Moho topography and lower crustal density in Southern Ontario from linearized gravity inversion
Abstract
We attempt to map both crustal thickness variations and density variations in the lower crust of Southern Ontario. Since both cannot be determined independently we constrain our dataset, consisting of complete Bouguer gravity values from the GSC and PACES, by estimates of crustal thickness from semblance-weighted receiver function analysis and from seismic refraction studies. Our linearized gravity inversion parametrizes the Moho as discrete rectangular blocks sitting at a reference depth (mean of the seismic estimates). We assume that the mass of one block is concentrated at its base and thus obtain a linear relationship between the height - or the density contrast - of the block and the vertical component of gravity measured at the surface. In a first step we leave the density contrast constant and only allow the thickness to vary. Our results show the following tentative correlations with surface structural units: thin crust (~30 km) beneath the mid-continent rift is flanked by thick crust (~48 km) on either side, the Ottawa-Bonnechere graben has thick (~45 km) crust, and so has the Composite Arc Terrane of the Grenville Orogen. Some of these results contradict seismic estimates of crustal thickness. In a second step we grid all available seismic thickness estimates, calculate the resulting gravity field, subtract the latter from Bouguer gravity data, and invert the difference for density variations. The results show a clear distinction between dense lower crust of the Central Gneiss Belt and lighter lower crust of the Composite Arc Terrane in the NW and SE of the Grenville Orogen, respectively. The Abitibi belt of the Superior craton also seems to have lower density lower crust. Residual gravity maps display mainly short-wavelength anomalies which we believe to be caused by shallow anomalies. Crustal thickness variations are not reflected in the generally flat surface topography of the area. Our results may show that Moho topography of orogenic belts can be sustained long after tectonic activity has ceased because of an increase in the density of the lower crust.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2005
- Bibcode:
- 2005AGUSM.T43D..04B
- Keywords:
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- 0905 Continental structures (8109;
- 8110);
- 0920 Gravity methods;
- 7205 Continental crust (1242);
- 8122 Dynamics;
- gravity and tectonics;
- 9350 North America