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M.C. Blake

Publications and source records attributed to M.C. Blake.

At least 37 records · Page 2Linked to original sources
USGS1986

Blueschist metamorphism of the Eastern Franciscan belt, northern California

Rocks of the Eastern Franciscan belt, northern California, are divided into two tectonostratigraphic terranes metamorphosed to the blueschist facies, both with a distinct lithologic association and deformational history. The easternmost terrane, the Pickett Peak terrane of Early Cretaceous isotopic age, consists of crenulated mica schist and gneissic to schistose metagraywacke, with lesser alkalic mafic metaigneous rocks and scarce metachert. The Pickett Peak terrane retains evidence of three periods of penetrative deformation, the first of which is characterized by segregation layering, and the second and third by crenulation cleavages. Blueschist-facies conditions persisted during the first two deformations. The Yolla Body terrane of Late Jurassic and Early Cretaceous paleontologic age lies structurally below and to the west of the Pickett Peak terrane. It is characterized by voluminous metagraywacke and lesser argillite, coherent interbedded radiolarian chert, and alkalic gabbroic dikes and sills. The Yolla Bolly terrane retains evidence for two phases of penetrative deformation that were coaxial with the second and third phases of deformation in the Pickett Peak terrane. The first phase of deformation (parallel to the second phase in the Pickett Peak terrane) was also accompanied by blueschist-facies metamorphism.

California↗
USGS1986

Southern hemisphere origin of the Cretaceous Laytonville Limestone of California

New paleomagnetic, paleontologic, and stratigraphic data from outcrops of the Laytonville Limestone (101 to 88 million years old) support a Southern Hemisphere orgin. A paleomagnetic megaconglomerate test is statistically significant and suggests magnetization at 14?? ?? 5?? south, predating Late Cretaceous to Eocene (70 to 50 million years ago) accretion. Rapid Kula plate movement or the existence and demise of a now vanished oceanic plate (or both) are required to accommodate the greater than 50?? of poleward displacement implied by the paleomagnetic data. This rapid motion brings into question the validity of a "speed limit" for absolute plate velocity based on present-day plate motions.

Science↗
USGS1985

Franciscan complex calera limestones: Accreted remnants of farallon plate oceanic plateaus

The Calera Limestone, part of the Franciscan Complex of northern California, may have formed in a palaeoenvironment similar to Hess and Shatsky Rises of the present north-west Pacific1. We report here new palaeomagnetic results, palaeontological data and recent plate-motion models that reinforce this assertion. The Calera Limestone may have formed on Farallon Plate plateaus, north of the Pacific-Farallon spreading centre as a counterpart to Hess or Shatsky Rises. In one model2, the plateaus were formed by hotspots close to the Farallon_Pacific ridge axis. On accretion to North America, plateau dissection in the late Cretaceous to Eocene (50-70 Myr) could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the Franciscan Complex. Partial subduction of the plateaus could have contributed to Laramide (70-40 Myr) compressional events3. ?? 1985 Nature Publishing Group.

Nature↗
USGS1978

Age measurements of potassium-bearing sulfide minerals by the 40Ar/39Ar technique

K-Ar ages have been determined for sulfide minerals for the first time. The occurrence of adequate amounts of potassium-bearing sulfides with ideal compositions K 3 Fe 10 S 14 (∼10 wt.% K) and KFe 2 S 3 (∼16 wt.% K) in samples from a mafic alkalic diatreme at Coyote Peak, California, prompted an attempt to date these materials. K 3 Fe 10 S 14 , a massive mineral with conchoidal fracture, gives an age of 29.4 ± 0.5m.y.( 40 Ar/ 39 Ar) , indistinguishable from the 28.3 ± 0.4m.y.( 40 Ar/ 39 Ar) and 30.2 ± 1.0m.y. 8 (conventional K-Ar) ages obtained for associated phlogopite (8.7 wt.% K). KFe 2 S 3 , a bladed, fibrous sulfide, gives a younger age, 26.5 ± 0.5m.y.( 40 Ar/ 39 Ar) , presumably owing to Ar loss.

Earth and Planetary Science Letters↗
USGS1978

Distribution and character of upper Mesozoic subduction complexes along the west coast of North America

Structurally complex sequences of sedimentary, volcanic, and intrusive igneous rocks characterize a nearly continuous narrow band along the Pacific coast of North America from Baja California, Mexico to southern Alaska. They occur in two modes: (1) as complexly folded but coherent sequences of graywacke and argillite that locally exhibit blueschist-grade metamorphism, and (2) as melanges containing large blocks of graywacke, chert, volcanic and plutonic rocks, high-grade schist, and limestone in a highly sheared pelitic, cherty, or sandstone matrix. Fossils from the coherent graywacke sequences range in age from late Jurassic to Eocene; fossils from limestone blocks in the melanges range in age from mid-Paleozoic to middle Cretaceous. Fossils from the matrix surrounding the blocks, however, are of Jurassic, Cretaceous, and rarely, Tertiary age, indicating that fossils from the blocks cannot be used to date the time of formation of the melanges. Both the deformation of the graywacke, with accompanying blueschist metamorphism, as well as the formation of the melanges, are believed to be the result of late Mesozoic and early Tertiary subduction. The origin of the melanges, particularly the emplacement of exotic tectonic blocks, is not understood.

Alaska, California, Oregon, Washington↗
USGS1975

Rutile and sphene in blueschist and related high-pressure facies rocks

Sphene and rutile are characteristic accessory minerals of blueschist facies metamorphic rocks. However, only sphene is present in the lowerr grade blueschist assemblages. In many areas of the world these blueschists pass into, or are in fault contact with, higher grade glaucophane schists, rutile-bearing amphibolites, and eclogites. The origin of the rutile may be related to a prograde metamorphic reaction that involves the alteration of sphene to rutile, but few data are available to support this hypothesis. It seems more probable that the rutile crystallized directly from ilmenite or titanium-rich clinopyroxene and magnetite in basalts and gabbros during the conversion of these rocks to amphibolites and eclogites containing omphacite, amphibolite, and garnet. Rutile in the higher grade blueschists, regardless of its origin, is a potential titanium resource and is currently being mined from eclogite and glaucophane-eclogite rocks in the Soviet Union. Resources are likely to be up to millions of tons contained Ti0 2 .

Open-File Report↗
USGS1975

The Oligocene volcanic center at Eureka, Nevada

A volcanic center covering an area of about 80 km 2 near Eureka, Nev., and active in the early Oligocene, is characterized by rhyolitic, rhyodacitic. and andesitic pyroclastic rocks, lava flows, and shallow intrusive bodies. These rocks were emplaced as intertonguing and interpenetrative units during a 5-m.y. interval; most of the volcanism was in the last 3 million years of this period (36 to 33 m.y. ago).

Nevada↗