Structural Geology
My structural geology work utilizes fault array analysis to statistically define and compare fault populations between different areas and/or time periods. My work primarily focuses on normal fault populations, but can be applied to various deformational structures. When combined with other geologic data (GPS, geochemical, geochronological, stratigraphic), statistical analysis of fault data can provide more robust evidence pertaining to an area’s deformational history.
Guatemala
Statistical fault analysis
When you measure fault orientations and offsets at an outcrop, you are most likely measuring an apparent offset. These measurements need to be transformed to true offsets to estimate strain. Additionally, you can’t forget about the effect of small, unobservable faults, which can accommodate up to 60% of strain (Walsh et a.,, 1991).
I created an R script (1Delongation) that combines multiple published analyses that will transform fault orientations and offsets and calculate the orientation of maximum elongation (for normal faulting) and the estimated strain that incorporates measured fault offsets and the estimated effect from small, unobservable faults. This is based on calculations from Titus et al., 2007; Xu et al., 2007; Xu et al., 2009; and Gross and Engelder, 2005. A more through explanation is provided in Garnier et al., 2019.
Guatemalan tectonics
Fault and GPS data indicates that Guatemala is shaped by the interactions of three tectonic plates: the North America, Caribbean, and the Central American Forearc (CAFA) plates. We conclude in Garnier et al., 2020 (in review) that these three plates meet in central Guatemala in the Guatemala City graben (GCG) region based on the following findings:
Active faulting (green arrows in figure) is observed within the Guatemala City graben region and in the extending Caribbean plate to the east, but not to the west. Only past faulting (red arrows) was observed in western Guatemala.
GPS data also indicates that E-W extension (yellow arrow) within the Caribbean plate is not observed beyond 40-50 km west of the Guatemala City graben. Additionally, a majority of the extension is accommodated across the Guatemala City graben.
Major faults terminate into the northern and southern ends of the Guatemala City graben, respectively the sinistral Motagua and dextral Jalpatagua faults. It makes sense that these faults would terminate into an extensional structure.
“Alicate” model for North America, Caribbean, and Central American Forearc (NAFCA) tectonics
Based on fault and GPS data, we suggest a new tectonic model for the NAFCA system where strain was once more distributed over a larger area (top panel in figure) and over time has localized into the Guatemala City graben (middle and bottom panels). We call this model the Alicate model, which is spanish for pliers. In the Alicate model, the Caribbean plate moves eastward in-between the upper and lower jaws of the North America/CAFA pliers. The extensional strain localization into the Guatemala City graben progressively transferred western Guatemala to the CAFA and/or North America plate.