A gravity constraint on the origin of highly extended terranes

Gravity provides a simple but fundamental constraint on the interpretation of highly extended terranes (HETs), including metamorphic core complexes. The Bouguer anomalies associated with HETs in the western Cordillera show no prominent, characteristic gravity signature. Instead, the largest Bouguer anomalies are positive by a few tens of milligals and are best explained by the contrast between bedrock and sedimentary fill or by shallow lithologic variations within the bedrock. In contrast to this lack of an associated anomaly, the mass removed by tectonic and erosional denudation of as much as 10 km of the upper crust in core complexes is gravitationally equivalent to about 1100 mGal. With complete isostatic compensation the predicted Bouguer anomaly would be small, but more importantly, the land surface would have subsided isostatically due to the inflow of dense mantle material replacing the lighter, denuded upper crust. Terranes that have been extended by 100% or more should evolve into deep troughs, perhaps more akin to the Red Sea or Gulf of California than to the mountainous core complexes. The most likely explanation for this observation is that compensation has taken place, not by inflow of dense mantle material, but by emplacement of material of crustal density, such as gabbro, derived from the mantle. In addition, this explanation would predict that products of crustal melting and mixing with basaltic magma would tend to be emplaced at intermediate to high levels in the crust. This general interpretation is supported by the lack of significant relief on the seismic reflection Moho beneath HETs. We therefore conclude that the lower plate of core complexes has been inflated or underplated by intrusions.