subordinate lensesof calc-silicateand granulite-ma- u-) atl o trix conglomerate.This is overlain by the Perry Mountain z l_t Formation of laminated gray to brown schists q\artz- 3. @ and q/ ites, followed by the Francestown Formation of pyrrho- tfr z tite-rich calc-silicates.The Upper Silurian consistsmain- f/t \z ly of the Warner Formation having a lower member ," />.- dominated by laminated green purple MHERST and calc-silicates, 10 km and an upper member of biotite-feldspar granulites with subordinate calc-silicate. The upper Warner appears to grade continuously into the lower Littleton, which on Fig. 17. Geologicindex map showinglocation of the Mo- Mount Monadnock contains distinctive lower and upper nadnock(Fig. 18),Hinsdale (Fig. 21), and Mount Grace(Fig. members with different bedding characteristicsand pro- 26) areassignificant to the theoryofthrust nappes.Also shows portions of quartzite. On Mount Monadnock a distinc- linesof crosssections in Figures24 and27. tive zone of seven quartzites in the upper member is a key to local structural interpretation. The Warner For- mation in New Hampshire appearsidentical to the Ma- Clough Quartzite, as previously mapped, contains mem- drid Formation in northwestern Maine, and the Frances- bers that may be transitional to units in the Rangeley town Formation appears to correlate eastward with the Formation farther east. In the Lake Warren region (Fig. extremely sulfide-rich pelites of the Smalls Falls Forma- 7, locality 9) Chamberlain (1985) has shown that con- tion. Sedimentological evidence in Maine suggeststhis glomeratestypical of the Clough occur betweenRangeley thick Silurian section, particularly its lower part, had its schists(shown as Partridge near locality 9 in Fig. 7) and sedimentary source to the west or northwest toward the overlying Francestown Formation, in the approximate Bronson Hill belt; whereas the overlying Littleton For- position normally occupied by the Perry Mountain. In mation was the distal part of a deltaic complex repre- the Derby Hill window area of the Monadnock quadran- senting early Acadian "flysch" derived from tectonic lands gle,P. J. Thompson (1985)has locateda lens of Clough- to the east (Hall et al., 1976). like conglomerate at the contact between the Rangeley The facies change between the thick Merrimack se- and Perry Mountain Formations. quenceto the east and the thin Bronson Hill sequenceto the west appearsto have been rather rapid along a zone Folds and thrusts of the Mount Monadnock area described as a "tectonic hinge" (Moench and Boudette, Chamberlain (personal communication, 1982) first 1970;Hatch et al., 1983).The higher Acadian fold nappes suggesteda major thrust east of the Bronson Hill anticli- of the Connecticut Valley region are mostly rooted on the norium based on a steep metamorphic gradient in the east side of the Bronson Hill anticlinorium, and might be Lovewell Mountain quadrangle, New Hampshire. This expectedto have a stratigraphy transitional between that contact has beentraced by P. J. Thompson ( I 985, I 988a) of the Bronson Hill anticlinorium and of the Merrimack through the Mount Monadnock area as the Chesham Pond belt. Recent work by J. B. Thompson (personal com- thrust (Fig. 18), and its relations with structural foatures munication) in the Skitchewaugnappe indicates that the of the underlying Monadnock sequenceare critical. 702 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

thrust. With effectsof later deformation removed, as in Figure 20, it can be seenthat the east-dipping fold axial surfacesare cut by a thrust that dips less steeply and is not merely the attenuatedlower limb of a recumbent fold. Such relationships appear in P. J. Thompson's detailed map of the Derby Hill tectonic window (Fig. 18) where axial surfacesofearlier folds are cut both by the Chesham Pond thrust and by a later related backthrust. Figure 20 is thus a model for the types of contact relationships ex- pected along thrust faults between previously folded se- quencesin the region. The strong development ofaugen schistsin the RangeleyFormation just above the Chesh- am Pond thrust (P. J. Thompson, 1985, Plate 4) is ex- tremely similar to the development of augen schists by dismemberment of pegmatites during synmetamorphic shearingon the Sgurr Beagslide in the ScottishHighlands (Powellet al., l98l). Beyond the Monadnock region, the Chesham Pond thrust may encircle Fall Mountain (Fig. 7) near Bellows Falls, (J. B. Thompson, C. P. Chamberlain, F. S. Spear, personalcommunications, 1983-1985; Spear and Cham- berlain, 1986)and the Ashuelot pluton (Figs. I l, 17,21) in the Hinsdalearea (Elbert, 1986;Armstrong, 1986).It is tentatively extended from the Monadnock quadrangle southward along the west margin of the Coys Hill pluton (: Cardigan pluton) through central Massachusetts(Fig. 17) and probably into Connecticut. A second proposed thrust, the Brennan Hill thrust, is exposedat a lower structural level in the western part of Fig. 18. Generalizedgeologic map of theMount Monadnock the Monadnock area (Fig. l8). This is a contact on the areaadapted from P.J.Thompson (1985), with locationsofcross east side of the Keene dome separatingstrata assignedto sectionsin Figure19 indicated. Patternsare similar to thosein the Lower Member of the Silurian Rangeley Formation keyto Figure19, but all Silurianstrata above Brennan Hill thrust above, from the Devonian Littleton Formation in a arelumped with the RangeleyFormation, and Silurianand Or- (Figs. dovicianstrata below thrust are lumped with the LittletonFor- BronsonHill sequencebelow 18, 19,20). This con- mation. The 7Q indicatesseven quartzites within the upper tact traces southward into the Mount Grace quadrangle, memberof theLittleton that havebeen traced across the summit in Massachusetts(Fig. 17), with consequencesdiscussed areaof Mount Monadnock. below. It traces northward into the Lake Warren area (Fig. 7, locality 9) where its consequencesare still being evaluated (Chamberlain et al., 1988). To the west near Structural data on Mount Monadnock itself (Figs. 18, Hinsdale, New Hampshire, west of the Ashuelot pluton 19) have led to the conclusion that the most prominent (Fig. l7), Elbert appearsto have identified the same sur- set of mapped isoclinal folds in bedding are folds of the face where a complexly deformed Monadnock sequence nappe stage. In their original setting, they were satellitic of strata appears to be in thrust contact with Partridge folds on the upper limb of a major nappe-stagesyncline, Formation in the core of the Bernardston nappe (Elbert, the Monadnock syncline, that was overfolded from east 1984, 1986, 1988).On the basisofthese considerations to west so that bedding faced west along the nappe-stage and direct lithic comparisonswith the Monadnock quad- axial surfaces(Fig. 20). Subsequently,by backfold- and rangle, the rocks mapped by Jasaitis (1983) as Littleton, dome-stagefolding, the originally west-facingrecumbent Clough, and Partridge in the Amherst inliers area (Fig. folds were overturned toward the east, so that the strata I 7), including outcrops of granulite-matrix conglomerate, now face east and southeast along the original nappe- are now assignedto the Rangeley Formation above the stage axial surfac€s (Fig. l9). Thus, the strata on the Brennan Hill thrust. mountain (P. J. Thompson, 1985, 1988b),which were once predominantly upside down on the lower limb of a Fold and thrust relations near Hinsdale. major anticlinal nappe (Fig. 20), are now predominantly New Hampshire right-side-up again becauseof the later folding (Fig. 19). Field relations in the Hinsdale area, New Hampshire Traced northward, the axial surfacesof the Monadnock (Fig. l7), are crucial for understanding the Brennan Hill syncline and satellitic nappe-stageanticlines and syn- thrust and as a stratigraphic key for other areas (Elbert, clines are clearly truncated upward by the CheshamPond 1988). The strata in this area (Fig. 2l) lie in a narrow ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 703

BBONSONHILL SEOUENCE(below Brennan Hill thrust) MONADNOCKSEOUENCE (above Brennan Hill thrust)

fr; ++-fl Mr HERMoNHoRNBLENDE GABBRo ++++ FITZWILLIAMGRANITE r+la*:*l + +r L*i + I (alsoabove thrust) ANDHARDWICK TONALITES l\\ 7 :\l SPAULDING |-_-_- | l--'--l GILEMTN , WAITSRIVER t---l KINSMANGRANITE AND ERVINGFORMATIONS LITTLETONFORMATION LITTLETONFORMATION Upper,schist and quarlzite I;rH FITCHFOBMATION (locallycombined Lower,schist on MAPS) CLOUGHOUARTZITE Fll_i-:=fjl r lj:::i. ::: ::l WABNERFORMATION F==== FORMATION (locallY E:-:l PARTRIDGEFORMATION FRANCESTON AMMONOOSUCVOLCANICS PERRYMOUNTAINFORMATION combined) l^^.^ ^ .l Upper,schist with iron fm. lenses Lower,bedded schist and quartzite - @ [t7i l l/ \l GNEISSESIN CORES OF DOMES |l..=rl RANGELEYFORMATION l\/\/\/\/\l

Fig. 19. Generalized geologic cross sections B-B' and C-C' acrossthe Mount Monadnock area adapted from P.J. Thompson (1985) and key to patterns used in Figures 18, 19, 21, 23, 24, 26, 27, 28, 30, and 31. Section C-C passesthrough summit of Mount Monadnock.

zone between the core of the Vernon dome to the west on the limbs of a tight isoclinal syncline, the Hinsdale and the Ashuelot pluton of Kinsman Granite to the east. syncline, that formed prior to the thrust and is truncated The first stratigraphic sequenceis the autochthon on the by it. In the southern part of Figure 21, much or all of Vernon dome including Ammonoosuc, Clough, and Lit- the west limb is removed by the thrust so that in many tleton. This is followed eastwardby the inverted sequence locations a downward or west-facingMonadnock stratig- oflittleton, Clough, and Partridgethat can be traced con- raphy is directly against Partridge across the thrust. In tinuously to the inverted limb at Bernardston (Fig. I l). the northern part of Figure 21, both limbs of the Hinsdale East of this is the trace of the Brennan Hill thrust, sepa- syncline are preserved, but the upward-facing strata of rating rocks of the Bronson Hill sequenceto the west the west limb are doubled by imbrication just above the from rocks of the Monadnock sequenceto the east. Lo- thrust. Thus, in this area early folds in rocks of the Mo- cally the Brennan Hill thrust cuts deep into the Bernards- nadnock sequenceare truncated below by the Brennan ton nappe, so that in one area the preserved core ofthe Hill thrust in much the same way that such folds are nappe includes only about 80 m of Partridge Formation truncated above by the CheshamPond thrust in the Mo- and 160 m of inverted Clough Quartzite. The rocks of nadnock area.These considerations lead to the schematic the Monadnock sequenceabove the thrust are arranged tectonic diagram of Figure 22. 704 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

Fig.20. Schematicrestored cross section through the Mount Monadnockarea showing the relationshipbetween early recurn- bent fold nappesand later thrusts.Adapted from P.J.Thomp- son(1985). Patterns generalized as in Figure18.

The strata of the Monadnock sequenceare best dis- Fig. 22. Schematicstructure diagram illustrating the rela- played in a thin west-facing, east-dipping inverted se- tionshipbetween early isoclinalfolds and later thrustsin the quence (Fig. at Biscuit Hill 23A). The stratigraphically Monadnock-Hinsdaleregion. Patterns for Partridgeof Bernards- lowest and structurally highest unit consists of gray schists ton nappeand autochthonsame as Figure16. Silurian strata of with conglomeratelenses of the RangeleyFormation, ap- Monadnocksequence are dotted.

parently in fault contact to the east off the map with the Kinsman Granite. This is stratigraphically overlain by thin Perry Mountain Formation with characteristic quartzite beds, some showing inverted graded bedding. Above this is an unusual zone, also assignedto the Perry Mountain, consistingof lensesand boudins of magnetite- garnet iron formation associatedwith massive gray schist. This is succeededby typical sulfidic calc-silicate of the

Fig.23. Detailed maps of two small areasat Biscuit Hill (A) Fig. 21. Detailed map of the Hinsdale area adapted from and south of SargentHill (B) near Hinsdale (seeFig. 2l). Adapt- Elbert (1988). Patterns slightly simplified compared to key in ed from Elbert (1988). For key to map patterns see Figure 19. Figure 19. Checkedareas in A are pegmatite. ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 705

Fig. 24. Restored cross section before Mesozoic faulting from Mount Monadnock through the Hinsdale area. Adapted from Elbert (1988). For key to patterns seeFigure 19.

FrancestownFormation with subordinate sulfidic pelite. Hinsdale areas (Elbert, 1986, 1988) must now be ex- The stratigraphically highest unit is the Warner Forma- plored (Robinsonet al., 1988b;Springston, 1990). In the tion, dominated by sharply bedded green and pink calc- western part of the Mount Grace area (Fig. 26) the in- silicatesnear the base,and grading into more feldspathic verted sequenceof Silurian Clough Quartzite overlying biotite granulite toward the top. The same thin sequence Devonian Littleton Formation is again confirmed as the occurs twice by thrust imbrication on the west limb of inverted limb of the Bernardston nappe. This can be di- the Hinsdale syncline near SargentHill (Fig. 23B), except rectly connected to the inverted limb with fossiliferous that the lensesof magnetite-garnetiron formation occur strata at Bernardston, when about 5 km of vertical dis- only in the upper of the two thrust slices. This might placement on the Mesozoic Connecticut Valley border indicate that the zones of iron formation are discontin- fault is restored (Fig.27). Further, the doubled-over lay- uously distributed or that the imbrication hasjuxtaposed ers of Clough Quartzite at Bliss Hill (Figs. 26, 28) are slices that had sites of deposition originally kilomerers confirmed as the synclinal hinge of the Bernardston nappe, apan. here plunging south. When the structure sectionsfrom the Monadnock area The large area of gray-weatheringschists surrounding and the Hinsdale areaare imaginatively connectedacross the Tully body of Monson Gneiss (Fig. 26), previously a restored Mesozoic Connecticut Valley border fault, assignedto the Littleton Formation, are now mainly as- making minimal adjustmentsfor the severedeformations signed to the Lower Silurian Rangeley Formation in di- related to formation of the gneissdomes, the result is as rect correlation with the Monadnock area. These include shown in Figure 24 (P. J. Thompson et al., 1987).If in- terpreted correctly, this shows the extreme variability in present thickness of the tectonic levels separatedby thrust faults as well as the complexity of structure that appar- ently was already present in the rocks before thrusting. The consequencesofthis interpretation have not yet been ghl- Bgllo!,s F_alls carried northward beyond the southwest flank of the Keene dome to the Bellows Falls region and beyond. To -;;n*;5-.=g:'5- these writers it seemsprobable that the recumbent fold BrennanHill Thrust*++ 4 at SkitchewaugMountain either representsa higher level fold nappe in the sequencebelow the Brennan Hill thrust or, more probably, a fold in the sequenceabove the Bren- nan Hill thrust. This second concapt has been used to construct a schematic diagram (Fig. 25) showing the nappe theory in its present state. Fig. 25. Schematic east-west cross section illustrating the The Mount Grace puzzle theory of fold and thrust nappesas developed in the 1980s by the authors. Adapted from Robinson et al. (1988b). Pattems for The implications for the Mount Grace area (Fig. 17, Partridge of Skitchewaug and Bernardston nappes and autoch- inset for Fig. 26) of the new concepts from the Monad- thon same as Figure 16. Silurian strata of the Monadnock se- nock (P. J. Thompson, 1985, 1988a)and Bernardston- quenceare dotted. 706 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

On the upper side of the thrust, Rangeley Formation predominates,but locally other Silurian units are present and near the northern termination of the main body of Monson Gneiss,the thrust appearsto cut down into pre- Silurian strata (Fig. 26). South and southwestof Bliss Hill (Fig. 28), the younger Silurian units include severallenses assignedto the Perry Mountain Formation becauseof their boudins of magrretite-garnetiron formation, one lens of Francestown Formation, and two lenses of Warner Formation. All of these are localized close to the thrust in such a way as to suggestthat the Rangeleywas struc- turally inverted above them, probably on the overturned limb of a nappe-stagerecumbent anticline subsequently followed by the thrust. Near Butterworth Ridge, north- west of the Tully dome, there is a narrow complex belt, shown as Perry Mountain on Figure 28, including con- fused representatives of all Silurian units including iron formation of the Perry Mountain (Springston, 1990). In this vicinity, the Brennan Hill thrust is apparently along the contact between Rangeley Formation and Partridge Formation (not shown at scaleof Fig. 28). On the north- west side of the northern tip of the Tully dome (Fig. 28), more extensive Warner Formation appears to be out of order betweenPerry Mountain Formation and Rangeley. Near the north end of the main body of Monson Gneiss the Brennan Hill thrust appearsto cut through the base of the Rangeley into the augen gneiss member of the Par- tridge, and some miles south, into the sulfidic schist member (Fig. 26). Near here the RangeleyFormation has a basal conglomerate against the augen gneiss member (Robinson et al., 1988b).East of the north tip of the main 72c15 body of Monson an earlier recumbent anticline with a Fig. 26. Generalizedgeologic map of the northeasternpart core of Monson Gneiss, the North Orange nappe, is be- ofthe Orangearea. From Robinson et d. (1988b).Line ofsec- lieved to be in the upper plate of the Brennan Hill thrust tion in part ofFigure27 runsalong east-west quadrangle bound- and has an exposedanticlinal hinge in the southeastcor- ary. For key to patternssee Figure 19. Heavy dashedpattern ner of the Mount Grace quadrangle (Fig. 26). Monson gneiss indicatesaugen memberof Partridge. Gneiss in the core of the anticlinal nappe also appearsin a continuous belt around the southern part of the Tully a few isolated small areasof conglomeratepreviously un- dome, with two moderately well exposedanticlinal hing- known or assignedto the Clough Quartzite. The west es, one on the northeast flank of the dome (Fig. 26) and margin of the Rangeley Formation is marked by Peter one south of Bliss Hill (Fig. 28). The placement of these Thompson's Brennan Hill thrust. This traces from the recumbent fold nappesin the upper plate of the Brennan Monadnock area southward into the Mount Grace quad- Hill thrust appearsto be necessitatedby the configuration rangle where it is unvoluted by the northward overtwn- of the thrust southwest of Butterworth Ridge (Fig. 28) ing of the main and Tully bodies of Monson Gneiss, and where RangeleyFormation is resting directly on Monson then passessouthward along the east margin of the main Gneiss in the core of the Tully dome. body toward Connecticut. To understand the outcrop pattern and structural evo- In the central part of the Mount Grace area (Figs. 26, lution of the Mount Grace area it is necessaryto recog- 28), on the lower side of the Brennan Hill thrust, the nize the complexity of the backfold-stageand dome-stage Bernardston nappe is mostly eliminated by the thrust from deformations that involuted the earlier structural features Bliss Hill southwestward for about 5 km. At different (Robinson et al., 1988b).This is illustrated in a structural placesin this distancethe thrust restson inverted Clough relief diagram (Fig. 29) showing the pattern and local Quartzite, on a thin strip of Partridge Formation in the overturning of even the simpler gneissdomes, a swirl in core of the nappe, and locally on Littleton Formation the pattern of dome-stage lineations, minor folds, and beneath the nappe. Around the Tully dome, as presently transport directions, and the northward overturning and interpreted, the thrust cuts much deeper,resting on Am- longitudinal transport of the main and Tully bodies of monoosuc Volcanics or directlv on the Monson Gneiss Monson Gneiss. In the core of the Pelham dome it has of the dome. been proved that the lineation is parallel to the transport ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION 707

Fig.27. Restoredcross section before Meso- zoic faulting from the Mount Grace quadrangle through Bernardston. Adapted from Robinson et al. (1988b). For key to pattems see Figure 19. Heavy dashed pattern indicates augen gneiss member of Partridge.

Keene do@

Vemon done

Tully body of Monsor Gneiss

l4aln body of Uonsoo Gneiss

Ii

( Approx. scale kiloneters ?elhan Kenpf teld olo2o etlcllne Fig. 28. Detailed map of the central part of the Mount Grace quadrangle showing distribution of rock units on either side of Fig.29. Schematicrestored structural relief diagram showing the Brennan Hill thrust and the complex involution of both the inferred flow pattern of major grreissbodies during the backfold Bernardstonnappe and the thrust by northward transport ofthe and dome stages. The effects of Mesozoic faulting have been main and Tully bodies of Monson Gneiss.From Robinson et al. removed. This deformation involuted both the axial surfaces of ( I 988b). For key to patternssee Figure I 9. Heavy dashedpattern the fold nappes and the later thrusts. Adapted from Robinson indicates augengneiss member of Partridge. et al. (1979, 1988b). 708 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

Fig. 30. Schematic east-westcross sectionsthrough the Mount relatedtight folding. Timing of the east-directedOak Hill recum- Grace area, showing the sequenceof structural development:(1) bent folds in the Keene gneissdome is unknown, but they prob- Formation of Bernardston and Nonh Orange fold nappes; (2) ably formed in the backfold stage. Adapted from Robinson et thrusting ofNorth Orangenappe onto Bernardstonnappe along al. (1988b). For key to patterns seeFigure 19. Dotted pattern for the Brennan Hill thrust; (3) involution of the fold and thrust Warner Formation is used for all Silurian strata above Rangeley nappes by northward longitudinal flow of the main and Tully in the Monadnock sequence. bodies of Monson Gneiss; (4) formation of gneiss domes and

direction of a set of dome-stage minor folds (Onasch, with a zircon age of 380 Ma (Ashwal et al., 1979) that 1973), but it is an oversimplification to apply this to all was itself deformed and metamorphosed in the dome the domes. For example the northward overturning and stage.The southeast-directedrecumbent folding may have transport of the main and Tully bodies had to precede been slightly earlier than the overturning and northward the main dome stagebecause strata inverted during this transport of the main and Tully bodies of Monson Gneiss. longitudinal transport were refolded by major anticlines With thesefeatures in mind, the progressivedevelopment and synclinesof the main dome stage. of the structure in the central part of the Mount Grace The V-shaped exposureof Partridge Formation within quadrangle is illustrated in a seriesof tectonic diagrams the south end of the Keene dome (Fig. 26) indicates an (Fig. 30) explained in the caption. early recumbent fold set, the Oak Hill folds, overturned from northwest to southeast, opposite to the sense of Regional implications and extensions the nappesdescribed here. The relative ageof this recum- Implications and extensionsbased on our proposedre- bent folding is uncertain, but it is tentatively assignedto visions of the nappe theory are under investigation. Do the backfold stage,in part becausethere are peak meta- the Brennan Hill and Chesham Pond thrusts extend morphic fabrics in some amphibolites that are parallel to northward from the vicinity of the Keene dome past Bel- the exposedsynclinal hinges of this fold system.Further, lows Falls (Spear and Chamberlain, 1986; Chamberlain similar southeast-directedrecumbent folds in the Pelham et al., 1988) to the latitude of Skitchewaug Mountain? dome (Ashenden,1973; Onasch, 1973; Michener, 1983) Rumble (1969, l97l) proposed that the Skitchewaug and its cover are truncated by the Belchertown intrusion nappe,or recumbent folds like it, can be identified at least '709 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION as far north as the northwest flank of the Owl's Head dome (Fig. l). Recently Moench and Hafner-Douglass KEENE DOME (1987) have proposedthat a large area ofrocks northwest of the domes in northern New Hampshire previously as- signedto the pre-Silurian, are in fact Silurian units ofthe Merrimack synclinorium transported across the axis of ff:=1=iY the anticlinorium in an enonnous allochthon with com- ,olr__-_- 1:S plex contact relations reminiscent of the Brennan Hill thrust. The general interpretation of the map pattern of /t'// i,=\ \ tt //'t the Merrimack synclinorium within central Massachu- '1./rAr\..-.\--,,,/-:\ -_:\:- ll u,.t/,_).-.\ setts is one west-directed recumbent ' // \ \.- of originally folds lt ",,lt[{'.r:'/t 7..1 \l \r subsequentlyoverturned to the east (Tucker, 1977; Znn r".tt et al., 1983; Peterson,1984). Within the synclinorium ,,lt,,"RsHug19it!..E; ,\tl:...,-lr//1 sequenceand its underlying basement in central Massa- tr,,'rr'. rrt,'r{tt ll eLUTOrrr'1ttvt"ttI\'.r\""2 chusetts,east ofand tectonically higher than the Chesh- I $ am Pond thrust, Berry has identified a group of imbri- rtu, ...r'i\Y, cated west-directedthrusts in previously folded strata, all subsequentlybackfolded into an eastwardoverturned po- -- = sition (Robinsonet al., 1986, 1989;Berry, 1989).Within \ \\',\ .\ central New Hampshire, Eusden (1988, Eusden et al., l$,.r.t. "'l) 1987) has proposed a dorsal zone separatingregions of west- and east-directednappes, and has rejectedthe con- cept that the east-directednappes are tectonically over- turned.

For-n aNn rHRUsr NAppESAND REGToNAL --,;;;ss METAMORPHISM I WEST NORTHFIELD Early on, a relationship was recognized between tec- RNARDSTON ANTICLI NE tonic level within the stack of nappes and the intensity of regionalmetamorphism (Thompson et al., 1968).Be- Z"|r',In causemany of the metamorphic rock fabrics are clearly \ NORTHFIELD related to structural features of the later gneiss-domestage, MESOZOICBASIN it was suggestedthat the inverted thermal regime pro- A//rt / P / l'1. duced by the nappes,and by the sheetlike intrusions in- C /.1t tercalatedwithin them, persistedinto the dome stage.It , t /,/-, has been suggestedthat the nappe stage began concur- rently with or soon after the intrusion of the Kinsman Fig. 3l. Generalizedgeologic map of the Bernardston-Hins- Granite at about 400 Ma. The Belchertown intrusion, dale areashowing the relationshipof stauroliteand sillimanite pluton features.See which cuts recumbent folds assignedto the backfold stage isogradsto the Ashuelot and to structural Figure21 for details.Adapted from Elbert(1988). For key to but was deformed and metamorphosed during the dome patternssee Figure 19. Pattern for RangeleyFormation is here has yielded zircon stage, a concordant age of 380 Ma appliedto all Silurianstrata of the Monadnocksequence. (Ashwal et al., 1979).These data imply that overhanging metamorphic thermal anomalies could have been pro- morphism followed by compression produced either by duced and persistedfor a period of about 20 m.y., a con- still higher nappes(Spear and Chamberlain, 1986; Spear, cept that is contrary to most modern views of heat flow 1986)or by backfolding(Schumacher et al., 1989).Rocks (Sleep,1979). closeto the gneissdomes have produced evidenceofearly Evidence for metamorphic discontinuities in the pile loading followed by pressurerelease thought to be related of nappes was one of the original reasonsfor proposing to the rise of the domes themselves(Schumacher et al., thrust nappes(Chamberlain, 1985),although most of the l 989). evidence given here is stratigraphic. Detailed petrologic A curious example of the ambiguity of evidenceis pro- studies of rocks closely juxtaposed acrossproposed thrusts vided in the Hinsdale area studied in detail by Elbert (Spearand Chamberlain, 1986; Spear, 1986;Elbert, 1987, (1987, 1988).In pelitic schistsof the low sillimanitezone 1988) have produced evidencefor diferences in P-7tra- immediately above and below the Brennan Hill thrust, jectories, based on assemblages,on pseudomorphs, and garnet-zoningprofiles suggestrather different P-T trajec- on garnet-zoning profiles. Deeper structural levels have tories and imply that all the garnet gowth, except the shown early heating followed by pressureincreases thought outer rims, took place when the rocks were in different to be causedby piling on of higher level nappes.Higher thermal regimes.The pattern of metamorphic zones(Fig. level nappes have given evidence of early high Z meta- 3l) shows a clear spatial relationship with the overriding 710 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

(apparently hot) mass of the Ashuelot pluton, but in de- Billings, M P. (1937) Regional metamorphism of the Littleton-Moosi- tail the isograds appear to cut cleanly across all the re- lauke area,New Hampshire. Bulletin of the Geology Societyof Amer- ica.48.463-566. cumbent folds and proposed thrusts, and even across -(1950) Stratigraphyand the study of metamorphic rocks. Geolog- contacts in the autochthon into the core of the Vernon ical SocietyofAmerica Bulletin, 61,435-448. dome. The exploration and resolution of such ambigu- - ( I 956) The geologyof New Hampshire, Part II: Bedrock geology, ities is beyond the scope of this paper. Obviously the 203 p. New Hampshire StatePlanning and Development Commission, relationships among metamorphic isograds,P-T trajec- Concord, New Hampshire. Billings, M.P., and Cleaves, A.B. (1934) Paleontology tories, heat sources, of the Littleton and tectonothermal evolution will be area, New Hampshire. American Joumal of Science,28, 412-438. subjects of researchfor those following in Jim Thomp- Boucot, A.J., and Arndt, R. (1960) Fossils of the Littleton Formation son's wake for some time to come. The results of such (Lower Devonian) of New Hampshire. U.S. GeologicalSurvey Profes- work, made possibleby the complex setting ofthe nappes sionalPaper 334-8, 4l-53. in the Connecticut Valley region, will be of very broad Boucot,A.J., and Rumble, D., III (1980)Regionally metarnorphosed (high sillimanite zone,granulite facies)Early Devonian brachiopodsfrom the interest to metamorphic petrologistsin general. Littleton Formation of New Hampshire. Journal of Paleontology,54, l 88-195. CoNcr,usroN Boucot, A.J., and Thompson, J.B., Jr. (1963) Metamorphosed Silurian The nappe theory in the Connecticut Valley region has brachiopodsfrom New Hampshire. Bulletin of the Geological Society of America, 74, 1313-1334. come a long way sinceThompson's first proposal and has Boucot, A.J., MacDonald, G.J.F., Milton, C., and Thompson, J.B., Jr. developedin ways unexpectedat the outset. Further sur- (1958) Metamorphosed middle Paleozoicfossils from central Massa- prisesof similar or greatermagnitude should be expected chusetts,eastern Vermont and westernNew Hampshire. Bulletin of the in the future. The theory has developed mainly through GeologicalSociety ofAmerica, 69, 855-870. (1985) closeattention to outcrops and faith that regional stratig- Chamberlain, C.P. Tectonic and metamorphic history of a high- gradeterrane, southwestern New Hampshire, 207 p. Ph.D thesis,Har- raphy can be used to decipher structural history. The vard University, Cambridge, Massachusetts metamorphic petrology of this region, though well de- Chamberlain,C.P., Thompson, J.B., Jr., and Allen, T. (1988) Stratigraphy veloped, has far to go before the full story is written, and and structure of the Fall Mountain and Skitchewaugnappes, south- obviously it will not come easily. Thompson has main- westernNew Hampshire. New EnglandIntercollegiate Geological Con- ference, 80th Annual Meeting, Keene, New Hampshire, Guidebook, tained that the way an outcrop is seendepends on what 32-39. is going on in the mind of the viewer, and we think that Dana, J.D. (1877) Note on the Helderberg Formation of Bernardston, will always be true. Massachusetts,and Vernon, Vermont. American Journal of Science, 14, 379-387. Acrxowr-rlcMENTS Elbert, D.C. (1984) Stratigraphic and structural reinterpretationsin rhe Bernardston-Northfield area, north-central Massachusetts. Geological The long-term researchrelated to this paper would not have been pos- Societyof America Abstracts with Programs, 16, 14. sible without the help and suggestionsof a wide variety of p€rsonstoo - (l 986) Recognitionand implications of a structurally inverted Mo- numerous to mention here. Field researchwas supportedby grants from nadnock-westernMaine stratigraphydirectly above the core ofthe Ber- the Earth SciencesSection of NSF including most recently gants EAR- nardston nappe, Hinsdale, New Hampshire. Geological Society of 8410370, 8608762, and 8804852 (to Robinson). kttering on Figures 5, America Abstracts with Programs, 18, 15. 8, 10, 14, and 28 was done by Marie Litterer. Jennifer A. Thomson as- - ( I 987) P-T path convergencebased on gamet Fe-enrichment growth sisted with references for the first draft. The manuscript and figures were during prograde staurolite breakdown in tectonically juxtaposed pelites greatly improved by the careful reviews and comments by John Rodgers from southwesternNew Hampshire. Geological Society of America and John B. Lyons. The final manuscript and figureswere preparedwhile Abstracts with Programs, 19, 653-654. Robinson was a guest with use of facilities at the Institute for Geology, -(1988) Tectonic and metamorphic evolution of the Bemardston Norwegian Institute ofTechnology, University ofTrondheim. To eachof nappe and the Brennan Hill thrust in the Bernardston-Chesterfield re- these persons and institutions we give our grateful acknowledgment. gion of the Bronson Hill anticlinorium. New England Intercollegiate Figures l, 7, I l, and 12 are adapted from earlier figuresand reprinted Geological Conference, 80th Annual Meeting, Keene, New Hampshire, here with the kind permission of John Wiley and Sons,Inc. Guidebook, 70-102. RnrnnnNcns crrED Elbert, D.C., Harris, A.G., and Denkler, K.E. (1988) Earliest Devonian conodonts from marbles ofthe Fitch Formation, Bemardston nappe, Armstrong, T.R. (1986) Subdivision ofgranitic rock rypesin the Ashuelot north-central Massachusetts.American Journal ofScience, 288, 684- pluton, southwestemNew Hampshire. Geological Society of America 700. Abstracts with Programs, 18, 2. Eusden,J.D., Jr. (1988) Stratigraphy,structure and metamorphism ofthe 'dorsal Ashenden, D.D. (1973) Stratigraphy and structure, northern portion of zone', central New Hampshire. New England Intercollegiate the Pelham dome, north-central Massachusetts, 132 p., 3 plates. M.S. GeologicalConference, 80th Annual Meeting, Keene,New Hampshire, thesis, Contribution no. 16, Department of Geology, University of Guidebook, 40-59. Massachusetts,Amherst, Massachusetts Eusden,J.D., Jr., Bothner, W.A., and Hussey, A.M. (1987) The Kear- Ashwal, L.D., ko, G.W., Robinson, Peter,Zartman, R.E., and Hall, D.J sarge-Central Maine synclinorium of southeasternNew Hampshire and (1979) The Belchertonn Quartz Monzodiorite pluton, west-central southwesternMaine: Stratigraphic and structural relations of an in- Massachusetts,a syntectonicAcadian intrusion. American Journal of verted section.American Joumal of Sciencn.287 . 242-264. Science.279.936-969. Field, M.T. (1975) Bedrock geologyof the Ware area,central Massachu- Berry, H.N., IV (1989) A new stratigraphicand structural interpretation setts, lE6 p. Ph.D. thesis,Contributionr.o.22, DepartmentofGeology of granulite-faciesmetamorphic rocks in the Brimfield-sturbridge area, and Geography,University of Massachusetts,Amherst, Massachusetts. Massachusettsand Connecticut, 330 p. Ph.D. thesis, University of Hadley, J.B. (1949) Bedrock geology of the Mount Grace quadrangle, Massachusetts.Amherst. Massachusetts. Massachusetts.U.S GeologicalSurvey GeologicQuadrangle Map GQ- Berry, W.B.N., and Boucot, A.J., Eds. (1970) Correlation of the North 3. scale1:31.680. American Silurian rocks. Geology Society of America Special Paper Hall, B.A., Pollock, S.G., and Dolan, K.M. (1976) Lower Devonian Se- r02,289 p. boomook Formation and Matagamon Sandstone,northern Maine: A 't ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION t1

flysch basin-margin delta complex. Geological Society of America Peterson,V.L. (1984) Structure and stratigraphy of the bedrock geology Memoir 148,57-63. in the Ashburnham-Ashby area, north-central Massachusetts,182 p. Harris, A.G., Hatch, N.L., Jr., and Dutro, J.T., Jr. (1983) Late Silurian M.S. thesis, Contribution no. 47, Department of Geology of Geology conodonts update the metamorphosed Fitch Formation, Littleton area, and Geography,University of Massachusetts,Amherst, Massachusetts. New Harnpshire. American Joumal of Science,283, 722-738 Powell,D., Baird, A.W., Charnley,NR., and Jordan,P.J (1981) The Hatch, N.L., Jr ( I 987) Lithofacies,stratigraphy, and structurein the rocks metamorphic environment of the Sgurr Beag Slide; a major crustal of the Connecticut Valley trough, eastem Vermont. New England In- displacement zone in Proterozoic Moine rocks of Scotland. Journal of tercollegiateGeological Conference, 79th Annual Meeting, Burlington, the GeologicalSociety ofLondon, 138, 661-673 vermont. Guidebook. 192-212. Richardson,C.H. (1931) The areal and structural geologyofSpringfield, Hatch, N.L., Jr., Moench, R.H., and Lyons, J.B. (1983) Silurian-Lower Vermont. Vermont StateGeologist lTth Report, 193-212. Devonian stratigaphy of easternand south-centralNew Hampshire: Robinson, P. (1963) Gneiss domes ofthe Orangearea, west-central Mas- Extensions from western Maine. American Joumal of Science,283, sachusettsand New Hampshire, 253 p. Ph D. dissertation, Harvard 739-7 6l. University, Cambridge, Massachusetts. Hatch, N.L., Jr., Robinson, P., and Stanley,R.S. (1988) Silurian-Devo- -(1967) Gneiss domes and recumbent folds of the Orange area, nian stratified rocks of the Connecticut Valley belt. U.S. Geological west-centralMassachusetts. In Robinson, Peter, Ed., Guidebook for Survey ProfessionalPaper 1366-D, D1-D23. field trips in the Connecticut valley ofMassachusetts,p. 17-47. New Heritsch, F. (1929) The nappe theory in the Alps (translatedby P.G.A. England Intercollegiate Geological Conference, 59th Annual MeetinS, Boswell),228 p. Methuen, I-ondon. Amherst, Massachusetts. Hitchcock, C.H. (1871) Helderbergcorals in New Hampshire. American -(1979) Bronson Hill anticlinorium and Merrimack synclinorium Journal of Science.2. 148-149. in central Massachusetts.In J.W. Skehan and P.H. Osberg,Eds., The -(1912) The Strafford quadrangle.Vermont StateGeologist Eighth Caledonidesin the U.S.A.: Geologicalexcursions in the northeastAp- Report, 100-145. palachians,p. 126-150. Project 27, CaledonideOrogen 1979, Weston Hitchcock, E. (1833) Report on the geology, mineralogy, botany, and Observatory,Weston, Massachusetts. zoology of Massachusetts,702 p. Amherst, Massachusetts. - (l 98 l) The basement-coverenigrna in the gneissdomes of central Hodgkins, C.E. (1985) Major and face element geochemistryand petrol- New England, U.S.A., Uppsala Caledonide Symposium (abs.). Terra ogy of the late Precambrian Dry Hill Gneiss, Pelham dome, central Cognita,1,70. Massachusetts,1 35 p. M S. thesis,Contribution no. 48, Department of Robinson, P., and Luttrell, G.W. (1985) Revision of some stratigraphic Geology and Geography,University of Massachusetts,Amherst, Mas- namesin central Massachusetts.U.S GeologicalSurvey Bulletin 1605- sachusetts A, A7l-A78. Hollocher, K.T. (1985) Geochemistryof metamorphosedvolcanic rocks Robinson, P., Tracy, R.J., and Ashwal, L.D. (1975) Relict sillimanite- in the Middle Ordovician Partridge Formation, and amphibole dehy- orthoclase assemblagein kyanite-muscovite schist, Pelham dome, west- dration reactions in the high-grade metamorphic zones ofcentral Mas- central Massachusetts(abs.). Eos, 56,466' sachusetts,275 p. Ph.D. thesis, Contribution no. 56, Department of Robinson, P., Thompson. J.B., Jr., and Rosenfeld,J.L. (1979) Nappes, Geology and Geography,University of Massachusetts,Amherst, Mas- gneiss domes, and regional metamorphism in western New Hampshire sachusetts. and central Massachusetts.In J.W. Skehanand P.H. Osberg,Eds., The Hollocher, Kurt, and Lent, A.D. (1987) Comparative petrology of am- Caledonidesin the U.S.A.: Geologicalexcursions in the northeast Ap- phibolites in the Monson Gneiss and the Ammonoosuc and Partridge palachians, p. 93-125. Intemational Geological Correlation Program Volcanics, Massachusetts.Northeastern Geology, 9, 145-152. Project2T, CaledonideOrogen, Weston Observatory,Weston, Massa- Jasaitis,R.A. (1983) Geology of pre-Mesozoicbedrock of the Amherst chusetts. area, west-centralMassachusetts, 96 p. M.S. thesis, Contribution no. Robinson, P., Field, M.T., and Tucker, R.D. (1982a) Stratigraphy and 46, Department of Geology and Geography,University of Massachu- structure ofthe Ware-Barrearea, central Massachusetts.New England setts. Amherst. Massachusetts. Intercollegiate Geological Conference, 74th Annual Meeting, Storrs, Iro, G.W., Zartman, R.E., and Brookins, D.G. (1984)Glastonbury gneiss Connecticut,Guidebook, 34 | -37 3. and mantling rocks (a modified Oliverian dome) in south-central Mas- Robinson, P., Tracy, R.J., Hollocher, K.T., and Dietsch, C.W. (1982b) sachusettsand north-central Connecticut:Geochemistry, petrogenesis, High grade Acadian regional melamorphism in south-central Massa- and radiometric age.U.S. GeologicalSurvey ProfessionalPaper 1295, chusetts. New England Intercollegiate Geological Conference, 74th An- 45 p. nual Meeting, Storrs, Connecticut,Guidebook, 289-339. Michener, S.R. (1983) Bedrock geologyof the Pelham-ShutesburySyn- Robinson, P., Hatch, N.L., Jr., and Stanley, R.S. (1984) The Whately cline, Pelham dome, west-centralMassachusetts, 101 p. M.S. thesis, thrust: A proposed structural solution to the stratigraphic dilemma of Contribution no. 43, Department ofGeology and Geography,Univer- the Erving Formation and associated Devonian strata, western Mas- sity of Massachusetts,Amherst, Massachusetts. sachusetts and adjacent Vermont. Geological Society of America Ab- Moench, R.H., and Boudette, E.L. (1970) Stratigraphy of the northwest stractswith Programs, 16, 59. limb of the Merrimack synclinorium in the Kennebago Lake, Rangeley Robinson, P., Tracy, R.J., Hollocher, K.T., Schumacher,J.C., and Berry, and Phillips quadrangles, western Maine. Guidebook for field trips in H.N., IV (1986)The central Massachusettsmetamorphic high. In Peter the Rangeley Lakes-Dead River Basin, Western Maine, New England Robinson and D.C. Elbert, Eds., Field trip guidebook: Regional Intercollegiate Geological Conference, 62nd Annual Meeting, Range- metamorphism and metamorphic phase relations in northwestern and ley, Maine, 1-25. central New England, p.195-266. Field Trip B-5, International Min- Moench, R.H., and Hafner-Douglass,K (1987) Stratigraphic definition eralogical Association, 14th General Meeting at Stanford University, of the Piermont allochthon, Sunday Mountain to Albee Hill, New Contribution no. 59, Department ofGeology and Geography,Univer- Hampshire (abs.).New England IntercollegiateGeological Conference, sity of Massachusetts,Amherst, Massachusetts. 79th Annual Meeting, Burlington, Vermont, Guidebook, 271. Robinson, P., Hatch N.L., Stanley,R.S. (1988a) The Whately thrust: A Moore, G.E., Jr. (1949) Structureand metamorphism of the Keene-Brat- structural solution to the stratigraphic dilemma of the Erving Forma- tleboro area,New Hampshire-Vermont. Bulletin of the GeologicalSo- tion. U.S. GeologicalSurvey ProfessionalPaper 1366-8, B1-B34. ciety ofAmerica, 60, 1613-1669. Robinson, P., Huntington, J.C., McEnroe, S.A., and Springston, G.E. Naylor, R.S. (1969) Age and origin ofthe Oliverian domes, central-west- (1988b) Root zone of the Bernardston nappe and the Brennan Hill ern New Hampshire. Bulletin of the GeologicalSociety of America, 80, thrust involuted by back folds and Sneiss domes in the Mount Grace 405-428. area, north-central Massachusetts. New England Intercollegiate Geo- Onasch, C.M. (1973) Analysis of the minor structural features in the north- logical Conference, 80th Annual Meeting, Keene, New Hampshire, central portion of the Pelham dome, 87 p. M.S. thesis, Contribution Guidebook, 293-334. no. 12, Department of Geology,University of Massachusetts,Amherst, Robinson, Peter, Tracy, R.J., Hollocher, Kurt, Berry, H.N., IV, and Massachusetts. Thomson, J.A. (1989) Basementand cover in the Acadian metamor- 712 ROBINSON ET AL.: NAPPES IN THE CONNECTICUT VALLEY REGION

phic high of central Massachusetts.In C.p. Chamberlain and p Rob- New Hampshire New England IntercollegiateGeological Conference, inson, Eds., Stylesofmetamorphism with depth in the central Acadian 80th Annual Meeting, Keene, New Hampshire, Guidebook, 274-280. high, New England, p. 69-140. Contribution no. 63, Department of Thompson, J.B., Jr, and Rosenfeld,J.L (1979)Reinterpretation ofnappes Geology and Geography,University of Massachusetts,Amherst, Mas- in the Bellows Falls-Brattleboro .rea, New Hampshire-Vermont. In sachusetts. J.W. Skehan and P.H. Osberg, Eds., The Caledonidesin the U.S.A.: Roll, M.A. (1986) Etrects of Acadian progade kyanite-zone metamor- Geological excursionsin the northeast Appalachians,p. 117-121. ln- phism on relict gamet from pre-Acadian granulite facies metamor- ternational Geological Correlation Program Project 27, Caledonide phism, Mount Mineral Formation, Pelham dome, MassachusettsGeo- Orogen, Weston Observatory, Weston, Massachusetts. programs, logical Society of America Abstracts with 18, 63. Thompson, J.8., Jr., Robinson, P., Clifford, T.N., and Trask, N.J., Jr. -(1987) Effects of Acadian kyanite-zone metamorphism on relict (1968) Nappes and gneissdomes in west-centralNew England. In E. granulite-facies assemblages,Mount Mineral Formation, pelham dome, Znn and W.S. White, Eds., Studiesof Appalachian geology:Northern Massachusetts,202 p. M.S. thesis,Contribution no. 60, Department of and maritime, p. 203-218,2 color plates. Wiley, New York. Geology and Geography,University of Massachusetts,Amherst, Mas- Thompson,J.B Jr., Cheney,JT., and Robinson,P. (1986) Meramor- sachusetts. phism on the east flank of the Green Mountain massif and Chester Rumble, D., III (1969) Stratigraphic,structural and petrologic studiesin dome. In Peter Robinson and D.C. Elbert, Eds., Field trip guidebook: the Mt. Cube area,New Hampshire-Vermont ph.D. dissertation.Har- Regional metamorphism and metamorphic phase relations in north- vard University, Cambridge, Massachusetts. western and central New England, p. 94-119. Field trip B-5, Inter- -(1971) Recumbent and reclined folds of the Mt. Cube area, New national Mineralogical Association, l4th General Meeting at Stanford Hampshire-Vermont. New EnglandIntercollegiate Geological Confer- University, Contribution no. 59, Department of Geology and Geog- ence, 63rd Annual Meeting, Concord, New Hampshire, Guidebook, raphy, University of Massachusetts,Amherst, Massachusetts. 43-52. Thompson, P.J. (1985) Stratigraphy,structure, and metamorphism in the Schumacher,J.C. (1988) Strati$aphy and geochemistryof the Ammo- Monadnock Quadrangle,New Hampshire, l9l p. Ph.D. thesis, Con- noosuc Volcanics, central Massachusettsand southwestern New tribution no. 58, Department ofGeology and Geography,University Hampshire. American Journal of Science,288. 6 I 9-663. of Massachusetts,Amherst, Massachusetts. Schumacher,J.C., and Robinson, Peter (1987) Mineral chemistry and - (1988a)Stratigraphy and structure ofthe Monadnock quadrangle, metasomaticgrowth of aluminous enclavesin gedrite-cordieritegneiss, New Hampshire. New England IntercollegiateGeological Conference, southwesternNew Hampshire. Joumal of Petrology,28, 1033-1073. 80th Annual Meeting, Keene,New Hampshire, Guidebook, 136-163. Schumacher,J.C., Schumacher,Renate, and Robinson, peter (1989) Aca- -(1988b) Geology of Mount Monadnock. New England Intercolle- dian metamorphism in central Massachusettsand southwesternNe\tr giate Geological Conference, 80th Annual Meeting, Keene, New Hampshire: Evidence for contrasting P-T trajectories. Geological So- Hampshire, Guidebook, 268-273. ciety SpecialPublication no. 34, 453-460, London. Thompson, P.J., Elbert, D.C., and Robinson, Peter (1987) Thrust nappes Sleep,N.H. (1979) A thermal constraint on the duration of folding with superimposedon fold nappes: A major component of early Acadian referenceto Acadian geology,New England (U.S.A.). Joumal of Ge- tectonicsin the central ConnecticutValley fegion, New England.Geo- ology,87, 583-589. logical Society of America Abstracts v/ith Programs, 19, 868. Spear,F.S. (1986) P-T paths in cenrral New England:The evolution ofa Trask, N J., Jr. (1964) Stratigraphyand structure in the Vernon-Chester- paired metamorphic belt. Geological Society of America Abstracts udth field area, Massachusetts,New Hampshire, Vermont, 99 p. Ph.D. dis- Programs,18,68. sertation, Harvard University, Cambridge, Massachusetts. Spear,F S.,and Chamberlain,C.P. (1986) Metamorphic and.tectomc evo- Trask, N.J., Jr., and Thornpson,J.8., Jr. (1967) Stratigraphyand structure lution of the Fall Mountain nappe complex and adjacent Merrimack of the Skitchewaugnappe in the Bernardstonarea, Massachusettsand synclinorium. In Peter Robinson and D.C. Elbert, Eds., Field trip adjacentNew Hampshire and Vermont. In Peter Robinson. Ed., Field guidebook:Regional metamorphism and metamorphic phaserelations trips in the Connecticut Valley of Massachusetts,p. 129-142. New in northwesternand Central New England,p. l2l-144 Field Trip B-5, England Intercollegiate Geological Conference, 59th Annual Meeting, International Mineralogical Association, 14th General Meeting at Stan- Amherst, Massachusetts. ford University, Contribution no. 59, Department ofGeology and Ge- Tucker, R.D. (1977) Bedrock geologyof the Barre area, cenrral Massa- ography, University of Massachusetts,Amhent, Massachusetts. chusetts, 132 p. M.S. thesis, Contribution no. 30, Department of Ge- Spear,F.S., and Rumble, Douglas, III (1986) Mineralogy, petrology, and ology and Geography,University of Massachusetts,Amherst, Massa- P-T evolution of the Orfordville area, west-centralNew Hampshire chusetts. and east-centralVermont. In Peter Robinson and D.C. Elbert. Eds.. Tucker, R.D., and Robinson, P. (1990) Age and serting ofthe Bronson Field trip guidebook: Regional metamorphism and metamorphic phase Hill magmatic arc: A re-evaluationbased on U-Pb zircon agesin south- relations in northwestern and Central New England, p. 57-93. Field ern New England. Bulletin ofthe Geological Society ofAmerica, 102, Trip B-5, International Mineralogical Association, l4th General Meet- 1404-t419. ing at Stanford University, Contribution no. 59, Department ofGeol- Zartman, R.E., and Leo, G.W. (1985) New radiometric ageson gneisses ogy and Geography,University of Massachusetts,Amherst, Massachu- of the Oliverian domes in New Hampshire and Massachusetts.Amer- setts. ican Journal of Science,285, 267-280. Springston,G.E. (1990) Stratigraphy and structural geologyofthe Roy- Zan,E-an, White, W.S., Hadley, J.B., and Thompson,J.B., Jr., Eds.,(1968) alston-Richmond area, Massachusetts-NewHampshire, 100 p. M.S. Studiesof Appalachian geology:Norrhern and maitime,475 p. Wiley, thesis,University of Massachusetts,Amherst. Massachuseus. New York. Thompson, J.8., Jr. (195a) Structural geologyofthe SkitchewaugMoun- Znn,E-an, Goldsmith, R., Ratcliffe, N.L., Robinson, P., and Stanley,R.S. tain area,Claremont quadrangle,Vermont-New Hampshire. New En- (1983) Bedrock geologicalmap of Massachusetts(two sheetsin color, gland Intercollegiate Geological Conference, 46th Annual Meeting, with cross sections,tectonic map, metamorphic map, explanation, black Hanover, New Hampshire, Guidebook, 93-174. and white referencesheet). U.S. Geological Survey, Washington, DC, - (1956) Skitchewaugnapp€, a major recumbentfold in the areanear scale1/250000. Claremont, New Hampshire (abs.).Bulletin of the Geological Society of America. 67. 1826-1827. M,cNuscRrFrREcETVED FEsnuARv 9, l990 - (l 988) The Skitchewaugnappe in the Mascoma area,west-central Mamrscnrpr AccEprEDFpsnueny 22, l99I