Geodynamic processes in North America

The interior part of the North American plate is subject to two important geophysical processes: glacial isostatic adjustment (GIA), and the intra-plate tectonic activities. GIA is the most major geophysical process and most of the Canadian landmass is currently experiencing vertical uplift associated with the GIA (except in Maritimes, south of Saskatchewan and Alberta). Plate tectonic is another geophysical phenomenon across the country which is more observed in four areas, including the western Canada subduction zone, the Queen Charlotte transform fault zone, the Yukon crustal deformation region, and eastern Canada region of high seismicity. Henton et al. (2006) have highlighted long term regions of interest for geodetic investigations across Canada including the Saint Lawrence seismic zone in eastern Canada, the active plate boundary above the Cascadia subduction zone along Canada’s west coast, and the active fault margin of the Queen Charlotte Islands as well as GIA-related deformation all over the Canadian landmass.

It is estimated that the last glacial maximum (LGM) has occurred about 20 kYBP (Richmond & Fullerton 1986). During the glaciation period, many areas in North America and Scandinavia peninsula were covered by ice sheets of up to four kilometres depth extended over thousands of kilometres. As a consequence of the ice weight, the lithosphere was depressed, and the resulting viscoelastic flow in the mantle caused a peripheral bulge (Mitrovica et al. 2001). The ice sheets began to melt and getting thinner since 10 kYBP. Therefore, the lithosphere began to rebound upwards to the regional isostatic equilibrium, the peripheral bulge began to migrate inward to the centre of uplift as it was gradually dissipated. This phenomenon is called glacial isostatic adjustment (GIA).

Figure 1. Observed vertical rates from the CBN (black dots) show a spatially coherent pattern of uplift consistent with the expected GIA signal (Henton et al. 2006)

GIA has clear evidences in the form of a three dimensional movements on the Earth crust at the vicinity of the LGM accompanied by change in gravity measurements in consequence of crustal uplift and mantle flow (Wahr et al. 1995). GIA is continuous at present, and the regions of highest uplift rates are generally consistent with areas of the thickest ice accumulation during the last period of continental glaciation (e.g., Peltier 1994; Dyke 2004). The uplift rates reaching 10 mm yr^-1 or more around Hudson Bay and decreasing further away (Figure 1). During the Wisconsin ice age (late Pleistocene), ice advanced over the Saint Lawrence valley and extended east into the Maritime Provinces and south into New England (e.g., Dyke 2004). Some GIA models indicate that the hinge line between uplift to the north and subsidence to the south is near to the Saint Lawrence valley (Tushingham & Peltier 1991; Peltier 2002). Horizontal velocities associated with GIA are also spatially coherent (typically directed radially outward from regions of highest uplift), but have smaller rates than vertical velocities (Henton et al. 2006). These first-order features of GIA-related crustal deformation are confirmed by various geodetic measurements in central and eastern North America (Lambert et al. 2001; Park et al. 2002; Sella et al. 2004).

Although more than 97% of the world’s earthquakes are caused by the continental movement of tectonic plates, the causes of earthquakes in eastern Canada are not well understood. Unlike plate boundary regions where the rate and size of seismic events is directly correlated with plates interaction, eastern Canada is located in a stable continental region within the stable North American plate, and therefore, has a relatively low rate of earthquake activity. Despite this fact, about 450 earthquakes occur in eastern Canada annually (NRCan 2011). On average, three events greater than magnitude 5 occur every decade.

It is hypothesized that the seismic activity in intra-plate zones is related to the regional stress fields, with the earthquakes concentrated in regions of crustal weakness (NRCan 2011). Horizontal strain rates can be directly related to the frequency of large earthquakes. Vertical motions may provide additional insight into the regional seismic process (Henton et al. 2006). Thus the occurrence of large earthquakes in active seismic regions of eastern Canada (e.g., Lower Saint Lawrence valley, Charlevoix, and Ottawa valley) can be better characterized through long term precise geodetic monitoring (Figure 2).

Figure 2. Historical and instrumental earthquakes in or near Canada between 1627-2010. Western coast of Canada apparently shows more seismic activity, where the oceanic Pacific plate is sliding to the north-west relative to North America (source: Earthquakes Canada website)

References

Dyke, A. S. (2004). An outline of North American deglaciation with emphasis on central and northern Canada. Developments in Quaternary Science, 2, 373-424. Elsevier.

Henton, J. A., Raymer, M. R., Ferland, R., Dragert, H., Mazzotti, S., & Forbes, D. L. (2006). Crustal motion and deformation monitoring of the Canadian landmass. Geomatica, 60(2), 173-191. Canadian Institute of Geomatics.

Lambert, A., Courtier, N., Sasagawa, G., Klopping, F., Winester, D., James?, T. S., & Liard, J. O. (2001). New constraints on Laurentide postglacial rebound from absolute gravity measurements. Geophysical Research Letters, 28(10), 2109. American Geophysical Union. doi:10.1029/2000GL012611

Mitrovica, J. X., Milne?, G. A., & Davis, J. L. (2001). Glacial isostatic adjustment on a rotating earth. Geophysical Journal International, 147(3), 562-578. Wiley Online Library. doi:10.1046/j.1365-246x.2001.01550.x

NRCan. (2011). Earthquake zones in Eastern Canada. Natural Resources Canada. Retrieved September 5, 2011, from http://earthquakescanada.nrcan.gc.ca/zones/eastcan-eng.php

Park, K. D., Nerem, R. S., Davis, J. L., Schenewerk, M. S., Milne?, G. A., & Mitrovica, J. X. (2002). Investigation of glacial isostatic adjustment in the northeast US using GPS measurements. Geophysical research letters, 29(11), 1509. American Geophysical Union. doi:10.1029/2001GL013782

Peltier, W. R. (1994). Ice age paleotopography. Science, 265(5169), 195. American Association for the Advancement of Science.

Peltier, W. R. (2002). Global glacial isostatic adjustment: palaeogeodetic and space-geodetic tests of the ICE-4G (VM2) model. Journal of Quaternary Science, 17(5-6), 491-510. Wiley Online Library. doi:10.1002/jqs.713

Richmond, G. M., & Fullerton, D. S. (1986). Summation of quaternary glaciations in the United States of America. Quaternary Science Reviews, 5, 183-196. doi:10.1016/0277-3791(86)90184-8

Sella, G. F., Stein, S., Wdowinski, S., Dixon, T. H., Craymer, M. R., & James?, T. S. (2004). Direct constraints on GIA motion in North America using GPS. AGU Spring Meeting Abstracts (Vol. 1, p. 3).

Tushingham, A. M., & Peltier, W. R. (1991). Ice-3G: a new global model of Late Pleistocene deglaciation based upon geophysical predictions of post-glacial relative sea level change. Journal of Geophysical Research, 96(B3), 4497-4523. American Geophysical Union.

Wahr, J., DaZhong, H., & Trupin, A. (1995). Predictions of vertical uplift caused by changing polar ice volumes on a viscoelastic Earth. Geophysical Research Letters, 22(8), 977-980. American Geophysical Union.