ALAN G JONES: SNORCLE Page

LITHOPROBE Slave-Northern Cordillera Lithosphere Evolution (SNORCLE) Transect
ElectroMagnetic activities

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Transect EM leaders.....

Dr. Alan G. Jones, Geological Survey of Canada
Prof. Ian Ferguson, University of Manitoba

Collaborators.....

Dr. Alan D. Chave, Woods Hole Oceanographic Institution
Dr. Rob Evans, Woods Hole Oceanographic Institution
Mr. Gary McNeice, Phoenix Geophysics

Financial Sponsors.....

Geological Survey of Canada
Natural Sciences and Engineering Research Council of Canada (Lithoprobe)
U.S. National Science Foundation
Indian and Northern Affairs Canada
Monopros
Kennecott
B.H.P.

Transect Overview.....

The SNORCLE transect consists of three (3) main lines:

Corridor 1: Yellowknife to Nahanni Butte: 800 km (measured)

Starts to the East on Archean Slave province at Tibbit Lake and continues west to Nahanni Butte crossing Proterozoic elements. Tibbit Lake is in the Yellowknife Greenstone Belt, so there will be no sampling of the Sleepy Dragon Complex or onto the Contwoyto Terrane to the East. Insufficient sampling of the Slave craton [see below for possible EM experiments to address this problem]. Profile crosses the Anton terrane to the west, which comprises the oldest known sialic crust. Profile crosses the PaleoProterozoic Talston Magmatic Arc and Wopmay Orogen, which includes such major features as the Great Bear magmatic zone, Hottah and Fort Simpson Terranes. Then crosses the (presumed) buried MesoProterozoic Racklan Orogen to the Cordilleran deformation front at Nahanni Butte

Corridor 1A: Great Slave Lake Shear Zone extension: 409 km (measured)

An extension to Corridor 1 runs from Fort Providence to Fort Smith crossing the Great Slave Lake Shear Zone (GSLsz). This feature is interpreted as a PaleoProperozoic continental transform structure with a reported offset of 300 - 700 km. Its role in the plate tectonic collisional processes is significant, and it is the most prominent feature on an aeromagnetic map of Canada. Due to it being under cover along Corridor 1A, there was little interest expressed by the geologists in the SNORCLE transect team, but the geophysicists rate determining its geometry high.

Corridor 2: Fort Nelson to Stewart over Watson Lake: 950 km (est.)

The transect begins to the East at Fort Nelson, just on the boundary of the Fort Simpson and Nahanni Terranes. It crosses the Nahanni Terrane and the Cordilleran deformation front, to give 200 km overlap with the western end of Corridor 1. The profile then runs virtually orogen-parallel for 300 km to the NW on Ancestral North America to the Tintina Fault at Watson Lake. The profile then crosses Cordilleran elements obliquely for 450 km to Stewart. In particular, this profile crosses the Stikine terrane, which is the largest Cordileran terrane and is arguably the most enigmatic in terms of its origin, motion, and tectonic history.

Corridor 3: MacMillan Pass to Skagway: 525 km (est.)

This profile runs NE-SW from MacMillan Pass on Ancestral North America, crossing the Tintina Fault, at about 1/2 way, then elements of the Northern Cordillera to Skagway (in the U.S.) just short of the Denali Fault. It traverses different Cordilleran terranes than does Corridor 2.

For more information, go to Lithoprobe's SNORCLE Home Page at http://www.litho.ucalgary.ca/transect_info/snorcle/index.html

EM Objectives.....

In terms of the general transect objectives that can be addresses by EM studies, these are:

1. Characterizing the deep crust and mantle between the oldest rocks in the world (Slave Province) to the youngest terranes on Earth.

This component will permit addressing the following transect objectives:

What kind of deep crust and mantle underlies the oldest rocks on Earth?
Is the tectonic style of Early Archean protocontiental nucleii unique?
Are deep structural and stratigraphic characteristics of Proterozoic orogens fundamentally different from those of Phanerozoic orogens, and if so, why?
How have deep Precambrian structures been significant in controlling the stratigraphic and structural evolution of Phanerozoic orogens?

This will be accomplished by a combination of contracted wide-band MT sites and long period LIMS sites installed and monitored by the bird-dogger plus an assistant, along the transects. The sites will initially be equi-spaced, but the surveys will be run in "dynamic" mode with in-field processing and modelling, permitting identification of critical infill site locations.

In addition, in order to address the Slave mantle root geometry characterization, a joint U.S.-Canadian project is proposed which will entail installing shallow-water MT equipment in helicopter or float-plane accessible lakes in the Slave. Additional industrial funding for this compoenent, as well as NSF funding, will be sought.

2. Characterising the geometry of the major vertical structures.

Knowing the current geometry of vertical structures permits one to address such transect objectives as:

Did plate tectonics operate in the Early Archean (GSLsz)?
Why do inter-related tectonic characteristics of the Canadian Cordillera, such as lithospheric extension, intracontinental strike-slip, and amount of contraction toward the craton, change from south to north?

EM studies have shown their superior effectiveness in determining the geometry of vertical structures to whole crustal depths over other geophysical methods (e.g., Fraser Fault study).

Closely-spaced (1 km) twenty MT stations in the vicinity of each of the three major vertical structures - the Great Slave Lake Shear Zone, the Tintina-Rocky Nountain Trench Fault and the Denali Fault - would be optimal. An EMAP survey, consisting of continuous E-field dipoles crossing the fault, would yield the best image (cf. San Andreas EMAP results), but is not affordable in the current budget.

3. In addition to these, auxiliary objectives of the EM program will be:

Does the Slave craton display strong mantle electrical anisotropy as exhibited by the Abitibi region of the Superior?
Does the mantle beneath the Slave craton to 1,000+ km depth display the same electrical conductivity as beneath the Superior craton?
Are there still fluids resident in the crust from the oblique subduction of the Kula plate which were cut off by the plate reorganization at 55 Ma?
Are current MT time series robust processing codes sufficient to remove the effects of highly non-uniform source fields?

EM Activities.....

Reduced budgeting over that requested means that not all activities can be carried out.
Highest priority be given to:

Corridor 1
Corridor 1A: GSZsz only
Corridor 2 (with reduced site-spacing in the 300 km along-strike region)
Corridor 3: Tintina only

And lower priority to (in priority order):

Corridor 3 except Tintina, with extension to cross the Denali fault
Corridor 1A except GSLsz
Industry targeted studies (industry funding will be sought for these studies)

This priority takes into consideration:

Emphasis on regional reconnaisance studies over targeted studies.
The proposed refraction program of two profiles from Yellowknife -> Nahanni Butte and Watson Lake -> Stewart
Crossing the Stikine terrane
Two crossings of the Tintina
One crossing of the GSLsz

The work is proposed to be carried out over three years, 1996, 1998 and 1999 as follows:

Year 1 activities: Corridor 1 and GSZsz. Very long period LIMS acquisition over the winter.
Year 2 activities: Slave craton characterization using shallow-water instruments (together with WHOI) Very long period LIMS acquisition over the winter. Possibly wide-band contract acquisition to east of Tibbit Lake over Sleepy Dragon complex (partial industry funding will be sought for this).
Year 3 activities: Corridor 2 and Tintina on Corridor 3.

EM Acquisition.....

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Locations of all MT sites on the Slave craton:
SNORCLE 1996 MT data acquisition (yellow circles)
1998 Winter Road experiment (green squares)
1998 Slave lakes experiment (blue stars)
Also shown are the locations of Michael Bostock's teleseismic stations (blue circles)

1996:

The first phase of lineitem acquisition of EM data along the SNORCLE transect began on August 1st, 1996, with a sixty (60) station magnetotelluric (MT) survey. The contractor (Phoenix Geophysics Ltd.) acquired the high frequency data pertinent to primarily the crust. In addition, longer period measurements were made at each location to image conductivity features in the subcrustal lithosphere down to the conducting asthenosphere.

In addition, a station was installed at Yellowknife within the seismic array. This station has had the 30,000 s high-pass telluric filters removed, and the electrodes were installed in buckets filled with saline solution, mud, and anti-freeze. It is intended for this station to operate for a year continuously.

1998:

Winter Road
The pilot phase winter road experiment took place during March 16 - April 15, 1998. Both wide-band and long period measurements were made at eleven locations along the winter road from the east end of the Ingraham trail (Tibbet Lake) to MacKay Lake, with three sites at the southern end of the line and the other eight on an east-west profile from northern Gordon Lake to MacKay Lake.

Slave Lakes
During late July, shallow water ocean-bottom MT instruments from Woods Hole Oceanographic Institution were deployed in ten lakes across the Slave Province.

Published Reports and Papers.....

1997 SNORCLE Transect Report
(For a copy of the Transect Report, contact the Lithoprobe Secretariate)
- Jones, A.G, I.J. Ferguson, N. Grant, B. Roberts & C. Farquharson, 1997.
  Results from 1996 MT studies along SNORCLE profiles 1 and 1A.
  Lithoprobe Publication No. 56, pp. 42-47.
1998 SNORCLE Transect Reports
(For a copy of the Transect Report, contact the Lithoprobe Secretariate)
- Wu, X., I. Ferguson and A.G. Jones, 1998.
  Electrical resistivity structure between the Nahanni terrane and Slave province.
  Lithoprobe Publication No. 64, pp. 93-102.
- Cassels, J. and A.G. Jones, 1998.
  Source field contamination of SNORCLE magnetotelluric data.
  Lithoprobe Publication No. 64, p. 103.
1999 SNORCLE Transect Reports
(For a copy of the Transect Report, contact the Lithoprobe Secretariate)
- Jones, A.G, I.J. Ferguson, G. McNeice, R. Evans and A. Chave, 1999.
  Electromagnetic studies of the Slave craton: preliminary results and ongoing experiments
  Lithoprobe Publication No. 69, pp. 56-71.
- Evans, R.L., A.G. Jones and A.D. Chave, 1999.
  Deep EM studies of the Slave craton.
  Contributed paper at:
  SNORCLE Transect Meeting, Calgary, March 5-7, 1999. Lithoprobe Publication No. 69, p. 72.
2000 SNORCLE Transect Reports
(For a copy of the Transect Report, contact the Lithoprobe Secretariate)
- Jones, A.G, I.J. Ferguson, R. Evans and A. Chave, 2000.
  The electric Slave craton.
  Contributed paper at:
  SNORCLE Transect Meeting, Calgary, February 25-27, 2000. Lithoprobe Publication No. 72, pp. 36-42.
- Wu, X., I.J. Ferguson and A.G. Jones, 2000.
  magnetotelluric response and geoelectric structure of southwestern Northwest Territories, Canada.
  Contributed paper at:
  SNORCLE Transect Meeting, Calgary, February 25-27, 2000. Lithoprobe Publication No. 72, pp. 43-55.
- Eaton, D.E., I. Asudeh and A.G. Jones, 2000.
  Mantle strain beneath the Great Slave Lake shear zone, NWT, from measurements of seismic and electrical anisotropy.
  Contributed paper at:
  SNORCLE Transect Meeting, Calgary, February 25-27, 2000. Lithoprobe Publication No. 72, pp. 56-63.
- Ledo, J., A.G. Jones and I.J. Ferguson, 2000.
  New SNORCLE magnetotelluric data: preliminary implications.
  Contributed paper at:
  SNORCLE Transect Meeting, Calgary, February 25-27, 1999. Lithoprobe Publication No. 72, pp. 124-127.
- Ledo, J. and A.G. Jones, 2000.
  Regional electrical conductivity of the southern Canadian Cordillera.
  Contributed paper at:
  SNORCLE Transect Meeting, Calgary, February 25-27, 2000. Lithoprobe Publication No. 72, pp. 225-231.
- Wu, X., I.J. Ferguson and A.G. Jones, 2000.
  Magnetotelluric response and geoelectric structure of Great Slave Lake shear zone along SNORCLE transect corridor 1a.
  Wu, X., I.J. Ferguson and A.G. Jones, 2000.