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Figure 2. A, Seismic “streamer" cable being deployed behind survey ship. The almost 2-milelong cable contains a series of hydrophones, which record echoes of acoustic waves reflected from the lake bottom and from rock layers in the subsurface and transmit the signals to recording equipment onboard. B, The seismicreflection surveying system in operation on Lake Superior. Air guns in four arrays marked by the orange floats are towed behind the ship about 30 feet beneath the surface. Powerful blasts (equivalent to the detonation of about 30 pounds of dynamite) of compressed air are released from the guns to generate sound waves. These sound waves, under suitable conditions, can penetrate to depths of 40 miles or more into the crust beneath the lakes and be reflected so that their echoes are recorded by hydrophones along the streamer cable that stretches for almost 2 miles behind the ship about 30 feet below the surface. Large masses of bubbles visible just behind the orange floats mark the position of an air gun blast.
Figure 3. Aeromagnetic map of part of Lake Huron and surrounding areas prepared by the Geological Survey of Canada. This map, which portrays the differing magnetic attraction of rocks in the subsurface, is colored so that areas of strongest attraction are shown in warm colors and areas of progressively lower attraction are shown by progressively cooler colors (see explanation on facing page; in nanoteslas). The data are further processed by computer to generate a false shaded-relief image in which areas of greatest attraction are shown as “hills” and “ridges" and areas of low attraction are shown as “valleys" and “basins." This "topography" is displayed as if it were being illuminated from the east. This technique enhances the visual display of the magnetic grain and gradients (slopes) in the magnetic field. Some of the more prominent features shown by the map are the Grenville Front, a major boundary between the Grenville Province on the east, about 1.1 billion years old, and older provinces on the west; major masses of intrusive granites (shown as outlined circular and elliptical features labeled GP) believed to have been formed about 1.5 billion years ago; and the boundary between the Hudsonian Province and the Central Plains Province.
Aeromagnetic surveying over Lake Huron and Lake Superior was conducted by an aircraft system owned and operated by the GSC, with support by the USGS. This type of surveying detects subtle changes in the Earth's magnetic field caused by differing magnetic properties in rocks beneath the aircraft. An area is mapped as the plane flies a series of parallel traverses and the plane's location and the measured magnetic field are recorded on magnetic tape. Later, computer processing and plotting are used to produce maps (fig. 3). The magnetic properties and other geophysical information can be used to
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Projection of these rock units onto land shows that they are volcanic flows of basalt alternating with layers of sedimentary rocks. Study of these units on land indicates they were deposited in a feature known as the Mid-Continent Rift, a major continental-scale structure that curves in a broad arc from Kansas through Lake Superior and into southern Michigan. The rift formed about 1.1 billion years (b.y.) ago as the North American Continent began to break apart along a line coincident with the current location of the rift. Rocks older than 1.1 b.y. were thinned by stretching and fracturing, and the basalts and sediments were deposited into the resulting depressions. The rifting process, had it continued, would have caused total separation of the continent on either side of the rift and the formation of an ocean basin like the present-day Atlantic Ocean. For reasons not yet fully understood, the rifting stopped before complete continental separation occurred, and as a result, the process of continental separation was “frozen into” the crust. Because of these circumstances, the seismic profiles not only are important in deciphering the geology of the immediate area but have much broader application in understanding the processes of continental rifting in general. The 18mile-thick rift fill shown on figure 4 is the greatest thickness of such rocks ever identified anywhere on Earth, and, therefore, the area seems to represent a unique preserved example of the later stages of continental rifting.
A general interpretation of the rift shows that it formed in 2.6-b.y.-old Archean rocks, which were originally about 25 miles thick but then were greatly thinned to current thicknesses of no more than 6 miles beneath the center of the rift (fig. 5). During attenuation, the enormous thickness of basalt and sedimentary rocks was deposited into the developing rift basin. The contact between the crust and underlying mantle, called the Mohorovicic discontinuity (commonly referred to as “Moho"), is also well shown in this profile (fig. 4) as an essentially horizontal contact across the entire rift.
This profile and others in Lake Superior and northern Lake Michigan have provided remarkable new data on the structure of the rift. The ongoing interpretation of the data by a large number of participants in GLIMPCE is revolutionizing our
"packages” of reflective rock units were detected to depths as great as 18 miles.
understanding both of the geologic history of the Lake Superior region and of continental rifts in general.
Aeromagnetic Mapping in Lake Huron
Figure 4. A portion of seismic reflection section F (see fig. 1 for location) that crosses the Mid-Continent Rift. The heavy black lines represent locations of rock interfaces that strongly reflect sound waves. The data are recorded as travel times (left-hand scale) that indicate the elapsed time, in seconds, from an air-gun blast to the recorded echo. These times can be approximately converted to kilometers (right-hand scale) by assuming an average sonic velocity of 6 kilometers per second. The section reveals an enormous graben (downdropped fault-bounded block) beneath the lake, far larger than previously predicted. The strongest reflections are caused by basalt flows and interlayered sedimentary rocks within the graben. Weaker but still prominent reflections are caused by sedimentary rocks. The prerift Archean rocks are largely transparent, producing strong reflections only locally. The crustmantle boundary (Moho) is clearly visible as a band of nearly horizontal reflections about 25 miles (40 km) deep.
The geology of Precambrian (more than 570 million years old) rocks beneath Lake Huron is critical to understanding the evolution of the Precambrian basement in North America because three major geologic provinces intersect beneath the lake. The Precambrian rocks are covered not only by the lake waters but also by a southwestward-thickening sheet of nearly flat-lying younger Paleozoic strata that increase in thickness from zero near the north shore to more than 9,000 feet in the southwest. Fortunately for our purposes, these rocks have almost no magnetic properties and are therefore "transparent" to the aeromagnetic technique. The strong magnetic character of rocks in Lake Huron (fig. 3), therefore, shows the nature of the underlying Precambrian rocks, although the general loss of detail toward the southwest is caused by increasing thickness of Paleozoic rocks and the consequent greater vertical distance between magnetic rocks
and surveying instruments. The new aeromagnetic map (fig. 3) clearly shows the boundaries of the three major provinces, as well as a variety of features within each province.
The Grenville Front, a sinuous northsouth feature, separates the 1.1-b.y.-old Grenville Province on the east from the Central Plains Province and Hudsonian Province on the west. The Central Plains Province underlies a large area of the midwestern United States but is nearly everywhere covered by younger strata. This province is generally poorly understood, because it has been studied only from drill cores and by aeromagnetic and other geophysical techniques. In general, it consists of a variety of metamorphic and igneous rocks roughly 1.7 b.y. old. Beneath Lake Huron, the Central Plains Province seems to be further complicated by the intrusion of large masses of granitic rocks that appear as circular to elliptical anomalies on the aeromagnetic map (fig. 3). At the two northernmost anomalies, where Paleozoic strata are thin to absent, the bedrock has been known for many years to consist of granitic and related rocks about 1.5 b.y. old. Similar-appearing anomalies farther south are therefore interpreted to be similar bodies of 1.5-b.y.-old granite.
Along the northern shore of the lake, sedimentary rocks of the Huronian Supergroup, more than 2.2 b.y. old, are exposed in a fold belt known as the Hudsonian Province. These rocks are generally weakly magnetic but, nevertheless, produce a distinct magnetic signature that can be traced southward beneath the lake. The contact between the Hudsonian and Central Plains Provinces can be located by the distinct change in magnetic signature between the two provinces.
Thus, a first step has been made at constructing a geologic map of Precambrian rocks beneath Lake Huron. Additional geologic and geophysical data gathered during planned future surveys will add additional detail and constraints to the interpretations that are now based solely on the aeromagnetic data.
Figure 5. Generalized sequence of events that formed the Mid-Continent Rift as it appears on line F. A, About 1.1 billion years ago, Archean rocks of the Superior Province, about 2.6 billion years old, began to pull apart laterally. Steep faults developed, and the area between the faults subsided. Eruption of large basalt flows began so that flows generally filled the graben as it subsided. B, The graben subsided uniformly for about 9 miles and was filled by basalt flows and interflow sedimentary rocks. C, The graben floor broke up as further subsidence continued so that older flows were folded and tilted. Volcanism ceased, but subsidence of as much as 6 miles continued as a thick, postvolcanic sedimentary section filled the graben.
Further GLIMPCE Activities
Sedimentary rocks with some interlayered
Basalt flows and interflow sedimentary
Archean crystalline rocks
GLIMPCE has made remarkable scientific progress in its first full year of operations, largely because of outstanding cooperation and coordination of efforts among many government and academic institutions in both the United States and Canada. Much work remains for the future, however, in order to complete interpretations of existing data, as well as to perform new surveys and mapping. At a planning workshop with 75 participants, long-range research goals were identified, and plans and objectives are being developed. Future surveys could include additional seismic
and aeromagnetic investigations and other types of geophysical surveys. Geological and geophysical studies are also needed on land to correlate exposed rocks near the shore with those inferred beneath the lakes in order to achieve a comprehensive understanding of the geologic evolution of the Great Lakes region.