The movement of the earth’s crust causes faults to be pushed onto their backs.
When the Earth’s tectonic plates experience tension, it can result in the formation of a specific type of fault. These faults, known as normal faults, allow for stretching in the Earth’s crust and are typically angled at approximately 60° from horizontal, as predicted by the established theory of mechanics. However, there have been observations of normal faults with much shallower angles worldwide. A recent study suggests that these unusual geological features could be caused by a flowing lower crust.
Per Anderson’s theory of faulting, which is a commonly accepted mechanical explanation for how the Earth’s crust fractures under pressure, a fault is expected to form at a 30° angle from the direction of the greatest compression. In cases where the crust is being stretched, this direction is typically vertical. As a result, normal faults are anticipated to occur at approximately a 60° angle from the horizontal.
However, in numerous locations, researchers have observed normal faults with shallower inclinations. One such instance can be found in the Menderes Massif in western Turkey. According to Oğuz Göğüş, a geophysicist at Istanbul Technical University and co-author of the study, the Menderes is among the most actively growing areas on the planet. The normal faults in this region are characterized by a 20° angle.
Geologists have long believed that a low-angle normal fault, also known as a detachment fault, can be created in two possible ways: by deviating from Anderson’s theory and fracturing at a shallow angle, or by initially forming at a 60° angle from the horizontal and then rotating. Recently, advancements in modeling methods have provided a greater understanding of the behavior of the Earth’s crust on a larger scale.
In the 1990s, previous research in western Anatolia suggested that there was rotation occurring due to the tilting of sedimentary layers above the area’s faults. However, the specific cause of this rotation has continued to be a topic of inquiry.
Steep to Shallow
This suggests that the lower crust must be able to move.
The Menderes fault has experienced significant stretching, resulting in the exposure of twin domes made of metamorphic rocks from the middle to lower layers of the Earth’s crust. This is one of the largest examples of such metamorphic core complexes in existence. The region has been expanded by at least 100%, causing heat to rise and the crust to become weaker due to the intense stretching and thinning.
According to John Singleton, a structural geologist at Colorado State University who was not involved in this study, the implication is that the lower crust must undergo flow. This flow occurs from areas that have not been stretched as much to areas that have undergone more stretching. The lower crust, although solid, is able to bend due to high temperatures and pressure below approximately 15 kilometers. As the crust is thinned through stretching, the weight on top is reduced and the lower crust is able to move upwards. Singleton explains that this buoyant and hot lower crust will rise similarly to a lava lamp.
The scientists utilized geological information from western Anatolia to replicate the motion of rocks within a section of crust with a vulnerable lower portion. According to Göğüş, their simulations indicate that as the crust is stretched, the lower portion may move both horizontally and vertically, while the upper portion develops faults or areas of high strain that align with the underlying flow.
According to numerical simulations, a moving lower layer of the Earth’s crust may have caused faults to rotate in the Menderes Massif region. The locations of the Moho, as determined by the recent study conducted by Bodur et al. and previous studies by Fichtner et al., Karabulut et al., and Zhu et al., are depicted by dashed lines. Image credit goes to Ömer Bodur.
Aligning the model with the Menderes
According to Göğüş, research suggests that in the Menderes region, the geological features and seismic imaging of the Earth’s crust indicate that faults were originally created at high angles and then underwent rotation. The study’s models provide an explanation for this phenomenon.
The scientists found that the previous data on the age and temperature patterns of rocks in the Menderes region support the findings of their model. The model’s forecasts also matched the information gathered from seismic tomography, a method that utilizes earthquake waves to create images of the Earth’s internal structure, similar to a computerized tomography (CT) scan of a human body.
One of the most intriguing aspects of this numerical simulation is its ability to be compared to the exposed rocks.
According to Göğüş, previous research has tackled this issue from various perspectives. However, there has been a lack of consistency between the models and observations. This new study is the first to successfully model rotational faults, utilizing evidence from geology, geophysics, and age.
Numerical simulations have value as they can connect known observations and research to processes that may still be uncertain. According to Robert Stern, a geologist at the University of Texas at Dallas who was not part of the study, one exciting aspect of these simulations is that they can be compared to real-life rock formations. This serves as a reality check that may not be possible when modeling other phenomena such as exoplanets. Stern also mentioned that this is a valuable demonstration of how well-designed numerical experiments can provide explanations for geological phenomena.
This study aids geologists in understanding the formation of the Menderes Massif and other metamorphic core complexes. The results and methods used in the study can offer fresh perspectives on the geological processes of the western United States, the Himalayas, and the entire Mediterranean region, according to Göğüş. This can contribute to the ongoing discussion about the connections between low-angle faults and metamorphic core complexes.
“I am a science writer named Rebecca Owen (@beccapox).”
Reference: Owen, R. (2023). Crustal movement causes faults to tilt, Eos, 104, https://doi.org/10.1029/2023EO230368. Published on 27 September 2023.
