Four maps are presented here that show the location and extent of magmatic fields between eastern Alaska and northern Mexico during the successive time intervals of 55–40, 40–25, 25–10, and 10–0 Ma, and four others show the distribution of metamorphic core complexes during the same Cenozoic time intervals. The maps are based on U.S. Geological Survey and Canadian Cordilleran data bases contining about 6000 isotopic dates and extensive literature review. For nearly 60 Ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate‐composition magmatic fields. The association is suggestive of a close link between magmatism and core complex formation, namely that magma directly and indirectly lowers the strength of the crust. Magmatism thus controls the location and timing of core complex formation. The stresses responsible may be inherited from Mesozoic crustal thickening, locally created by uplift and magmatic thickening of the crust, and imposed by the global pattern of plate motions and driving forces. Since the Miocene, rates of magmatism, extension, and core complex formation have declined. The modern Basin and Range province is not a suitable model for the situation that existed during major magmatic culminations. The singular event of early Miocene time, the merging of two large magmatic fields, extinguishing the Laramide magmatic gap, explains several disconnected observations: the hyperextension episode of the Colorado River corridor, rapid reorientation of stress patterns across much of western North America, and subsequent rapid tectonic movements in California. Magma‐triggered breakup of western North America lithosphere coincided with development of the San Andreas transform system. Thermal destruction of the Laramide magmatic gap created a California “microplate” about 22 Ma ago that moved rapidly away from North America. Thus two plate tectonic processes, thermal destruction of the lithosphere “bridge” and northward growth of a transform system, interacted to produce Miocene and later tectonic patterns and events.

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During the summer of 1967, three high-frequency, high-gain, and highly portable seismographs were operated at seventy-eight sites throughout Iceland. Over 990/0 of the more than 1000 events recorded were found to lie in nine regions with radii of less than about 5 km. Although most of the events were not greater than 4 km deep, six were of the order of 5 to 15 km deep, and one may have been as much as 40 km deep. One large earthquake swarm was recorded from Myrdalsjiikull in south-central Iceland, where four events less than magnitude 5 were reported by the U. S. Coast and Geodetic Survey (U.S.C.G.S.) in early 1967. No earthquake greater than magnitude 4.5 has been reported since 1958 from the other regions of high microseismicity, suggesting that these microearthquakes were not simply aftershocks. Three events of magnitude 4 to 5 did occur, however, in each of two seismic regions after the initial recording period. Thus, some of the microearthquakes may have been foreshocks. A close correspondence was found between areas of major hydrothermal activity and high microearthquake activity. The highest activity recorded was in the Krafla volcanic region in northeastern Iceland, which has not been active since 1746. This activity had a b value of 0.83 ± 0.16 over 1% units of magnitude. The focal mechanisms were consistently similar and gave a solution with one nearly vertical nodal plane striking north-south. Eight of the nine zones of microseismirity lie on an east-west line near 64·N. When considered in relation to adjoining active seismic zones, reported historic seismicity of Iceland, and the location of the mid-Atlantic ridge and areas of active rifting and volcanism, the existence of a transform fauIt is suggested, following the methods used by Sykes (1967) to outline such faults on the sea floor with larger earthquakes. Magnetic data and some geologic features support this hypothesis. Seismic refraction data are not in disagreement with it. Many of the tectonic features of Iceland do not, however, readily fit into this framework. If sea-floor spreading is active in Iceland, it is more complicated in detail than previously suggested.

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