Abstract
Understanding deep crustal structure can provide us with insights into
tectonic processes and how they affect the geological record. The deep
crustal structure can be studied using a variety of seismological
techniques such as receiver function analysis, and surface and body wave
tomography. Using models of crustal structure derived from these
methods, it is possible to delineate tectonic boundaries and regions
that have been affected by similar processes. However, often velocity
models are grouped in a somewhat subjective manner, potentially meaning
that some geological insight may be missed. Cluster analysis, based on
unsupervised machine learning, can be used to more objectively group
together similar velocity profiles and, thus, put additional constraints
on the deep crustal structure. In this study, we apply hierarchical
agglomerative clustering to the shear wave velocity profiles obtained by
Gilligan et. al. (2016) from the joint inversion of receiver functions
and surface wave dispersion data at 59 sites surrounding Hudson Bay.
This location provides an ideal natural laboratory to study Precambrian
tectonic processes, including the 1.8Ga Trans-Hudson Orogen. We use Ward
linkage to define the distance between clusters, as it gives the most
physically realistic results, and after testing the number of clusters
from 2 to 10, we find there are 5 main stable clusters of velocity
models. We then compare our results with different inversion parameters,
clustering schemes (K-means and GMM), as well as results obtained for
profiles from receiver functions in different azimuths and found that,
overall, the clustering results are consistent. The clusters that form
correlate well with the surface geology, crustal thickness, regional
tectonics, and previous geophysical studies concentrated on specific
regions. The profiles in the Archean domains (Rae, Hearne, and Superior)
were clearly distinguished from the profiles in the Proterozoic domains
(Southern Baffin Island and Ungava Peninsula). Further, the crust of
Melville Peninsula is found to be in the same cluster as the crust of
the western coast of Ungava Peninsula, suggesting a similar crustal
structure. Our study shows the promising use of unsupervised machine
learning in interpreting deep crustal structures to gain new geological
insights.