Sam A Johnstone, Ph.D.

I study how erosion and tectonics shape Earth’s surface. I use laboratory techniques that record the cooling of minerals carried toward Earth’s surface by the competing action of tectonic uplift and erosion. I pair these measurements with comparisons between models and measurements of Earth’s topography. I aim to better understand fault histories and how erosion modifies Earth’s surface.

Biography

Background:

I study how erosion and tectonics shape Earth’s surface. During my Ph.D., I studied how processes of erosion shape landscapes mantled by soil, focusing on the dynamics introduced by the breakdown of rocks beneath this soil mantle.  For my M.Sc., I focused on laboratory techniques that record the cooling of minerals carried toward Earth’s surface by the competing action of tectonic uplift and erosion.   At the USGS I am integrating these fields to infer fault histories.

Extension in the crust presents both hazard and opportunity. Active faulting in zones of extension places nearby communities at risk. However, over geologic time, these same faults provide fluid pathways that control the distribution of natural resources, generate regions of high heat flow that may be exploited for geothermal energy, and control the formation of sedimentary basins that provide reservoirs for petroleum and water resources. Understanding how these fault systems evolve has long been a concern of geologists.

Post-doctoral resesarch:

In investigations of fault system evolution over millions of years, low-temperature thermochronology has proven to be an invaluable tool. Thermochronology measures the cooling histories of minerals. With this technique we can track the exhumation of rock, i.e., its path toward the surface resulting from tectonics and/or erosion, as recorded by the thermal history of mineral grains.

As part of my work at the USGS I am developing a facility to measure the thermal histories recorded by common accessory minerals (see 'Multimedia' below).  Trace amounts of Uranium (U), Thorium (Th), and other radioactive elements are present in a variety of minerals. Alpha particles, a heavy isotope of Helium (He), are produced as part of the decay of these elements.  However, these alpha particles are able to leak out of the crystals hosting the radioactive parent elements that produced them at a rate that depends on temperature. Therefore, within a mineral the concentration of He relative to its parent nuclides depends on the history of temperatures that mineral has been exposed to. We can then model different cooling histories to determine those cooling histories that best characterize the observations from individual minerals. While thermochronology has most commonly been applied to studies of long-term mountain belt evolution, other studies have shown that it can also be used for constraining fault slip, mineralization, and even brief but extreme temperature perturbations caused by events like wildfires.

At the scale of extensional sedimentary basins and their bounding ranges, we have long recognized the relationship between topography and the distribution of normal faults. More recently, techniques have been developed to investigate the longitudinal elevation profiles of rivers with increasing detail to infer information about present and past rates of fault slip. These techniques are based on the idea that rivers will erode at a rate that depends on the slope of the river and the discharge through that river. Over time, vertical tectonic movement will steepen the river until it can incise at a rate that counteracts this tectonic movement. Therefore the slope of a river relative to its discharge should encode some information about erosion rate, and by inference tectonic rates. Depending on the river in question, river profiles may record information about tectonic uplift rate over the last few million years.

During my post-doctoral work at the USGS I will be integrating observations from low-temperature thermochronology and models of the development of landscape morphology (see 'Multimedia' below') to develop a more complete picture of the history of fault slip in the Sangre De Cristo Mountains. The Sangre de Cristo Mountains and the normal faults bounding their eastern margins are among the principal structures defining the Rio Grande Rift and the southern Rocky Mountains.  This region and surrounding areas host major porphyry molybdenum deposits as well as a large fraction of the principal rare earth element districts identified within the U.S.

Other active research:

In addition to this work, I am developing tools that utilize topographic data to aid in primary geologic interpretations.  Thanks to technological developments we are developing increasingly accurate maps of Earth’s topography. These maps allow us to more accurately image the features that define Earth’s surface, including those that present hazards to individuals  - such as faults and landslides.  Beyond aiding in our identification of these features, we can extract quantitative information about features from topography. For example, using new topographic data we can rapidly collect information about the size and relative smoothness of fault scarps from existing mapping, which may be related to the magnitude and timing of the earthquakes that formed them. Similarly, the construction of alluvial fans may produce an initially rough surface that smoothens over time after sedimentation on that fan has stopped.  It is possible then to calibrate the decay in roughness that results from that smoothing to estimate an age of a fan surface from topographic data alone (see 'Multimedia' below).