Marine Geology, 93 (1990) 317-329 317 Elsevier Science Publishers B.V., Amsterdam - - Pr inted in The Nether lands

Geological and Geophysical Evidence in Support of the Hypothesis of Anticlockwise Rotation of Northern Alaska

ASHTON F. EMBRY

Geological Survey of Canada, 3303-33rd St. NW, Calgary, Alta. T2L 2A7 (Canada)

(Received Janua ry 10, 1989; revision accepted April 6, 1989)

A b s t r a c t

Embry, A.F., 1990. Geological and geophysical evidence in support of the hypothesis of anticlockwise rota t ion of nor thern Alaska. In: J.R. Weber, D.A. Forsyth, A.F. Embry and S.M. Blasco (Editors), Arctic Geoscience, Mar. Geol., 93: 317-329.

The most popular hypothesis for the origin of the Amerasia Basin of the Arctic Ocean is tha t it formed by the anticlockwise rota t ion of nor the rn Alaska and adjacent nor theas te rn Siberia away from the Canadian Arctic Islands in Cretaceous time. Because there is no obvious pa t te rn of magnetic stripes in the Amerasia Basin, and the geological evidence presented to support the anticlockwise rota t ion model has been ra ther general and unconvincing, the hypothesis has been repeatedly challenged and a l ternat ive hypotheses for the origin of the Amerasia Basin have been proposed.

Recently released seismic reflection data from the Chukchi Sea and the Canadian Arctic Islands, in combinat ion with surface and well data, have allowed regional s t ructura l and s t ra t igraphic t rends to be established both in the Canadian Arctic Islands and nor thern Alaska Chukchi Sea region. To test the validity of the anticlockwise rota t ion model, Upper Devonian s t ruc tura l and depositional trends, Carboniferous-Jurass ic basin axes and Lower Triassic facies belts have been plotted on Har land et als.'* Arctic pre-Cretaceous plate reconst ruct ion which used the anticlockwise rota t ion hypothesis as its basic premise. In all cases coherent and reasonable tectonic and depositional reconstruct ions were obtained. Furthermore, seismic lines and well logs from the nor thwes tern Alaska -Chukch i Sea region and the southwestern Sverdrup Basin, areas which are juxtaposed in the reconstruction, were compared and found to be remarkably similar. Overall, the available geological and geophysical data support the anticlockwise rota t ion model.

I n t r o d u c t i o n

Two main basins, the Amerasia and Eurasia, comprise the present Arctic Ocean and they are separated by a narrow ridge of continental crust, the Lomonosov Ridge. The Eurasia Basin is the northern extension of the North Atlantic Ocean and it formed by seafloor spreading away from the axial Nansen Ridge in Tert iary time (Vogt and Avery, 1974). The spreading history of the basin is reasonably

Geological Survey of Canada Contr ibut ion 56688. "1984.

well known due to the recognition of clearly defined magnetic anomalies.

The Amerasia Basin is a triangular-shaped oceanic basin which includes two prominent bathymetric highs, the Alpha Ridge complex and the Chukchi Cap. The Alpha Ridge com- plex appears to be oceanic in origin (Asudeh et al., 1987) but the nature of the Chukchi Cap is unknown. Heat flow and depth to oceanic basement studies indicate a Cretaceous age for the Amerasia Basin (Lawver and Baggeroer, 1983) and thus it formed earlier than the Eurasia Basin.

The origin of the Amerasia Basin is not well

0025-3227/90/$03.50 © 1990 Elsevier Science Publishers B.V.

understood and a number of hypotheses for its formation have been proposed over the last 30 years. The first and most popular hypothesis is that the basin originated by the anticlockwise rotation of northern Alaska and adjacent Siberia away from the Canadian Arctic Islands (Carey 1958; Rickwood, 1970; Tailleur, 1973). The pole of rotation is interpreted to have been located in the Mackenzie Delta region. In this model both the Canadian Arctic margin and Alaskan-Siber ian margin are rift margins with the Lomonosov margin being a transform margin (Fig.l). Although this hypothesis has been repeatedly advocated, little convincing geological or geophysical evidence has been provided by its supporters. There is no obvious pattern of magnetic anomalies which would allow the spreading history to be interpreted and the plate movements to be reconstructed. Furthermore, in regard to geological evidence, most workers have noted only general geologi- cal similarities between Arctic Alaska and the Canadian Arctic Archipelago (e.g., Grantz et al., 1979). However, such generalities can be accounted for in other hypothetical plate tectonic models and are not convincing evi- dence for the anticlockwise rotation of Alaska.

Reasonably persuasive data which support the anticlockwise rotat ion model are recently published palaeomagnetic data from northern

Alaska (Halgedahl and Jarrard, 19,n7). How ever, due to the problems associated with determining and interpreting palaeomagnetic results from northern Alaska (e.g. very steep magnetic inclinations and resetting of pa laeo magnetic directions) these data ar~ not re garded as conclusive.

As a result of this lack of solid evidence to support the rotation of Alaska, other plate tectonic models to explain the origin of the Amerasia Basin have been proposed. Most of these (Herron et al., 1974; Churkin et al., 1980; Jones, 1982) are incompatible with existing geological and geophysical data and they have not been adopted by other workers. These hypotheses are reviewed by Lawver et al. (1985) and are not considered further here.

A serious rival to the anticlockwise rotation of Alaska model is the southward translation of Alaska model (Bally, 1976; Christie, 1979; Kerr, 1981; Dutro, 1981). This model proposes that the Amerasia Basin opened by the rifting and southward translation of northern Alaska and adjacent northeastern Siberia away from the Lomonosov Ridge-Barents Sea region. In this model the Lomonosov margin and the Alaskan-Siber ian margin are rift margins with the Canadian Arctic margin being a transform margin (Fig.2). Recently a number of workers have refined this model and its popularity

Fig.1. Anticlockwise rotation of northern Alaska and adjacent northeastern Siberia model for the opening of the Amerasia Basin (stippled).

Fig.2. Southward translation of northern Alaska model for the opening of the Amerasia Basin (stippled).

ANTICLOCKWISE ROTATION OF NORTHERN ALASKA 319

seems to be increasing (Crane, 1987; Smith, 1987; Hubbard et al., 1987). However, as is the case for the anticlockwise rotat ion of Alaska model, only broad generalities have been offered in support of this model. Furthermore the validity of this model depends on the existence of a major left-lateral strike-slip fault through the northern Yukon and northern Alaska (Kaltag Fault, Fig.2). Present geological and geophysical data available for the northern Yukon appear to deny the existence of such a fault (Bamber and Waterhouse, 1971; Poulton, 1982; Lane, 1988). However, the purpose of this paper is not to argue against the southward translation of Alaska model, a task much better accomplished by those working in the northern Yukon. Rather it is to provide specific geologi- cal and geophysical data which are supportive of the anticlockwise rotation of Alaska model.

The data and interpretations presented in this paper are based on my studies of the geology of the Canadian Arctic Archipelago over the past twenty years and on published information for northern Alaska and adjacent offshore areas. It is only within the last six years that critical seismic reflection data have become publically available for the northern Alaska and Canadian Arctic areas. These data have provided the key to reconstructing palaeogeographic and structural trends in both areas and such trends are ideal for testing the anticlockwise rotat ion of Alaska model. Various structural and palaeogeographic trends have been plotted on present-day geog- raphy and also on the plate reconstruction of Harland et al. (1984) which restores northern Alaska and adjacent northeastern Siberia against the Canadian Arctic Islands. This reconstruction was chosen because it uses the anticlockwise rotat ion of Alaska as its basic premise and because it was published before the geological data used to test the validity of the model were compiled.

Geological trends

Previous workers have noted that the Phan- erozoic column of northern Alaska is similar to

that of the Canadian Arctic Islands and that the two areas have similar geological histories (Tailleur and Brosg6, 1970; Grantz et al., 1979; Smith, 1987). These authors indicated that these similarities were evidence for a closer positioning of the two areas before the Amera- sia Basin had opened in Cretaceous time. However, as noted in the previous section, just how these two areas were formerly juxtaposed is unclear.

Figure 3 illustrates a Devonian Cretaceous correlation chart for northern Alaska and the Canadian Arctic Islands, and, as shown, the stratigraphic similarities between the two areas are striking. Embry (1989) elaborates on these similarities and concludes that the two areas were tectonically and depositionally linked throughout the Devonian to Cretaceous inter- val. The Devonian to Cretaceous succession of Arctic North America can be divided into three megasequences, each having been deposited under a distinctive tectonic setting:

(1) Middle-Upper Devonian clastic strata deposited in foreland basins which flanked areas of orogenesis (Ellesmerian Orogeny).

(2) Carboniferous-early Middle Jurassic clas- tics and carbonates deposited in basins which developed by rifting and thermal subsidence along the area previously occupied by the Devonian orogenic belt (e.g. Sverdrup Basin of Arctic Canada and Hanna Trough of Arctic Alaska).

(3) Early Middle Jurassic-Cretaceous clastic strata deposited in a foreland basin which flanked the Brooks Range Orogen (Colville Trough), in previously established extensional basins, and in new extensional basins related to rifting and seafloor spreading in the Amera- sian Basin (e.g. Dinkum Graben, offshore northeastern Alaska, and Banks Basin, south- western Arctic Islands).

These three megasequences are approxi- mately equivalent to the three tectonic se- quences of Lerand (1973) (Franklinian, Elles- merian and Brookian) which have been com- monly recognized in the stratigraphic succes- sion of northern Alaska (e.g. Bird and Mole- naar, 1987).

As indicated above, the first two megase- quences developed prior to the formation of the Amerasia Basin. Thus, geological trends recog- nized in Devonian- lower Middle Jurassic strata within Arctic North America are well

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suited to test the validity of various plate tectonic restorations. The geological trends which are used herein to test the anticlockwise rotat ion of Alaska model are: (1) Upper Devo nian strat igraphic and structural trends; (2) Carboniferous-Middle Jurassic depocentre trends, and (3) Lower Triassic facies trends.

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Both nor thern Alaska and the Canadian Arctic Islands were affected by Devonian orogenesis (Ellesmerian Orogeny). A clastic wedge derived from the orogenic highlands and deposited in an adjacent, tectonically linked foreland basin is present in both areas (Embry and Klovan, 1976; Nilsen and Moore, 1982). Figure 4 i l lustrates the general s t ructural and depositional trends for the Devonian orogenic belts and their related clastic deposits. A large area of highly deformed Lower Palaeozoic- Proterozoic s trata is present in the northern part of the Canadian Arctic Islands. A foreland basin with up to 5000m of Middle Uppe r Devonian clastic s trata occurs to the south. Pa laeocurrent and facies trends indicate a general longitudinal (southwesterly) t ransport of sediment in the basin. Structural trends within the highly deformed strata and the clastic wedge form an overall curvil inear pat tern with northwester ly trends occurring in the west. These northwester ly trends are approximately perpendicular to the Amerasia Basin margin. On Banks Island to the south, Middle-Upper Devonian' clastic s trata and underlying carbonates are very gently de- formed. The s tructures trend nor th - sou th and deformation increases to the west. However, seismic reflection data for the continental shelf

Fig.3. Upper Palaeozoic and Mesozoic correlation chart for northern Alaska and Sverdrup Basin (Canadian Arctic Islands). Data for northern Alaska from Bird and Molenaar (1987), Hubbard et al. (1987) and Thurston and Theiss (1987). Data for Sverdrup Basin from Embry (in press). M.H. = Murray Harbour Fro.; R.P. = Roche Pt. Fm.; H.B. = Hoyle Bay Fro.; P.B. = Pat Bay Fm.; S.P. = Sandy Pt. Fro.; K . C . = K a r e n Creek Fm.

ANTICLOCKWISE ROTATION OF NORTHERN ALASKA 321

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c l a s t i c s t r a t a , A r c t i c N o r t h A m e r i c a ( f rom E m b r y , 1988).

adjacent to Banks Island indicate that defor- mation of the Lower Palaeozoic and older strata remains mild upto the shelf edge, which is at the limit of seismic resolution (J. Dietrich, pers. commun., 1985).

In northern Alaska and adjacent northern Yukon, Devonian and older strata have been severely deformed during Cretaceous and Ter- tiary. Consequently Devonian structural and stratigraphic trends are not very clear. A broad area of Lower Palaeozoic-Proterozoic strata which were deformed in Late Devonian time extends from the Chukchi Sea to the northern Yukon (Tailleur and Brosg~, 1970; Bell, 1974; Grantz et al., 1982). A thick Upper Devonian clastic wedge occurs south of the belt of highly deformed strata and palaeocur- rent data indicate derivation from the de- formed strata (Nilsen and Moore, 1982). Struc- tural trends appear to be east-west and to wrap around an eastern terminus of the deformed belt in northern Yukon and adjacent Northwest Territories.

In the Chukchi Sea area seismic reflection

data have revealed the presence of an isolated wedge of mildly deformed rocks interpreted to be Devonian clastic strata and underlying carbonates (Grantz et al., 1982; Craig et al., 1985; Thurston and Theiss, 1987). This area of clastic strata is surrounded by highly deformed lower Palaeozoic strata. Structures in the clastics have a nor th-south trend (W. Masterson, pers. commun., 1988).

As shown on Fig.4 there are a number of Devonian structural and stratigraphic trends which are truncated by the Alaskan and Canadian margins of the Amerasia Basin. These include the following:

(1) Two separate areas of Ellesmerian oro- genesis with the orogenic front being approxi- mately perpendicular to the continental mar- gin in various areas.

(2) A north-south, gently deformed foreland basin on Banks Island and the adjacent conti- nental shelf with no adjacent orogenic belt.

(3) An apparently isolated, wedge-shaped area of mildly deformed Devonian clastic strata in the Chukchi Sea.

The Devonian trends of Fig.4 were plotted on the Harland et al. (1984) reconstruction result- ing in a coherent structural and depositional pattern (Fig.5). The Ellesmerian Orogenic Belt appears as a continuous entity with a sinuous trend from northern Ellesmere Island to the northern Yukon. The mildly deformed, fore- land basin on Banks Island becomes paired with the orogenic belt of northern Alaska. Also within this reconstruction the isolated wedge of Devonian clastic strata of the Chukchi Sea becomes part of the regional foreland basin. Furthermore it is apparent that the Upper Devonian clastic wedge of northern Alaska was deposited on the opposite side of the orogenic belt compared with the Devonian

clastic wedge of the Arctic Islands. This observation counters Nilsen's (1981) interpre- tat ion that the anticlockwise rotation of Alaska model is untenable because the Devo- nian clastic strata of Alaska have very differ- ent palaeocurrent directions compared with those of the clastic wedge in the Canadian Arctic Islands.

As an additional check on the validity of the proposed reconstruction of Devonian trends a published seismic line from the Chukchi Sea which images the Devonian clastic strata and underlying carbonates (Craig et al., 1985, plate 1) was compared with an unpublished, but publically available, seismic line on southern Prince Patrick Island (Fig.6). The location of

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Fig.5. Distribution and structural trends of highly deformed lower Palaeozoic strata and Devonian clastic strata plotted on pre-Cretaceous Arctic reconstruction of Harland et al. (1984) (from Embry, 1988). A coherent depositional and structural picture results from using this reconstruction. 1 = seismic line from Chukchi Sea and 2 = seismic line from Prince Patrick Island (see Fig.6).

ANTICLOCKWISE ROTATION OF NORTHERN ALASKA

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Fig.6.A. Seismic line from eastern Chukchi Sea (see Figs.5 and 7 for location). Geological interpretation from Craig et al. (1985). B. Seismic line from central Prince Patrick Island, Canadian Arctic Islands (see Figs.5 and 7 for location). Geological interpretation from T. Brent and C. Harrison (pers. commun., 1987). Stratigraphic units: /=post-Devonian strata; 2 = Middle-Upper Devonian clastics; 3 = Lower Palaeozoic carbonates (mainly Ordovician Silurian); 4= Lower Palaeozoic carbonates and clastics (mainly Cambrian). These remarkably similar seismic lines are reasonably close and closely coincide in structural and depositional strike on the pre-Cretaceous Arctic reconstruction of Harland et al. (1984) (Fig.5).

these lines are plotted on present-day geogra- phy (Fig.7) and on the Harland et al. (1984) reconstruct ion (Figs.5 and 8). Figure 5 suggests that the two areas crossed by the seismic lines were quite close in the Devonian and that the strike directions of the two lines would have

been approximately coincident. There is a remarkable similarity between the two seismic lines, as might be expected if the proposed reconstruct ion is valid. On both lines a thin interval of post-Devonian strata unconfor- mably overlies a thick interval (2-3 s two-way

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Fig.7. Basin axes for Sverdrup Basin (Canadian Arctic Islands), Hanna Trough (Chukchi Sea) and Arctic Alaska (northern Alaska). I---South Meade No. 1; 2= seismic line of Fig.6a; 3= seismic line of Fig.6b; 4= Sandy Point L-46.

Fig.8. Basin axes for Sverdrup Basin, Hanna Trough and Arctic Alaska plotted on pre-Cretaceous Arctic reconstruction of Harland et al. (1984). Note the juxtaposition of the Sverdrup Basin and Hanna Trough axes on this reconstruction suggesting that these basins were once part of the same through-going seaway, l f S o u t h Meade No. 1; 2=se ismic line of Fig.6a; 3--seismic line of Fig.6b; 4--Sandy Point I~46.

ANTICLOCKW1SE ROTATION OF NORTHERN ALASKA 325

travel time or about 4000-6000 m of strata) of interpreted Devonian clastic strata. In both areas these clastic s trata are characterized by interval velocities of 4-4.5 km s- 1 (Craig et al., 1985; T. Brent, pers. commun., 1988). The clastics overlie Lower Palaeozoic carbonates in which two reflectors about 1 s apart can be identified. It is worth noting that the Chukchi Sea seismic line compares more closely with the southern Prince Patrick seismic line than any other Arctic Canada seismic line outside the southern Prince Patrick area.

In summary, Devonian structural and depo- sitional trends in Arctic North America sup- port the anticlockwise rotat ion of Alaska model.

Carboniferous-Middle Jurassic trends

A new tectonic regime was initiated throughout Arctic North America in earliest Carboniferous (Embry, 1989). Extensional basins developed by rifting and thermal subsid- ence along the area previously occupied by the Devonian orogenic belt. Thick intervals of Carboniferous-Middle Jurassic strata were deposited in these basins. In the Canadian Arctic Archipelago the main depocentre was the Sverdrup Basin (Balkwill, 1978). The axis of the basin has been delineated using outcrop data and subsurface well and seismic data. The axis trends southwest in the eastern and central parts of the basin and then swings northwest in the western part (Fig.7) (Embry, in press). Land areas which supplied sediment to the basin were present to the east, south and northwest. The basin axis is apparently trun- cated at the margin of the Amerasia Basin.

In northern Alaska and the adjacent Chukchi Sea, Carboniferous-Middle Jurassic strata were deposited in a basin with an axis which trends approximately east-west on land (Arctic Alaska Basin) and which then turns northward in the Chukchi Sea (Hanna Trough of Grantz et al., 1982; Central Chukchi Basin of Thurston and Theiss, 1987) (Fig.7). Land areas which provided sediment lay to the north and west. As was the si tuation in Canada, the basin

axis appears to be truncated at the margin of the Amerasia Basin.

Figure 3 shows that the Carboniferous-Mid- dle Jurassic successions of the basins of northern Alaska and the Canadian Arctic Islands are very similar and I suggest that the now-separated basins were at one time parts of a single entity. The two basin axes have been plotted on the Harland et al. (1984) reconstruc- tion and, as shown on Fig.8, the axes are almost perfectly aligned. Thus it would indeed appear as if the Hanna Trough is the westward continuation of Sverdrup Basin. To further test this conjecture the Triassic-lower Middle Jurassic succession of a well in northwestern Alaska (South Meade No. 1) was compared with an equivalent succession penetrated in a well in the southwestern Sverdrup Basin (Sandy Point L-46). The present-day and reconstructed locations of these wells are shown in Figs.7, 8 and 11. Note that on the reconstruction (Figs.8 and 11) the well locations are reasonably close together and in approximately the same tec- tonic position (basin margin).

Figure 9 illustrates the correlation of the Triassic-lower Middle Jurassic successions of the two wells. The stratigraphy in the two wells is very similar and distinctive lithologi- cal units and log markers can be readily correlated. Overall it has been found that the Triassic-lower Middle Jurassic strata of the southwestern Sverdrup Basin are more similar to equivalent strata in northwestern Alaska than they are to equivalent s trata in the eastern Sverdrup Basin (see Embry (in press) for stratigraphic comparisons between the eastern and western parts of the Sverdrup Basin).

Another test of the validity of positioning northern Alaska-Chukchi Sea region against the Canadian Arctic Islands in pre-Cretaceous time is to plot facies trends for a given interval and ascertain whether a reasonable palaeogeo- graphic pattern is obtained. Such a test was performed for Lower Triassic strata. In the Sverdrup Basin, thick Lower Triassic sand- stones of fluvial to shallow-shelf origin (Bjorne Formation) occur along the eastern and

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southern basin flanks. Shale and sittstone of outer shelf to slope origin (Blind Fiord Forma- tion) occur farther basinward (Fig.10). Shelf sandstones reappear in the Lower l 'riassic succession along the northwestern basin flank and the northwestern edge of the basin is interpreted to be present on the continental shelf (Embry, in press). In the southwestern part of the basin the Lower Triassic facies and thickness trends are northwesterly, perpendi- cular to the Amerasia Basin margin (Embry, in press).

In the northern Alaska-Chukchi Sea region, f luvial-nearshore sandstones (Ledge Sand- stone, Ivishak Formation), which were derived from the north, occur along the North Slope (Hubbard et al. 1987, fig. 12). The sandstones become finer grained southwards and eventu-

Fig.9. Correlation of Lower Triassic-Middle Jurassic subsurface sections from northern Alaska (South Meade No. 1) and southwestern Sverdrup Basin (Sandy Point L- 46). Well locations are in Fig.7. The wells are reasonably close together and are in the same tectonic-deposit ional position on the reconstruction of Harland et al. (1984) (Figs.8 and 11). G.l.=Grosvenor Island Fm.; M.S.= Maclean Strait Fro.; McC.I.=McConnell Island Fro.; for other abbreviations, see Fig.3.

-----7

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~ F l e v l a l - nearshore shelf sandstone dominant

~ Offshore shelf - basin shale - slltstone dominant

,ana

ANTICLOCKWISE ROTATION OF NORTHERN ALASKA 327

al ly c h a n g e fac ies to sha le and s i l t s tone ( K a v i k Member , I v i s h a k F o r m a t i o n ) of of fshore she l f to bas ina l or ig in (Fig.10). In the C hukch i Sea area , s e i s m i c - s t r a t i g r a p h i c r e l a t i onsh ips indi- ca te t h a t sands were der ived f rom bo th the eas t and wes t wi th a cen t r a l sha le t r o u g h o c c u r r i n g a long the axis of H a n n a T r o u g h (Gran tz et al., 1982; T h u r s t o n and Theiss , 1987) (Fig.10). The land a r ea to the wes t was cal led C h ukch i P l a t f o r m by Gran t z et al. (1982) and T h u r s t o n and The iss (1987).

The above-descr ibed Lower Tr iass ic facies t rends were p lo t ted on the H a r l a n d et al. (1984) r e c o n s t r u c t i o n and, as i l l u s t r a t ed on F ig . l l , a c o h e r e n t p a l a e o g e o g r a p h i c p ic tu re appeared . Two s igni f icant land a reas which p rov ided sed iment to the bas ins were present . The C a n a d i a n Shield and f l ank ing Arc t ic P l a t f o r m were the d o m i n a n t source a r ea s and the th ickes t , mos t widespread s ands tones f r inge this l andmass . A second l andmass occurs to the n o r t h w e s t of the Arc t ic I s l ands and inc ludes

the Chukch i P la t fo rm. This land area , n a m e d C r o c k e r l a n d by E m b r y (1989), a lso p rov ided c las t ics to the Sve rd rup Bas in and H a n n a Trough . A deep-wa te r a r ea d o m i n a t e d by shale and s i l t s tone follows the bas in axes.

Conclus ion

In r ecen t years , sufficient da t a h a v e become ava i l ab l e to a l low reg iona l s t r u c t u r a l and depos i t iona l t rends to be es tab l i shed for the D e v o n i a n - M i d d l e J u r a s s i c success ions in bo th the n o r t h e r n A l a s k a - C h u k c h i Sea reg ion and the C a n a d i a n Arc t ic Is lands . These t rends a re older t h a n the f o r m a t i o n of the A m e r a s i a Bas in (Cre taceous) and some are a t a h igh ang le to the p re sen t A m e r a s i a Bas in marg ins . Such geologica l t r ends a l low a g iven p la te t ec ton ic model for the or ig in of the A m e r a s i a Bas in to be e v a l u a t e d in r ega rd to i ts success or fa i lure in p rov id ing r e a s o n a b l e p a l a e o g e o g r a p h i c and t ec ton ic r econs t ruc t ions . I t has been found

Fig.11. Early Triassic palaeogeography of Arctic North America on pre-Cretaceous Arctic reconstruction of Harland et al. (1984). The established facies belts neatly link up on this reconstruction, as does the northern land area (Crockerland) which has been identified in the western Chukchi Sea and north of the Canadian Arctic Islands. 1 = South Meade No. 1; 2= Sandy Point L-46.

328 ~ !, ~i~-

that the plate tectonic model which restores northern Alaska and adjacent northeastern Siberia against the Canadian Arctic Islands provides a reasonable restoration of various geological trends in Arctic North America. The following trends become aligned to form a through-going pattern on the above restora- tion: (1) the tectonic front of the Ellesmerian Orogeny, (2) the Carboniferous--Middle Juras- sic basin axes, and (3) Lower Triassic facies trends.

In summary, the hypothesis that the Amera- sis Basin opened by the antictockwise rotation of northern Alaska and adjacent northeastern Siberia away from the Canadian Arctic Islands is supported by geological and geophysical data from northern Alaska and the Arctic Archipelago.

A c k n o w l e d g e m e n t s

The Geological Survey of Canada supported this research through the Frontier Geoscience Program and allowed its publication. I would like to thank Polar Continental Shelf Project for substantial logistical support during field studies. Discussions with J. Dixon, M. Cecile, and A. Grantz on the origin of the Amerasia Basin have been enjoyable and enlightening. T. Brent and C. Harrison advised on the interpretation of the Prince Patrick seismic line. L. Lane, S. May, J. Sweeney and D. Forsyth critically read the manuscript and suggested worthwhile changes. The manu- script was expertly processed by Billie Chiang. Paul Wozniak kindly drafted the figures.

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