#listric

Piecing together an outcrop

This is a wall of sedimentary rocks exposed in Utah that has had a geologist take a pen to it. The digitally sketched lines mark specific beds that you can track across the outcrop as a method of reconstructing the motion on the faults.

Take a look at the rocks above and below the red line. Everywhere you see it, the red line marks the top of a gray/white colored sandstone unit. The purple line just above that marks a coal bed that sits atop a sequence of more fine-grained mudstones.

That sequence of rocks repeats at different elevations throughout this outcrop with faults in-between. Some of the layers have been tilted by the faults, some have not. The faults found here dip both ways, a commonly seen feature of faults produced by the fact that rocks often break at different angles.

These rocks have been extended, pulled apart by tectonic forces that created normal faults at this spot. See the fault that curves towards flat at the bottom of the outcrop? That’s a common shape for normal faults, they bend horizontal with depth in a geometry we call “Listric faulting”.

The state of Utah has been pervasively pulled apart over the past 30 million years as part of the Basin and Range extension (http://tmblr.co/Zyv2Js1JkP4qs ).

-JBB

Basins and Ranges

Here’s a cool fact: 20 million years ago, Nevada was half as wide as it is now. Literally.

In a post earlier this week we showed a very small normal fault of the Basin and Range province in the western U.S. This satellite photo shows the Basin and Range at its true extreme. The state of Nevada, and several others, is crossed by a series of ranges that trend almost north-south, with valleys/basins in-between

The continent being pulled apart created this topography. As the continent is pulled apart, normal faults have grown, with the rocks dropping down to fill in the space required by extension. The ground dropped down by these faults has filled in with sediments shed from the higher-standing basins, creating mountain ranges, fairly flat basins filled with sediment, and sharp, sometimes spectacular boundaries between them.

-JBB

adj. relating to or characteristic of a list of the life of a major group: listering listrics.