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At least 649 records · Page 36Linked to original sources
USGS2004

Age and evolution of the Precambrian crust of the Tobacco Root Mountains, Montana

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger (metamorphosed) mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90–1.77 Ga. The oldest U-Pb age reported here (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak Metamorphic Suite, although younger ages of clearly detrital grains suggest the protolith was deposited subsequent to 3.2 Ga. Alternatively, a Pb-Pb age of ca. 2.45 Ga from a single subhedral zircon from this quartzite suggests the quartzite, and perhaps other Spuhler Peak Metamorphic Suite lithologies, may have formed in the Proterozoic. An Archean age, however, seems most compatible with the Archean Sm-Nd model ages of mafic and metasedimentary components of the Spuhler Peak Metamorphic Suite and the age distribution of zircons from the quartzite, which is very similar to the age distribution present in Archean quartzites in the region. The Spuhler Peak Metamorphic Suite lies in tectonic contact with volumetrically dominant, Archean, quartzofeldspathic gneisses and intercalated metasedimentary rocks. The protoliths of these gneisses were apparently emplaced 3.2–3.4 Ga, and are interpreted to be the basement upon which the intercalated (meta)sedimentary rocks were deposited. U-Pb analyses of zircons from anatectic leucosome near the boundary between the gneisses and the Spuhler Peak Metamorphic Suite, however, yield a significant population of 1.77 Ga grains, which are interpreted to have crystallized from the leucosome. All other grains are Archean (to 3.48 Ga) and interpreted to derive from the metasedimentary source of the leucosome. In addition, U-Pb analyses of zircons extracted from a granulite facies mafic dike that cuts across Archean gneissic banding indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga. Structural, petrologic, and geochronologic data suggest all lithologies experienced granulite facies metamorphism at ca. 1.77 Ga and that the Spuhler Peak Metamorphic Suite was tectonically emplaced after 2.06 Ga, but before 1.77 Ga. This Paleoproterozoic tectonic activity is most likely a result of burial during terrane collision (e.g., the juxtaposition of the Wyoming and Hearne provinces) and/or to postcollisional mafic underplating.

Montana↗
USGS1971

Plate tectonics and magmatic evolution

The validity of the general idea of plate tectonics is accepted; the magmas evolved along the spreading ridges are thought to be largely tholeiitic basalt, although alkalic olivine basalt and ultramafic rocks of several kinds have also been dredged from them. The ultramafics may be residual from the partial melting of pyrolite while the tholeiite was being formed at shallower depths, or they may possibly be fragments of the mantle raised by the injection of sills. Bouvet and Jan Mayen Islands, both on the crest of the Mid-Atlantic Ridge, are largely composed of alkali basalt with very minor differentiates of trachyte and even rhyolite that may be readily accounted for by differentiation at a high level in the volcanic edifice. Iceland, though, has so much granite and rhyolite widely distributed that it seems likely, as suggested by several students, that its basement is sialic. The volcanic islands tend to be more alkalic the farther they are from the ridges; perhaps they rose from deeper sources in areas of low heat flow and are not related to plate margins. If the African Rifts are incipient plate margins, it is noteworthy that the magmas associated with them are wholly different from the tholeiites of the oceanic ridges. They are among the most highly alkaline of any rocks known. The magmatic activity at the subduction zones, where the plates are being destroyed, is very different. There are three varieties of these plate junctions: continental against oceanic, oceanic against oceanic, and continental against continental. In both the junctions involving oceanic crust the material being consumed includes a variable thickness of sediment, underlain by 5 or 6 km of tholeiitic basalt overlying the downgoing mantle. These rocks are much less refractory than the pyrolite of the mantle and must surely compose a large part of material parental to the magmas formed along the subduction zones, the andesites, granodiorites, and granites. There is nowhere the tremendous volume of intermediate rocks that would have had to be formed if these voluminous magmas had been products of crystallization differentiation from a basaltic magma. The presently most active of the continent-continent junctions is along the Himalayas where India is underthrusting the continent of Asia; here there is no evidence of magmatism except along the transcurrent faults at either end of the main range. But there are large volcanic and plutonic masses that have no obvious relation to the plate boundaries active in Mesozoic and Cenozoic time. The Eogene volcanics of the San Juans and the Neogene volcanics of the Yellowstone are more than 1,500 km from any obvious subduction zone, and these regions of magmatic activity seem no more closely related to subduction zones than are the Tertiary igneous rocks of West Texas, the Cretaceous tuffs and plutons of Arkansas, the Cretaceous intrusives of the Monteregian Hills, and the minor Tertiary intrusives of Virginia.

Washington↗
USGS1976

Plate tectonic model for the evolution of the eastern Bering Sea Basin

The eastern Bering Sea Basin, composed of the Aleutian and Bowers Basins, is flanked to the north by Mesozoic foldbelts that probably represent zones of plate subduction in Mesozoic time. Present plate subduction occurs 400 to 1,000 km farther south, at the Aleutian Trench. North-south magnetic lineations that formed at an oceanic spreading ridge, probably in Mesozoic time (117 to 132 m.y. ago), have been identified in the Aleutian Basin. The orientation and age of those anomalies can be explained by reconstructing Kula-Farallon Pacific plate motions during late Mesozoic–early Tertiary time. In Mesozoic time, subduction of the Kula plate occurred north of the Aleutian Trench near the present location of the Bering Sea continental margin. At about 70 m.y. B.P. (Late Cretaceous), the zone of subduction shifted south to the present location of the Aleutian Trench, thereby trapping a fragment of oceanic plate imprinted with north-south magnetic lineations within the eastern Bering Sea Basin. A stable basin framework has prevailed behind the Aleutian arc since early Tertiary time.

GSA Bulletin↗
USGS1981

Early evolution of the Bering Sea by collision of oceanic rises and North Pacific subduction zones

Three major bathymetric features exist in the Bering Sea: Shirshov Ridge, Bowers Ridge, and Umnak Plateau. New refraction data over Umnak Plateau and previous geophysical data across Bowers Ridge indicate that a thickened welt of crustal material is present beneath both features. The crustal structure is transitional between oceanic and continental types. Various models for the origin of these features have been investigated. One that has not been proposed previously assumes that the protostructures of Bowers Ridge and Umnak Plateau could have formed outside of the present Bering Sea. According to this model, before formation of the Aleutian Ridge in late Mesozoic or earliest Tertiary time, these protostructures moved into their present Bering Sea positions. Prior to the arrival of these two structures in the Bering Sea, oceanic crust was subducted along the Bering continental margin connecting Alaska and Siberia. The collision of the Umnak Plateau protostructure with the southeastern edge of the margin may have caused subduction to terminate here and move southward. The new southerly position of subduction beneath the Aleutian Ridge was therefore controlled by late Mesozoic or early Tertiary locations of Umnak Plateau, Bowers Ridge, and possibly, the north-trending Shirshov Ridge farther to the west.

GSA Bulletin↗
USGS1985

The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin

The Dunbar dome in northeastern Wisconsin is a critical structural feature in the early Proterozoic Penokean orogen. It provides exposures of gneisses (Dunbar Gneiss) that structurally underlie the voluminous metavolcanic rocks of northeastern Wisconsin, and exposures of abundant granitoid rocks ranging from tonalite to granite. The granitoid rocks cut both the gneisses in the core and the supracrustal (cover) metavolcanic rocks and were emplaced essentially along the core-cover boundary. The Dunbar Gneiss is calc-alkaline and was derived from volcanic and intrusive rocks of intermediate composition. The various intrusive rocks have calcic, calc-alkaline, and alkali to alkali-calcic compositions, and they progress with time to more SiO 2 and K 2 -rich compositions. U-Pb zircon ages indicate that accumulation of the layered rocks in the core and cover, deformation and metamorphism, and intrusion of the granitoid rocks spanned a relatively short time, ∼1865–1835 Ma. We interpret the dome as being a large-scale, fold-interference structure resulting from polydeformation modified by diapirism. Northeast-oriented folds (F 3 ) and a related mylonitic foliation (S 3 ), nearly confined to the dome, are superposed on northwest-oriented folds (F 2 ) that developed during regional deformation. In the core-cover boundary, these structures are obliterated by a zone of intense deformation—a mylonitic foliation and a steeply plunging stretching lineation—as much as 500 m wide, which we interpret as resulting from diapirism. Metamorphic zoning is concentric: amphibolite facies in inner parts of the mantle and greenschist facies in the outer part of the mantle. The Wisconsin magmatic terrane, as represented by the rocks in the Dunbar dome, differs from the epicratonic, early Proterozoic sedimentary-volcanic sequence (Marquette Range Supergroup) in Michigan, to the north, in stratigraphy, structure, and volume and composition of igneous rocks. Whereas the basalts in northern Michigan are compositionally similar to continental rift basalts, the volcanic rocks in the Dunbar dome have over-all island-arc compositional affinities. The over-all calc-alkaline compositions of the intrusive rocks are similar to those in magmatic arcs formed at convergent plate-margin settings. Accordingly, we interpret the Wisconsin magmatic terrane as an oceanic-arc complex that was sutured to the North American continent during development of the Penokean orogen. Similar interpretations based on broad regional observations have been proposed previously.

Michigan, Wisconsin↗
USGS1985

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Alaska↗
USGS1989

The structural geometry and evolution of foreland thrust systems, northern Virginia

Seismic reflection data reveal that the structural geometry of the central Appalachians of northern Virginia consists of three distinct thrust systems. Each thrust system is characterized by a unique internal geometry.The Blue Ridge thrust sheet is a composite thrust sheet composed primarily of imbricated Precambrian crystalline rocks. It over-rode Cambrian-Ordovician carbonates and formed a sheared, basement-cored fault-bend fold. Thrusts within the sheet may be Taconic and earliest Alleghanian, whereas final thrusting and emplacement of the sheet were probably slightly younger but still early Alleghanian. The North Mountain thrust sheet is characterized by imbricated Cambrian-Ordovician carbonates that are deformed into large-amplitude mode II fault-bend folds and fault-propagation folds. Rocks of this sheet were transported more than 60 km across a similar section of carbonates. The leading edge of the North Mountain thrust sheet was deformed into a fold with a mode II fault-bend fold geometry and was juxtaposed against middle Paleozoic rocks. The middle Paleozoic rocks occur in a ramp across which displacement along the North Mountain thrust was transferred to a higher detachment. More than 60 km of cover rocks displaced during the emplacement of the North Mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western Valley and Ridge province or were backthrust above the sheet. The timing for imbrication and emplacement of the North Mountain thrust sheet is probably Main Phase Alleghanian. The Lower Carbonate duplex extends from beneath the Blue Ridge and North Mountain thrust sheets, westward across the western Valley and Ridge province. The Lower Carbonate duplex is characterized by imbricated Cambrian-Ordovician carbonates that form low-amplitude fault-bend folds. This thrust system also probably formed during Main Phase Alleghanian deformation.

Virginia↗
USGS1991

Evolution of a Permo-Triassic sedimentary melange, Grindstone terrane, east-central Oregon

The Grindstone terrane in east-central Oregon is one of the few areas in western North America where large blocks of unmetamorphosed Devonian, Mississippian, and Permian limestones are inter mixed with Permian and Lower Triassic radiolarian chert and Pennsylvanian?, Permian, and Triassic volcaniclastic rocks. Although originally described as parts of a coherent succession, we interpret the Grindstone rocks to be a sedimentary mélange composed of Paleozoic limestone slide and slump blocks that became detached from a carbonate shelf fringing a volcanic knoll or edifice in Late Permian to Middle Triassic time and were intermixed with Permian and Triassic slope to basinal clastic and volcaniclastic rocks in a forearc basin setting. Paleogeographic affinities of the Grindstone limestone faunas and volcaniclastic debris in the limestone and clastic rocks all indicate deposition in proximity to an island-arc system near the North American craton. The Grindstone terrane deposits are unconformably overlain by Upper Triassic to Middle Jurassic sequences of the Izee terrane. Although lithologic and faunal differences indicate that the Grindstone and Izee terranes together represent a tectonic block separate from the adjacent Baker terrane, all three terranes were juxtaposed by Late Triassic or Early Jurassic time.

Oregon↗
USGS1997

Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons

The Solitario is a large, combination laccolith and caldera (herein termed “laccocaldera”), with a 16-km-diameter dome over which developed a 6×2 km caldera. This laccocaldera underwent a complex sequence of predoming sill, laccolith, and dike intrusion and concurrent volcanism; doming with emplacement of a main laccolith; ash-flow eruption and caldera collapse; intracaldera sedimentation and volcanism; and late intrusion. Detailed geologic mapping and 40 Ar/ 39 Ar dating reveal that the Solitario evolved over an interval of approximately 1 m.y. in three distinct pulses at 36.0, 35.4, and 35.0 Ma. The size, duration, and episodicity of Solitario magmatism are more typical of large ash-flow calderas than of most previously described laccoliths.

Texas↗
USGS1998

Structural and kinematic evolution of the Yukon-Tanana upland tectonites, east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly

The Yukon-Tanana terrane, the largest tectonostratigraphic terrane in the northern North American Cordillera, is polygenetic and not a single terrane. Lineated and foliated (L-S) tectonites, which characterize the Yukon-Tanana terrane, record multiple deformations and formed at different times. We document the polyphase history recorded by L-S tectonites within the Yukon-Tanana upland, east-central Alaska. These upland tectonites compose a heterogeneous assemblage of deformed igneous and metamorphic rocks that form the Alaskan part of what has been called the Yukon-Tanana composite terrane. We build on previous kinematic data and establish the three-dimensional architecture of the upland tectonites through kinematic and structural analysis of more than 250 oriented samples, including quartz c-axis fabric analysis of 39 samples. Through this study we distinguish allochthonous tectonites from parautochthonous tectonites within the Yukon-Tanana upland. The upland tectonites define a regionally coherent stacking order: from bottom to top, they are lower plate North American parautochthonous attenuated continental margin; continentally derived marginal-basin strata; and upper plate ocean-basin and island-arc rocks, including some continental basement rocks. We delineate three major deformation events in time, space, and structural level across the upland from the United States-Canada border to Fairbanks, Alaska: (1) pre-Early Jurassic (>212 Ma) northeast-directed, apparent margin-normal contraction that affected oceanic rocks; (2) late Early to early Middle Jurassic (>188-185 Ma) northwest-directed, apparent margin-parallel contraction and imbrication that resulted in juxtaposition of the allochthonous tectonites with parautochthonous continental rocks; and (3) Early Cretaceous (135-110 Ma) southeast-directed crustal extension that resulted in exposure of the structurally deepest, parautochthonous continental rocks. The oldest event represents deformation within a west-dipping (present coordinates) Permian-Triassic subduction zone. The second event records Early to Middle Jurassic collision of the arc and subduction complex with North American crust, and the third event reflects mid-Cretaceous southeast-directed crustal extension. Events one and two can be recognized and correlated through southern Yukon, even though this region was affected by mid-Cretaceous dextral shear along steep northwest-striking faults. Our data support a model of crustal assembly originally proposed by D. Tempelman-Kluit in which previously deformed allochthonous rocks were thrust over parautochthonous rocks of the attenuated North American margin in Middle Jurassic time. Approximately 50 m.y. after tectonic accretion, east-central Alaska was dissected by crustal extension, exposing overthrust parautochthonous strata.

Alaska↗
USGS2000

Influence of El Niño–Southern Oscillation (ENSO) events on the evolution of central California's shoreline

Significant sea-cliff erosion and storm damage occurred along the central coast of California during the 1982–1983 and 1997–1998 El Niño winters. This generated interest among scientists and land-use planners in how historic El Niño–Southern Oscillation (ENSO) winters have affected the coastal climate of central California. A relative ENSO intensity index based on oceanographic and meteorologic data defines the timing and magnitude of ENSO events over the past century. The index suggests that five higher intensity (relative values 4–6) and 17 lower intensity (relative values 1–3) ENSO events took place between 1910 and 1995. The ENSO intensity index correlates with fluctuations in the time series of cyclone activity, precipitation, detrended sea level, wave height, sea-surface temperature, and sea-level barometric pressure. Wave height, sea level, and precipitation, which are the primary external forcing parameters in sea-cliff erosion, increase nonlinearly with increasing relative ENSO event intensity. The number of storms that caused coastal erosion or storm damage and the historic occurrence of large-scale sea-cliff erosion along the central coast also increase nonlinearly with increasing relative event intensity. These correlations and the frequency distribution of relative ENSO event intensities indicate that moderate- to high-intensity ENSO events cause the most sea-cliff erosion and shoreline recession over the course of a century.

California↗
USGS1985

Early Tertiary marine fossils from northern Alaska: Implications for Arctic Ocean paleogeography and faunal evolution

Marine mollusks and ostracodes indicate a post-Danian Paleocene to early Eocene (Thanetian to Ypresian) age for a fauna from the Prince Creek Formation at Ocean Point, northern Alaska, that also contains genera characteristic of the Cretaceous and Neogene-Quaternary. The life-association of heterochronous taxa at Ocean Point resulted from an unusual paleogeographic setting, the nearly complete isolation of the Arctic Ocean from about the end of the Cretaceous until sometime in the Eocene, in which relict Cretaceous taxa survived into Tertiary time while endemic taxa evolved in situ; these later migrated to the northern mid-latitudes. Paleobiogeographic affinities of the Ocean Point association with mild temperate faunas of the London Basin (England), Denmark, and northern Germany indicate that a shallow, intermittent Paleocene seaway extended through the Norwegian-Greenland Sea to the North Sea Basin. Early Tertiary Arctic Ocean paleogeography deduced from faunal evidence agrees with that inferred from plate-tectonic reconstructions.

Geology↗
USGS1987

Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton.

Colorado, New Mexico, Wyoming↗
USGS1991

Archean and Proterozoic crustal evolution: Evidence from crustal seismology

Seismic-velocity models for Archean and Proterozoic provinces throughout the world are analyzed. The thickness of the crust in Archean provinces is generally found to be about 35 km (except at collisional boundaries), whereas Proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high-velocity (>7.0 km/s) layer at the base. We consider two models that may explain these differences. The first model attributes the difference to a change in the composition of the upper mantle. The higher temperatures in the Archean mantle led to the eruption of komatiitic lavas, resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt. Proterozoic crust developed above fertile mantle, and subsequent partial melting resulted in basaltic underplating and crustal inflation. In the second model, convection in the hot Archean mantle is considered to have been too turbulent to sustain stable long-lived subduction zones. By the Proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed, and the high-velocity basal layer was formed by basaltic underplating.

Geology↗
USGS1994

Origin and evolution of the Seattle Fault and Seattle Basin, Washington

Analysis of seismic reflection data reveals that the Seattle basin (Washington) is markedly asymmetric and consists of ∼9-10 km of Eocene and younger deposits. The basin began as a discrete geologic element in the late Eocene (∼40 Ma), the result of a reorganization in regional fault geometry and kinematics. In this reorganization, dextral offset on the Puget fault south- east of Seattle stepped eastward, and the Seattle fault began as a restraining transfer zone. North-vergent reverse or thrust faulting on the Seattle fault forced flexural subsidence in the Seattle basin to the north. Offset on the Seattle fault and subsidence of the Seattle basin have continued to the present.

Geology↗
USGS2007

The Pennsylvanian-early permian bird spring carbonate shelf, Southeastern California: Fusulinid biostratigraphy, paleogeographic evolution, and tectonic implications

The Bird Spring Shelf in southeastern California, along with coeval turbidite basins to the west, records a complex history of late Paleozoic sedimentation, sea-level changes, and deformation along the western North American continental margin. We herein establish detailed correlations between deposits of the shelf and the flanking basins, which we then use to reconstruct the depositional history, paleogeography, and deformational history, including Early Permian emplacement of the regionally significant Last Chance allochthon. These correlations are based on fusulinid faunas, which are numerous both on the shelf and in the adjoining basins.

Special Paper of the Geological Society of America↗