Geoscience Reference
In-Depth Information
10.4.4 The Keweenawan Rift system
The Keweenawan or Mid-continent Rift system is a 100-km-wide, 2000-km-
long, extinct (
1100 Ma old) rift system extending from Kansas to Michigan
in the U.S.A. (see Fig. 10.39). Beneath Lake Superior, the rift bends by 120
◦
;
it has been suggested that this is the location of an ancient triple junction. The
rift is delineated by the high gravity and magnetic anomalies associated with the
thick sequence of basaltic lavas it contains. Seismic-refraction data from the rift
∼
Figure 10.61.
(a) An unmigrated seismic-reflection record section from Lake
Superior, crossing the Keweenawan Rift, shown to approximately true scale for
6kms
−1
material. Ba denotes reflection from pre-rift basement; M, reflection from
crust-mantle boundary. (b) A line drawing of the migrated version of the reflection
record section shown in (a). Vertical lines indicate the crust-mantle boundary. (From
Behrendt
et al
.(1988).)
