Southern Margin Of Laurentia Research Articles

Late Paleozoic Flexural Extension and Overprinting Shortening in the Southern Ozark Dome, Arkansas, USA: Evolving Fault Kinematics in the Foreland of the Ouachita Orogen

AbstractFaults and folds on the southern flank of the Ozark dome in northern Arkansas, USA, record flexural extension in a foreland area followed by shortening in response to the late Paleozoic Ouachita orogeny. Map‐scale structures and an analysis of fault‐slip data collected systematically during geologic mapping demonstrate that most deformation in the area accommodated north‐south extension as the southern margin of Laurentia was flexed beneath the thrust load of the Ouachita belt, probably during Middle Pennsylvanian. Extension was concentrated in northeast‐ and west‐northwest‐trending structural zones having sets of discontinuous, often en echelon normal and strike‐slip faults and associated monoclinal folds. Reactivation of basement weaknesses that underlie these zones is indicated by their close match to oblique‐rift models in which both the proportions of normal and strike‐slip faulting and the internal extension directions vary with orientation of the zones. Subsequent propagation of north‐south Ouachita shortening into the foreland formed small‐offset strike‐slip and sparse reverse faults that overprinted older extensional structures. Strike‐slip faults were concentrated in reactivated northeast‐trending structural zones. In two areas, reverse faults and local anticlines were developed in the footwalls of older normal faults, both near intersections of northeast‐ and west‐northwest‐trending structural zones. These are interpreted as areas of incipient inversion due to compressional stress concentrations at fault‐block corners. Spatial overlap of areas of north‐south shortening and fluid flux marked by silicification or lead‐zinc mineralization indicates that regional fluid flow of brines was coeval with and may have enhanced inversion during Late Pennsylvanian to early Permian.

Detrital zircon geothermochronology reveals pre-Alleghanian exhumation of regional Mississippian sediment sources in the southern Appalachian Valley and Ridge Province

AbstractThe Black Warrior foreland basin records sedimentation associated with the development of intersecting Ouachita and Alleghanian thrust belts along the southern margin of Laurentia. Mississippian–Pennsylvanian units in the Black Warrior basin are interpreted to be sourced from either the northern Appalachians and mid-continent or more regionally from the southern Appalachians or nearby Ouachita thrust belt. We present detrital zircon U-Pb ages and Th/U values from Paleozoic units that indicate zircon from the Mississippian Hartselle Sandstone are temporally and chemically compatible with being sourced from the southern Appalachians. Zircon mixing models suggest sediment was primarily recycled from Cambrian, Ordovician, and Devonian strata in the Appalachian Valley and Ridge, with minor influx from Piedmont units. A ca. 415 Ma zircon population requires additional input from the Maya Block of the Yucatan Peninsula or similar outboard terranes. We present zircon (U-Th)/He analysis and thermal history modeling of Paleozoic units, which detail pre-Alleghanian exhumation in the Appalachian Valley and Ridge. Both the Cambrian Chilhowee Group and Pennsylvanian Pottsville Formation exhibit (U-Th)/He dates ranging from 507 to 263 Ma with a Mississippian subset (353–329 Ma, n = 4), which indicates rapid cooling and inferred exhumation during Late Devonian–Early Mississippian Neoacadian tectonism. We propose a Mississippian drainage system that transported material along southern Appalachian structural fabrics to the juncture between Appalachian and Ouachita thrust belts followed by a sediment-routing rotation toward the Black Warrior foreland. This interpretation honors chemical-age zircon data, accounts for metamorphic grains in thin section petrography, and matches Mississippian–Pennsylvanian Black Warrior foreland lithostratigraphic relationships.

Amsassia(calcareous alga) from the Lower Ordovician (Tremadocian) of western Newfoundland, and the biologic affinity and geologic history of the genus

AbstractModular coral-like fossils from Lower Ordovician (Tremadocian) thrombolitic mounds in the St. George Group of western Newfoundland were initially identified asLichenariaand thought to include the earliest tabulate corals. They are here assigned toAmsassia terranovensisn. sp. andAmsassia? sp. A from the Watts Bight Formation, andA.diversan. sp. andAmsassia? sp. B from the overlying Boat Harbour Formation.Amsassia terranovensisn. sp. andA.argentinafrom the Argentine Precordillera are the earliest representatives of the genus.Amsassiais considered to be a calcareous alga, possibly representing an extinct group of green algae. The genus originated and began to disperse in the Tremadocian, during the onset of the Great Ordovician Biodiversification Event, on the southern margin of Laurentia and the Cuyania Terrane. It inhabited small, shallow-marine reefal mounds constructed in association with microbes. The paleogeographic range ofAmsassiaexpanded in the Middle Ordovician (Darriwilian) to include the Sino-Korean Block, as well as Laurentia, and its environmental range expanded to include non-reefal, open- and restricted-marine settings.Amsassiaattained its greatest diversity and paleogeographic extent in the Late Ordovician (Sandbian–Katian), during the culmination of the Great Ordovician Biodiversification Event. Its range included the South China Block, Tarim Block, Kazakhstan, and Siberia, as well as the Sino-Korean Block and Laurentia, and its affinity for small microbial mounds continued during that time. In the latest Ordovician (Hirnantian), the diversity ofAmsassiawas reduced, its distribution was restricted to non-reefal environments in South China, and it finally disappeared during the end-Ordovician mass extinction.UUID:http://zoobank.org/ef0abb69-10a6-46de-8c78-d6ec7de185fe

Sandstone petrographic and mudstone REE and Nd-isotopic evidence for Middle Pennsylvanian arrival of Gondwana sediments in the Fort Worth Basin

The Fort Worth Basin in north-central Texas is a foreland basin of the Ouachita Orogen that was formed during the late Paleozoic Laurentia-Gondwana collision, and its Paleozoic strata contain key information about the tectonic processes on the southern margin of Laurentia. Here we study sandstone petrography, mudstone rare-earth element (REE) and Nd isotope geochemistry of the Paleozoic clastic rocks in this basin. These data were integrated with previously published detrital zircon UPb data to determine Paleozoic sediment provenance in the Fort Worth Basin and response of the sedimentary system to the Laurentia-Gondwana collision. Sandstone composition changed from the Cambrian (Q90F6L4)–Middle Pennsylvanian (Q89F4L7) to the Late Pennsylvanian-earliest Permian (Q84F2L14), suggesting an increase of input from a recycled orogen after the Middle Pennsylvanian. The mudstones have REE patterns of average upper crust. Compared to the Cambrian-Lower Pennsylvanian mudstones, the Middle Pennsylvanian-lowermost Permian mudstones have more depleted light REE, larger Eu anomalies, and lower εNd values, suggesting a change of sediment provenance after the Early Pennsylvanian. This change is best explained by the arrival of clastic grains from Gondwana, most likely the Sabine Peri-Gondwana terrane, during the Middle Pennsylvanian. Our new provenance data support the previous interpretation that Gondwanan detritus contributed sediment to the Fort Worth Basin during the Middle Pennsylvanian-early Permian, most likely through erosion by transverse rivers linking the high Ouachita hinterland and foreland as the collision of Laurentia and Gondwana advanced to the east of the Fort Worth Basin.

Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny

New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476–1470 Ma Wolf River granitic batholith was not an isolated igneous event, but was accompanied by regional metamorphism, deformation, and sedimentation. Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval, which were deposited closely following the 1.65–1.63 Ga Mazatzal orogeny. In Baraboo Interval strata, muscovite parallel to slatey cleavage, in hydrothermal veins, in quartzite breccia, and in metamorphosed paleosol yielded 40Ar/39Ar plateau ages of 1493–1465 Ma. In addition, U–Th–total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488 ± 20 Ma, and recrystallized hematite in folded metapelite gave a mean U/Th–He age of 1411 ± 39 Ma. Post-Baraboo, arkosic polymictic conglomerate, which contains detrital zircon with a minimum peak age of 1493 Ma, was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith. This episode of magmatism, regional deformation and metamorphism, and sedimentation, which is designated herein as the Baraboo orogeny, provides a midcontinental link between the Picuris orogeny to the southwest and the Pinware orogeny to the northeast, completing the extent of early Mesoproterozoic (Calymmian) orogenesis for 5000 km along the southern margin of Laurentia. This transcontinental orogen is unique among Precambrian orogenies for its great width (~1600 km), the predominance of ferroan granites derived from partial melting of lower continental crust, and the prevalence of regional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.

The Alamos Metamorphic Complex, evidence of late Paleozoic collision between Laurentia and Gondwanan blocks in northwestern Mexico

The Alamos Complex is a metamorphosed volcano-sedimentary sequence exposed south of the late Paleozoic Ouachita–Marathon–Sonora orogen in northwestern Mexico, which in turn, is regarded as a continuation of the Alleghenian–Variscan mountain chain. Mineral assemblages in metavolcanic rocks indicate greenschist facies metamorphism. Thermobarometric studies yield an average metamorphic temperature of 380 °C, whereas andalusite in metapelites indicates pressure less than 0.4 ± 0.05 GPa. The main deformation phase generated pervasive foliation, which in turn was isoclinal to close folded and a second axial planar cleavage was developed. Metavolcanic rocks of the Alamos Complex can be separated in two suites: (1) an anorogenic suite, similar to enriched middle ocean basalts (E-MORB) in the Tesia region, and (2) an orogenic suite, characteristic of volcanic-arc setting in the Mocuzari region. U–Pb detrital zircon geochronology of one sample from Tesia yielded a maximum depositional age averaging 395 ± 28 Ma. One sample from the Mocuzari region yielded a maximum depositional age averaging 1104 ± 24 Ma, with the largest peak at 1428. A clastic sequence correlated with the Middle Triassic–Lower Jurassic Barranca Group, nonconformably cover the Alamos Complex. The basal conglomerate of that group yields a maximum depositional age averaging 372 ± 23 Ma. Accordingly, the Alamos Complex must be in part deposited after the early Devonian time and before the Middle Triassic. The main sources of detritus for the complex are the Meso- and Paleoproterozoic rocks of southwestern Laurentia, whereas the main source of detritus for the Barranca Group is the Rio Fuerte Formation exposed ~ 100 km south of the study area. The metamorphic event is ascribed to the late Paleozoic collision of Gondwanan blocks against the southern margin of Laurentia.

Early Cambrian alkaline volcanism on the southern margin of Laurentia: evidence in the volcaniclastic units from the Puerto Blanco Formation in the Caborca block, NW Mexico

ABSTRACTVolcaniclastic units are exposed at the base of the Puerto Blanco Formation in the Caborca region, northwestern Mexico. The lower unit reveals the presence of Early Cambrian mafic volcanism in this region. It consists of a volcano-sedimentary sequence represented by tuffaceous conglomerates, agglomerates, lapillistones, tuffs, and altered mafic volcanic flows. Petrographic analysis classified the volcanic clasts as albite-sphene-calcite-actinolite granofels, with a moderate to intense hydrothermal alteration, precisely characterized by EPMA analysis. Albite-actinolite geothermometry indicates temperatures from 400 to 500°C, suggesting metamorphic conditions in the upper temperature greenschist facies. Geochemistry analysis shows a high TiO2 basic–ultrabasic volcanism that originated the volcanic clasts. Rock protoliths were studied using immobile trace elements, which classified them as OIB-type alkaline basalts with the characteristic spider hump-shaped pattern, situated in an anorogenic intracontinental tectonic setting with enriched mantle signatures. 40Ar/39Ar geochronology shows metamorphic ages of 52.58 ± 2.0 and 91.67 ± 0.55 Ma, consistent with the emplacement of Laramidic granitoids identified in the region. Possible correlations of this alkaline volcanism include the Southern Oklahoma Aulacogen and the late stages of the rifting of north western Laurentia represented in western United States.

The syn-orogenic sedimentary record of the Grenville Orogeny in southwest Laurentia

Late Mesoproterozoic sedimentary sequences in southwest Laurentia range in age from 1340 to 1035Ma and record regionally extensive intracratonic sedimentation before and during the Grenville Orogeny in southern Laurentia. This paper examines the specific links between intracratonic sedimentation and orogenesis and develops a new tectonic model for basin formation throughout southwest Laurentia during the Late Mesoproterozoic. New detrital zircon, muscovite, and biotite ages refine the provenance, depositional age, and regional correlations of Late Mesoproterozoic strata throughout southwest Laurentia. These data provide a new view of the Grenville orogen in southern Laurentia as it evolved from a continental margin arc to a continent-continent collision based on the record of inboard sedimentation. Deposition of all Late Mesoproterozoic sequences in southwest Laurentia was facilitated by far-traveled, continental-scale rivers that flowed north off the developing Grenville orogen. Sediment accumulation took place during four discrete basin forming events that can be directly linked to changes in the style of convergence and collision along the southern margin of Laurentia. The oldest basin system (lower Apache and Pahrump Groups) records distal back arc basin sedimentation at 1340–1320Ma, synchronous with widespread magmatism along the southern margin of Laurentia. Unconformably overlying shallow marine carbonate-bearing sequences (lower Unkar Group and correlatives) were deposited between 1255 and 1230Ma in a regionally extensive retroarc basin with clastic sediment sourced in part from an active continental arc along the southern margin of Laurentia. Shallow marine and terrestrial siliciclastic sequences (upper Unkar Group and correlatives) were deposited at 1140–1100Ma and were sourced from unroofing thrust nappes in the orogen to the south in an extensive foreland basin system during continent-continent collision. Finally, coarse-grained siliciclastic deposits of the 1060–1035Ma Hazel Formation represent a proximal foreland basin recording the final stages of the Grenville Orogeny in southern Laurentia.

New fossil discoveries in the Chilhowee Group (southern Appalachians, USA): evidence for the Ediacaran-Cambrian transition, 'Cambrian Agronomic Revolution', and earliest trilobites at the southern margin of Laurentia

New fossil discoveries in the Chilhowee Group (southern Appalachians, USA): evidence for the Ediacaran-Cambrian transition, 'Cambrian Agronomic Revolution', and earliest trilobites at the southern margin of Laurentia

Carbon cycling across the southern margin of Laurentia during the Late Ordovician

The Guttenberg Isotope Carbon Excursion (GICE), a positive carbon isotope excursion that occurs near the base of the Katian Stage, is thought to be a global event possibly related to Late Ordovician cooling. Documenting how much regional and global variability exists in carbon isotopic trends prior to and during the GICE is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle, paleoclimate, and chemostratigraphic correlation during the Ordovician. To investigate carbon isotopic trends along the southern margin of the Laurentian carbonate platform during the Late Ordovician, we measured bulk carbonate δ13Ccarb and δ18Ocarb as well as organic carbon δ13Corg values from four locations in Alabama. These sections are excellent study sites because they are well exposed, contain the regionally well-correlated Deicke and Millbrig K-bentonites, and are in a region where δ13C trends have not been studied.Carbonate δ13Ccarb results from one section (i.e. Fort Payne) in northeastern Alabama record a 1.5‰ positive excursion above the Millbrig K-bentonite. We interpret the 1.5‰ positive excursion as a primary feature and identify it as the GICE. The characteristic GICE excursion is absent in the other study sections and we attribute this absence to the presence of unconformities (either non-depositional or erosional) in these sections. We find neither evidence for sea level control on δ13C trends nor conclusive evidence for δ13CDIC gradients along the southeast margin of Laurentia's epeiric sea. Combined with the proximity to the Iapetus Ocean, these results suggest that carbon isotopic trends in Alabama are a reliable proxy for open ocean conditions. Therefore, despite the absence of GICE excursion in all but one of our study sections, increasing δ13C values in Alabama prior to the GICE provides evidence for a steady 13C enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric CO2 during the early Late Ordovician

Reactivation of the Archean-Proterozoic suture along the southern margin of Laurentia during the Mazatzal orogeny: Petrogenesis and tectonic implications of ca. 1.63 Ga granite in southeastern Wyoming

The presence of ca. 1.63 Ga monzogranite (the “white quartz monzonite”) in the southern Sierra Madre, southeastern Wyoming, is anomalous given its distance from the nearest documented plutons of similar age (central Colorado) and the nearest contemporaneous tectonic margin (New Mexico). It is located immediately south of the Cheyenne belt–a ca. 1.75 Ga Archean-Proterozoic tectonic suture. New geochronological, isotopic, and geochemical data suggest that emplacement of the white quartz monzonite occurred between ca. 1645 and 1628 Ma (main pulse ca. 1628 Ma) and that the white quartz monzonite originated primarily by partial melting of the Big Creek Gneiss, a modified arc complex. There is no evidence that mafic magmas were involved. Open folds of the ca. 1750 Ma regional foliation are cut by undeformed white quartz monzonite. On a regional scale, rocks intruded by the white quartz monzonite have experienced higher pressure and temperature conditions and are migmatitic as compared to the surrounding rocks, suggesting a genetic relationship between the white quartz monzonite and tectonic exhumation. We propose that regional shortening imbricated the Big Creek Gneiss, uplifting the now-exposed high-grade rocks of the Big Creek Gneiss (hanging wall of the thrust and wall rock to the white quartz monzonite) and burying correlative rocks, which partially melted to form the white quartz monzonite. This tectonism is attributed to the ca. 1.65 Ga Mazatzal orogeny, as foreland shortening spread progressively into the Yavapai Province. Mazatzal foreland effects have also been described in the Great Lakes region and have been inferred in the Black Hills of South Dakota. We suggest that the crustal-scale rheologic contrast across the Archean-Proterozoic suture, originally developed along the southern margin of Laurentia, and including the Cheyenne belt, facilitated widespread reactivation of that boundary during the Mazatzal orogeny. This finding emphasizes the degree to which crustal heterogeneities can localize subsequent deformation in accretionary orogens, producing significant crustal melting in the distal foreland–a region not typically associated with orogenic magmatism.

Minas Fault Zone: Late Paleozoic history of an intra-continental orogenic transform fault in the Canadian Appalachians

The Minas Fault Zone (MFZ) defines the boundary between the Avalon and Meguma terranes in the Canadian Appalachians and is exposed in mainland Nova Scotia and southern New Brunswick. These terranes originated along the Gondwanan margin, but had accreted to Laurentia by the middle Devonian. The surface trace of the MFZ is adjacent to the southern margin of the LateDevonian–Permian Maritimes Basin. The Late Devonian–Late Carboniferous evolution of the MFZ involves several episodes of oblique dextral shear that resulted in basin formation and inversion and at various times the zone was the focus of magmatism, regional fluid flow and mineralization. In the Late Devonian–Early Carboniferous, asymmetric rifting accompanied by dextral shear produced two coeval sequences: the Horton Group, which is dominated by continental clastic strata, and the Fountain Lake Group, which consists predominantly of bimodal volcanic rocks that overlie high-level plutons emplaced along active shear zones. The overall tectonic environment may have been dominated by dextral transtension along the southern margin of Laurentia, which corresponded with the northern flank of the Rheic Ocean. A major change in the evolution of the Minas Fault Zone occurred in the Late Mississippian–Early Pennsylvanian and produced the E-W Chedabucto Fault, clockwise rotation of pre-existing structures, local zones of transtension and transpression, as well as regional fluid flow and extensive mineralization. This major change may reflect the onset of Laurentia–Gondwana oblique collision, the effects of which continued into the latest Carboniferous with coeval development of flower structures and pull-apart basins in zones of local transpression and transtension.

Petrogenesis of Mesoproterozoic granitic plutons, eastern Llano Uplift, central Texas, USA

The Llano Uplift of central Texas is a gentle structural dome exposing ∼ 1370 to 1230 Ma metaigneous and metasedimentary rocks of Grenville affinity along the southern margin of Laurentia. The metamorphic rocks were subsequently intruded by ∼ 1119 to 1070 Ma late syn- to post-tectonic granites collectively known as the Town Mountain Granite (TMG). The eastern most of the TMG, the Marble Falls (MF), Kingsland (KL), and Lone Grove (LG) plutons, are metaluminous to marginally peraluminous, high-K, calc-alkaline, ferroan, biotite-calcic amphibole granites [Fe/(Fe + Mg) = 0.71–0.92 and 0.78–0.91 for biotite and calcic amphibole, respectively] displaying distinct variation trends with increasing silica content. They are chemically and texturally zoned and have mineralogical and chemical characteristics similar to A-type granites; i.e., 1) Fe-rich biotites, calcic amphiboles, accessory fluorite, and sporadic rapakivi texture, 2) high K 2O (> 4 wt.%), 3) low Al 2O 3 (< 16 wt.%) and CaO (< 3 wt.%), 4) high Fe/(Fe + Mg), 5) enrichments in Zr, Nb, REE, Ga/Al, and 6) depleted Eu. However, in contrast to typical A-type granites (having low Sr and Ba) the MF, KL,and LG plutons are enriched in Sr and Ba; i.e., up to 229 ppm and 1090 ppm, respectively. On granite discrimination diagrams [(K 2O + Na 2O)/CaO vs. Zr + Nb + Ce + Y (ppm) and Zr (ppm) vs. Ga/Al⁎10,000] the KL and MF plutons plot within the A-type field, whereas the LG pluton compositions are divided between A-type and fractionated granite fields (I-, S- and M-types). On tectonic discrimination diagrams (Y vs. Nb ) the MF and KL granites plot in the “within-plate” granite field, but the LG pluton plots across several fields including “within-plate” and “volcanic arc plus syn-collisional” fields. Consequently the tectonic classification on a geochemical basis for the LG pluton is unclear. Based on thermal metamorphic mineral assemblages, normative Q–Ab–Or plots, and Q–Ab–Or–H 2O experimental data (Johannes and Holtz, 1996), crystallization temperatures and pressures are estimated to range from 750 to 850 °C and 200 to 500 MPa, respectively. The assemblage of titanite + magnetite + quartz suggests crystallization at low f O2 [confirmed by Fe/(Fe + Mg) vs. [4] Al microprobe analyses of calcic amphibole] and a water content of less than 1.5 wt.% (Wones, 1989). Like other Town Mountain-type plutons, the MF, KL, and LG granites display comparable iron contents at similar alkali and silica enrichments. Melting models (Ba vs. Sr) suggest the MF, KL, and LG plutons may have evolved from the partial melting (anatexis) of juvenile, tonalitic, lower crustal rocks, followed by plagioclase and pyroxene dominated fractionation. Nd isotopic data for the MF pluton ( ε Nd = + 3.4 at 1.06 Ga; Patchett and Ruiz, 1989) and whole-rock δ 18O values for the MF, KL, and LG plutons (+ 7.0 < δ 18O >+10.1‰; Rangel et al., 2008) suggest that the magmas in the eastern Llano Uplift may contain a significant mantle component, whereas relatively high δ 18O values (+ 9.3 to + 9.7‰; Bebout and Carlson, 1986) for other coeval TMG rocks suggest that a significant crustal component is involved. Whole-rock and trace-element chemistry indicate that the MF and KL plutons, along with the coarser grained textures of the LG pluton, are ‘A-type’ granites. However, with no coeval mafic dikes, syenitic compositions, or volcanic rocks it is clear that the TMG plutons do not represent anorogenic granites. The available evidence is most compatible with emplacement of the TMG plutons in a post-orogenic (Grenville), relaxation and extensional (i.e., slab breakoff) setting.

The Gulf of Mexico and the southern margin of Laurentia

[Mickus et al. (2009][1]; p. 387 in this issue of Geology ) call fresh attention to the origin of the Gulf of Mexico, and particularly to the geodynamic processes that formed its rifted margin north of the oceanic crust forming its central expanse. I cannot hide my sympathy for the perspectives of

Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas

Research Article| January 01, 2008 Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas S. Mosher; S. Mosher 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar J.S.F. Levine; J.S.F. Levine 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar W.D. Carlson W.D. Carlson 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (1): 55–58. https://doi.org/10.1130/G24049A.1 Article history received: 01 May 2007 rev-recd: 05 Sep 2007 accepted: 06 Sep 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. Mosher, J.S.F. Levine, W.D. Carlson; Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas. Geology 2008;; 36 (1): 55–58. doi: https://doi.org/10.1130/G24049A.1 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 SocietyGeology Search Advanced Search Abstract The Llano uplift of central Texas, United States, exposes the core of a Mesoproterozoic orogenic belt that formed along the southern margin of Laurentia during Grenville time. A new collisional model is proposed that reconciles differences in structural stacking, apparent tectonic transport, and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high-pressure eclogitic rocks, emplacement of ophiolitic rocks, and subsequent late-stage to postcollisional plutonism. Our model proposes that subduction with southward polarity resulted in collision of an exotic arc with Laurentia, emplacement of ophiolitic rocks, and telescoping of the intervening basinal sediments, followed by overriding of the arc and margin of Laurentia by a southern continent with transport toward Laurentia. The model further proposes that convergence led to subduction of the Laurentian margin, resulting in high-pressure metamorphism, but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from Laurentia, in a manner similar to that proposed for the Alpine orogeny. Slab breakoff resulted in upwelling of the asthenosphere, leading to intrusion of juvenile granitic plutons. Subduction along strike caused continued contraction that waned with time. The eastern uplift records continent-arc-continent collision, whereas the western uplift records continent-continent collision; the two regions also expose different crustal levels in the orogen. The striking similarity with Phanerozoic orogens, including emplacement of ophiolites and formation of high-pressure rocks, implies that plate tectonic processes including subduction were active prior to the Neoproterozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Olivos Olistostrome: Remnant of a Late Permian Oceanic Basin along the Southwestern Margin of Laurentia, Chihuahua, Mexico

We report on the finding of an olistostrome containing Late Permian basalts (257 ± 7 Ma K-Ar, whole-rock) with MORB affinity embedded within deep-water Jurassic sediments (Upper Bajocian—Middle Callovian) in south-central Chihuahua, northern Mexico, at the southwestern margin of the North American craton (Laurentia). The largest Paleozoic outcrops in northern Mexico occur in the Chihuahua, Caborca, and Cortez terranes; the Los Olivos area is a small outlier with no discernible relation with either of those outcrops. Study of the Sierra de Olivos, in south-central Chihuahua, offers key elements to decipher the Late Paleozoic and Mesozoic evolution of the southern North American craton. Elements of oceanic crust in the olistostrome of Los Olivos are massive and pillowed basalts. Their chemistry shows that they are K-poor (K2O ~0.1%) and TiO2 rich (~0.8%); they are extremely depleted in elements such as Rb, Sr, and Zr, and trace-element compositions place them in the N-MORB and IAT fields of tectonic discrimination diagrams. The basalts, together with associated granitic tectonic blocks exhibiting a well-developed mylonitic texture, lie at the southern margin of Laurentia, and south of the inferred trace of the Mojave-Sonora megashear. They also lie along one of the inferred traces of the Ouachita suture. The Permian basaltic slabs were tectonically emplaced within the Middle Jurassic sedimentary sequence, but we infer that they were deposited as gravity slides. Their contacts with less competent rocks were reactivated, facilitating tectonic displacement during the Hidalgoan orogeny. The deep-marine Middle Jurassic sequence is informally assigned to the Pelayo Formation, which is covered by deep-water Cretaceous sedimentary facies of the Mezcalera Formation and by Tertiary volcanic rocks of the Sierra Madre Occidental magmatic arc. If the radiometric age determination of the oceanic basalts correctly dates the emplacement of these lavas, our study provides evidence on the existence of a Late Paleozoic—Mesozoic oceanic plate, subducted collision of either Gondwanic or Pacific elements with Laurentia, or during accretion of the Guerrero terrane (e.g., the Mezcalera plate). Characteristics of the markedly epiclastic Middle Jurassic units that host the olistostrome supports the assumption of an active margin setting for Jurassic deposition at the southern extreme of the Laurentia craton.

U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada

U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analysis of detrital zircon in 11 sandstone samples from the late Paleo-proterozoic Dubawnt Supergroup of the Baker Lake and Thelon Basins are presented here. Alluvial fan and braided stream facies of the ca. 1.84–1.78 Ga Baker Sequence yield age spectra with significant groupings at 2.70 and 2.60 Ga, reflecting provenance chiefly from local sources in the northwestern Hearne domain of the western Churchill Province. Age spectra are consistent with sed-imentological and stratigraphic information, which indicate provenance from the proximal uplifted shoulders of an intracontinental rift that developed in response to inboard effects of the Hudsonian orogeny (collision of Superior craton into western Churchill Province). The unconformably overlying 1.76–1.74 Ga Whart Sequence exhibits a more varied provenance and has an abundance of 2.0–1.9 Ga detrital zircon. These ages match those of synorogenic plutons in the Taltson-Thelon orogen (collision of Slave craton into the western Churchill Province). Provenance from the Taltson-Thelon orogen is supported by easterly paleocurrents and Rb-Sr cooling ages from mafic dikes that indicate reactivated collision of the Slave craton and related uplift and denudation coeval with deposition of the Whart Sequence. The depositional age of the Whart Sequence is constrained by intercalated rhyolite flows with conventional 207 Pb/ 206 Pb crystallization ages of 1757.6 ± 3.4 Ma and 1753.0 ± 1.7 Ma (2σ). The thermal effects of this collision also are recorded in isotopic data from basement rocks throughout the western Churchill Province and in syndepositional to postdepositional faults that parallel the main boundary faults of the Slave indentor. Late reactivation and associated metamorphism, faulting, and basin development could be the result of the inboard effects of subduction and related collision along the southern margin of Laurentia. The detrital zircon geochronology of the Barrens Sequence supports recycling of local supracrustal sequences and weathering of underlying granitic gneisses. Zircons of Hudsonian affinity (ca. 1.9–1.8 Ga) are nearly absent, which argues against a distant eastern source region for the Barrens Sequence. The Dubawnt Supergroup detrital zircon age data set shows concentrations ca. 2.75–2.6 Ga, 2.1–1.9 Ga, and 1.85–1.75 Ga, which are concordant with the known ages of major juvenile crust–forming events leading up to final amalgamation of Late Archean and Paleoproterozoic supercontinents. Detritus of this age was shed cratonward and captured by interior basins, such as the Baker Lake and Thelon Basins, during major collisional events that reworked the margins of the western Churchill Province (e.g., Hudsonian and Thelon-Taltson orogenies).

Proterozoic history and crustal evolution in southwestern Colorado: Insight from U/Pb and Sm/Nd data

New U/Pb age constraints and Sm/Nd isotope data further establish the timing of magmatic and tectonic events from 1.8 to 1.4 Ga in southwestern Colorado, and give insight into the interaction of crust and mantle systems during the assembly and evolution of orogenic belts along the southern margin of Laurentia. Proterozoic orogenic events in the Needle Mountains led to crustal assembly, thickening and tectonic reworking that was punctuated by periods of relative crustal stability. Pre-1.75 Ga arc successions in southwestern Colorado were produced and assembled into juvenile crust in an orogenic period that culminated in regional deformation and metamorphism that accompanied intrusion of synorogenic plutonic rocks from 1.73 to 1.71 Ga. Emplacement of post-orogenic plutonic rocks at ∼1.70 Ga was followed by uplift and erosion, and the deposition and deformation of siliciclastic sedimentary sequences. Paleoproterozoic crust underwent local melting and thermal metamorphism during invasion of felsic to mafic magmas at ∼1.43 Ga that coincided with a pulse of intracratonic deformation. Invasion and melting of first-cycle 1.8–1.7 Ga crust at ∼1.71 and ∼1.43 Ga involved heat and volatiles from mantle melts that were a catalyst for crustal melting and magmatic recycling. Sm/Nd isotopic signatures of 1.8–1.4 Ga volcanic and plutonic rocks in southwestern Colorado lend evidence to the emplacement of melts from depleted mantle and juvenile crust in the production continental crust, with minor, if any, contamination by crust older than 1.8 Ga. The Proterozoic magmatic record in southwestern Colorado is consistent with continental growth by the addition of juvenile crust extracted from the mantle, followed by periodic intracrustal growth during orogenic pulses in which mantle magmas underplate and melt the crust with consequent thickening.

The uranium mineralization potential of the Paleoproterozoic Sioux Basin and its relationship to other basins in the southern Lake Superior region

The Paleoproterozoic Sioux Basin and the Baraboo, Barron, Flambeau, McCaslin, and Waterloo quartzites in northcentral USA are all supermature quartz arenites that are among the world's oldest redbeds. These quartzites were deposited on a stable craton at ca. 1750 Ma, following exhumation and uplift of Penokean crust and major orogenic activity related to the amalgamation of Archean microcontinents at ca. 2.0–1.8 Ga to form Laurentia. 40Ar/ 39Ar ages of 1615–1543 Ma from diagenetic muscovite in the Sioux Basin provide the first geochronological evidence for Mazatzal-related deformation and low-grade metamorphism of the quartzites during subsequent convergence on the southern margin of Laurentia. One muscovite sample from the Sioux Basin has a 40Ar/ 39Ar age of 1465 Ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40Ar/ 39Ar ages for muscovite from the Baraboo and Waterloo quartzites. Thus, the Sioux Basin and other quartzites in the region have experienced a similar tectonic and fluid history. The Sioux Basin has similar sedimentologic characteristics as other Paleoproterozoic basins in North America, including the uranium-rich Athabasca Basin, Canada, that is host to some of the world's largest and highest-grade unconformity-type uranium deposits. We compared key characteristics, such as the stratigraphy, character of diagenetic fluids, paleoaquifer characteristics, and composition of leachable Pb in the sandstones from the Sioux Basin with the Athabasca Basin in order to evaluate the fluid history of the basin and the potential of the Sioux Basin to host unconformity-type uranium mineralization. Based on the hydrogen and oxygen isotopic composition of muscovite (M1) that formed during diagenesis at 150–200 °C, the pore fluids that were present during peak diagenesis in the Sioux Basin had δ 18O and δD values of −0.5 to +5.2‰ and −32 to −46‰, respectively, similar to those of the fluids associated with uranium mineralization and alteration in the Athabasca Basin. However, fluid flow in the Sioux Basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers, in contrast to the extensive paleoaquifers in the Athabasca Basin that promoted large-scale diagenetic reactions and alteration that led to the formation of unconformity-type uranium mineralization. In addition, the non-radiogenic isotopic composition of leachable Pb in the Sioux Basin sandstones indicates that the majority of the Pb is from a normal crustal source, inconsistent with mobilization of radiogenic Pb from a uranium-rich source, such as a uranium deposit. Although the Sioux Basin shares similar general characteristics to the Athabasca Basin, the results of this study suggest that it has a lower potential than the Athabasca Basin to host significant high-grade unconformity-type deposits.

Kinematic Constraints on Rodinia Reconstructions from the Core of the Texas Grenville Orogen

Mesoproterozoic metamorphic rocks exposed in the Llano Uplift, central Texas, were involved in a ∼1.15–1.12 Ga Grenville‐age collisional orogenic event along the southern margin of Laurentia. This event is characterized by polyphase ductile deformation synchronous with transitional amphibolite‐, granulite‐ to eclogite‐facies metamorphism. Results of detailed structural mapping in the southeastern part of the Llano Uplift show that these rocks record a deformational history involving northeast‐directed ductile thrusting and regional‐scale polyphase folding, all related to northward collision of a southern continent and an arc with the southern margin of Laurentia. We use the structural, metamorphic, and kinematic history of the southeastern Llano uplift to critically evaluate collisional models for the Grenville orogen in Laurentia and discuss the implications for reconstructions of Rodinia.