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J.T. Hagstrum

Publications and source records attributed to J.T. Hagstrum.

26 records · Page 2Linked to original sources
USGS1993

Deposition of Franciscan Complex cherts along the paleoequator and accretion to the American margin at tropical paleolatitudes

Red radiolarian cherts from three localities within the Franciscan subduction complex of northern California contain three components of remanent magnetization which are best isolated by progressive thermal demagnetization. The first component, usually removed by 300 °C, has an in situ direction similar to the present axial-dipole field and is probably a recently acquired thermoviscous overprint. A second component, generally removed between 300 and 630 °C, has constant (normal) polarity and direction within each section and is interpreted to have been acquired by low-temperature chemical alteration during subduction and accretion at the continental margin. The third component, isolated between ∼560 and 680 °C, has both normal and reversed polarities, passes a fold test, and is inferred to have been acquired during or soon after deposition. The available paleomagnetic, biostratigraphic, and geochemical data indicate deposition of these cherts along the paleoequator (0°-2°N or S paleolatitude) between Pliensbachian and Oxfordian time as the oceanic plate moved eastward, relative to North America, beneath the equatorial zone of high biologic productivity. Between Bathonian and Cenomanian time, the chert sequences apparently moved progressively away from the paleoequator (2°-15°N or S), and were soon after accreted to the American continental margin. Plate reconstruction models for the Farallon plate corroborate low-paleolatitude trajectories from ridge crest to subduction zone (for example, from 3°S to 11°N), and they imply subsequent northward translation of the Franciscan Complex (> 4,000 km) by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates.

Geological Society of America Bulletin↗
USGS1992

Jurassic ash-flow sheets, calderas, and related intrusions of the Cordilleran volcanic arc in southeastern Arizona: Implications for regional tectonics and ore deposits

Volcanologic, petrologic, and paleomagnetic studies of widespread Jurassic ash-flow sheets in the Huachuca-southern Dragoon Mountains area have led to identification of four large source calderas and associated comagmatic intracaldera intrusions. Stratigraphic, facies, and contact features of the caldera-related tuffs also provide constraints on the locations, lateral displacements, and very existence for some major northwest-trending faults and inferred regional thrusts in south-eastern Arizona. For example, the intricate Cochise thrust system, as mapped by others in the southern Dragoon Mountains, consists instead of primary depositional contacts within caldera-fill megabreccia, and the inferred regional thrusts do not exist, at least as previously interpreted. Silicic alkalic compositions of the Jurassic caldera-related, ash-flow tuffs; bimodal associated mafic magmatism; and interstratified coarse sedimentary deposits provide evidence for synvolcanic extension and rifting within the Cordilleran magmatic arc. Gold-copper mineralization is associated with subvolcanic intrusions at several of the Jurassic calderas.

Arizona↗
USGS1987

Structural reinterpretation of the Ajo mining district, Pima County, Arizona, based on paleomagnetic and geochronologic studies

The Ajo mining district of southern Arizona is divided into two main structural blocks by the Gibson Arroyo fault. The eastern Camelback Mountain block contains the Late Cretaceous-early Tertiary porphyry copper deposit which has been previously thought to be associated with the displaced apex of a large intrusion exposed by deeper erosion in the western Cardigan Peak block. However, unpublished U-Pb data support a mid-Tertiary age for the western intrusion. Paleomagnetic and geologic evidence indicates that the ore deposit has been tilted to the south a total of approximately 120 degrees : 68 degrees before and another 55 degrees after emplacement of the overlying Locomotive Fanglomerate and Ajo Volcanics. The Ajo Volcanics have K-Ar ages (24-25 m.y.) slightly older than those from the large intrusion of the Cardigan Peak block (20-23 m.y.). Paleomagnetic directions are consistent with the southward tilting of the volcanic rocks, but they suggest that the remanent magnetization and perhaps the K-Ar dates of the western intrusion were reset by the emplacement of dikes younger than the Locomotive Fanglomerate and Ajo Volcanics. These and other geologic relations indicate the following sequence of mid-Tertiary events in the district: (1) emplacement of the western intrusion, (2) movement along the Gibson Arroyo fault, (3) unroofing and perhaps tilting of the pluton [asymp] 70 degrees to the south along with the Camelback Mountain block, (4) syntectonic depositions of the Locomotive Fanglomerate and the Ajo Volcanics, (5) continued uplift and tilting to the south totaling 40 degrees to 60 degrees , (6) intrusion of the youngest dikes with attendant alteration and remagnetization of the host rocks, and (7) minor (?) oblique movement along the Gibson Arroyo fault.

Economic Geology↗
USGS1986

A paleomagnetic and stable isotope study of the pluton at Rio Hondo near Questa, New Mexico: Implications for CRM related to hydrothermal alteration

Paleomagnetic and rock magnetic data combined with stable isotope data from the middle Tertiary pluton along the Rio Hondo in northern New Mexico suggest that its magnetic remanence has both thermal (TRM) and high-temperature chemical (CRM) components. Oxygen isotope temperatures indicate that magnetite associated with the more rapidly cooled higher levels of the pluton, and with mafic inclusions and cogenetic rhyolitic dikes sampled at lower levels of exposure, ceased subsolidus recrystallization and isotopic exchange above its Curie temperature (580°C) in the presence of a magmatic fluid. Continued cooling imparted a TRM to these portions of the pluton. The more slowly cooled granodiorite at lower levels has quartz-magnetite isotopic temperatures that are below the Curie temperature of magnetite implying that its magnetization is high-temperature CRM. Sub-Curie isotopic temperatures for other granitic plutons in the western U.S.A. suggest that CRM may be commonly derived from subsolidus interactions between magnetite and magmatic fluids in plutonic rocks. A meteoric-hydrothermal system generated by the cooling Rio Hondo pluton, and not by younger adjacent intrusions, resulted in limited alteration along zones of high permeability near the southern margin of the Rio Hondo pluton, and in more prevasive alteration of the pluton to the north. The meteoric-hydrothermal alteration occurred at relatively high temperatures (> 350°C) and, with the exception of local chloritization, caused little visible alteration of the rocks. The isotopic ratios indicate that little of the magnetite could have grown from or exchanged with a meteoric-hydrothermal fluid.

Earth and Planetary Science Letters↗
USGS1983

Paleomagnetic results from seven middle Paleozoic plutons in the Appalachian Piedmont of North and South Carolina

Seven isotopically dated plutons of similar middle Paleozoic age (approx. 400 Ma) were sampled for paleomagnetic study. The results from 12 sites (152 samples) within these plutons show two types of remanence behavior that appear fairly consistent with rock composition. The more leucocratic rocks (9 sites) tend to yield single-component magnetizations, of generally lower coercivities and blocking temperatures relative to the other units, that are considered to represent a recent viscous overprint, The mesocratic to melanocratic rocks show both single- and multicomponent magnetizations, the latter giving characteristic directions isolated through remagnetization-plane analysis. Three sites are considered reliable and yield paleomagnetic poles, of inferred Early Devonian age, that may reflect a 30? southward displacement from stable North America. One large site (25 samples) gives a remagnetized pole position of inferred Carboniferous age consistent with poles from North America, perhaps indicating that the Piedmont may have become part of the North American plate by this time.

Open-File Report↗