A STRUCTURAL HISTORY BY THE MOUNT STUART BATHOLITH, NORTH CASCADES, WASHINGTON

The North Cascades Crystalline Core have been afflicted by many compressional and tensional tectonic events since the Late Cretaceous Period. The small-scale faults recorded within the Cretaceous Mount Stuart Batholith (MSB) near Leavenworth, Washington act as a key to these events. In the summer of 2018, over 900 fault-slip datums were recorded along road cuts within the MSB. The fault-slip data was collected using a mobile Brunton smartphone application. Compiling the collected data in a concise table allowed for graphical kinematic analysis in Stereonet and FaultKin software programs (Almendinger, 2019) following the methods of Marrett and Almendinger (1990).

The data was subdivided into regions and fault types. By subdividing this data, several trends and patterns became evident. First, the data revealed a pattern of transtension, transpression and compressional tectonics, notably in the Tumwater Canyon region. The data in this region correlated strongly with the Leavenworth Fault, a series of N-S dextral faults and local reverse faults. The other regions of the MSB were not as influenced by local large-scale faulting in the surrounding geologic units; however, they were consistently influenced by normal faulting or extensional (transtensional) tectonic activity. The cross cutting relationships of these faults was not collected. However, fault surface analyses indicated that the faults were predominantly formed under the brittle regime with very small amounts of displacement. On several occasions, the observed fault zones were wider than a centimeter indicating larger displacements, and on rare occasions greater than a meter. The MSB is a more cohesive body than the surrounding metamorphosed and sedimentary rock units, and appears to have acted as a stress dampener of the large faults in the surrounding formations. Further research regarding the timing of the small-scale faults in the MSB may help to correlate activity with the regional tectonics of the North Cascades Crystalline Core.