Abstract

The thrusting mapped on the surface in northeastern Nevada in recent years has now been proven to be segments of a regional thrust network. Wildcat wells exploring for oil and gas have encountered multiple thrust complexes in the subsurface, which now indicates horizontal thrust sheets are present under northeastern Nevada. This information in northeastern Nevada suggests that these same horizontal thrusts are present in south central Idaho and northwestern Utah.

In more regional studies, it is now indicated that most of the Basin and Range Province is superimposed on a horizontal stacked shortened sedimentary complex which has suffered extension in late Cenozoic time. This late Tertiary Cenozoic extensional tectonicsm allowed many previous thrust surfaces to be used again in the back-to-the-west extension of the near surface stacked plates. Many of the normal faults mapped on the surface are now proving to be normal-listric faults with movement along the older east vergent upcutting listric thrust faults.

Most of the mountain ranges and valleys are now believed to have had eastward directed movement, emplacing older sediments over younger sediments. It is believed that a major regional detachment (Decollement) is present at approximately minus 40,000 ft or deeper. Horizontal movement along this deep detachment displaced all sediments down to and including the pre-Cambrian clastic sediments in some areas.

The COCORP line number 1, shot in west central Utah, has indicated major horizontal displacement of Paleozoic and pre-Cambrian sediments over Mesozoic sediments. The published COCORP preliminary depth section indicates horizontal displacement over Mesozoic sediments.