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At least 145 records · Page 8Linked to original sources
USGS1999

40Ar/39Ar thermochronology of mesoproterozoic metamorphism in the Colorado Front Range

A low-pressure metamorphic episode in the Colorado Front Range has been identified by the presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages. The overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding ~525??C. Twenty-eight 40Ar/39Ar dates on hornblende, muscovite, biotite, and microcline were used to infer the age and thermal conditions of metamorphism. Muscovite and biotite 40Ar/39Ar ages fall mainly in the interval 1400-1340 Ma, consistent with cooling through the closure temperature interval of micas (~400??-300??C) after about 1400 Ma. In contrast, hornblende apparent ages (T(c)~500??-550??C) between 1600 and 1390 Ma reflect variable retention of radiogenic argon. Forward modeling of argon diffusion shows that the distribution of hornblende and mica ages is consistent with the partial resetting of argon systematics ca. 1400 Ma by a thermal pulse reaching maximum temperatures around 550??C and decaying within <20 m.yr. These temperatures match the conditions inferred from the overprinting assemblage; thus, muscovite and biotite ages are interpreted to date the cooling phase of this metamorphic event. This late metamorphism is broadly coeval with the intrusion of ca. 1400-Ma granitic plutons in the study area and throughout the southwestern United States. However, thermal effects are observed far from pluton margins, suggesting pervasive, regional crustal heating rather than restricted contact metamorphism. Our results suggest that ca. 1400-Ma metamorphism and plutonism are manifestations of a regional thermal episode that both partially melted the lower crust and pervasively metamorphosed middle crustal rocks.

Journal of Geology↗
USGS1999

An ancient depleted mantle sample from a 42-Ma dike in Montana: Constraints on persistence of the lithosphere during Eocene Magmatism

Recent models for the Cenozoic tectonic evolution of the western margin of North America propose that delamination of ancient lithosphere accompanied asthenospheric upwelling, magmatism, and uplift subsequent to Laramide deformation. On the basis of the age of an alkaline dike in south-central Montana, thermometry of mantle xenoliths from the dike, and Sr, Nd, and Pb isotopic compositions of the dike and a xenocryst, we show that refractory lithosphere, derived from ancient, depleted mantle, remained in place under the Wyoming Craton as late as 42 Ma. The Haymond School Dike, a camptonite, yields a 40Ar/39Ar plateau date of 41.97 ?? 0.19 Ma (2??). Paleomagnetic data are consistent with this date and indicate intrusion during chron C19r. The dike has Sr, Nd, and Pb isotopic compositions similar to those of other Eocene alkaline rocks from central Montana. A clinopyroxene megacryst from the dike has ??42 = 17, and 87Sr/86Sr = 0.70288, indicating that it derives from ancient, depleted mantle isotopically distinct from the source of the host camptonite. Thermometry of xenoliths from the dike shows pyroxene populations that formed at 880?? and 1200??C. Combining thermometry with previous estimates of the regional Eocene geotherm inferred from xenoliths in kimberlites, and with the Al-in-orthopyroxene barometer, we infer that lithospheric mantle remained intact to depths of 110-150 km as late as 42 Ma. Eocene magmatism was not accompanied by complete removal of ancient lithosphere.

Journal of Geology↗
USGS2000

Timing of the Acadian Orogeny in northern New Hampshire

New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian, (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.

Journal of Geology↗
USGS2000

Turbidite megabeds in an Oceanic Rift Valley recording jokulhlaups of late Pleistocene glacial lakes of the western United States

Escanaba Trough is the southernmost segment of the Gorda Ridge and is filled by sandy turbidites locally exceeding 500 m in thickness. New results from Ocean Drilling Program (ODP) Sites 1037 and 1038 that include accelerator mass spectrometry (AMS) 14C dates and revised petrographic evaluation of the sediment provenance, combined with high-resolution seismic-reflection profiles, provide a lithostratigraphic framework for the turbidite deposits. Three fining-upward units of sandy turbidites from the upper 365 m at ODP Site 1037 can be correlated with sediment recovered at ODP Site 1038 and Deep Sea Drilling Program (DSDP) Site 35. Six AMS 14C ages in the upper 317 m of the sequence at Site 1037 indicate that average deposition rates exceeded 10 m/k.yr. between 32 and 11 ka, with nearly instantaneous deposition of one ~60-m interval of sand. Petrography of the sand beds is consistent with a Columbia River source for the entire sedimentary sequence in Escanaba Trough. High-resolution acoustic stratigraphy shows that the turbidites in the upper 60 m at Site 1037 provide a characteristic sequence of key reflectors that occurs across the floor of the entire Escanaba Trough. Recent mapping of turbidite systems in the northeast Pacific Ocean suggests that the turbidity currents reached the Escanaba Trough along an 1100-km-long pathway from the Columbia River to the west flank of the Gorda Ridge. The age of the upper fining-upward unit of sandy turbidites appears to correspond to the latest Wisconsinan outburst of glacial Lake Missoula. Many of the outbursts, or jokulhlaups, from the glacial lakes probably continued flowing as hyperpycnally generated turbidity currents on entering the sea at the mouth of the Columbia River.

Journal of Geology↗
USGS2000

Diagenesis of the Purington Shale in the Illinois Basin and implications for the diagenetic state of sedimentary rocks of shallow Paleozoic basins

The clay minerals, micas, and feldspars of the Pennsylvanian-age Purington Shale have been more diagenetically active than generally recognized. They have undergone diagenetic changes comparable to those of Cenozoic and Mesozoic sedimentary rocks buried three to four times as deeply and heated to approximately twice the temperature. The Purington Shale on the Western Shelf of the Illinois Basin has experienced maximum burial of ???1.0 km and was never heated to more than 60??C, except for a very brief time at about 80??C. The illite/smectite (I/S) of the Purington Shale has ???90% illite. The chemical compositions of the feldspar assemblage in the 0.09-0.063-mm fraction differ from most modern detrital suites of feldspars but are similar to diagenetic sequences described by others from much greater burial depths and are extensively, but not completely, albitized. The simplest interpretation of the apparently advanced stage of diagenesis is that the diagenetic processes have been operating at less pressure and lower temperature but for a longer time; an example of low-temperature, time-dependent diagenesis. By similar reasoning, the apparently anomalously advanced maturity of all of the sedimentary rocks in the relatively shallow Illinois Basin is explained. This generalization should be considered for all shallow Paleozoic basins and should influence (1) exploration for petroleum, (2) use of modal and chemical analyses for determining provenance, and (3) use of chemical composition of shales as proxy for crustal evolution.

Journal of Geology↗
USGS2001

Late cenozoic fluvial stratigraphy of the New Jersey piedmont: A record of glacioeustasy, planation, and incision on a low-relief passive margin

Late Cenozoic fluvial deposits and erosional landforms in the New Jersey Piedmont record two episodes of valley incision, one in the Late Miocene and one in the Early Pleistocene, separated by periods of planation and fluvial deposition. The upland erosion surface and a fluvial gravel are the remnants of a low-relief Late Miocene landscape. Late Miocene incision was followed by deposition of a fluvial plain and cutting of straths in the Pliocene. Early Pleistocene incision produced the present valleys, which contain Middle to Late Pleistocene fluvial deposits. The two incisions correspond to permanent glacioeustatic lowering during expansion of the Antarctic ice sheet in the Middle to Late Miocene and development of Northern Hemisphere ice sheets in the Late Pliocene. Bordering Coastal Plain marine deposits indicate that the upland erosion surface was formed during a rising sea-level trend between the Late Oligocene and Middle Miocene. The Pliocene plain and straths formed during a period of rising sea level in the Early Pliocene. The stratigraphic record indicates that the oldest preserved landforms are no older than Late Miocene, that landscape planation in coastal regions of low-relief passive margins can be achieved in <20 m.yr., and that these surfaces can be incised and dissected in <5 m.yr.

Journal of Geology↗
USGS2001

SHRIMP-RG U-Pb zircon geochronology of mesoproterozoic metamorphism and plutonism in the southwesternmost United States

Mesoproterozoic intrusive and granulite‐grade metamorphic rocks in southern California have been inferred to be exotic to North America on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks. Ion microprobe geochronology indicates that zircons in granulite‐grade gneisses, dated at 1.4 Ga using conventional methods, are composed of 1.68–1.80‐Ga cores and 1.19‐Ga rims. These Early Proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1.19‐Ga San Gabriel anorthosite complex. The lack of a 1.4‐Ga metamorphic event suggests that Proterozoic rocks in this region, rather than being exotic to North America, may in fact be a midcrustal window into Mesoproterozoic crustal evolutionary processes in southwestern North America.

California↗
USGS2002

Emsian synorogenic paleogeography of the Maine Applachians

The Acadian deformation front in the northern Appalachians of Maine and New Hampshire can now be closely located during the early Emsian (Early Devonian; 408-406 Ma). Tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408-Ma time scale tie point for the early Emsian. The deformation front lay between a belt of Lower Devonian flysch and molasse that were deposited in an Acadian foreland basin and had not yet been folded and a belt of early Emsian plutons that intruded folded Lower Devonian rocks. This plutonic belt includes the newly dated Ore Mountain gabbro (U/Pb; 406 Ma), which hosts magmatic-sulfide mineralization. Along the deformation front, a 407-Ma pluton that locally truncates Acadian folds (Katahdin) was the feeder to volcanic rocks (Traveler Rhyolite; 406-407 Ma) that are part of the foreland-basin succession involved in these same folds. The Emsian igneous rocks thus define a syncollisional magmatic province that straddled the deformation front. These findings bear on three alternative subduction geometries for the Acadian collision.

Journal of Geology↗
USGS2003

Extensive deposits on the Pacific plate from Late Pleistocene North American glacial lake outbursts

One of the major unresolved issues of the Late Pleistocene catastrophic-flood events in the northwestern United States (e.g., from glacial Lake Missoula) has been what happened when the flood discharge reached the ocean. This study compiles available 3.5-kHz high-resolution and airgun seismic reflection data, long-range sidescan sonar images, and sediment core data to define the distribution of flood sediment in deepwater areas of the Pacific Ocean. Upon reaching the ocean at the mouth of the Columbia River near the present-day upper continental slope, sediment from the catastrophic floods continued flowing downslope as hyperpycnally generated turbidity currents. The turbidity currents resulting from the Lake Missoula and other latest Pleistocene floods followed the Cascadia Channel into and through the Blanco Fracture Zone and then flowed west to the Tufts Abyssal Plain. A small part of the flood sediment, which was stripped off the main flow at a bend in the Cascadia Channel at its exit point from the Blanco Fracture Zone, continued flowing more than 400 km to the south and reached the Escanaba Trough, a rift valley of the southern Gorda Ridge. Understanding the development of the pathway for the Late Pleistocene flood sediment reaching Escanaba Trough provides insight for understanding the extent of catastrophic flood deposits on the Pacific plate .

Journal of Geology↗
USGS2004

Geochemical discrimination of five pleistocene lava-dam outburst-flood deposits, western Grand Canyon, Arizona

Pleistocene basaltic lava dams and outburst‐flood deposits in the western Grand Canyon, Arizona, have been correlated by means of cosmogenic 3 He ( 3 He c ) ages and concentrations of SiO 2 , Na 2 O, K 2 O, and rare earth elements. These data indicate that basalt clasts and vitroclasts in a given outburst‐flood deposit came from a common source, a lava dam. With these data, it is possible to distinguish individual dam‐flood events and improve our understanding of the interrelations of volcanism and river processes. At least five lava dams on the Colorado River failed catastrophically between 100 and 525 ka; subsequent outburst floods emplaced basalt‐rich deposits preserved on benches as high as 200 m above the current river and up to 53 km downstream of dam sites. Chemical data also distinguishes individual lava flows that were collectively mapped in the past as large long‐lasting dam complexes. These chemical data, in combination with age constraints, increase our ability to correlate lava dams and outburst‐flood deposits and increase our understanding of the longevity of lava dams. Bases of correlated lava dams and flood deposits approximate the elevation of the ancestral river during each flood event. Water surface profiles are reconstructed and can be used in future hydraulic models to estimate the magnitude of these large‐scale floods.

Arizona↗
USGS2004

Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

Journal of Geology↗
USGS2005

Paleoproterozoic metamorphism in the northern Wyoming province: Implications for the assembly of Laurentia

U-Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high-grade metamorphism at ???1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ???3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 ??m) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well-defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ???100 m.yr. younger than the proposed time of Wyoming-Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ???1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia. ?? 2005 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS2006

Timing of magmatism following initial convergence at a passive margin, southwestern U.S. Cordillera, and ages of lower crustal magma sources

Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali-calcic Fe- and Sr-enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal-miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo-Triassic intrusion ages, and a prolonged postemplacement thermal history within the long-lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc. ?? 2006 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS2006

Calcite-graphite thermometry of the Franklin Marble, New Jersey Highlands

We present new stable-isotope data for the Mesoproterozoic Franklin Marble from outcrops along an 80-km traverse parallel to and across strike of the structural grain of the western New Jersey Highlands. Calcite and dolomite from marble have an average ??13C of 0.35??? ?? 0.73??? PDB (n = 46) and a more limited range than other Mesoproterozoic marbles from the Adirondacks and the Canadian Grenville Province. The small range of ??13C values from the New Jersey samples is consistent with the preservation of a primary marine isotopic signature and limited postdepositional isotopic modification, except proximal to Zn or Fe ore deposits and fault zones. Fractionations between calcite and well-formed graphite (??13C[Cal-Gr]) for analyzed Franklin Marble samples average 3.31???. ?? 0.25??? (n = 34), and dolomite-graphite fractionations average 3.07??? ?? 0.30??? (n = 6). Taken together, these indicate an average temperature of 769?? ?? 43??C during metamorphism associated with the Ottawan Orogeny in the New Jersey Highlands. Thus, carbon isotope fractionations demonstrate that the Franklin Marble was metamorphosed at granulite facies conditions. Metamorphic temperatures are relatively constant for the area sampled and overprint the metamorphosed carbonatehosted Zn-Fe-Mn ore deposits. The results of this study support recent work proposing that pressure and temperature conditions during Ottawan orogenesis did not vary greatly across faults that partition the Highlands into structural blocks. ?? 2006 by The University of Chicago. All rights reserved.

Journal of Geology↗
USGS1997

Depositional processes in large-scale debris-flow experiments

This study examines the depositional process and characteristics of deposits of large‐scale experimental debris flows (to 15 m 3 ) composed of mixtures of gravel (to 32 mm), sand, and mud. The experiments were performed using a 95‐m‐long, 2‐m‐wide debris‐flow flume that slopes 31°. Following release, experimental debris flows invariably developed numerous shallow (∼ 10 cm deep) surges. Sediment transported by surges accumulated abruptly on a 3° runout slope at the mouth of the flume. Deposits developed in a complex manner through a combination of shoving forward and shouldering aside previously deposited debris and through progressive vertical accretion. Progressive accretion by the experimental flows is contrary to commonly assumed en masse sedimentation by debris flows. Despite progressive sediment emplacement, deposits were composed of unstratified accumulations of generally unsorted debris; hence massively textured, poorly sorted debris‐flow deposits are not emplaced uniquely en masse. The depositional process was recorded mainly by deposit morphology and surface texture and was not faithfully registered by interior sedimentary texture; homogeneous internal textures could be misinterpreted as the result of en masse emplacement by a single surge. Deposition of sediment by similar, yet separate, debris flows produced a homogenous, massively textured composite deposit having little stratigraphic distinction. Similar deposit characteristics and textures are observed in natural debris‐flow deposits in China. Experimental production of massively textured deposits by progressive sediment accretion limits interpretations that can be drawn from deposit characteristics and casts doubt on methods of estimating flow properties from deposit thickness or from relations between particle size and bed thickness.

Journal of Geology↗
USGS1997

The Bishop Tuff: New insights from eruptive stratigraphy

The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1‐F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1‐F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2‐F8), Lack phenocrystic pyroxenes, and contain few or no Glass Mountain‐derived rhyolite lithic fragments. Earlier packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain‐derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less‐welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to ≥1 mm/s of dense‐welded massive ignimbrite, equivalent to ≥2.5 mm/s of non‐welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in ≥35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south‐central part of the caldera (F1‐8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre‐eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.

California↗
USGS1998

Early proterozoic evolution of the saskatchewan craton and its allochthonous coyer, trans-hudson Orogen

The composition, chronology, and structural relations of the Saskatchewan Craton and enveloping mylonitic rocks exposed in basement windows of the Glennie Domain, Trans‐Hudson Orogen, have been determined by geochemical, geochronologic, and structural studies accompanying detailed field mapping. Basement windows lie along the hinge zone of a regional crustal culmination and consist mostly of 2.4–2.5 Ga felsic plutonic rocks enveloped by the Nistowiak Thrust. The Nistowiak Thrust is a folded, 1–2 km thick, upper amphibolite facies mylonite zone formed during emplacement of the Flin Flon–Glennie Complex across the Saskatchewan Craton. It is likely correlative to the Pelican Thrust, which envelops basement windows in the Hanson Lake Block ∼100 km to the east. An internal high strain zone within the overlying nappe pile, the Guncoat Thrust, is composed primarily of mylonitized porphyroclastic pelitic and psammitic migmatites. U‐Pb geochronological results suggest calc‐alkaline plutonism from 1889–1837 Ma, thrust stacking, peak metamorphism and associated anatexis between 1837 and 1809 Ma, isotopic closure of titanite at 1790–1772 Ma, and intrusion of late granitic rocks at 1770–1762 Ma. This is in agreement with ages from the Hanson Lake Block, and La Ronge, Kisseynew, and Flin‐Flon domains in Saskatchewan and Manitoba, and from the Ungava‐Baffin portion of Trans‐Hudson Orogen, suggesting broadly synchronous thermotectonic processes along a strike length of 2000 km. We speculate that the Saskatchewan Craton, rather than representing an exotic continental fragment, rifted from the Superior and/or Hearne Provinces at ca. 2.1 Ga and that the Trans‐Hudson Orogen is an internal orogen. In this scenario the Maniwekan Ocean, developed between the Rae‐Hearne and Superior cratons, opened and closed about similar pole(s) of plate motion.

Journal of Geology↗
USGS1998

Paleoslope analysis of slump folds in the devonian flysch of Maine

Ancient submarine slide and slump deposits in the Devonian flysch of central and northern Maine show considerable variation in fold style, from symmetric to asymmetric to sheath geometries. Building on earlier work by Farrell and Eaton, we suggest that the spectrum of fold styles reflects the degree of simple shear within each slump deposit. We present a stereographic approach to paleoslope analysis that exploits fold hinge attitudes, axial surface attitudes, sheath axes, vergence, S‐ and Z‐asymmetry—depending on the style of slump folding. Our paleoslope determinations from widely scattered locations across the Devonian foreland basin in Maine show a regionally consistent pattern of westerly to northwesterly slopes.

Journal of Geology↗