GEOPHYSICAL EVIDENCE FOR MAGMATIC MODIFICATION OF THE CRUST IN THE ROCKY MOUNTAIN REGION

A variety of recent geophysical studies document that magmatic modification of the crust in the Rocky Mountain region has been widespread and locally extensive. The initial accretionary events that formed the crust were accompanied by intrusion (~1.6 Ga). The crust then experienced intrusion and burial by huge volumes of granitic and rhyolitic rocks 1.4 - 1.3 Ga. Regional extension and mafic magmatism occurred at ~1.1 Ga, contemporaneous with the Grenville orogeny that completed the formation of the supercontinent in this region. Coincident, short wave-length gravity and magnetic anomalies delineate mafic intrusions in the upper crust of a variety of ages with 1.1 Ga probably being the main period of intrusion. The break-up of Rodinia that followed soon thereafter involved considerable magmatism with the massive upper crustal mafic complex in the Southern Oklahoma aulacogen being the best example. Felsic intrusions are harder to delineate, because the negative gravity anomalies they cause can be indistinguishable from those caused by sedimentary basins. This effect is well documented in the Taos-Mora area where a ~1.6 Ga intrusion and Phanerozoic sediments combine to produce a prominent gravity low. The Mid-Tertiary batholithic intrusions associated with the Datil-Mogollon and San Juan volcanic fields produce intense gravity lows that are evidence of large scale modification of the upper crust. The Valles caldera certainly represents significant magmatic modification of the crust, but not at the scale of these features. From a deeper crustal point of view, the origin of the underplated (?), high velocity lower crust is a major question. The massive amount of 1.4 Ga magmatism in the upper crust suggests a causal relationship, but this hypothesis needs much more documentation and testing.