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Robert J. Fleck

Publications and source records attributed to Robert J. Fleck.

45 records · Page 3Linked to original sources
USGS2004

Location, Age, and Tectonic Significance of the Western Idaho Suture Zone (WISZ)

The Western Idaho Suture Zone (WISZ) represents the boundary between crust overlying Proterozoic North American lithosphere and Late Paleozoic and Mesozoic intraoceanic crust accreted during Cretaceous time. Highly deformed plutons constituted of both arc and sialic components intrude the WISZ and in places are thrust over the accreted terranes. Pronounced variations in Sr, Nd, and O isotope ratios and in major and trace element composition occur across the suture zone in Mesozoic plutons. The WISZ is located by an abrupt west to east increase in initial 87Sr/86Sr ratios, traceable for over 300 km from eastern Washington near Clarkston, east along the Clearwater River thorough a bend to the south of about 110? from Orofino Creek to Harpster, and extending south-southwest to near Ola, Idaho, where Columbia River basalts conceal its extension to the south. K-Ar and 40Ar/39Ar apparent ages of hornblende and biotite from Jurassic and Early Cretaceous plutons in the accreted terranes are highly discordant within about 10 km of the WISZ, exhibiting patterns of thermal loss caused by deformation, subsequent batholith intrusion, and rapid rise of the continental margin. Major crustal movements within the WISZ commenced after about 135 Ma, but much of the displacement may have been largely vertical, during and following emplacement of batholith-scale silicic magmas. Deformation continued until at least 85 Ma and probably until 74 Ma, progressing from south to north.

Open-File Report↗
USGS2002

Age and tectonic significance of volcanic rocks in the northern Los Angeles Basin, California

Volcanic rocks, mostly basalts and some andesites, are interbedded with middle Miocene strata and are overlain by younger rocks throughout the greater part of the Los Angeles Basin, California. Roughly correlative flows, previously dated radiometrically (or paleontologically) at about 16.4 to 10.7 Ma, crop out in five separate regions around the basin perimeter. Los Angeles Basin volcanic rocks have special meaning because they offer clues to tectonomagmatic events associated with onset of clockwise transrotation of the western Transverse Ranges region and to the timing and locus of the initial basin opening. Whole-rock 40 Ar/ 39 Ar dating of near-tholeiitic olivine basalts of the Topanga Formation (Hoots, 1931) from three sites in the easternmost Santa Monica Mountains, combined with 87 Sr/ 86 Sr dating of fossil carbonates from interstratified marine beds at nine sites, establish a new age of 17.4 Ma for these oldest known Topanga-age volcanics of the Los Angeles Basin. We also record three new 40 Ar/ 39 Ar ages (15.3 Ma) from andesitic flows of the lower Glendora Volcanics at the northeast edge of the basin, 70 km east of the Santa Monica Mountains. A whole-rock determination of 17.2±0.5 Ma for nearby altered olivine basalt in the unfossiliferous Glendora volcanic sequence is questionable because of a complex 40 Ar/ 39 Ar age spectrum suggestive of 39Ar recoil, but it may indicate an older volcanic unit in this eastern area. We hypothesize that the 17.4-Ma volcanics in the eastern Santa Monica Mountains are an early expression of deep crustal magmatism accompanying the earliest extensional tectonism associated with rifting. The extremely thick younger volcanic pile in the western and central parts of the range may suggest that this early igneous activity in the eastern area was premonitory. Paleomagnetic declination data are needed to determine the pre-transrotational orientation of the eastern Santa Monica Mountains volcanic sequence. The new age determinations do not yield unequivocal support for either of two proposed explanations of possible age trends of Miocene volcanic rocks in southern California but underscore the need for further work.

California↗
USGS2001

Origin, extent, and thickness of quaternary geologic units in the Willamette Valley, Oregon

Stratigraphic and chronologic information collected for Quaternary deposits in the Willamette Valley, Oregon, provides a revised stratigraphic framework that serves as a basis for a 1:250,000-scale map, as well as for thickness estimates of widespread Quaternary geologic units. We have mapped 11 separate Quaternary units that are differentiated on the basis of stratigraphic, topographic, pedogenic, and hydrogeologic properties. In summation, these units reflect four distinct episodes in the Quaternary geologic development of the Willamette Valley: 1) Fluvial sands and gravels that underlie terraces flanking lowland margins and tributary valleys were probably deposited between 2.5 and 0.5 million years ago. They are the oldest widespread surficial Quaternary deposits in the valley. Their present positions and preservation are undoubtedly due to postdepositional tectonic deformation - either by direct tectonic uplift of valley margins, or by regional tectonic controls on local base level. 2) Tertiary and Quaternary excavation or tectonic lowering of the Willamette Valley accommodated as much as 500 m (meters) of lacustrine and fluvial fill. Beneath the lowland floor, much of the upper 10 to 50 m of fill is Quaternary sand and gravel deposited by braided channel systems in subhorizontal sheets 2 to 10 m thick. These deposits grade to gravel fans 40 to 100 m thick where major Cascade Range rivers enter the valley and are traced farther upstream as much thinner valley trains of coarse gravel. The sand and gravel deposits have ages that range from greater than 420,000 to about 12,000 years old. A widely distributed layer of sand and gravel deposited at about 12 ka (kiloannum, thousands of years before the present) is looser and probably more permeable than older sand and gravel. Stratigraphic exposures and drillers' logs indicate that this late Pleistocene unit is mostly between 5 and 20 m thick where it has not been subsequently eroded by the Willamette River and its major tributaries. 3) Between 15,000 and 12,700 years ago, dozens of floods from Glacial Lake Missoula flowed up the Willamette Valley from the Columbia River, depositing up to 35 m of gravel, sand, silt, and clay. 4) Subsequent to 12,000 years ago, Willamette River sediment and flow regimes changed significantly: the Pleistocene braided river systems that had formed vast plains of sand and gravel evolved to incised and meandering rivers that are constructing today's fine-grained floodplains and gravelly channel deposits. Sub-surface channel facies of this unit are loose and unconsolidated and are highly permeable zones of substantial groundwater flow that is likely to be well connected to surface flow in the Willamette River and major tributaries. Stratigraphic exposures and drillers' logs indicate that this unit is mostly between 5 and 15 m thick.

Oregon↗
USGS1999

Digital Geologic Map of the Nevada Test Site and Vicinity, Nye, Lincoln, and Clark Counties, Nevada, and Inyo County, California

This digital geologic map of the Nevada Test Site (NTS) and vicinity, as well as its accompanying digital geophysical maps, are compiled at 1:100,000 scale. The map compilation presents new polygon (geologic map unit contacts), line (fault, fold axis, metamorphic isograd, dike, and caldera wall) and point (structural attitude) vector data for the NTS and vicinity, Nye, Lincoln, and Clark Counties, Nevada, and Inyo County, California. The map area covers two 30 x 60-minute quadrangles-the Pahute Mesa quadrangle to the north and the Beatty quadrangle to the south-plus a strip of 7.5-minute quadrangles on the east side-72 quadrangles in all. In addition to the NTS, the map area includes the rest of the southwest Nevada volcanic field, part of the Walker Lane, most of the Amargosa Desert, part of the Funeral and Grapevine Mountains, some of Death Valley, and the northern Spring Mountains. This geologic map improves on previous geologic mapping of the same area (Wahl and others, 1997) by providing new and updated Quaternary and bedrock geology, new geophysical interpretations of faults beneath the basins, and improved GIS coverages. Concurrent publications to this one include a new isostatic gravity map (Ponce and others, 1999) and a new aeromagnetic map (Ponce, 1999).

Open-File Report↗
USGS1994

The Death Valley turtlebacks reinterpreted as Miocene­ Pliocene folds of a major detachment surface

Determining the origin of extension parallel folds in metamorphic core complexes is fundamental to understanding the development of detachment faults. An excellent example of such a feature occurs in the Death Valley region of California where a major, undulatory, detachment fault is exposed along the well-known turtleback (antiformal) surfaces of the Black Mountains. In the hanging wall of this detachment fault are deformed strata of the Copper Canyon Formation. New age constraints indicate that the Copper Canyon Formation was deposited from ~6 to 3 Ma. The formation was folded during deposition into a SE-plunging syncline with an axial surface coplanar with that of a synform in the underlying detachment. This relation suggests the turtlebacks are a folded detachment surface formed during large-scale extension in an overall constrictional strain field. The present, more planar, Black Mountains frontal fault system may be the result of out-stepping of a normal fault system away from an older detachment fault that was deactivated by folding.

The Journal of Geology↗
USGS1990

Chapter 21: Neodymium, strontium, and trace-element evidence of crustal anatexis and magma mixing in the Idaho batholith

Variations in initial 143 Nd/ 144 Nd in Late Cretaceous plutonic rocks along the South Fork of the Clearwater River (SFCR) supplement results of Sr and O studies, which demonstrate large-scale mixing in magmas forming the western margin of the Idaho batholith. These marginal or border phases of the batholith span the terrane boundary between Proterozoic crust of North America and late Paleozoic-Mesozoic intraoceanic arc terranes (WSD terranes), delineated by the Western Idaho suture zone (or WISZ). ɛ Nd (t) values in Early Cretaceous and older, pre-accretionary plutons of the WSD range from +3 to +7.6, and average +5.7. Proterozoic orthogneisses and metasedimentary rocks range from -7.4 to -13.7 and -10.45 to -15.7, respectively. ɛ Nd (t) in Late Cretaceous plutons of the SFCR decreases abruptly from west to east near the WISZ, varying inversely with ɛSr (t) . Although Sr isotopic evidence (Fleck and Criss, 1985) is consistent with a binary mixing model, Sm-Nd results modify those conclusions, suggesting that SFCR plutons may be divided into three groups. Group 1 plutons occur in a narrow zone (<4 km width) along the suture zone (WISZ). These bodies probably represent at least three-component mixtures of very high-Sr, arc-type magmas, one or more Proterozoic crustal components that may include lower crust, and a high-Nb, high-Zr component. Group 2 plutons are characterized by high ɛSr (t) .and nearly constant, low ɛNd (t) . These bodies are thought to represent mixtures of deep-seated partial melts of two different Proterozoic lithospheric types, possibly representing upper and lower crust. Plutons belonging to Group 3 have ɛNd (t) .values <-14 and probably incorporated substantial amounts of Proterozoic metasedimentary rocks, but mixing components are poorly defined. Trace-element variations in SFCR rocks also reflect the arc terrane-continental crustal boundary as Nb, Zr, and Nd increase dramatically, whereas Sr, Rb/Nb, and Sm/Nd exhibit coincident decreases east of the WISZ. Modeling of these variations with the isotopic variations in Nd and Sr supports mixing, but precludes contamination-bulk-assimilation models. Correlated ɛNd, ɛSr, and δ 18 O within the SFCR favors mixing of crustal and subcrustal magmas rather than derivation of the melts entirely from subcontinental lithosphere.

Idaho↗
USGS1980

Rubidium-strontium geochronology and plate-tectonic evolution of the southern part of the Arabian Shield

Rubidium-strontium studies of Precambrian volcanic and plutonic rocks of the Arabian Shield document an early development of the Arabian craton between 900 and 680 m.y. (million years) ago. Geologic studies indicate an island-arc environment characterized by andesitic (dioritic) magmas, volcaniclastic sedimentation, rapid deposition, and contemporaneous deformation along north or northwest-trending axes. Magmatic trends show consistent variation in both composition and geographic location as a function of age. The oldest units belong to an assemblage of basaltic strata exposed in western Saudi Arabia that yield an age of 1165:!:110 m.y. The oldest andesitic strata studied yield an age of 912:!:76 m.y. The earliest plutonic units are diorite to trondhjemite batholiths that range from 800 to 9,00 m.y. in age and ,occur along the western and southern parts of Saudi Arabia. Younger plutonic units, 680 to 750 m.y. in age, range from quartz diorite to granodiodte and become more abundant in the central and northeastern parts of the Arabian Shield. Initial 'Sr/ 86 Sr ratios for both dioritic groups range from 0.7023 to 0.7030 and average 0.7027. The absence of sialic detritus in sedimentary units and the evidence for an island-arc environment suggest the early development of the Arabian craton at a convergent plate margin between plates of oceanic lithosphere. Active subduction apparently extended from at least 900 m.y. to about 680 m.y. Subsequent to this subduction-related magmatism and tectonism, called the Hijaz tectonic cycle, the Arabian craton was sutured to the late Precambrian African plate in a collisional event. This period of orogeny, represented in Arabia and eastern Africa by the Mozambiquian or Pan-African event, extended from some time before 650 m.y. to at least 540 m.y. and perhaps 520 m.y. B.P. Although the tectonic processes of subduction and continental collision during the 900+ to 500-m.y. period require similar directions of plate convergence, the differences in magmatic and tectonic. styles of Hijaz orogenesis from those of the Pan-African and the temporal break between them in much of the southern part of the Arabian Shield support division into at least two events. As defined by the ages of major plutonic units, the axis of magmatic and tectonic activity migrated eastward or northeastward during the Hijaz cycle, the predominantly dioritic plutonic rocks becoming younger and more siliceous to the east. Granodiorite to granite pl}.1tonism of the Pan-African event, however, shows no geographic bias, being distributed throughout the Arabian Shield. Although the Hijaz diorites and Pan-African granitic rocks exhibit strong contrasts in composition and age differences as great as 250 m.y. in the westernmost parts of the area, the two groups are less distinct compositionally and nearly the same age in the eastern part.

Professional Paper↗
USGS1976

Geochronology of the Arabian Shield, western Saudi Arabia: K-Ar results

An orogenic event, correlated with the Pan-African event in eastern Africa, affected the Arabian Peninsula between 510 and 610 m.y. ago and is well-recorded geochronologically. The event probably included two thermal pulses or maxima, the first occurring between 560 and 610 m.y. ago and the second between 510 and 540 m.y. ago. The earlier pulse, the more severe one, included the majority of the igneous activity and metamorphism. During the last part of the 510- to 610-m.y. period, left-lateral strike-slip faulting occurred along a set of northwest-trending en echelon fracture zones, whose composite displacement may be as large as 240 km. At least one and probably more orogenic events affected the Arabian Peninsula before the Pan-African event, but only minimum ages can be assigned to these, because thermal effects of the 510- to 610-m.y. event have reset K-Ar ages. Major diorite-granite batholiths, however, formed before 760 m.y. ago.

Arabian Shield↗