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Seismic refraction data were collected along a 320‐km‐long “Transect” line in southern Alaska, crossing the Prince William, Chugach, Peninsular, and Wrangellia terranes, and along several shorter lines within individual terranes. Velocity structure in the upper crust (less than 9‐km depth) differs among the four terranes. In contrast, layers in the middle crust (9‐ to 25‐km depth) in some cases extend across projected terrane boundaries. The following observations can be made: (1) An intermediate‐velocity layer (6.4 km/s) at 9‐km depth extends across the deep projection of the suture between the Chugach and Peninsular terranes, suggesting that the northern Chugach and southern Peninsular terranes are detached and rest on a deeper terrane of unknown origin. (2) The top of a gently north dipping sequence of low‐ and high‐velocity layers (5.7–7.8 km/s), more than 10 km thick, extends from near the surface in the southern Chugach terrane to more than 20‐km depth beneath the southern Peninsular terrane. This sequence, truncated by the suture between the Prince William and Chugach terranes, is interpreted to be an underplated “terrane” made up of fragments of the Kula plate and its sedimentary overburden that were accreted during subduction in the late Mesozoic and/or early Tertiary, during or between times of accretion of the Prince William and Chugach terranes. (3) A thick crustal “root”, with a laminated sequence at its top, extends from a depth of 19 km to as much as 57 km beneath the northern Peninsular and Wrangellia terranes. This root extends across the deep projection of the suture between the Peninsular and Wrangellia terranes, although resolution of this apparent crosscutting relationship is relatively poor. This root may represent tectonically or, possibly, magmatically emplaced rocks. The lower crust beneath the Prince William, Chugach, and southern Peninsular terranes includes a north dipping, 3‐ to 8‐km‐thick section of subducting oceanic crust.

Results from recent studies of the Kane and Oceanographer fracture zones suggest that significant variations in crustal thickness are associated with parts of these fracture zones, with total crustal thicknesses in places less than half the thickness of normal oceanic crust. The extent of this anomalous crust and its origin are still very poorly constrained by available geophysical data. Here we report results from a 75 km long refraction line shot in late 1977 over three Hawaii Institute of Geophysics ocean bottom seismometers deployed within the Vema transform valley near its western intersection with the Mid‐Atlantic Ridge (10°53′N, 43°39′W). We show that the apparent crustal thickness beneath this portion of the Vema transform valley is about 5 km, not substantially different from crustal thicknesses typically associated with oceanic crust. However, the Vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the Kane and Oceanographer fracture zones. It is characterized by low compressional wave velocities, a relatively uniform velocity gradient throughout most of the crust, and the absence of a typical layer 3 refractor. This anomalous crust is interpreted as intensely fractured, hydrothermally altered and serpentinized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls. The large variation in Moho depths found in these large‐offset, slowly slipping transforms is attributed to differences in the depth and extent of serpentinization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer. One important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor.

Two recent developments in southwestern archaeology are brought together in this paper. First, theoreticians have begun to argue that the archaeological record should be viewed as the product of selection-driven evolution. Second, tree-ring research has produced a highly detailed history of climate for a large area of the northern Southwest. We view the record of climatic oscillations and extreme events as a record of the strength of selection favoring stabilization of specialized agricultural strategies in the arid northern Southwest. Published data from Black Mesa provide a cultural record of sufficient precision to permit comparison with the climatic record, while new data from Vermillion Cliffs, southern Utah, document one local end-product of an evolutionary sequence shaped to an important degree by the long-term variability of climate. Anasazi occupation of many regions failed to persist through the “Great Drought” of the 1270s. From a local perspective, this extreme climatic event caused adaptations shaped by selection prior to the 1270s to fail; from a broader temporal-spatial perspective, however, the drought must be seen as part of the selective regime that shaped subsequent human adaptation to the northern Southwest.

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