SEISMIC ANALYSES IN THE VICINITY OF THE CHEYENNE BELT

Recent receiver function analyses of portable seismic arrays in the Dinosaur National Park area image a thick crustal welt just south of the Cheyenne Belt - a suture recording the accretion of Proterozoic island arcs to an Archean craton. Crustal thicknesses away from the suture are found to agree with previous studies, but in an ~50km wide zone just south of the suture, crustal thickness increases by approximately 10 km.

This thick crust appears to be typical of the Cheyenne Belt in other locations. Gravity shows a similar signature across the Cheyenne Belt, and the CD-Rocky Mountain array shows a similar feature to the east (Zurek and Dueker, this meeting).

Long lived Moho displacements such as this may be more common than typically realized. Examples include the Redbank thrust of central Australia, the Main Uralian fault zone, and the suture between the Svecofennia and Skellefte arcs in Sweden and Finland. Further examination of the Cheyenne Belt may therefore lead to a more globally useful model of continental evolution.

If this welt is of normal crustal composition, a large gravity anomaly would be expected; such a gravity anomaly does not exist. This implies some form of buoyancy compensation. Also, if this welt is associated with the suture, then this welt has persisted for approximately 1.8 Ga. Such persistence suggests an abnormally strong lithosphere.

Our preliminary model involves localized compensation of the welt by an imbricated mantle wedge or eclogotized oceanic crust. This type of model may explain both the gravity signature and temporal longevity of such a structure.