Thrust faults shove older rock on top of younger rock, breaking geology's rule
Normally the oldest rock layers lie deepest. Thrust faults flip that order, pushing ancient strata up and over younger ones, sometimes for kilometres. Geologists in the Alps, Scotland, Scandinavia and Canada each worked this out in the 1880s. Some thrusts never reach the surface and stay hidden until they rupture, as Los Angeles learned in 1994.
A thrust is a reverse fault that dips at 45 degrees or less. When the fault plane is very shallow, often under 15 degrees, and the upper block has travelled a long way, geologists call it an overthrust. Erosion can then cut a hole through the upper sheet, exposing the rocks beneath in a small patch called a window, or strip most of it away, leaving isolated remnants perched on the lower block, known as klippen.
Hidden versions are the dangerous ones. A blind thrust ends before reaching the surface, leaving little trace, and the destructive Northridge earthquake of 1994 came from one nobody had mapped. Even exposed thrusts are hard to spot, because their low angle produces subtle offsets and repeated layers that are easy to miss on flat, worn-down land.
Thrusts tend to travel along weak layers such as mudstone or salt, called decollements, then climb steeply through stronger rock at ramps angled about 15 to 30 degrees before flattening again. Motion over a ramp bends the overlying rock into a fault-bend fold. Where two weak layers sandwich a strong one, repeated ramp formation slices the strong unit into a stack of fault-bounded pieces called horses, producing a lozenge-shaped duplex. Duplexing is an efficient way to shorten the crust by thickening it rather than crumpling it.
Large overthrusts form where crust is squeezed hard, in belts built by colliding continents or by material piling up at subduction zones; the Himalayas, Alps and Appalachians are riddled with them. At ocean trenches, sediment scraped off a sinking plate must thicken by up to 200 percent, achieved by stacking thrust upon thrust in chaotic mixed rock. Key early studies included the Glarus Thrust in the Alps and the Moine Thrust in the Scottish Highlands, and in 1884 Archibald Geikie coined the term thrust-plane for faults that seem to push rock almost horizontally forward.
Source: Thrust fault
