Gravity Studies

Integration of gravity models with the seismic and geochemical studies will provide additional constraints on the composition and structure of the crust and upper mantle. For instance, seismic studies will have difficulty distinguishing eclogite and dunite due to their similar seismic velocity and Vp/Vs ratios. However, these two rock types can be distinguished based on their density. Therefore, long wavelength gravity anomalies may provide the crucial piece of information needed to distinguish lithologies that have similar seismic properties but distinct densities.

We plan to augment the existing Canadian gravity database, which is comprised of marine surveys and stations with a ca. 12 km interval on land, with data that will be collected on the Ewing during the active seismic data acquisition, and also by taking new gravity readings along the in-land portions of the main transects during the seismic deployment. These new readings will be merged with the existing Canadian database and then modeled. Software and expertise for reducing gravity readings with standard methods is available at the participating institutions.

Gravity models, constrained by the seismic results will be used as an integrative tool to construct lithospheric scale cross-sections. Initially, 2.5-D gravity modeling will be performed along the Douglas Channel transect by UTEP (Miller/Andronicos) and the Burke/Dean transect by UBC (Clowes). Various software packages are available for such work (e.g. Cady, 1980; Webring, 1985); both groups have extensive experience with such modeling.

Once well-constrained 2.5-D density models have been developed, they will be integrated with the geology and the 2-D and 3-D seismic velocity models to create 3-D density models, the calculated responses from which will be compared with the observed gravity. The objective is to determine 3-D density variations, within the crust and especially within the mantle that should distinguish between a mantle root that has foundered and one that may be present over the entire study area. This research will be undertaken by UBC (Clowes), who has experience in carrying out 3-D forward and inverse gravity modeling (e.g. Roy and Clowes, 2000; Roy and Clowes, in prep.) Clowes’ contribution in this part of the proposal is firm whether the Canadian NSERC proposal is funded or not, as existing sources of funding can be used. The analytical procedure followed is based on the developments described by Li and Oldenburg (1998). In generating the 3-D model, Clowes will collaborate closely with Hole and his research results based on the 3-D seismic traveltime studies. We anticipate significant 3-D variations in the gravity field between the two transects that will need to modeled with care based on the existing gravity data (Figure 18).

Throughout the 2.5-D and 3-D gravity studies, the crustal portions of the models will be constrained by the active-source results and surface geology. The calculated values from this modeling effort represent the crustal gravity signature. The active source upper mantle velocity models and the teleseismic velocity models will then be used to constrain possible density variations due to temperature in the mantle. Given possible error estimates for the velocity models and range of possibilities for densities based on the seismic data, a range of gravity models will have to be explored. However, should the gravity calculated from models based on the range of seismic constraints still produce a significant misfit of the calculated gravity anomaly to the long wavelength portion of the observed anomaly, then we will be able to confidently attribute the misfit to the presence of anomalously dense ultramafic residue beneath the CPC. Thus gravity studies represent an essential component of our proposal to determine the presence or absence of an ultramafic root.