Abstract

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.

Magnetic stripes of the sea floor imply that plate drifting has been steady, as might be inferred from the immense inertia of a plate as large as North America and at least as thick as 100 to 150 km. The association of most current seismic and magmatic activity with plate boundaries suggests a genetic relation between plate motion, mountain uplift, orogeny, and magmatism. Yet, in any one locality, the phenomena of orogeny, mountain uplift, and magmatism are sporadic and episodic—not uniform, as plate motion seems to have been.

This seeming inconsistency is examined in this paper. First, although deformation (orogeny), mountain uplift, and magmatism are commonly associated, they need not be. Any one of them may take place without the others, and dating one does not necessarily date the others. Extrapolating the age of an orogeny from the age of a pluton is especially hazardous, even locally. Second, in any local area each of the processes is recorded episodically and does not proceed monotonically as might at first glance be expected if steady plate motion is the driving mechanism.

Because of this local independence of magmatic behavior from that expected, I have examined the timing of magmatism in the whole Cordillera from Alaska to Baja California for the interval since the close of the Paleozoic, utilizing 1,224 dates from more than 80 references. The dates were restricted to those of rocks more siliceous than diorite or latite, for such rocks can hardly have been derived directly from the mantle. So siliceous a lava therefore implies a parental pluton in the underlying crust.

This record shows that, except for two epochs in the Triassic, there has not been a 5-m.y. interval without siliceous magmatism somewhere along the Cordillera since the end of the Paleozoic. As the data bear no relation to magma volume but merely record the number of rocks dated, the graph is biased by the inclusion of many dates from the much studied Oligocene ash flows of the southwestern United States, even though an effort was made to avoid including more than one date for any individual body. The graph suggests that the apparently increasing activity from the Triassic to a peak in the Oligocene (while drift was proceeding) is merely a result of geologic perspective, older rocks being neither as well preserved nor as well exposed as younger. In short, though magmatism has been episodic at any one locality, taking the Cordillera as a whole, it has been proceeding steadily along with the westward drift of the continent.

The local episodicity must be related to local properties of crust and varying properties of the underlying mantle as the crust drifts over it. Both crust and mantle are known to be heterogeneous. Varying mantle densities far below the Low Velocity Zone are reflected in a relief of more than 120 m in the geoid. The response of various crustal segments drifting over so heterogeneous a mantle would thus be expected to vary from place to place and from time to time.

Inasmuch as several students have suggested a relation between drift and epeirogeny, the graph of magmatic dates has been examined for inflections that may correlate with times of expanding or retreating seas, as reported by Gill and Cobban (1966, 1973). No correlation is apparent, either with major or minor migrations of the shore lines. Of course this does not disprove the suggested relation because the graph does not represent volumes of magma, but it: certainly offers no support to it.

Epeirogenic movement and the present high stand of this and other circum-Pacific continents may result from the buoyancy of the vast quantities of sial carried down Benioff zones. Some of this has been converted to magma and forms the huge Mesozoic and Tertiary batholiths around the Pacific rim; some has been returned to the crust as metamorphic rocks such as the Franciscan of California, and much more has probably been added to the lower crust by metamorphic flowage, there to be redistributed as the crust moved over an uneven Low Velocity Zone and a lower mantle of varying density and thus varying gravitational pull. Thickening of the crust by these processes of adding sial derived from the subducted oceanic crustal material has increased the freeboard of all continents overlying Benioff zones active since the present drift pattern was established in the early Mesozoic. It is not a result of world-wide orogeny, either Laramide or Alpine, but of the buoyancy of the sialic fraction stripped from the descending plates.

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