Eastern Peninsula Research Articles

Deformation of granitic rocks across the brittle-ductile transition

A microstructural analysis has been carried out on mylonites and mylonitic gneisses of the Eastern Peninsular Ranges Mylonite Zone, which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies. Composite planar fabrics in the form of C and S planes are found at all metamorphic grades. Fractured feldspars, kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites. At mid-upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery. Biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns. At epidote-amphibolite grade and above, recovery processes and annealing recrystallization predominate in all minerals. Residual orthoclase porphyroclasts show strain-related myrmekite formation along those sides of the grains that face the instantaneous shortening direction. Myrmekite formation due to replacement reactions cannot explain this geometry. It is proposed that the myrmekites formed due to a combination of exsolution, replacement and strain-enhanced diffusion.

Borrego Springs–Santa Rosa mylonite zone: A Late Cretaceous west-directed thrust in southern California

The Borrego Springs shear-zone section of the Eastern Peninsular Ranges Mylonite Zone (EPRMZ) contains unequivocal evidence for ductile shearing with an east-over-west sense of movement. Field evidence includes the presence of S and C planes and shear bands. All lines of microstructural evidence are internally consistent and support the field observations. It appears that the data are compatible only with post–mid-Cretaceous west-directed thrusting on the EPRMZ.

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone, Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.

Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California

Research Article| April 01, 1973 Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California RICHARD LEE ARMSTRONG; RICHARD LEE ARMSTRONG 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 Search for other works by this author on: GSW Google Scholar JOHN SUPPE JOHN SUPPE 2Department of Geology, University of California, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD LEE ARMSTRONG 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 JOHN SUPPE 2Department of Geology, University of California, Los Angeles, California 90024 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (4): 1375–1392. https://doi.org/10.1130/0016-7606(1973)84<1375:PGOMIR>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RICHARD LEE ARMSTRONG, JOHN SUPPE; Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California. GSA Bulletin 1973;; 84 (4): 1375–1392. doi: https://doi.org/10.1130/0016-7606(1973)84<1375:PGOMIR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract K-Ar dates for 123 mineral separates from 91 samples of granitic rocks, and 3 samples of Orocopia and Vitrefax Schist from Nevada, western Utah, the Mojave Desert, the Colorado Desert, and the northern and eastern Peninsular Ranges of southern California, range in age from 218 to 19 m.y.; however, most of the dates lie between 160 and 50 m.y. A majority of the hornblende and muscovite dates are nearly concordant with, but generally a few m.y. older than, coexisting biotite dates; some are highly discordant.The oldest plutons gave Late Triassic to Early Jurassic dates (El Paso and San Bernardino Mountains, 218 to 194 m.y.). Jurassic igneous activity reached a culmination slightly before 150 m.y. ago over a large area in northern Nevada and western Utah and supplied ash to the Morrison Formation of the Colorado Plateau. Several plutons of the northeastern Mojave Desert yielded similar dates.A similar major culmination of Mesozoic pluton emplacement in early and middle Late Cretaceous time is represented by scattered plutons in Nevada and probably by most of the plutons in the Peninsular Ranges and southern deserts of California. Earliest Tertiary dates were obtained from far-southeastern California (50 to 66 m.y.).Most dates in the Cordilleran batholith belt of the Southwest, which parallels the edge of the late Mesozoic continent, are between 160 and 75 m.y. and are essentially coeval with metamorphism in the parallel high-pressure, low-temperature Franciscan belt to the west (approximately 150 to 75 m.y.). About 75 m.y. ago, this pattern abruptly changed to a Laramide one with abundant 75 to 50 m.y. dates in a belt running through southern Arizona into southeastern California at a high angle to the edge of the continent. Only very sparse dates of this age are found in Nevada. Abundant Laramide dates are not encountered again until the Idaho, Boulder, and Coast Range batholiths. These important changes in Cordilleran tectonics may be effects of a change from dominantly dip-slip subduction during Franciscan time to dominantly north-northeast strike-slip subduction during Laramide time. Furthermore, the initiation of rifting of eastern North America in Triassic time, changes in spreading rate, and reorientation of spreading directions in the Atlantic Ocean appear to correlate with the beginning of widespread pluton emplacement in western North America, changes in the rate of magma generation in the Cordillera, and shifts of the Cordilleran tectonic pattern, respectively, during Mesozoic time. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Effect of Soil Factors on the Distribution of Rhizobium japonicum Serogroups

AbstractThe soybean variety ‘Kent’ was planted at 19 sites representing two soil types located on Maryland's Eastern Peninsula. Routine soil tests including soil organic N were performed and correlated with serological distribution of R. japonicum. Nodule samples reacted with 16 Beltsville antisera with no serogroup occupying more than 20% of the total. Beltsville serogroup 110 was positively correlated with several organic N fractions, soil NH4+, and soil NO3‐ for the Sassafras soil. Serogroup 123 showed a positive correlation with soil pH and exchangeable Mg. When the Matapeake soil was considered, serogroup 94 was positively correlated with soil N status much like 110 was for the Sassafras soil.

Origin, distribution and phylogenetic affinity of the species of Mangifera L.1

SUMMARY The problem of the origin of the genus Mangifera has been discussed in the present paper, with evidence from a thorough analysis of the phytogeographical data and phylogenetic taxonomy of the component species, and also from the climatology (past and present) and geological history of the areas concerned. The phylogenetic taxonomy is based on an exhaustive analysis of the morphological data, with corroborative evidence from cytological, anatomical and pollen morphology studies on the species of the genus, accumulated in the Asiatic herbaria. The phylogenetic taxonomy indicates that the two groups of species under Sections I and II have differentiated early either from a common ancestor or from some of the present species (e.g. M. duperreana or M. cochinchinensis). The species under Section I have undergone further differentiation into mainly three groups: (1) the pentamerous, (2) the tetramerous and (3) the intermediate having both penta- and tetramerous flowers. It appears likely that the second and third groups have evolved from the first group through reduction of floral parts. The stamens have undergone gradual reduction, and series of stages from the presence of five fully developed stamens to those with one developed stamen and complete absence of staminodes are found in the flowers of the various species. The genus had its origin somewhere in the Eastern Peninsula (comprising Burma-Siam-Indo-China and Malaya), during the Eocene or an earlier period in the Cretaceous. The species have then spread to India and Ceylon in the west, and to Eastern Malaysia and the Philippines through the Sunda Archipelago. The two main centres of species differentiation are the Assam-Burma-Siam-Indo-Chinese region and the Malay Peninsula. Other smaller centres are the Sunda Islands and the Philippines. The most interesting feature of distribution is the paucity of the species (only seven species out of forty-one) in Eastern Malaysia. Out of the seven species, four belong to Section I and three to Section II. Only one species of Section I is spread over both Eastern and Western Malaysia, but all the three species of Section II are present in both the regions. It, therefore, suggests that the species of Section II had a more primitive origin. They had consequently more time to spread from Western to Eastern Malaysia and to adjust themselves to the Equatorial Monsoon Region of Malaysia. The paucity of the species in Eastern Malaysia is attributable to the unstable archipelagic condition of the region and to the absence of any direct land connexion with the Asiatic mainland since the Cretaceous, when the Angiosperms originated. The actual process of species-formation is difficult to postulate, but from a cytogenetic analysis of three species of Section I, it is indicated that they had an allopolyploid origin. The numerous varieties of M. indica and the two allied species have further differentiated by gene mutations and hybridization. The mode of origin of the species of Section II could not be established as fresh materials for an examination of these species could not be secured from Malaysia during World War II. But the species of Section I, except M. duperreana, which are morphologically very much allied, are indicated to have similar mode of origin through hybridization and gene mutations; the distinctive genetic constellation being preserved through isolation. The overlapping and contiguous nature of the areas of distribution of a majority of these species also indicates that hybridization had a major role to play in the process. The range of variation in the genus is limited, and there occur groups of species with almost similar characters. The limited range of variation in the morphological, cytological, pollen and anatomical structures suggests a stabilization of the characters in the species during the earlier epochs since their origin in the early Tertiary. The diversification and evolution that are now taking place among the species are due to gene-mutations and natural hybridization, which are always occurring in nature. Selection is ultimately playing its role in the survival of the variants.

Are there Negritos in Celebes?

PROF. FLOWER, in his interesting lecture on “The Pygmy Races of Men” (Journ. Anthr. Inst. vol. xviii. p. 82, 1888, and NATURE, vol. xxxviii. p. 67), after having spoken of the Negritos in the Philippines, says, apparently on the authority of Quatrefages: “As the islands of these eastern seas have become better known, further discoveries of the existence of a small Negroid population have been made in Formosa, in the interior of Borneo, Sandalwood Island (Sumba), Xulla, Bouron, Ceram, Flores, Solor, Lomblem, Pantar, Ombay, the eastern peninsula of Celebes, &c.”