Magnetic and gravity anomalies, with typical amplitudes of 1,000 γ and 50 mgal, respectively, occur over the Great Valley of California. Aeromagnetic data were combined to make a composite aeromagnetic map of most of the Great Valley and adjoining areas. Good correlation between local details of the gravity and magnetic anomalies suggests that dense, magnetic rock is the source of the anomalies. A nearly continuous magnetic high and weak gravity highs occur where serpentinite is exposed along the Coast Range thrust fault. Magnetic highs, but no associated gravity highs, occur where serpentinite crops out in the western Sierra Nevada metamorphic belt. The metavolcanic rocks are relatively non-magnetic. Major magnetic highs with strong associated gravity highs are caused by gabbro at three places in the western Sierra Nevada. Two of these gabbro occurrences are probably part of ophiolite complexes. Gravity, magnetic, and drill-hole data were used to construct a map of basement rock types. Gabbro and similar mafic rocks are abundant beneath the crest of the Great Valley anomalies. Ultramafic rocks are very rare in drill holes that reach basement. A major break in the anomaly patterns suggests a possible east-trending fault in the basement rocks near Fresno. A two-dimensional crustal model was made across central California through the use of seismic refraction data and gravity and magnetic modeling. Magnetic rock comes to within 2.5 km of the surface just east of the center of the Great Valley and dips steeply to the west beneath the western side of the valley. If gabbro is the source of the anomalies, then sialic crust must be virtually nonexistent beneath the Great Valley. Analogy with the ophiolite complexes of the Sierra Nevada foothills suggests that the source of the Great Valley anomalies is a tectonically emplaced fragment of oceanic crust. In Late Jurassic time, an eastward-dipping subduction zone in the western Sierra Nevada foothills became detached and stepped westward to the present position of the Coast Range thrust fault, leaving behind a fragment of oceanic crust. I propose that this fragment, covered by Tithonian and younger strata, causes the Great Valley magnetic and gravity anomalies.

Eclogitic rocks occur in a restricted area, some 13 mi north of Fairbanks, as conformable bands and lenses intercalated with amphibolite, impure marble, and pelitic schist. The structural style of the eclogite-bearing terrane is characterized by northwest-trending, isoclinal recumbent folds that have been deformed by open or overturned folding along northeast-trending axes. Crystalline schist masses south of the eclogite-bearing terrane show only the northeast-trending folds and contain mineral assemblages of the greenschist facies. Mineral assemblages from the crystalline schists, which are intimately associated with the eclogitic rocks, are of the lower amphibolite facies. No basic igneous analogs were found for the eclogitic rocks, and their bulk compositions are very different from those reported for other eclogites. On the ACF plot, the calcite-rich variants appear to have been derived from marls, and the calcite-free varieties are compositionally similar to subgraywackes rather than mafic igneous rocks. The eclogitic rocks and associated amphibolites are characterized by the following essential mineral assemblages: (a) garnet-clinopyroxene (± calcite, quartz, sphene); (b) garnet-clinopyroxene-amphibole (± calcite, quartz, sphene); and (c) garnet-amphibole (± calcite, quartz, plagioclase, epidote, rutile). Mica and plagioclase feldspar also occur in some variants. The garnets in these eclogites are compositionally similar to those from “group C” eclogites as defined by Coleman and others (1965), and many plot within the field defined by the above authors for garnets from eclogites within blueschist terrane. The eclogitic clinopyroxenes are true omphacites, averaging 27 percent of the jadeite component. When shown on White’s (1964) diagram, these clinopyroxenes also plot in a field defined by other clinopyroxenes from group C eclogites. The group C affinity of these eclogitic rocks is also reinforced by the garnet-pyroxene tie-line relations in the MgO-CaO-FeO system, and even more conclusively by K D data on garnet-clinopyroxene pairs. Based on experimental data from various authors, and thermodynamic considerations, these eclogitic mineral assemblages were probably crystallized at temperatures of 540° to 590°C at 5.5 to 7.5 kb. K 40 /Ar 40 mica and hornblende dates indicate that earlier deformation of the eclogite-bearing terrane occurred in Paleozoic time, while the later event was of Cretaceous age.

Work on the Precambrian rocks of the Beartooth Mountains has been mainly in terrain dominated by gneiss. We undertook the current project in the belief that an intensive study of an area where the rocks perhaps were not as thoroughly reconstituted by metamorphic and igneous activity (as is the case farther east) and where metasedimentary rocks are exposed in substantial amounts might reveal a somewhat more detailed picture of Precambrian events than has emerged from earlier studies in the region. The North Snowy block constitutes such an area. The northwestern part of the Beartooth Mountains is divisible into structural blocks separated by major faults, and the rocks therein are treated as seven formations composing the North Snowy Group. The formations, from oldest to youngest, are the Barney Creek Amphibolite, George Lake Marble, Jewel Quartzite, Davis Creek Schist, Mount Delano Gneiss, Mount Cowen Gneiss, and Falls Creek Gneiss. All are newly named and described herein. The Barney Creek Amphibolite, George Lake Marble, Jewel Quartzite, and Davis Creek Schist are older than the Mount Delano Gneiss. Pb 207 /Pb 206 ratios from zircons indicate that the age of the Mount Delano Gneiss is about 3,000 m.y., the Mount Cowen Gneiss is about 2,565 m.y., and K-Ar dates indicate that the age of the Falls Creek Gneiss is about 2,200 m.y. The dominating structural feature in the North Snowy block is a fragmented nappe that extends for 22.5 km before disappearing beneath the Paleozoic sedimentary cover. Its axial surface strikes northeast and dips northwest, bisecting an amphibolite core with flanks made up of mirror-image repetitions of metasedimentary and gneiss units; throughout the nappe structure the main foliation parallels the axial surface. Five sets of deformation structures were recognized in the field and designated D 1 through D 5 . Folds in D 1 , the earliest structure for which evidence was recognized, trend northwesterly. These large to small passive-flow folds, characterized by isoclinal form, possess an axial-plane schistosity that is the major structural element deformed in younger events. D 2 folds trend north, deform the earlier schistosity, and approach isoclinal form; their axial planes are parallel to the gross compositional layering. Deformation structures in D 3 were divided into several sets (D 3a to D 3f ). The rocks were essentially flat lying (that is, recumbent folds and nearly horizontal gross layering) during episodes D 1 through D 3 . D 4 folds have northeast trends and steep axial planes; they are of concentric character and overprint D 3 and older structures. D 5 folds are of diverse trend and are decidedly subordinate. They overprint, kink, and shear all earlier structures. There is a general progression from more mafic to more silicic gneiss through an 800-m.y. time span: Mount Delano to Mount Cowen to Falls Creek. Zircon morphology, relict igneous texture, rotated metamorphic xenoliths, and relict intrusive contacts support an origin for the gneisses of the North Snowy block by intrusion and consolidation followed by continuing metamorphism parallel to older lines (that is, synkinematic). The only rock attributable to granitization is the sheared gneiss unit, which apparently was formed through feldspathization of schist by late magmatic fluids emanating from tonalitic melt. A study of zircon morphology follows. (1) The detrital zircons of the schists retained their rounded sedimentary form following metamorphism in the amphibolite or perhaps hornblende granulite facies. Two colors of detrital zircon, one rimming the other in part, suggest derivation from a complex terrain in which at least two “events” had occurred. (2) In paragneiss formed by feldspathization of schist (granitization), the zircon entirely retained its sedimentary character. No overgrowths or outgrowths were formed. Relict, rounded overgrowths exist only on detrital zircons and had formed in an earlier cycle. (3) The zircon in the orthogneisses retained its igneous character. Minor quantities of detrital zircon assimilated in the igneous gneisses retained their sedimentary character—no outgrowths, overgrowths, or partial refaceting have been observed. Some outgrowths occur on igneous zircons; they grew during the respective igneous crystallization phases. (4) The igneous character of the zircon suites in the several gneisses supports field and fabric data suggesting intrusive igneous origin. Migmatites appear to be subordinate, as are gneisses produced by feldspathization of schists. Similarity of zircon morphology in gneisses throughout the Beartooth Mountains, as well as in the Little Belt Mountains and perhaps in all areas of basement rocks of southwestern Montana, generally indicate a predominantly igneous mode of origin, although metasomatic processes have operated in varying degrees. Concepts regarding structural evolution of the Beartooth Mountains have changed. Our view, based on the multiplicity of data enumerated in the text, indicates that there were at least four bona fide orogenies widely separated in time, each differing in its grade of metamorphism and effect on the geometry of structures. The first three were accompanied by widespread granitic intrusion. The four orogenies are, from oldest to youngest, the Pine Creek (circa 3,000 m.y.), the Beartooth (circa 2,600 m.y.), and two unnamed orogenies (circa 2,200 m.y. and 1,700 m.y.). The latter events were followed consecutively by locally intense mylonitization (circa 1,176 m.y.), locally strong development of concentric upright folds (D 4 or post-1,176 m.y.), and major wrench movements on west-northwest- and north-northeast-trending faults. The basement blocks of the northwestern part of the Beartooth Mountains have had a long history of development. Most of them were outlined by shear zones at least as early as 1,700 m.y. ago; the Pine Creek Lake-Mount Cowen block was split by the Marten Peak reverse fault during D 4 (post-1,176 m.y.); major wrench movements accentuated the westerly and northerly borders early in D 5 (prediabase) and to some extent were further accentuated after these young diabases were intruded (late D 5 ). We conclude that these predominantly wrench movements were all Precambrian. Fractures of the area are grouped into four sets. Sets 1 (northwest) and 4 (northeast) are interpreted as extensional ac and be fractures, respectively, having formed in D 5 but in part expressing stress systems involving northwest-southeast shortening of D 3 to D 4 time. Sets 2 and 3 formed also in D 5 but represent conjugate shears; tension associated with this system was apparently largely taken up along sets 1 and 2. Most of the major faults that had their origins in Precambrian time have been reactivated during Late Cretaceous and early Cenozoic time (Laramide orogeny). These later movements were apparently largely dip slip, the minimum stratigraphic displacements ranging from perhaps 900 to more than 1,800 m. Dip-slip movement apparently began as early as Late Cretaceous and seems to have ceased on most of the faults prior to volcanism during the middle Eocene. The Deep Creek fault, however, has continued to be active up to the Holocene as shown by scarplets in glacial material and alluvial fans.

Previous structural interpretations of the Sonoma Range in north-central Nevada have concluded that the Tobin thrust fault—regarded as the equivalent of the Golconda thrust fault—is younger than other thrust faults of post-Triassic age in the range. However, thrust emplacement of the distinctive oceanic upper Paleozoic rocks of the Golconda allochthon over a large region in western and north-central Nevada, and perhaps even beyond, seems to have taken place prior to deposition of Triassic strata in the region. Hence, the structural relationships in the Sonoma Range that bear on the age of the Golconda thrust fault have been questioned. Restudy of the critical part of the Sonoma Range in the vicinity of Clear Creek shows that the oldest faults in the area that bound rocks of the Golconda allochthon and therefore may represent the Golconda thrust fault are, in fact, segments of a single fault that has been displaced by several successive slices of the Clear Creek thrust fault, the north end of which cuts Triassic rocks exposed in the northwestern Sonoma Range. Furthermore, the geometry of rocks displaced since Triassic time on the Clear Creek system of thrust faults suggests that the faults regarded as parts of the Golconda thrust fault in the Sonoma Range are offset segments of the type Golconda thrust fault as exposed about 15 km to the northeast. Consequently, the Golconda thrust fault in its type locality, as well as in the Sonoma Range, is evidently older than faults that cut Triassic rocks, and its age relationships do not conflict with the generally accepted Late Permian or Early Triassic time of emplacement of the Golconda allochthon. Integrated into this structural reinterpretation of the Sonoma Range are several other conclusions and findings of more than local significance, including the following: (1) Prior to emplacement of the Golconda allochthon, lower Paleozoic rocks in the Sonoma Range area, such as the Harmony and Valmy Formations and perhaps the Preble Formation, were intricately deformed and faulted together, presumably during the middle Paleozoic Antler orogeny. (2) Coarse clastic detritus derived from the Harmony and Valmy Formations occurs in the Golconda allochthon of the Sonoma Range, which suggests that it was originally deposited along the North American continental margin. (3) Radiometric ages of plutonic rocks in the Sonoma Range suggest that post-Triassic displacement, perhaps as gravity slides, of parts of the Golconda allochthon on the Clear Creek system of thrust faults took place between about 170 and 100 m.y. ago.