A Transect Through the Foreland and Transitional Zone of Western

A TRANSECT THROUGH THE F O R E L A N D AND VERMONT TRANSITIONAL ZONE OF W E S T E R N by Rolfe S Stanley, K a t h e r i n e Leonard, and B a rbar a D e p a r t m e n t of G e o l o g y U n i v e r s i t y of V e r m o n t Burling ton, Vermont, 05405 Strehle INTRODUCTION Western Vermont is u n d e r l a i n by three d i s t i n c t i v e sequences of rocks that range in age from Late P r o t e r o z o i c to M i d d l e O r d o v i c i a n and are typical of the w e s t e r n part of the A p p a l a c h i a n Mountains The lowest most sequence, which rest with pr o f o u n d u n c o n f o r m i t y on the M i d d l e P r o t e r o z o i c of the G r e e n M o u n t a i n and L i n c o l n massifs, l a r g e l y consists of m e t a w ac kes , mafic v o l c a n i c rocks and phyllites that r e p r e s e n t a rift clastic sequence These rocks grade upward into s i l i c i c l a s t i c and c a r b o n a t e rocks of the p l a t f o r m sequence, w h i c h in turn are o v e r l a i n by M i d d l e O r d o v i c i a n shales of the f o r e l a n d basin sequence The b o u n d a r y b e t w e e n the two sequences is the base of the Ch e s h i r e F o r m a t i o n (fig 1) North of Burlington, Vermont the p l a t f o r m sequence grades into shales, breccias and c o n g l o m e r a t e s of the ancient p l a t f o r m m a r g i n and e a s tern basin These s e q u e n c e s have been s t u died by a n u m b e r of w o rker s in the •past (Cady, 1945; Cady and others, 1962; Hawley, 1957; Erwin, 1957; Welby, 1961; Stone and Dennis, 1964, for example) and are re ceiving current a t t e n t i o n by M e h r t e n s (1985; in press) and her students (Gregory, 1982; Myrow, 1983; Teetsel, 1985; Bulter, 1986; MacLean, 1986) Agnew (1977), Carter (1979), Tauvers (1982), DiPietro (1983) and D o r s e y and others (1983) have r e e x a m i n e d parts of the rift clastic sequence while D o o l a n and his students are c u r r e n t l y working in the same sequence in northern V e rmont and Quebec F i g u r e s and i l l u s t r a t e the r e p r e s e n t a t i v e s t r a t i g r a p h i c columns for w e s t e r n V e r m o n t north of the l a t i t u d e of the L i nc oln massif w h e r e the field c o n f e r e n c e will be held Additional information can be found in W e l b y (1961) and Doll and others (1961) The structure of western Vermont is dominated by major, n o r t h - t r e n d i n g folds and i m b r i c a t e thrust faults w h i c h are well d i s p l a y e d on the G e o l o g i c Map of V e r m o n t (Doll and others, 1961) The rift clastic and p l a t f o r m s e q u e n c e s have each been di splaced westward on m a j o r thrust faults that extend through much of Vermont The larger of the two, the C h a m p l a i n thrust, extends from s o u t h e r n Quebec to Albany, New York and places the older p l a t f o r m s e q u e n c e over the y o u n g e r foreland b a s i n sequence Estimates of w e s t w a r d d i s p l a c e m e n t range from 15 km to 100 km The smaller of the two, the H i n e s b u r g thrust, places t r a n s i t i o n a l and rift clastic rocks over the p l a t f o r m se q u e n c e and as such forms a bo u n d a r y b e t w e e n the s y n c l i n o r i a l rocks of w e s t e r n V e r m o n t and the Green Mountain anticlinorium Do rsey and others (1983) have d e m o n s t r a t e d that the H i n e s b u r g thrust d e v e l o p e d a l o n g the o v e r t u r n e d limb of a large r e c u m b e n t fold ( f a u l t - p r o p a g a t i o n fold of Suppe, 1985) and t h e r e f o r e is quite d i f f e r e n t from the g e o m e t r y of ■the Champlain 1—1 cd £ •H 00 •H T3 1—1 00 43 £ cd cd o £ •H T3 O cd £ £ •H CJ* 43 in £ Cm O O 4-3 rH •H G S O •H • •• Q /? kô ã ã i" ãft v V Oft ãããã II* M l 'i 111 I * • C-:7V 1* • • • m*J•• : sV • §• •• I •• ft • ft• P ‘ M • • •• • / / I ift■• w.x# r ft / • ft ■ ! " 1! 'hi" M iitiM U ip J ii h i h a i !! "i'i 'iH i i t - ' - I"I'liiMlmh ,IIIi'lAs i. O U % ID \ • c ft • fto * c W m £ -H Cd G O •H •H 03 O Pi ft ft a //:' i " 1i S & m ' f / f f i ‘ i i # 11 ' I i i '1S $ ! lift/ JC I rH >» c CO H Z o s CO UJ GC => h O X DC h cc UJ > m Q Z < u CO CO Q Ui h o cc < oJ Hi X cc X UJ Q h- h < < o z X CO CO h >- X < DC DC < CD LL H CO X CC X I— O CO at DC O CO CO UJ CO CD 05 LL 88 / A -5 On the n o r t h e a s t side of the q u a r r y (fig 6) a syncline and an associated blind , s y n f o r m a l thrust fault are truncated by the m a j o r thrust fault that is c o n t i n u o u s across the north wall of the quarry The origin of this s t r u c t u r e is not clear, but it is thought to be a s s o c i a t e d with a duplex or ramp below the level of the qu arry floor STOP - "THE BEAM" - THIS IS A S U P E R B O U T C R O P THAT SERVES AS A F I E L D L A B O R A T O R Y FOR R E S E A R C H AND T E A C H I N G OF F O R E L A N D D EFORMATION P L E A S E STUDY IT USE YOUR C A ME RAS BUT NOT YOUR HAMMERS REFER TO F I G U R E S AND The outcrop is located in the Cumberland Head Formation a p p r o x i m a t e l y miles west of the e x pos ed front of the C h a m p l a i n thrust fault or a p p r o x i m a t e l y 4600 feet b e l o w the restored w e s t w a r d p r o j e c t i o n of the thrust surface The m a j o r q u e s t i o n s that will be d i s c u s s e d are: How ramp faults form ?, 2.Are there criteria to d e t e r m i n e if i m b r i c a t e thrust faults d e v e l o p toward the foreland or h i n t e r l a n d ?, 3* What is the r e l a t i o n b e t w e e n fa u l t i n g and cleavage development ?, 4* What processes are involved in the f o r m a t i o n of fault zones ?, 5* Are there c r i t e r i a that indicate the r e l a t i v e i m p o r t a n c e and d u r a t i o n of m o t i o n a l o n g a fault zone ?, Is there evidence that abnormal pore pressure existed during f a u l t i n g ?, and f i na lly What is the s t r u c t u r a l e v o l u t i o n of the imbricate faults ? The first six questions will be largely a d d r e s s e d by direct e v i d e n c e at the outcrop The last q u e s t i o n will be a n s w e r e d by p a l i n s p a s t i c a l ly r e s t o r i n g the i m b r i c a t e d and cleaved s e q u e n c e to its u n d e f o r m e d state (fig 8) THE OUTCROP Five i m b r i c a t e thrust faults and a s s o c i a t e d ramps are exposed in p r o f i l e s e c t i o n in a foot thick bed of m i c r i t e that extends 45 feet a l o n g an a z i m u t h of N 80 E (fig 7a) The i m b r i c a t e fault can be f u r t h e r c l a s s i f i e d as a c e ntra l d u p l e x of three horsts that are s e p a r a t e d from two simple ramps at e i t h e r end of the outcrop by a p p r o x i m a t e l y feet of flats The m i c r i t e bed is s u r r o u n d e d by at least feet of w e l l - c l e a v e d calcareous shale Bedding plane faults are p r es ent along the u p p e r and lower surface of the m i cri te w h e r e they merge with ramp faults that cut across the m i cr ite bed at an angle of a p p r o x i m a t e l y 30 degrees The lower bedding plane fault or floor thrust is r e l a t i v e l y p l a n a r and the fault zone is thick In c o m p a r s i o n the u p p e r b e d d i n g plane faults are folded in the ramp areas, cut by ramp faults, and the fault zones are thin The u p p e r b e d d i n g plane fault forms the roof thrust for the central duplex A l o n g the i n t e r v e n i n g flats the u p p e r faults are g e n e r a l l y p l a n a r a l t h o u g h they are cut by the S1 c l e a v a g e in many places The lower b e d d i n g plane fault is the m a j o r d e c o l l e m e n t across the outcrop Older b e d d i n g planes faults are also present throughout the shale and are offset by the p e n e t r a t i v e S1 cleavage All the fault s u r f a c e s are covered by layers of sparry calcite that v a r y in t h i c k n e s s from s e v eral mm to 4-6 cm The thickest zone is 89 join sills hi gher in layers of the fault fault zone the fault zone and A -5 zone, are more common in the lowest in the shale d i r e c t l y b e n eath the 2) Discontinuous, dark, styolitic clay laminae with concentrations of quartz adjacent to or w i t h i n the laminae are interlayered with the calcite in all but thinnest zones The laminae are identical in a p p e a r a n c e to the se lvedges on the S1 c l e a v a g e surfaces Some of the clay laminae are c o n t i n u o u s with chips of shale which are either completely enclosed within a c a lci te layer or occur at the b o u n d a r y b e t w e e n two calcite veins These shale chips p r e s e r v e v a r y i n g degrees of pr e s s u r e solution For example, some of the chips are s i m ilar to the u n d e f o r m e d shale in m i c r o l i t h o n s away from fault zones w h e r e a s others have dark, thin selvedg es w i t h i n the chip and along their edges 3) A relative pl anar s u r f a c e d e c o r a t e d with shale laminae cuts older surfaces in the f a u l t - z o n e deposit and is c o n t i n u o u s for or 10 feet along the floor fault Such surfaces as this are called slip s u r f a c e s 4) The size of the s p a r r y c a l cite is di r e c t l y p r o p o r t i o n a l to vein width Most grains are bladed in form, but their long axes is not prefferentially alined The calcite in all the layers is twinned with the g r e a t e s t d e n s i t y o c c u r i n g in the t h i nner layers b e t w e e n shale laminae where the grains are turbid and small The larger grains in thicker layers n e a r e s t the planar slip surface are g e n e r a l l y more twinnned than are those grains in veins f u r ther away 5) The s l i c k e n l i n e s on grooves rather than c a l cite each of fibers the vein layers are formed by 6) The calcite-shale layers tend to be more parallel in s e c t i o n s cut parallel to the s l i c k e n l i n e s rather than they are in s e c t i o n s cut p e r p e n d i c u l a r to the s l i c k e n l i n e s where the layers are i r r e g u l a r or anastomose It is clear from the f o r e g o i n g i n f o r m a t i o n that the calcite has been i n t r u d e d along the faults a f t e r the initial c o h e s i o n had been b r o k e n along the s h a l e - s h a l e or s h a l e - m i c r i t e contacts where the s t r e n g t h contrast is the g r e a t e s t and the cohesive s t r e n g t h the weakest Once such a zone has developed and is filled with calcite, it becomes a zone of w e a k n e s s S u b s e q u e n t failure likely o c c u r r e d along the c a l c i t e - s h a l e i n t e r f a c e and re s u l t e d in scabs and chips of the shale b e i n g i n c o r p o r a t e d into the fault zone Other shale fragments may have been sheared in along faults or carried in along thick veins D u r i n g renewed m o v e m e n t the calcite and quartz were dissolved from the shale to form the black s e l v e d g e s which are i n t e r l a y e r e d with the calcite and de c o r a t e the slip surfaces These b o u n d a r i e s then formed weak planes along w h i c h s u b s e q u e n t m o v e m e n t o c c u r r e d w i t h i n the fault zones As the fault zone thickened m o v e m e n t could occur along pl anar surfaces (slip surfaces) which s m o o t h e d out the i r r e g u l a r g e o m e t r y formed by 94 A-5 ramp zones and broad folds in the i n t e r v e n i n g flat regions of the micrite bed Movement was no lo nger restricted to the c a l c i t e - s h a l e b o u n d a r i e s of thin fault zones, but could occur along any f a v o r a b l y s i t u a t e d c a l c i t e - s e l v e d g e boundary The r e s u l t i n g clay se l v e d g e then acted as a ca t a l y s t that f a c i l i t a t e d s o l u t i o n of c a lci te from the selvedge-calcite boundary of the surrounding veins This process resulted in the stylolitic form of the selvedge We suggest that p r e f e r r e n t i a l s o l u t i o n in the d i r e c t i o n of fault movement produced the slickenline grooves and the s t y l o l i t i c s e l v e d g e s best seen in se c t i o n s cut p e r p e n d i c u l a r to the slickenlines B e cau se the floor fault was c o n t i n u a l l y active d uring the e v o l u t i o n of the i m b r i c a t e system, It is not s u r p r i s i n g to find evidence for re p e a t e d vein i n j e c t i o n in the form of n u m e r o u s c r o s s c u t t i n g veins in the thick fault zone deposit D u r i n g each of these events the influx of fluid and the s u b s e q u e n t c r y s t a l l i z a t i o n was r e l a t i v e l y rapid so that sparry calcite formed ther than fibered calcite As the fault zone thickened with layers of calcite and clay selvedge, new veins could form a l o n g any surface of w e a k n e s s w i t h i n the fault zone rather than being c o n f i n e d to the outer b o rder s with the c o un try rock As m o v e m e n t c o n t i n u e d across the fault zone, the calcite in the older veins became h e a v i l y twinned and s e v e r e l y strained R e p e a t e d s o l u t i o n of calcite a l o n g their b o u n d a r i e s with the ad j a c e n t clay s e l v e d g e s r e duce d their t h i c k n e s s and pr o d u c e d the common o b s e r v a t i o n that calcite in m a n y of the t h inne r veins are h e a v i l y twinned The s c e nario that has been i n f e r r e d from the fault zone fabrics and the relative age r e l a t i o n s a m o n g the faults s u g g e s t s that fault m o v e m e n t was i n t e r m i t t e n t with each event o c c u r i n g rapidly During the i n t e r v e n i n g time d e f o r m a t i o n in the fault zone may have been r e s t r i c t e d to t w i n n i n g in the calcite Shortening The s h o r t e n i n g across the o u t c r o s s is c o n v e n i e n t l y re c o r d e d by the folds in the ramps areas and s t r u c t u r a l o v e r l a p across the faults The d i s p l a c e m e n t on each of the faults in the m i c r i t e bed ranges from to 19 inches w h i c h adds up to a total d i s p l a c e m e n t of 52.8 inches or 4-4 feet over a p r e s e n t h o r i z o n t a l d i s t a n c e of 35 feet The five a n t i c l i n e s over ramps and the broad folds along the flats a c cou nt for a p p r o x i m a t e l y inches of a d d i t i o n a l s h o r t e n i n g so that the total s h o r t e n i n g equal 57.8 inches or 4.8 feet These values c o r r e s p o n d to a s h o r t e n i n g of 13*7 percent In the shale the c o r r e s p o n d i n g s h o r t e n i n g is p r o v i d e d by volume r e d u c t i o n across the c l e a v a g e surfaces In order to see if the s h o r t e n i n g d e t e r m i n e d from the m i c r i t e bed is c o m p a r a b l e to the s h o r t e n i n g in the shale, an i n d e p e n d e n t e s t i m a t e was made for the shale by d e t e r m i n i n g the p e r c e n t a g e of i n s o l u b l e materia l Samples of suitable material from four different microlithons where i m m e r s e d in h y d r o c h l o r i c acid until all the soluble m a t e r i a l was elimina ted The final a v e rage residue was 36 p e r cent of the 95 A—5 or i g i n a l mass (a range of 32$ to 39$ for samples) The nu mber of cl e a v a g e selvedges across a p r esen t width of 26 feet was then counted The total w i d t h of the s e l v e d g e s (a range of 1.16 to 1.78 foot) was then m u l t i p l i e d by 2.8 to given an e s t i m a t e of the original width now represented by the cleavages (3*25 to feet) The original length of the p r e s e n t 26 foot w i d t h was then e s t i m a t e d to be 29*3 to 31 feet S h o r t e n i n g was then c a l c u l a t e d to be in the range of 7*3 to 10.6 percent This d i f f e r e n c e in s h o r t e n i n g values in the "beam" and the shale is not c o n s i d e r e d to be s i g n i f i c a n t b e c a u s e the m e t h o d for e s t i m a t i n g s h o r t e n i n g in the shale is less a c c u r a t e than the method for the m i c r i t e bed For example, the n u m b e r of cleavage surfaces in the shale and their thickness were underestimated since many thin cleavage selvedges were overlooked in the originally count F u r t h e r m o r e , if the lowest value of 32 p e r c e n t were used along with all the e x i s t i n g data the total s h o r t e n i n g would be a little over 14$Thus the range of values overlaps the shortening value c a l c u l a t e d for the m i c r i t e bed We therefore conclude that the formation of the cleavage and consequent shortening in the shale occurred during imbricate f a u l t i n g in the m i c r i t e bed A l t h o u g h this c o n c l u s i o n may seem i n t u i t i v e l y obvious, it does have i m p o r t a n t i m p l i c a t i o n s for the e v o l u t i o n of the cleavage B e c a u s e we have a l r e a d y p roven that the fsults represe nt a t i m e - t r a n s g r e s s i v e se q u e n c e that d e v e l o p e d from east to west, we must also conclude that the cl e a v a g e in the surrounding shales must have developed in a similar mannner Unlike our e a rlie r c o n c l us i on, this r e l a t i o n is far from obvious from the r e l a t i o n s w i t h i n the c l e a v e d shale In fact it is the e x i s t e n c e of the m i c r i t e bed and its fault g e o m e t r y that allows us to conclude that the cleavage in the shale is indeed a time t r a n s g r e s s i v e phenomena The e v o l u t i o n of the cl e a v a g e and its r e l a t i o n to i m b r i c a t e f a u l t i n g is best shown by r e t r o d e f o r m i n g the faulted micrite bed to its original predeformational condition (fig 8) The next p r o b l e m is the or igin of Sr, the rotated cleavage, and St the f i n e l y spaced cl e a v a g e b e l o w the floor thrust Because Sr is s i m p l y the dominant S1 c l e a v a g e t h r o u g h o u t the shale and is only p r e s e n t near the floor and roof faults, it had to form after the ramp faults d e v e l o p e d and d u r i n g s u b s e q u e n c e d i s p l a c e m e n t on the roof and floor faults D u r i n g this time the S1 cl e a v a g e near these faults is rotated in simple shear in the , d i r e c t i o n of fault displacement The fact that the Sr c l e a v a g e below the floor fault is r o t ated more than it is a l o n g the roof fault is c o n s i s t e n t with our e a r l i e r c o n c l u s i o n that the floor thrust was ac tive t h r o u g h o u t d e f o r m a t i o n whereas the i n d i v i d u a l roof faults are short lived The St cleavage, w h i c h is only p r ese nt along the floor thrust, c l e a r l y formed after Sr b e c a u s e it cuts across Sr at a low angle and is not folded or rotated Its a b s e n c e along the roof faults is c o n s i s t e n t with their short h i s t o r y of d i s p l a c e m e n t St is a true fault zone cleavage b e c a u s e it is r e s t r i c t e d to a thin region below 96 A -5 fi xed point floor thrust roof thrust r a mp f a u l t •• • V RETRODEFORMED S E C T I O N OF T H E CUM BERLAND HEAD FORMATION SOUTH HERO,VERMONT St anl ey , 1987 Figure 97 A-5 the floor fault cleavage the is in the Furthermore, St orientation predicted by Ramsay equation Tan 20'=2/gamma the (Ramsay and Huber, 1984) w h e r e gamma of 36 d e gr ees is the rotation of S1 (Sr) as it is traced into the fault zone This relation proves that St is the result of simple shear along the floor fault Are St and Sr time t r a n s g r essive? B e c a u s e we have d e m o n s t r a t e d e a r l i e r that the floor fault and the r e s p e c t i v e roof faults are time t r a n s g r e s s i v e , it must follow that both Sr and St are also time t r a n s g r e s s i v e to the west This c o n c l u s i o n suggests that the amount of r o t a t i o n of Sr and the intensity of St should also in c r e a s e to the east where the d i s p l a c e m e n t on the floor fault has been longer We could detect no such r e l a t i o n which, in turn, may suggest that there is a limit beyond w h i c h Sr can be rotated EVOLUTION OF S T R U C T U R E S The e v o l u t i o n of the i m b r i c a t e faults and the various cleavages d e s c r i b e d in the f o r e g o i n g s e c t i o n is i l l u s t r a t e d in a series of r e t r o d e f o r m e d sections in figure S e c t i o n shows the "beam" in its p r esen t state Section is developed by reversing the d e f o r m a t i o n a s s o c i a t e d with the y o u n g e s t ramp fault at the w e s t e r n part of the outcrop For example, the rocks of the h a n g i n g wall block (B, s e c t i o n 2) are u n f o l d e d as they are returned to their original position east of the footwall block (A, section 2) D u r i n g the time r e p r e s e n t e d by s e c t i o n the active floor fault in the e a s t e r n part of the d i a g r a m climbs s e c t i o n along the ramp fault below block C and c o n t i n u e s a l o n g the top of block A and B As a result the St and Sr c l e a v a g e s b e l o w b locks A and B are absent F u r t h e r m o r e , the S1 c l e a v a g e is shown to be more abundant in the u p p e r plate than the lower plate b e c a u s e it is a c t i v e l y moving S e c t i o n s 3, and show the r e t r o d e f o r m a t i o n c o n t i n u i n g to east and are c o n s t r u c t e d in the same m a n n e r as s e ct ion Thus e v o l u t i o n of the i m b r i c a t e s y s t e m and its a s s o c i a t e d s t r u c t u r e s be seen by s t u d y i n g di a g r a m s t h roug h the the can STOP - CLAY POINT (fig 9) - Clay Point is l o c ated several 1000 feet west of the trace of the C h a m p l a i n thrust fault (fig and 5) • The rocks consists of m e d i u m to dark gray, noncalcareous shales, dolomitic s i l t s t o n e s and beds of brown-weathered d o l o m i c r i t e of the I b e r v i l l e F o r m a t i o n , a foreland basin flysch deposit This sequence is r h y t h m i c a l l y layered with the base of each cycle m a r k e d by y e l l o w i s h - b r o w n w e a t h e r e d , dark gray laminat ed s i l t s t o n e w h i c h contains ripple l a m i n a t i o n s (Hawley, 1972) These beds grade u p w a r d into dark grey, well cleaved shale that are m a r k e d by thin layers of l i n e a t e d calcite These layers represent b e d d i n g plane faults (Tb, fig* 9a) w h i c h are i d e n t i c a l to the b e d d i n g plane faults at the "beam" Like the bedding, these faults are folded into a w e s t - f a c i n g , o v e r t u r n e d a n t i c l i n e that is cut by a prominant, east-dipping, pressure solution cleavage (S1, fig* 9b) This cl e a v a g e has the same o r i e n t a t i o n as the S1 cleavage at the "beam" (STOP 3) and is part of the same g e n e r a t i o n a l t h o u g h it 9J: A -5 W A TE R 31 | s t tig p h ic to p s DEFORMED IBERVILLE FORMATION A T C L A Y POINT, VERM ONT Figure 9a N F i g u r e 9b the - Lower h e m i s p h e r e equal area p r o j e c t i o n o r i e n t a t i o n of s t r u c t u r a l e l e m e n t s at Clay Point 99 showing A-5 formed e a r l i e r because it is s i t u a t e d f a r ther to the east and c l o s e r to the C h a m p l a i n thrust fault The o v e r t u r n e d limb of the a n t i c l i n e is cut by a series of thrust faults (Tn through Tn-4) that are c l ea rly i n f l u e n c e d more by the cleavage than they are by the bedding The sense of d i s p l a c e m e n t on these faults is provided by the rotated S1 cl e a v a g e adjacent to the fault surface We believe that faults of this type that are controlled by the c l e a v a g e are y o u n g e r than the faults seen at the "beam" This r e l a t i o n can be seen at the east end of the "beam" where a y o u n g e r fault has d e v e l o p e d from the older ramp and is cl i m b i n g through the upper plate shale The other features that you should study are: # 1) C a lcit e fractures (fig 9a) M a n y of these fractures c o n t a i n cross fibers and in d i c a t e slow e x t e n s i o n parallel to the fiber dir ecti on Several of the veins are co m p o u n d with an outer b o r d e r of fiber calcite and an inner core of sparry calcite, which indicates rapid extension These veins th erefore opened more r a pid ly after an e a r lier period of slow extension 2) S e v e r a l of the faults on w e s t e r n side of the outcrop have two d i r e c t i o n s of s l i c k e n l i n e s A l t h o u g h this fact suggests a change in fault m o t i o n during the evolution of the s t r u c t u r e at Clay Point, a n o t h e r e x p l a n a t i o n is s u g g e s t e d by the fact that many of these faults are b e d d i n g plane faults (Tb) D u r i n g folding the older n o r t h w e s t e r l y s l i c k e n l i n e d i r e c t i o n was rotated out of the plane containing the transport direction (deformation plane) S u b s e q u e n t m o v e m e n t along the same n o r t h w e s t e r l y d i r e c t i o n p r o d u c e d the new s l i c k e n l i n e s w h i c h cut across the older direction 3) A w e s t - t r e n d i n g the cross section Mesozoic dike 4) S t u d y the laminate c a lcit e fabric p e r p e n d i c u l a r and p a r a l l e l cut the anticline just north a l o n g the faults Look at to the s l i c k e n l i n e direction of the STOP - THE C H A M P L A I N T H R U S T F A U L T AT LONE ROCK POINT, B U R L I N G T O N , V E R M O N T - The f o l l o w i n g d i s c u s s i o n is r e p r i n t e d from The C e n t e n n i a l Field Guide, Volume 5, of the G e o l o g i c a l S o c i e t y of A m e r i c a in 986 All the figure n u m b e r s for this stop refer to those figures in the reprint The r e p r i n t e d d i s c u s s i o n appears in A p p e n d i x STOP - THE H I N E S B U R G T H R U S T F A U L T AT HINES BU R G, V E R M O N T - This is the classic and best e x p o s e d locality for the H i n e s b u r g thrust fault It contains many fault r e la ted fabrics that have r e c e n t l y been studied by Strehle (1985) and p u b l i s h e d by Strehle and S t a n l e y (1986) in a b u l l e t i n of the V e r m o n t G e o l o g i c a l Su rvey (Studies in V e r m o n t G e o l o g y N o 3)This p u b l i c a t i o n also contains an a l y s i s of other fault zones of w e s t e r n V e r m o n t w h i c h will be seen during this NEIGC The reader is r e f e r r e d to this paper or an e a rl ier NEIGC trip by G i l l e s p i e and others (1972) The Hinesburg thrust fault separates 100 the C a m b r i a n - O r d o vician rocks A-5 of the platform sequence from the older, hi ghly deformed m e t a m o r p h i c rocks of the e a s t e r n h i n t e r l a n d As shown in figure 4, the H i n e s b u r g thrust fault d e v e l o p e d a l o n g the overturned, s h ear ed limb of a large r e c u m b e n t fold This fault p r o b a b l y broke out from the o v e r t u r n e d limb of a f a u l t - p r o p a g a t i o n fold (Suppe, 1985) and t h e r e f o r e is s i m i l a r in o r i g i n to the A r r o w h e a d M o u n t a i n thrust fault To the south the H i n e s b u r g thrust fault dies out somewhere in the overturned limb of the Lincoln massif (Tauvers, 1982; DiPietro, 1983; D e l l o R u s s o and Stanley, 1986) At the M e c h a n i c s ville locality the lower 40 m of the Ch e s h i r e Quartzite is s t r u c t u r a l l y o v e r t u r n e d along the base of the upper plate ofthe H i n e s b u r g thrust fault Higher up the cliff the quartzite grades into the Fairfield Pond Formation of T a u vers (1982) The lower plate rocks, w h i c h are poorly exposed, c o ns ist of c a r b o n a t e s of the Lower O r d o v i c i a n B a s c o n Forma ti on S l i v e r s of dark gray ph y l l i t e of the B r o w n e l l M o u n t a i n P h y l l i t e are found at s e v e r a l local it i es a l o n g the fault trace Chlorite, mu scov i te, and s t i l p n o m e l a n e are p r e s e n t in the q u a r t z i t e M u s c o v i t e and ch l o r i t e are p r esent in the schist The following features sh ould be studied here: 1) The change in fabric as the fault surface is a p p r o a c h e d q u a r t z i t e grades from a p r o t o m y I o n i t e to an u l t r a m y l o n i t e a l o n g f a u l t surface 2) The folds are pr e s e n c e of east-over-west asymmetrical related to simple s h e a r a l o n g the fault 3) The p r o m i n e n t m i n e r a l and quartz clusters lineation consisting of folds The the These elongate quartz f 4) "Z" shaped quartzite quartz 5) Rare east-dipping 6) Late fractures and veins shear that are associated with beds of bands associated en e c h e l o n fr a c t u r e The interpretation of these structures and the f a bri cs are d i s c u s s e d in S t r e h l e and S t a n l e y (1986) 101 arrays thin sections REFERENCES Agnew, P.C., 1977, Reinterpretation of northwestern Vermont: M S.thesis, Burlington, Vermont, 82p the Hinesburg thrust in University of Vermont, Butler, R G., Jr., 986, S e d i m e n t a t i o n of the U p p e r C a m b r i a n Danby F o r m a t i o n of w e s t e r n Vermont; an e x ampl e of mixed s i l i c i c l a s t i c and carbonate p l a t f orm sedimentation: M S thesis, University of Vermont, Burlington, Vermont, 137p Cady, W M., 1945, S t r a t i g r a p h y and s t r u c t u r e G e o l o g i c a l S o c iety of A m e r i c a Bulletin, v of w e s t - c e n t r a l 56, p 515-587 Vermont: # Cady, W M., Albee, A L., and Murphy, J F., 1962, B e d r o c k g e o l o g y of the L i nc oln M o u n t a i n q u a d r a n g l e , Vermont: U S G e o l o g i c a l Survey G e o l o g i c Q u a d r a n g l e Map, GQ - 164, scale 1:62,500 Carter, C M., 1979, I n t e r p r e t a t i o n of the H i n e s b u r g thrust north of the La m o i l l e River, n o r t h w e s t e r n Vermont: M S Thesis, U n i v e r s i t y of Vermont, Burling ton, Vermont, 84 p D i P i e t r o , , 1985, Geology of the Starksboro area, V e r m o n t G e o l o g i c a l S u r v e y S p e c i a l B u l l e t i n No 4, 14 p Doll, C G., Cady, W M., Thompson, 1961, Centennial Geologic Map of V e r m o n t G e o l o g i c a l Survey, Scale Vermont: J B., Jr., and Billings, M P., Vermont: M o n t p e l i e r , Vermont, 1: 250, 000 Dorsey, R L , Agnew, P C., Carter, C M., R o s e n c r a n t z , E J., Stanley, R S., 1983, B e d r o c k g e o l o g y of the M i l t o n quadran gle, n o r t h w e s t e r n Vermont: V e r m o n t G e o l o g i c a l S u r v e y Special B u l l e t i n No 3, 14 p Erwin, R.B, 957, The g e o l o g y S o u t h Hero Island, Vermont: 9, p • of the limestone of Isle La Motte V e r m o n t G e o l o g i c a l Survey B u l l e t i n Gregory, G J., 1982, P a l e o e n v i r o n m e n t s of Cambrian) of n o r t h w e s t e r n Vermont: M Vermont, Burlington, Vermont, 180p the D u n h a m S thesis, and No D o l o m i t e (Lower U n i v e r s i t u y of Barton, T E and Frank, T K Gillespie, R P Stanley, R S structural along the and chronology 1972, Superposed folds s o u t h e a s t e r n part of the H i n e s b u r g S y n c l i n o r i u m : ±n_ Doolan, B.D., and Stanley, R S., eds , G u i d e b o o k to field trips in Vermont, New E n g l a n d I n t e r c o l l e g i a t e G e o l o g i c a l C o n f e r e n c e 64th Annual Meeting, U n i v e r s i t y of Vermont, B u r l i n g t o n , Vermont, p 151 - 166 Hawley, David, 1957, O r d o v i c i a n shales and s u b m a r i n e slide b r e c c i a s of n o r t h e r n C h a m p l a i n V a l l e y in Vermont: G e o l o g i c a l S o c i e t y of A m e r i c a Bulletin, v 68, p 155-194 1972, Sedimentation 102 characteristics md tectonic A-5 deformation of M i d d l e Ordovician shales of n o r t h w e s t e r n Vermont north of M a l l e t t s Bay: i_n Doolan, B.D., and Stanley, R S., eds , G u i d e b o o k to field trips in Vermont, New England I n t e r c o l l e g i a t e G e o l o g i c a l C o n f e r e n c e 64th A n n u a l Meeting, U n i v e r s i t y of Vermont, Burling ton, Vermont, p 151 - 166 Leonard, Katherine, 1985, F o r e l a n d folds and thrust belt d e f o r m a t i o n c h r o n o l og y, O r d o v i c i a n l i m e s t o n e and shale, n o r t h w e s t e r n Vermont: M S thesis, U n i v e r i s t y of Vermont, B u r l i n g to n, Vermont, 120p McHone, J G and Bulter, J R , 1984, M e s o z o i c igneous p r o v i n c e s New England and the o p e n i n g of the Atlantic: G e o l o g i c a l S o c i e t y A m e r i c a Bulletin, V 95, p* 757-765* of of McLean , 1986, Depositional e n v i r o n m e n t s and stratigraphic relationships of the Glens Falls Limestone, C h a m p l a i n Valley, Vermont and New York: M S thesis, University of Vermont, B u r l i n g t o n , Vermont, 170p M e h r t e n s , C J., 1985, V e r m o n t Geology, v The C a m b r i a n p l a t f o r 4*, p* E1-E16 S e d i m e n t ology Myrow, Paul, 1983, M S west-central Vermont: B u r l i n g t o n , Vermont, 177p* R a m s a y , J G and Huber, M structural geology, v o l London, 306p I., Stanley, R 1980, Mesozoic S* , s i g n i f i c a n c e in w e s t e r n Vermont: in of the thesis, 1984, Strain northwestern Vermont: Ch e s h i r e Formation m University of Veront The techniques of an l aysis: Academic faults Vermont and their Geology, v modern Press environmental 1, p 22-32 - , 1987, The C h a m p l a i n thrust fault, Lone Rock Point, Burlington, Vermont: Geological Society of' America, Centennial Field Guide for the N o r t h e a s t Section, p 67-72 Stanley, R S., Ra tcli f fe , N M.,1985, Tectonic synthesis of the Taconic orogeny in w e s t e r n New England: Geological Society of A m e r i c a Bulletin, v.96, p 1227-1250 Sutter, J.F., Ra tcli f fe , N.M., and Mukasa, S.B , 985 ^ A r / ^ A r and K-Ar data bearing on the n etamorphic and tectonic history of in w e s t e r n New England: G e o l o g i c a l S o c i e t y of A m e r i c a Bulletin, Tauvers P R., 1982, B e d r o c k g e o l o g y of the L i n c o l n area V e r m o n t G e o l o g i c a l S u r v e y S p e c i a l B u l l e t i n No 2, p Stone, S W quadrangle, 79p* Strehle, B A Vermont: and Dennis,” J G., 964, The g e o l o g y of the M ilton Vermont: Vermont Geological S u r v e y B u l l e t i n No 26, and Stanley, R S., 103 986, A comparison of fault zone fabrics in n o r t h w e s t e r n Vermont: V e rmon t G e o l o g i c a l in V e rmo nt G e olo gy No- 3, 36 p and plates Survey, A-5 Studies % uppe, John, 1985, Inc -, p Principles of structural eetsel, M B., 1984, Sedimentation shales in northwestern Vermont: Vermont, Burlington, Vermont, 97p elby, C W., 1961, Vermont: V e r mont of M the S geology: Taconic thesis, B e d r o c k g e o l o g y of the central G e o l o g i c a l S u r v e y B u l l e t i n No APPENDIX - LONG 104 ROCK POINT Prentice-Hall foreland basin University of Champlain Valley 14, 296p - REPRINT of A -5 G e o lo g ic a l S o ciety o f A m e ric a C e n te n n ia l Field G u i d e — N o rth e a s te rn S e c tio n , 1987 50 The Champlain thrust faulty Lone Rock Point, Burlington, Vermont R olfe S Stanley, Department o f Geology, University o f Vermont, Burlington, Vermont 05405 LOCATION The 0.6 mi (1 km) exposure of the Champlain thrust fault is located on the eastern shore of Lake Champlain at the north end o f Burlington Harbor The property is owned by the Episcopal Diocesan Center Drive several miles (km) north along North Avenue (Vermont 127) from the center o f Burlington until you reach the traffic light at Institute Road, which leads to Burlington High School, The Episcopal Diocesan Center, and North Beach Turn west toward the lake and take the first right (north) beyond Burlington High School The road is marked by a stone archway Stop at the second building on the west side o f the road, which is the Administration Building (low rectangular building), for writ­ ten permission to visit the field site Continue north from the Administration Building, cross the bridge over the old railroad bed, and keep to the left as you drive over a small rise beyond the bridge Go to the end o f this lower road Park your vehicle so that it does not interfere with the people living at the end o f the road (Fig 1) Walk west from the parking area to the iron fence at the edge o f the cliff past the outdoor altar where you will see a fine view o f Lake Champlain and the Adirondack Mountains From here walk south along a footpath for about 600 ft (2 0 m) until you reach a depression in the cliff that leads to the shore (Fig 1) SIG NIFICANCE This locality is one o f the finest exposures o f a thrust fault in the Appalachians because it shows many o f the fault zone fea­ tures characteristic of thrust faults throughout the world Early studies considered the fault to be an unconformity between the strongly-tilted Ordovician shales o f the “Hudson River Group” and the overlying, gently-inclined dolostones and sandstones o f the “Red Sandrock Formation” (Dunham , M onkton, and W i­ nooski formations of Cadv 1945) which was thou 2ht to be Silurian because it was lithically similar to the Medina Sandstone o f N ew York Between 1847 and 1861, fossils o f pre-Medina age were found in the “Red Sandrock Formation” and its equivalent “Quebec Group’ in Canada Based on this information, Hitchcock and others ( Figure Location m ap o f the Cham plain thrust fault at Lone Rock Point, Burlington, Vermont The buildings belong to the Episcopal Di­ ocesan Center The road leads to Institute Road and Vermont 127 (North Avenue) The inferred change in orientation of the fault surface is based on m easured orientations shown by the dip and strike symbols The large eastward-directed arrow marks the axis of a broad, late syn­ cline in the fault zone The location of Figures and are shown to the left o f “Lone Rock Point.” The large arrow points to the depression referred to in the text northern Vermont (D o ll and others, 1961; Coney and others, 1972) Leonard’s work has shown that the earliest folds and faults in the Ordovician sequence to the west in the Champlain Islands are genetically related to the development o f the Champlain major fault of regional extent W e now know that it is one o f thrust fault In southern Vermont and eastern N ew York, Rowley and slices others (1 9 ), Bosworth (1 ), Bosworth and Vollmer (1981), crust exposed and Bosworth and R ow ley (1984), have recognized a zone o f late thrust post-cleavage faults (Taconic Frontal Thrust o f Bosworth and associated faults (C h a m p lain th ru st result o f field studies by Keith (1923, 1932), Clark (1934), Cady R ow ley, 1984) along the western side o f the Taconic Mountains (1945), W elby (1961), D oll and others (1961), Coney and others R ow ley (1983), Stanley and Ratcliffe (1983, 1985), and Ratcliffe (1972), Stanley and Sarkisian (1972), Dorsey and others (1983), (in Zen and others, 1983) have correlated this zone with the Leonard (1985) Recent Ando Champlain thrust fault If this correlation is correct then the the Champlain thrust zone w ould extend from Rosenberg, Canada, have shown m etam orphic to the Catskill Plateau in east-central N ew York, a distance of thrust fault dips eastward phosed rocks o f the Green Mountains This geometry agrees vith 199 mi (3 km), where it appears to be overlain by Silurian and and D evonian rocks The C O C O R P line through southern Vermont sections across R S Stanley shows an east-dipping reflection that roots within Middle Proter­ ozoic rocks o f the Green Mountains and intersects the earth’s surface along the western side o f the Taconic Mountains (Ando and others, 1983, 1984) The relations described in the foregoing paragraphs suggest that the Champlain thrust fault developed during the later part of the Taconian orogeny o f Middle to Late Ordovician age Subse­ quent movement, however, during the middle Paleozoic Acadian orogeny and the late Paleozoic Alleghenian orogeny can not be ruled out The importance of the Champlain thrust in the plate tectonic evolution o f western N ew England has been discussed by Stanley and Ratcliffe (1983, 1985) Earlier discussions can be found in Cady (1969), Rodgers (1970), and Zen (1972) R E G IO N A L G EO LO G Y In Vermont the Champlain thrust fault places Lower Cam­ brian rocks on highly-deformed M iddle Ordovician shale North o f Burlington the thrust surface is confined to the lower part o f the Dunham Dolom ite At Burlington, the thrust surface cuts upward through 2,275 ft (7 0 m) o f the Dunham into the thickbedded quartzites and dolostones in the very lower part o f the M onkton Quartzite Throughout its extent, the thrust fault is located within the lowest, thick dolostone o f the carbonatesiliciclastic platform sequence that was deposited upon Late Proterozoic rift-clastic rocks and M iddle Proterozoic, continental crust o f ancient North America At Lone R ock Point in Burlington the stratigraphic throw is about 8,850 ft (2,700 m), which represents the thickness o f rock cut by the thrust surface T o the north the throw decreases as the thrust surface is lost in the shale terrain north o f Rosenberg, Canada Part, if not all, o f this displacement is taken up by the Highgate Springs and Philipsburg thrust faults that continue northward and becom e the “Logan’s Line” thrust o f Cady (1969) South o f Burlington the stratigraphic throw is in the order o f 6,000 ft (1 ,8 0 m) A s the throw decreases on the Champlain thrust fault in central Vermont the displacement is again taken up by m ovem ent on the Orwell, Shoreham, and Pinnacle thrust faults Younger open folds and arches that deform the Champlain slice may be due either duplexes or ramps along or beneath the Champlain thrust fault T o the west, numerous thrust faults are exposed in the Ordovician section along the shores o f Lake Champlain (H awley, 1957; Fisher, 1968; Leonard, 1985) One o f these broad folds is exposed along the north part o f Lone Rock Point (Fig 2) Based on seismic reflection studies in Vermont, duplex formation as described by Suppe (1 ) and Boyer and Elliot (1 ) indeed appears to be the mechanism by which major folds have developed in the Champlain slice North o f Burlington the trace o f the Champlain thrust fault is relatively straight and the surface strikes north and dips at about 15° to the east South o f Burlington the trace is irregular because the thrust has been more deformed by high-angle faults and broad folds Slivers o f dolostone (Lower Cambrian Dunham D olom ite) and limestone (Lower Ordovician Beekmantown Group) can be found all along the trace o f the thrust The limestone represents # 106 A-5 Figure A sketch o f the Cham plain thrust fault at the north end of Lone Rock Point showing the large bend in the fault zone and the slivers of Low er O rdovician limestone The layering in the shale is the SI cleav­ age It is folded by small folds and cut by many generations of calcite veins and faults The sketch is located in Figure fragments from the Highgate Springs slice exposed directly west and beneath the Champlain thrust fault north o f Burlington (D oll and others, 1961) In a 3.3 to 10 ft (1 to m) zone along the thrust surface, fractured clasts o f these slivers are found in a matrix o f ground and rewelded shale Estimates o f displacement along the Champlain thrust fault have increased substantially as a result o f regional considerations (Palmer, 1969; Zen and others, 1983; Stanley and Ratcliffe, 1983, 1985) and seismic reflection studies (Ando ami others, 1983, 1984) The earlier estimates were less than mi (15 km) and were either based on cross sections accompanying the G eo­ logic M ap o f Vermont (D o ll and others, 1961) or simply trigon­ om etric calculations using the average dip o f the fault and its stratigraphic throw Current estimates are in the order o f 35 to 50 mi (6 to km) Using plate tectonic considerations, R ow ley (1 ) has suggested an even higher value o f 62 mi (100 km) These larger estimates are more realistic than earlier ones consid­ ering the regional extent o f the Champlain thrust fault Lone Rock Point A t Lone R ock Point the basal part o f the Lower Cambrian Dunham D olostone overlies the M iddle Ordovician Iberville Formation Because the upper plate dolostone is more resistent than the low er plate shale, the fault zone is well exposed from the northern part o f Burlington Bay northward for approximately mi (1.5 km; Fig 1) The features are typical o f the Champlain thrust fault where it has been observed elsewhere The Champlain fault zone can be divided into an inner and outer part The inner zone is 1.6 to ft (0.5 to m) thick and consists o f dolostone and limestone breccia encased in welded, but highly contorted shale (Fig 3) Calcite veins are abundant O ne o f the most prominent and important features o f the inner fault zone is the slip surface, which is very planar and continuous throughout the exposed fault zone (Fig 3) This surface is marked by very fine-grained gouge and, in som e places, calcite slicken­ lines W here the inner fault zone is thin, the slip surface is located A -5 Champlain thrust fault Lone Rock Point F ig u re V iew o f th e C h a m p la in th ru st fault lo o k in g east a t the southern end of Lone Rock Point (Fig 1) T h e ac c o m p a n y in g line d w in g locates by n u m b e r the important features discussed in the text: 1, the c o n tin u o u s p la n a r slip surface; 2, lim estone slivers; 3, A hollow in the base o f the dolostone is filled in w ith lim estone a n d d o lo sto n e breccia; 4, F a u lt mullions decorate the slip surface at the base o f the d o lo sto n e; 5, a sm all dik e o f sh ale has been injected between the breccia and the dolostone along the interface between the Dunham D olom ite and the Iber­ ville Shale Where the inner fault is wider by virtue o f slivers and irregularities along the basal surface of the Dunham D olom ite, the slip surface is located in the shale, where it forms the chord between these irregularities (Fig 3) The slip surface represents the surface along which most o f the recent m otion in the fault zone has occurred As a consequence, it cuts across all the irregu­ larities in the harder dolostone o f the upper plate with the excep­ tion o f long wave-length corrugations (fault mullions) that parallel the transport direction As a result, irregular hollow s along the base of the Dunham D olom ite are filled in by highly contorted shales and welded breccia (Fig 3) The deformation in the shale beneath the fault provides a basis for interpreting the movement and evolution along the Champlain thrust fault The compositional layering in the shale o f the lower plate represents the well-developed SI pressuresolution cleavage that is essentially parallel to the axial planes o f the first-generation o f folds in the Ordovician shale exposed below and to the west o f the Champlain thrust fault (Fig 4) As the trace of the thrust fault is approached from the west this cleavage is rotated eastward to shallow dips as a result o f west­ ward movement o f the upper plate (Fig 4) Slickenlines, grooves, and prominent fault mullions on the lower surface o f the dolo­ stone and in the adjacent shales, where they are not badly deformed by younger events, indicate displacement was along an azimuth of approximately N 60°W (Fig 4; Hawley, 1957; Stanley and Sarkesian, 1972; Leonard, 1985) The SI cleavage at Lone Rock Point is so well developed in the fault zone that folds in the original bedding are largely destroyed In a few places, however, isolated hinges are preserved and are seen to plunge eastward or southeastward at low angles (Fig 4) As these FI folds are traced westward from the fault zone, their hinges change orientation to 107 the northeast A similar geometric pattern is seen along smaller faults, which deform S I cleavage in the Ordovician rocks west o f the Champlain thrust fault These relations suggest that FI hinges are rotated towards the transport direction as the Champlain thrust fault is approached The process involved fragmentation o f t N Figure Lower hemisphere equal-area net showing structural elements associated with the Cham plain thrust fault The change in orientation of the thrust surface varies from approximately N 20°W to N14°E at Lone R ock Point The orientation o f S cleavage directly below the thrust is the average o f 40 m easurem ents collected along the length of the expo­ sure S 1, however, dips steeply eastward in the Ordovician rocks to the west of the Cham plain thrust fault as seen at South Hero and Clay Point w here F I hinges plunge gently to the northeast N ear the Champlain thrust fault F I hinges (small circles) plunge to the east M ost slickenlines in the adjacent shale are approxim ately parallel to the fault mullions shown in the figure R S Stanley the FI folds since continuous fold trains are absent near the thrust Much o f this deformation and rotation occurs, however, within 300 ft (100 m) o f the thrust surface Within this same zone the SI cleavage is folded by a second generation of folds that rarely developed a new cleavage These hinges also plunge to the east or southeast like the earlier FI hinges The direction o f transport inferred from the analysis o f F2 data is parallel or nearly parallel to the fault mullions along the Champlain thrust fault Stanley and Sarkesian (1 ) suggested that these folds developed during late translation on the thrust with major displacement during and after the development o f generation folds New information, however, suggests that the F2 folds are simply the result o f inter­ nal adjustment in the shale as the fault zone is deformed by lower duplexes and frontal or lateral ramps (Figs 1, 2) The critical evidence for this new interpretation is the sense of shear inferred from F2 folds and their relation to the broad undulations mapped in the fault zone as it is traced northward along Lone Rock Point (Fig 1) South o f the position o f the thick arrow in Figure 1, the inferred shear is west-over-east whereas north o f the arrow it is east-over-west The shear direction therefore changes across the axis o f the undulation (marked by the arrow) as it should for a synclinal fold R E FE R EN C E S C ITED Ando, C J., Cook, F A., Oliver, J E., Brown, L D., and Kaufman, S., 1983, Crustal geometry of the Appalachian orogen from seismic reflection studies, in Hatcher, R D., Jr., Williams, H., and Zietz, I., eds., Contributions to the tectonics and geophysics of mountain chains: Geological Society of America Memoir 158, p 83-101 Ando, C J., Czuchra, B L., Klemperer, S L., Brown, L D., Cheadle, M J., Cook, F A., Oliver, J E., Kaufman, S., W alsh, T., Thompson, J B., Jr., Lyons, J B., and Rosenfeld, J L., 1984, Crustal profile of mountain belt; C O C O R P Deep Seismic reflection profiling in New England Appalachians and implications for architecture of convergent mountain chains: American Association of Petroleum Geologists Bulletin, v 68, p 819-837 Bosworth, W., 1980, Structural geology of the Fort Miller, Schuylerville and portions of the Schagticoke ^-m inute Quadrangles, eastern New York and its implications in Taconic geology [Ph.D thesis]: Part 1, State University of New York at Albany, 237 p Bosworth, W., and Rowley, D B., 1984, Early obduction-related deformation features of the Taconic allochthon; Analogy with structures observed in modem trench environments: Geological Society of American Bulletin, v 95, p 559-567 Bosworth, W., and Vollmer, F W , 1981, Structures of the medial Ordovician flysch of eastern New York; Deformation of synorogenic deposits in an overthrust environment: Journal of Geology, v 89, p 551-568 Boyer, S E., and Elliot, D., 1982, Thrust systems: American Association of Petroleum Geologists Bulletin, v 66, p 1196-1230 Cady, W M., 1945, Stratigraphy and structure of west-central Vermont: Geologi­ cal Society of America Bulletin, v 56, p 515-587 , 1969, Regional tectonic synthesis of northwestern New England and adja­ cent Quebec: Geological Society of America M emoir 120,181 p Clark, T H., 1934, Structure and stratigraphy o f southern Quebec: Geological Society of America Bulletin, v 45, p 1-20 Coney, P J., Powell, R E., Tennyson, M E., and Baldwin, B., 1972, The Champlain thrust and related features near Middlebury, Vermont, in Doolan, B D., and Stanley, R S., eds., Guidebook to field trips in Vermont, New England Intercollegiate Geological Conference 64th annual meeting: Burl­ ington, University of Vermont, p 97-116 Doll, C G., Cady, W M., Thompson, J B., Jr., and Billings, M P., 1961, Centennial geologic m ap of Vermont: Montpelier, Vermont Geological Sur­ vey, scale 1:250,000 ' Dorsey, R L., Agnew, P C., Carter, C M., Rosencrantz, E J., and Stanley, R S., 1983, Bedrock geology of the Milton Quadrangle, northwestern Vermont: Vermont Geological Survey Special Bulletin No 3, 14 p Fisher, D W., 1968, Geology of the Plattsburgh and Rouses Point Quadrangles, New York and Vermont: Vermont Geological Survey Special Publications No 1, 51 p Hawley, D., 1957, Ordovician shales and submarine slide breccias of northern Champlain Valley in Vermont: Geological Society of America Bulletin, v 68, p 155-194 Hitchcock, E., Hitchcock, E., Jr., Hager, A D., and Hitchcock, C., 1861, Report on the geology of Vermont: Clairmont, Vermont, v 1, 558 p.; v 2, p 559-988 Keith, A., 1923, Outline o f Appalachian structures: Geological Society of Amer­ ica Bulletin, v 34, p 309-380 , 1932, Stratigraphy and structure of northwestern Vermont: American Journal of Science, v 22, p 357-379, 393-406 Leonard, K E., 1985, Foreland fold and thrust belt deformation chronology, Ordovician limestone and shale, northwestern Vermont [M.S thesis]: Bur­ lington, University of Vermont, 138 p Palmer, A R., 1969, Stratigraphic evidence for magnitude of movement on the Cham plain thrust: Geological Society of America, Abstract with Programs, Part 1, p 47-48 Rodgers, J., 1970, The tectonics of the Appalachians: New York, Wiley Inter­ science, 271 p Rowley, D B., 1982, New methods for estimating displacements of thrust faults affecting Atlantic-type shelf sequences; With applications to the Champlain thrust, Vermont: Tectonics, v 1, no 4, p 369-388 , 1983, Contrasting fold-thrust relationships along northern and western edges o f the Taconic allochthons; Implications for a two-stage emplacement history: Geological Society o f America Abstracts with Programs, v 15, p 174 Rowley, D B., Kidd, W S.F., and Delano, L L., 1979, Detailed stratigraphic and structural features o f the Giddings Brook slice o f the Taconic Allochthon in the Granville area, in Friedman, G M., ed., Guidebook to field trips for the New York State Geological Association and the New England Intercolle­ giate Geological Conference, 71st annual meeting: Troy, New York, Rensse­ laer Polytechnical Institute, 186-242 Stanley, R S., and Ratcliffe, N M., 1983, Simplified lithotectonic synthesis o f the pre-Silurian eugeoclinal rocks of western New England: Vermont Geological Survey Special Bulletin No , 1985, Tectonic synthesis o f the Taconic orogeny in western New England: Geological Society of America Bulletin, v 96, p 1227-1250 Stanley, R S., and Sarkesian, A., 1972, Analysis and chronology of structures along the Champlain thrust west of the Hinesburg Synclinorium, in Doolan, D L , and Stanley, R S., eds., Guidebook for field trips in Vermont, 64th annual meeting o f the New England Intercollegiate Geological Conference, Burlington, Vermont, p 117-149 Suppe, J., 1982, Geometry and kinematics of fault-related parallel folding: Amer­ ican Journal o f Science, v 282, p 684-721 W elby, C W , 1961, Bedrock geology o f the Champlain Valley of Vermont: Vermont Geological Survey Bulletin No 14, % p Zen, E-an, 1972, The Taconide zone and the Taconic orogeny in the western part o f the northern Appalachian orogen: Geological Society of America Special Paper 135,72 p Zen, E-an, ed., Goldsmith, R., Ratcliffe, N M., Robinson, P., and Stanley, R S., compilers, 1983, Bedrock geologic m ap of Massachusetts: U.S Geological Survey; the Com m onwealth of Massachusetts, Department o f Public Works; and Sinnot, J A., State Geologist, scale 1:250,000 108 ... fracture arrays in the footwall and h a n g w a l l that are t r u n c a t e d by the ramp 92 A -5 faults The presence of sparry calcite and cavities in the fractures i ndicates that the fractures... o r e , the roof fault of the hangingwall block is folded and cut by S1 cleavage At the junction of the ramp fault and the floor fault, the q u a s i p l a n a r slip surfaces in the c a l c i... the w e s t e r n part of the foreland in South Hero Island and end along the western margin of the hinterland at the Hinesburg thrust fault at Mechanicsvil e , Vermont The trip across central