Continental ridges in the Arctic Ocean: Lorex constraints

Tectonophysics,

217

89 (1982) 2 17-231

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

CONTINENTAL

RIDGES IN THE ARCTIC OCEAN: LOREX

CONSTRAINTS

*

J.F. SWEENEY ‘,+*, J.R. WEBER ’ and SM. BLASCO ’ ’ Earth Physics Branch, Department of Energy Mines and Resources, Ottawa, Ont., Kid

0Y3

(Canada)

’ Atlaniic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, N.S., BZY 4A2 (Canada)

(Final version received March 10, 1982)

ABSTRACT Sweeney, J.F., Weber, J.R. and Blasco, SM., 1982. Continental ridges in the Arctic &eat-t: LOREX constraints. In: G.L. Johnson and J.F. Sweeney (Editors), Structure of the Arctic. Tectonophysics, 89: 217-237.

Recent multidisciplinary geophysical measurements over the Lomonosov Ridge close to the North Pole support the widely held belief that it was formerly part of Eurasia. The known lithologies, ages, P-wave velocity structure and thickness of the crust along the outer Barents and Kara continental shelves are similar to permitted or measured values of these parameters newly acquired over the Lomonosov Ridge. Seismic, gravity and magnetic data in particular show that the ridge basement is most likely formed of early Mesozoic or older sedimentary or low-grade metasedimentary rocks over a crystalline core that is intermediate to basic in composition. Short-wavelength magnetic anomaly highs along the upper ridge flanks and crest may denote the presence of shallow igneous rocks. Because of the uncertain component of ice-rafted material, seafloor sediments recovered from the ridge by shallow sampling techniques cannot be clearly related to ridge basement lithology without further detailed analysis. The ridge is cut at the surface and at depth by normal faults that appear related to the development of the Makarov Basin. This and other data are consistent with the idea that the Makarov Basin was formed by continental stretching rather than simple seafloor spreading. Hence the flanking Alpha and Lomonosov ridges may originally have been part of the same continental block. It is suggested that in Late Cretaceous time this block was sheared from Eurasia along a trans-Arctic left-lateral offset that may have been associated with the opening of Baffin Bay. The continental block was later separated from Eurasia when the North Altantic rift extended into the Arctic region in the Early Tertiary. The data suggest that the Makarov Basin did not form before the onset of rifting in the Artic.

* Contribution from the Earth Physics Branch No. 964, LOREX contribution No, 9. ** Present address: Pacific Geoscience Centre, P.O. Box 6000, Sidney, B.C., V8L 4B2 (Canada) 0040- 195i/82/~-~/$02.75

0 1982 Eisevier Scientific ~blis~ng

Company

INTRODUCTION

It is generally distinct

stages.

long-standing basins,

believed Consensus

that the Arctic regarding

remains

uncertain.

basin

the evolution

(Fig. 1). The age and mode

however,

Ocean

The

agreement from the area of ambiguity. Since the early 1960’s when Heezen

developed

in at least two

of the Eurasia

of origin

Basin

of the Canada

Lomonosov

Ridge

divides

and

(1961)

and

Ewing

has been

and

Makarov

the area

Wilson

of

(1963)

Siberia

/

REENLAND

Fig. 1. Arctic ‘Eurasia

seafloor

nomenclature

Basin’. Dot indicates

500 m contour.

and gross bathymetry.

initial position

of LOREX

Fram and Nansen camps.

Contour

together

basins

interval

is

I

are called

km. Also shown ib

219

Fig. 2. Schematic 1978).

map of the anamalous

magnetic

field in the Eurasia

Basin (modified

from Coles et al.,

recognized

that the Eurasia

Basin is the extension

of the North Atlantic

seafloor into

the Arctic, it has been commonly assumed that the Lomonosov Ridge was originally part of the outermost Barents and Kara continental shelves and that it was split off and transported centred should

along

to its present

position

the Nansen-Gakkel

be a sliver of continental

in the Arctic

Ridge.

In other

Ocean words,

by seafloor

accretion

the Lomonosov

Ridge

material.

There are two major reasons for believing this. The first is the overall morphology and geophysical character of the ridge. It is relatively narrow and quite linear with few lateral irregularities

(Fig. 1). The flanks are steep and the crest appears

relatively

flat and smooth. These characteristics suggested to Dietz and Shumway (1961) that the Lomonosov Ridge may be a fault block rather than a volcanically constructed or an

otherwise

thermally

(Ostenso,1962;

Karasik

1978) magnetotelluric Weber,

generated

feature.

Reconnaissance

gravity.

magnetic

et al.. 1971: Coles et al., 1978), heat flow (Judge and Jessop. (DeLaurier,

1978) and plumbline

deflection

1974) data from the ridge show that it can be composed

material, although interpretation strained by other measurements.

of individual

parameters

The second and more compelling argument presence within the Eurasia Basin of a magnetic

(Lillestrand

and

of continental-type

has generally

been uncon-

for its continental nature is the anomaly pattern that is approxi-

mately symmetric about the Nansen-Gakkel Ridge (Fig. 2). Heat flow patterns and present seismicity along the ridge plus its seismic continuity with the mid-Atlantic Ridge to the south indicate that the Nansen-Gakkel Ridge is an accreting margin and the Eurasia Basin a product of activity along this axis over the last 50-70 Ma (e.g.. Pitman and Talwani. 1972; Vogt et al., 1979). That is, prior to the inception of the Eurasia continental With carried

the Lomonosov

margin

of western

this background out during

structure an

Basin,

continental

long

margin.

this experiment the Lomonosov

was adjacent

the Lomonosov

fragment

to break

Several of the marine

are presented Ridge

of the polar

Ridge

off

geological

what processes so cleanly

Details

(LOREX).

as to logistics,

on ridge

allowed

from

and geophysical

here and, from them, the nature

are assessed.

Experiment

to collect data bearing

and. from this. to discover

narrow

to or part

Eurasia.

in mind,

the spring of 1979, was designed

and composition

apparently

Ridge

returns

from

and mode of origin of instrumentation

methods of data collection can be found in Weber (1979). Blasco et al. (1979) (1980), Aagaard ( 198 1) and Mair and Forsyth ( 1982). THE LOREX

such

its parent

and Coles

DATABASE

It was decided that a multidisciplinary corridor of data collected completely across the ridge offered the best chance to achieve the stated scientific goals. Two factors dictated the location of the initial camp sites. The first is the transpolar current that carries the pack ice obliquely across the ridge toward Greenland. The

221

second is the time between

first light in mid-March

in early June. This 2.5 month region

of about

positioned

5 km/day

upstream this starting

(Weber

and the start of the melt season

plus the average drift rate for pack ice in this

and Sweeney,

of the most narrow

Basin close to the North From

window

1977) required

that the camps be

part of the ridge. That is, in the Makarov

Pole (Fig. 1).

point

the camps drifted

successfully

across the ridge (Fig. 3a,

b). Bathymetry and gravity measurements were recorded continuously at each camp and about 250 spot readings were made on helicopter traverses along lines normal to the ridge. Near bottom currents were measured in the crestal region (Pounder, 1980; Aagaard, 1981). High resolution shallow seismic reflection profiling and intermediate depth reflection profiling of the sediment column and basement were conducted along the drift path of the Main Camp (Fig. 3a; Blasco et al., 1979; Overton, 1980). In addition, two pseudo-reversed crustal refraction profiles, one normal (Fig. 3b) and one parallel to the trend of the ridge, were carried out at predetermined sites over the feature (Mair and Forsyth, 1982). Aeromagnetic data were collected at low altitude (300m) along and across the Lomonosov Ridge from the LOREX camps to the Lincoln Shelf (Hood and Bower, 1980). Magnetic total field measurements were also taken along the drift paths of the LOREX satellite camps (Coles, 1980) and magnetic induction and magnetotelluric at all three ice camps (Camfield et al., 1980). Morphology Ridge

data were collected

and seabed geology

morphology

derived

from

these data

confirm

earlier

reconnaissance

de-

scriptions. The Lomonosov Ridge rises sharply from the abyssal depths of the Fram (4200 m) and Makarov (3900 m) basins and crests at 950- 1500 m below sea-level in the region studied (Weber, 1979; Fig. 3b). Average slopes on the Makarov flank are steeper,

up to 14’, than those on the Fram

flank, less than 7” (Weber,

1980). Near

surface samples of the seabed were obtained by gravity coring, grab sampling and dredging along the drift track of the Main Camp (Fig. 3a). Overall, sediments recovered from abyssal depths are much. finer grained than those retrieved from the ridge itself. Makarov Basin sediments, however, contain far more silt-rich interbeds than Fram Basin sediments which are composed mainly of firm clay layers (Blasco et al., 1979). Sediments recovered from the Lomonosov Ridge contain almost no recognizable turbidites and have a large sand-sized fraction, dominantly quartz with minor feldspar. Dredge recrystalized dolomitized

samples from the ridge included fingernail-sized pieces of chalk, a manganese nodule and a fragment of biotite schist.

Bottom photographs show a coarse gravel pavement over much of the crest and Makarov flank of the ridge. Three types of micro-fossil populations have been recognized within the top few centimeters of a core recovered from the Fram flank of the ridge in 1721 m of water (site B25 in Fig. 3b): Late Cenozoic pollen, mid-Cretaceous dinoflagellates and Upper Devonian spore fragments that have been

. ‘. .

. . .. .

possible

Contour

interval

~~-

seafloor

lineaments.

r

7---

location

~~~~

magnetic Short-wavelength

seismic (C-C’), site is marked.

of crustal

~~~~~

and core B25 retrieval

100 m. Profile

sites ( LI, I_?) are indicated

bathymetq.

meter mooring

lines indicate

Current

Fig. 3b. LOREX

_

(M-M’) magnetic

anomaly

i

~___~

by

models highs indicated

(G-G’)

-DEPTHS 1N METRES

and gravity

,

*

,J.!

X . Dashed

are given.

. ..-.

I

I

I

J

t

.

?,

I x

‘b,

*..

225

reworked undergone

several

times (Blasco

significant

thermal

et al., 1979). The dinoflagellates

have been measured

flow to the east-southeast Makarov suggest

Basin.

(Aagaard, (toward

The indicated

Basin

thereby

producing

1980, 1981; Pounder,

current

1981). The currents Greenland)

current

that close to the North

over the top of the Lomonosov

spores

have

alteration.

At the top of the Fram flank (Fig. 3b), near-bottom cm/s

and

diagonally

pattern

speeds in excess of 12

are pulsed

plus seawater

temperature

Pole cold water from the Eurasia Ridge and then sinking

a downslope

current

along

and appear

to

across the ridge into the

adiabatically

the Makarov

profiles

Basin is spilling into Makarov flank

(Aagaard,

1980).

Seismic structure

As reported elsewhere (Blasco indicate that less than 40m of overlies the Fram flank of the sediment-free. The crest appears fault blocks with (sediment-free) mably

overlain

by about

et al., 1979; Weber, 1980) shallow seismic returns stratified conformable, unconsolidated sediment ridge while the Makarov flank appears to be to be made up of several slightly tilted enechelon scarps facing the Makarov Basin and tops confor-

75 m of stratified

unconsolidated

sediments

(Fig. 4). The

sediments are truncated by the faults which extend into the underlying basement rocks. In the Fram and Makarov basins sediments are stratified, flat-lying and at least 1100 m thick close to the ridge. These unconsolidated abut the ridge flanks and show little evidence 1979).

of internal

deposits deformation

unconformably (Blasco et al.,

Basement beneath the Makarov Basin exhibits significant relief on the shallow seismic records and it outcrops at about 20 km and also between 50 and 60 km from

0

‘;;I-

Fig. 4. Generalized

LOMONOSOV RIDGE

shallow

seismic structure (line drawing from original data in Blasco et al., 1979;

Weber, 1980). Fault traces penetrate near-surface the seismic records.

I

basement and are dashed where weakly established

in

the ridge (Fig. 3b). The arch-like results

from contouring

series of seamounts Structures liminary

for example,

within

analysis

appearance

of widely

Lomonosov

of records

1980). This may indicate

spaced

of the outermost data.

Much

bathymetric

less regular

feature

topography,

a

may be present. Ridge

basement

from the intermediate a preponderance

rocks

are not apparent

reflection

of high angle

experiment structures

in pre(Overton.

or perhaps

highly contorted and discontinuous structural pattern or. less likely, significant structure within the ridge core. More detailed interpretation

a

a lack of of these

records is presently underway (A. Overton, pers. commun., 1981). Details of the LOREX crustal refraction experiment are reported elsewhere in this issue (Mair and Forsyth, 1982). Results are described briefly here for a profile extending across the Lomonosov Ridge into the adjacent D.A. Forsyth, pers. commun.. 1980, 198 1; Fig. 5a). Close to the ridge the Fram crust of the Makarov

Basin crust is thicker

Basin, chiefly because

deep basins (J.A. Mair and by 3-4 km overall

of differences

in the extent

than

the

of the 6.6

km/s layer. Each basin contains about 1 km of unconsolidated sediments (2.7 km/s) that overlie a few kilometers of 4.7 km/s basement (Layer 2?) with the remainder of the crust formed by rocks with P-wave velocities crust beneath velocity units. of the feature of the crustal

the crest of the ridge is made up entirely of The 4.7 km/s layer, up to 6 km thick, forms while the 6.6 km/s unit extends to a depth of root is skewed toward the Makarov Basin and,

Fig. 5a. P-wave velocity crustal to mdicate location

of at least 6.6 km/s

diminished

model (from Mair and Forsyth,

resolution

of refraction

data beyond

(Layer 3?). The

the same two basement the topographic portion about 27 km. The mass although a fault trace is

1982). Moho dashed

beneath

Fram Basin

base of ridge flank. Velocities in km/s.

Profile

given in Fig. 3b.

b. Two-dimensional Profile location

free-air

gravity

given in Fig. 3b.

model based on P-wave velocity

horizons

in (a). Dewties

in g/cm’.

227

not evident be offset Basin.

on the records, below

More

the boundary

the two crustal

the ridge crest with the downthrown

detailed

aspects

of crustal

existence

of enechelon

between

the ridge and the deep basins,

Magnetic

between

fault blocks within

and conductivity

structure

block and

ridge basement

cannot

layers appears

toward

to

the Makarov

composition,

such

or differences

in rock type

be resolved

with the refraction

as the data.

structure

Previous high-level reconnaissance surveys show the ridge to be associated with an irregular magnetic anomaly pattern of low relief save for local highs along its crest and Makarov flank close to the North Pole (e.g., Coles et al., 1978). The low-level Hood and Bower (1980) aeromagnetic returns support this regional picture and, in addition, show a spatial correlation in Makarov Basin between positive magnetic anomalies and basement relief as determined from LOREX bathymetric, gravity and shallow seismic work (Figs. 3b, c and 4). A zone of linear magnetic highs in excess of 1000 nT lies parallel south of the LOREX

to the ridge crest along

the Makarov

flank just

traverse.

Coles (1980) showed that a strongly positive magnetic anomaly of up to 400 nT is present over the ridge along the LOREX drift path. A preliminary magnetic crustal model constrained to be below the mid-crustal shows that a tabular region of highly susceptible

P-wave velocity horizon (Fig. 5a) rocks (0.06 SI) at depths between 7

and 17 km can account for the observed anomaly data (Fig. 6). This tabular zone extends, at depth, well into the Makarov Basin. The base of the model is loosely constrained and could be extended to include the crustal root beneath the ridge with appropriate lowering of the rock susceptibility. A shallower model top, however, appreciably

Fig. 6. Proposed

degrades

magnetic

the fit to the observed

source

within

the crust

1980). The model total field (solid line) is compared measurements

(dot-dash

line). Arrows

indicate

field (R.L. Coles, pers. commun.,

below

the Lomonosov

with LOREX

Ridge (modified

data (dashed

base of ridge. Profile location

1981).

from Coles,

line) and 1970 airborne

given in Fig. 3b.

Shallower

magnetic

short-wavelength topographic

sources within positive

maxima

the ridge are indicated

anomalies

on the Fram

that flank

coincide, near

by the presence

in two instances,

the crestal

region

of small with

(Fig.

local

3b; Coles,

1980). Camfield

et al. (1980) have reduced

netic and geoelectric points

become

electrical within appear

fields to transfer

evident.

currents

First,

the time varying functions

component

and apparent

of the geomag-

resistivities

the ridge does not have a conducting

flow parallel

to the ridge in the adjacent

and two

core; induced

deep seas rather

the ridge. Second, spatial changes in magnetotelluric to correlate with the depth of the highly conducting

than

apparent resistivities seawater. A simple

two-layer two-dimensional model of the ridge involving seawater and the underlying rock was constructed from LOREX bathymetric data. The modeled electrical response agrees reasonably well with measured variations except in the Fram Basin adjacent to the base of the ridge where the model response falls off more rapidly than the observations. It has not proved possible to decrease this mismatch by adding model inhomogeneities of reasonable conductivity beneath the Fram Basin. The large observed response over the basin may arise from the three-dimensional form of the ridge on the Fram side (Figs. 1 and 3b) or from problems of incomplete separation of variation fields from internal R.D. Kurtz, pers. commun., 1981. 1982). Electrical

conductivity

anomalies

and external

are typically

currents.

attributed

(P.A. Camfield,

to variations

in rock

temperature, to changes in rock porosity or to the concentration and continuity conducting minerals within the earth. Heat flow from 21 measurements along Main Camp drift path show a rather undistorted 60-70 75-85 present

mWm--2 mWmp2 thermal

pattern

in Makarov Basin, 60-65 mWme2 in Fram Basin (A.!!?. Judge, pers. regime

appears

to have a rather

of the

over the region of interest:

on the ridge flanks and crest. commun., 1981). That is, the deep-seated

subcrustal

Electrical inhomogeneity beneath the Fram Basin, if any, may therefore to temperature variations within the earth.

source.

be unrelated

Gruvity

The free-air gravity anomaly field is dominated by a prominent high, between 60 and 90 mGai, that extends along the crest of the Lomonosov Ridge (Fig. 3~). This anomaly is flanked by similarly trending lows, generally between -20 and - 50 mGa1, centred just beyond the ridge flanks in each basin. Secondary highs in Makarov Basin coincide with bathymetric and basement relief (Fig. 3b). As with bathymetric representation of these features, ellipticity of gravity anomaly contours results from the spacing of the collected data (Fig. 3a). A crustal density model (Fig. 5b) was constructed with density blocks defined by P-wave velocity horizons derived from the refraction and shallow-reflection studies (e.g., Fig. 5a). In the absence of firm control of Moho depth in Fram Basin far from

229

the

ridge,

the crust-mantle

documented Ridge.

shallowing

No attempt

associated

was made

with basement

the unconsolidated Remaining

boundary

(Jackson

and

to account

was

inclined

very

slightly

Reid,

1980)

toward

the

for the small positive

relief in the Makarov

sediment

block densities

Basin (see Weber,

layer and seawater were adjusted

were assigned

in the Fram

its

gravity

anomalies

1980). The mantle, fixed density

until a good fit between

observed gravity fields was achieved. This simple approach worked well except

to reflect Nansen-Gakkel

Basin

values.

the calculated

and

where calculated

free-air values were consistently much less than the observed anomaly. The position of this discrepancy close to the base of the ridge coincides with the zone where electromagnetic

observations

cannot

be fitted

with a two dimensional

model.

It

appears that crust beneath the Fram Basin either is thinner than the seismic refraction model suggests (Weber, 1980) or is more dense close to the ridge than crust below

the Makarov

Basin.

Pursuing

the latter

option,

reduced

rock porosity

yields increased rock density and, because geostatic pressures rapidly eliminate pore space as depth of burial increases, significant reductions in pore volume are possible mainly

within

rocks of the upper

crust. Accordingly,

a zone representing

reduced

porosity was added to the Fram Basin gravity model in Layer 2 (Fig. 5b). A density of 3.0 g/cm’ was considered a reasonable maximum for unporous Layer 2 material and significantly higher densities were tried but, despite greatly improved fits, the calculated gravity anomaly remained well below the observed field in all cases. To adjust this, an additional block (density = 3.14 g/cm3) was added to the gravity model within the lower crust. The existence of either of these blocks is not supported by other LOREX data sets, although a similar deep zone (Layer 3b) is known elsewhere in the Arctic in oceanic crust of the Canada Basin (Mair and Lyons,

1981). The

beneath

the Fram Basin (Fig. 5a).

crustal

refraction

experiment

could

not

detect

such

a layer

SYNTHESIS

Geophysical parameters provide gross compositional properties of rocks within the core of the feature. Ridge structure is indicated mainly by the reflection and refraction results and, to some extent, by aero- and surface magnetic field data. The average age of rocks that make up the Lomonosov Ridge cannot be well bracketed but some tentative comparisons can be made. Composition Near-bottom seawater velocity patterns suggest that, in the study area, pulsed competent currents are presently scouring much of the crest and Makarov flank of the Lomonosov Ridge (Aagaard, 1981). The flow may pick up finer material from the upper parts of the ridge and carry it down along the Makarov flank into the

abyss

where

observed

it is deposited.

in bottom

finer-grained

nature

flanks. Second,

of abyssal

the silt-rich

by scouring

episodes

the adjacent

ridge.

The fraction

This current

photographs

and sediments

interbeds

can explain bottom

relative

in Makarov

related to variations

of recovered

behaviour recovered

much

sediments.

is the

to those of the ridge crest and Basin may be turbidites

in the velocity of currents

or photographed

of what is First

crestal

material

produced

descending

derived

from

from local

sources is unclear. The detritus is abundant, widespread and similar in texture and composition along the drift track. Slumping or scouring of ridge material may be a significant component of this elastic deposit but the present crestal seabed also is probably armoured with ice-rafted debris from which the finer fraction has been removed by near-bottom currents. .Support for this contention is the similarity in composition between recovered material from the ridge and ice-rafted debris recovered elsewhere from Arctic seafloor sediments of Alpha Ridge and in Canada Basin (Clark et al., 1980). Ice-rafted material together with atmospheric dust may account for as much as 80% of the sediment deposited in the Arctic Basin over the last 5 Ma (Mullen et al., 1972). Near affected millions

the Lomonosov Ridge, deposition by currents appears to have strongly the pattern of sediment accumulation within the Makarov Basin for many of years. The unconformable contact between basement rocks and the

overlying undeformed horizontally stratified material indicates that major sedimentation not only postdates development of the ridge but also is not the result of pelagic processes

(Blasco et al.. 1979; Fig. 4).

Local igneous

bodies

along or close to the ridge crest may be indicated

distribution

of short-wavelength

positive

1980; Coles.

1980; Fig. 3b). The large amplitude

flank,

for example,

could be evidence

dikes

that

the trend

contribute ridge.

parallel

little, however.

The high magnetic

magnetic

anomalies

magnetic

(Hood

and

by the Bower,

high along the Makarov

of a major dike or a series of closely spaced

of the ridge.

Sources

to the long-wavelength susceptibility

needed

within

magnetic to generate

the upper

crust

high associated an acceptable

may

with the crustal

model indicates that much of the lower crust beneath the ridge and nearby Makarov Basin may be composed of intermediate to basic rocks. A deeper model base with correspondingly reduced rock susceptibilities does not appreciably alter this contention. Gravity and refraction crustal models yield a density and minimum P-wave velocity for the lower crust of 2.95 g/cm3 and 6.6 km/s, respectively. Both are reasonable

values for the suggested

If the upper average density

crust beneath and minimum

range of rock types.

the ridge is relatively nonmagnetic, then its modeled P-wave velocity of 2.50 g/cm’ and 4.7 km/s respec-

tively indicate that, overall, it is composed neither of highly magnetic rocks such as FeTi-rich basalts, nor of lithologies with high P-wave velocities including igneous intrusives and high grade metamorphic rocks, nor of dense rocks including dolomites and

crystalline

rocks

with low porosity.

Rock

types

that

satisfy

the geophysical

231

constraints gories

fall mainly

but porous

indicated, related

into

the sedimentary

crystalline

rock fragments

to its basement

or low-grade

rocks such as vesicular

recovered

metasedimentary

basalt

from the Lomonosov

cate-

are not excluded.

As

Ridge can not be reliably

lithology.

Structure

Normal faulting, both at the surface and deep within the crust below the ridge crest, is indicated by the combined seismic data (Figs. 4 and 5a). Downthrown blocks lie to the south toward the Makarov Basin in all cases. Fault strike and the existence of lateral offsets cannot be determined with assurance, nor can the large apparent offset at depth be physically connected to the set of smaller enechelon displacements close to the surface. If the ridge is a series of imbricated fault blocks, then it is reasonable ridge trend more or however, by the 45” corridor (Sobczak,

to suppose because of its linear character that offsets within the less parallel to its strike. Structural trends may be complicated, change in orientation of the ridge axis adjacent to the LOREX 1977; Fig. 1). Subparallel with this direction, three regional

lineaments

9O”W are tentatively

at about

identified

based on alignment

of valleys,

embayments, arches and peaks in seafloor morphology (Fig. 3b). These proposed features partially transect the ridge but do not appear to continue into the Makarov Basin.

Closer

to the Laptev

faults proposed

Shelf, the ridge may be cut by highly

on the basis of patterns

in aeromagnetic

data (Karasik

oblique

major

et al., 1971).

Along the ridge further toward North America, intense magnetic anomalies over the Makarov flank are aligned with the ridge axis (Hood and Bower, 1980). In other words,

the structural

complex. Linear Lomonosov

magnetic

character anomalies

Ridge (Taylor

of the ridge in the Makarov

appears

variable

Basin appear

et al., 1981). As indicated

earlier,

and

may be quite

to be aligned several

with the

LOREX

data

sets show that, near the ridge, the magnetic anomalies coincide with basement relief and do not represent reversals. The Makarov Basin may not have been formed by simple seafloor spreading. Further evidence for this may be the unrealistically young age of 18 Ma estimated for the basin crust by Taylor et al. (1981) using an empirical seafloor age versus depth relation (Sclater et al., 1971). Such a recent history of accretion is precluded by the absence of significant heat flow anomalies (Judge and Jessop, 1978) and the lack of a topographic spreading ridge remnant within the Makarov Basin. Although Taylor et al. questioned its applicability to small ocean basins, the Sclater et al. relation may fail in basins not produced by a spreading ridge. Instead, Makarov Basin may represent a zone of thinned or stretched continental crust in which a combination of fault blocks and igneous features form the basement surface. The magnetic crustal model allows the possibility that continental-type rocks may be present below part of the basin (Fig. 6; Coles, 1980). The

extent

and distribution

revealed

of basement

by additional

structure

The time of origin of the Lomonosov that comprise elastic

it. Given

sediments

average

density

within

the Makarov

Basin would

be

shallow seismic and gravity profiling.

its presumed

rocks, and

then

P-wave

Ridge places a minimum

origin,

a crude

velocity

age on the rocks

if the ridge is composed

estimate

of their

determined

of shelf-type

age is provided

for the upper

crust

by the

below

the

feature (see Fig. 5). Comparison of these values with both rock density and P-wave velocity versus age data from Birch (1942) indicates that ridge basement may be made up of early Mesozoic is necessarily and Mesozoic

tentative

or older elastic rocks. This age assignment,

and it applies

successions

Kara shelves (Harland, faces the Eurasia

Barents

and

1973a, b) and such rocks may form much of the margin

that

Basin.

are present

such as it is,

only to the given rock types. Thick Paleozoic in archipelagos

This is in keeping

along

with the above

the outer

density-velocity

age

estimates. PERSPECTIVES

What is known now about the Lomonosov Ridge that wasn’t known before LOREX? The structural information has been most definitive and it shows that the ridge has undergone a period of tensional faulting. The time of faulting is not clear but it postdates an interval of undisturbed sedimentation along what is now the ridge crest. The significance of the structures is equally ambiguous but the southerly direction of downfaulting indicates that the offsets may be related to the development of the Makarov Basin. This suggestion is preliminary given our ignorance of the timing of events and the uncertain extent of the structures in question. Local causes,

such as faulting

associated

with the nearby

zone of changing

ridge orienta-

tion, are by no means precluded. This leaves open the question compositional

and age constraints

of ridge origin. provided

Leaving

by LOREX

the uncertainties

aside, the

data favor a crustal

block

containing early Mesozoic or older elastic sedimentary or low-grade metasedimentary rocks in its upper part and intermediate to basic crystalline rocks in its lower part. A relatively deep crustal root and the absence of a high P-wave velocity (7.3-7.6 km/s) basal layer is considered to be typical of submarine plateaus with continental as opposed to oceanic affinities (Carlson et al., 1980). The P-wave velocity structure measured along the outer Barents and Kara shelves is similar to that determined for the Lomonosov Ridge (Eldholm and Talwani, 1977; Beliayevsky et al., 1968; Mair and Forsyth, 1982). The thick Paleozoic and Mesozoic sediments of these outer shelves are among the rock types permitted for the upper crust below the ridge. In short, it is likely that the ridge was once part of the Eurasian polar shelf as Wilson

(1963) suggested.

233

The

compositional

Lomonosov

constraints

taken

by themselves,

Ridge crust could be a pile of vesicular

rocks and therefore correspondence

oceanic

between that,

Eurasia

before

overlying

It is the morphology

the measured

polar shelves of western one considers

in nature.

however,

basalts

properties Basin

that

the

a core of basic

of the ridge and the

of its crust and those of the outer

that make this possibility

the Eurasia

allow

existed,

less likely, especially the Lomonosov

when

Ridge

was

juxtaposed with the Barents and Kara shelves. How the ridge became separate is less clear. If the faults within the ridge are not related to the opening of the Eurasia Basin, then the separating event may not have involved rifting. Yet the accretionary origin of the Eurasia Basin requires large scale movement of the ridge normal to the line of separation. One major difference between present and early events in the Arctic Basin is the location of the pole of rotation between the Eurasia and North American plates. Prior to the opening of the Eurasia

Basin

significant

separation

between

the two plates

took

place

in Late

Cretaceous and possibly earlier time about a pole in northern Greenland (Fig. 7; Pitman and Talwani, 1972). Separation continues today about a pole in Siberia. Close

to this pole, focal mechanism

taking place along the modern If the present

solutions

plate boundary

plate dynamics

indicate

that left-lateral

zone (Chapman

can be applied

shearing

and Solomon,

to the past, then similar

is

1976).

displace-

ments may have occurred between .the impinging plates close to their former pole of rotation in northern Greenland (Fig. 7). That is, during Cretaceous time before the Eurasia

Basin existed,

the terrain

from Eurasia and was displaced that may have been delineated

about

to become

the Lomonosov

Ridge

sheared

left-laterally along a newly formed plate boundary by earlier events: an episode of shearing between

eastern North America and western Europe in the Triassic (Roy, 1972) and the Jurassic welding of a continental block to Eurasia along a zone to the east of the Cherskiy foldbelt (Fujita and Newberry, 1982; Fig. 7). Shearing between the ridge and Eurasia continued until the outset of the Tertiary when the rotation pivot began 1972) thereby allowing the North its move into Siberia (Pitman and Talwani, Atlantic now

rift to extend into the Arctic region.

the Nansen-Gakkel

Eurasia

as the spreading

Ridge proceeded

The route of the proposed

and

Seafloor

accretion

the Lomonosov

(Wilson,

shear around

Ridge

began along what is was separated

from

1963). Greenland

is conjectural

(Fig. 7). To the

east of Greenland, initial rifting to open the North Atlantic was accompanied by right-lateral transform movements (Eldholm and Talwani, 1977), in apparent contradiction Greenland

to the sense of offset indicated by the present and Ellesmere Island, on the other hand,

scenario. The zone between has long been thought to

represent a’ major left-lateral offset (e.g., Wilson, 1963) that was created in Late Cretaceous or Early Tertiary time by the opening of Baffin Bay. The shearing of the Lomonosov Ridge from Eurasia and the origin of Baffin Bay may therefore be genetically linked. This suggestion, which implies that Baffin Bay was initiated prior to the opening of the Eurasia Basin, is tentative because the available constraints are

234

LATE CRETACEOUS Fig. 7. Cartoon zones Talwani

(1972).

A = Alpha

clear

Jackson

Cretaceous

Late Cretaceous Present

Ridge,

EE = Eurasia

not

of proposed

are dashed.

EB = Baffin

to present

history

pole of rotation

pole position

and plate

Bay, C‘B = Canada

Basin. M = Makarov

about

PRESENT

70 Ma

of Lomonosov

for opening

boundary

zone from

(L). Plate boundary

Ridge

of North

Atlantic

from

Pitman

and

Chapman

and Solomon

(1976).

foldbelt.

E = Ellesmere

Island,

Basin, C‘S J Cherskiy

Basin.

the age relationship

between

the two basins

et al., 1979; Vogt et al., 1979) and estimates

(Srivastava,

of the amount

1978;

of left-lateral

displacement along the Greenland-Ellesmere zone vary widely (Christie et al., 1981). As proposed, this two stage history of ridge movements relative to Eurasia is a consequence of the transition from a compressional to a tensional tectonic regime in Arctic Atlantic

regions

produced

to the opposite

al., 1978). As a product

by the migration of crustal

extension,

Ridge may thereby have been created movements associated with compression of the Makarov

of the pole of opening

side of the Arctic at the end of Mesozoic the Makarov

for the North time (Sweeney

et

flank of the Lomonosov

distinctly after the shearing stage of ridge in the Arctic. This implies the non-existence

Basin when the ridge sheared

from Eurasia

and it suggests that the

initial Lomonosov block may have included what is now the Alpha Ridge, considered by several workers beginning with King et al. (1966) to be a continental fragment. The Makarov Basin may have been produced by stretching and collapse of continental crust when the newly formed Lomonosov block was split in two by processes as yet undefined. Basement rocks of these two major Arctic submarine ridges may therefore have a common origin and the continental sliver initially sheared

from Eurasia

may have been much

broader

than

the present

Lomonosov

Ridge. This scheme explains how a relatively slender linear fragment can be split from a continent and be carried large distances away, but additional information is needed

235

on several fronts

to more critically

on the mode of origin Alpha

Ridge, on the character

bordering

evaluate

of the Makarov

continents

the story. Better constraints

Basin, on the nature

of the junction

between

and on the connections,

the submarine

if any, between

Bay and the development

of the Arctic Basin. These problems

for future Arctic

basement

seafloor

geophysical

are needed

of the crust below the ridges and the

the genesis of Baffin remain

central

targets

studies.

ACKNOWLEDGEMENTS

Dave Halliday generated

reduced

computer

benefited

the gravity

models

from critiques

data

of crustal

and discussions

crustal P-wave velocity R.D. Kurtz regarding

and helped

density

collect

structure.

it. Suzanne

The

first

Juneau

author

with J.A. Mair and D.A. Forsyth

(JFS)

regarding

models, with P.A. Camfield, R.L. Coles, J.M. DeLaurier and electrical and magnetic crustal structure and with B.D.

Bornhold and D.L. Clark regarding glacial-marine sedimentation. C.J. Yorath and M.R. Dence suggested several improvements to a preliminary version of the manuscript. Tactical and logistic support for LOREX was provided by the Polar Continental Shelf Project, the Canadian Squadron of the Canadian Armed Forces.

Forces

Base

at Alert

and

the

435th

EOS,

Trans.

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