Donald T. Secor

Delamination in collisional orogens

During continental collision, the weight of the downgoing slab may lead to extensional shearing in the descending plate, permitting separation of the downdip, dense oceanic lithosphere from the adjacent lighter continental lithospher. Separation may occur by means of a lithosphere-penetrating normal-sense shear zone, by means of pure shear necking of the lithosphere, or by some combination of these mechanisms. During separation, the continental crust attached to the lower plate may undergo an episode of extensional tectonics. The proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism. If the continents continue to converge following delamination, a compressive tectonic regime will be reestablished in the crust. The subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat Moho. The likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime, as well as underplating associated with syntectonic plutonism, may account for the seismic complexities observed at Moho depths beneath some mountain belts.

A model for the development of crenulations in shear zones with applications from the Southern Appalachian Piedmont

Abstract Two sets of crenulations are associated with a major, Alleghanian, dextral shear zone which deformed stratigraphic and structural boundaries in the eastern Piedmont of South Carolina. Both sets of crenulation planes are oblique to the boundaries of the shear zone. The morphologies and orientations of the crenulation sets and their spatial distributions indicate that they are related to slip along foliation planes, and that they serve to compensate for displacement components of foliation slip normal to the overall movement direction in the shear zone. The crenulations act to maintain the initial thickness of the shear zone. Our evaluation of the recent literature on shear zones suggests that crenulations related to foliation slip are abundant and constitute a reliable sense of shear indicator.

The Carolina Zone: overview of Neoproterozoic to Early Paleozoic peri-Gondwanan terranes along the eastern Flank of the southern Appalachians

Abstract The Carolina Zone is an amalgamation of mainly Neoproterozoic–Early Paleozoic metaigneous-dominated terranes that are clustered along the eastern flank of the southern Appalachians. These terranes are distinguished from other divisions of the orogen by a commonality in gross geologic content and by their close spatial association. They are considered exotic relative to Laurentia on the basis of stratigraphic and tectonic evolution, paleontology, and position in the orogen analogous to that of exotic terranes in the northern Appalachians. They are probably peri-Gondwanan in origin. Within this first-order identity, the terranes exhibit remarkable heterogeneity, with respect to deposition, magmatism, and tectonothermal overprint. The depositional–magmatic history of the zone is viewed in three broad stages, including: (I) pre-600 Ma, (II) ca. 590–560 Ma, and (III) younger than ca. 550 Ma. Although each stage records significant felsic volcanism, there are few compelling stratigraphic linkages between terranes. Stage III plutonism may form a link between the two largest terranes in the zone. The isotopic evolution of the zone reflects the stratigraphic heterogeneity; isotopically juvenile magmatism in some terranes is coeval with more crustally evolved magmatism in others. The tectonothermal history of the zone is heterogeneous, producing a patchwork of suprastructural and infrastructural elements of different ages. Major tectonothermal events responsible for this pattern span the Neoproterozoic–earliest Cambrian, the Late Ordovician–Silurian, and the late Paleozoic. Evidence for regionally extensive events in the zone is sparse and such a fundamental concept as its time of accretion to Laurentia is speculative. The central Piedmont shear zone, a late Paleozoic ductile thrust that defines the western limit of the Carolina Zone, marks the final emplacement of the zone against Laurentian elements.

Confirmation of the Carolina Slate Belt as an Exotic Terrane

An assemblage of Middle Cambrian Atlantic faunal province trilobites has been found in the rocks of the Carolina slate belt near Batesburg, South Carolina. Geologic and paleomagnetic data suggest that the Carolina slate belt and the adjacent Charlotte belt constitute an exotic terrane that was accreted to North America in early to middle Paleozoic time.

Character of the Alleghanian orogeny in the southern Appalachians: Part III. Regional tectonic relations

Geological and geochronological studies in the eastern Piedmont of the southern Appalachians have documented a polyphase late Paleozoic deformational chronology (D 2 –D 4 ) that is contemporaneous with the Alleghanian orogeny recorded in the western Appalachian foreland. The scale and vergence of D 3 anticlinoria in the eastern Piedmont are similar to those of anticlinoria in the western Appalachians which are related to ramping of underlying thrust surfaces. We suggest that the Appalachian decollement extends southeastward beneath the entire Piedmont Province and that all of the Piedmont rocks were displaced at least 175 km northwestward relative to North America during the Alleghanian orogeny. Late Paleozoic deformational effects in the eastern Piedmont thus are considered cogenetic with Alleghanian foreland deformation. We interpret the Alleghanian orogeny to be the result of oblique, dextral convergence and collision of Gondwana and Laurentia.

Character of the Alleghanian orogeny in the southern Appalachians: Part II. Geochronological constraints on the tectonothermal evolution of the eastern Piedmont in South Carolina

A nearly concordant U-Pb zircon age of 550 ± 4 Ma is interpreted to closely date crystallization of the epizonal Little Mountain metatonalite in the southeastern part of the Charlotte belt in South Carolina. This confirms field studies which indicate that the Charlotte belt contains a plutonic metaigneous complex that developed as a sub-volcanic-arc infrastructure, contemporaneous with vulcanism manifested in the Carolina slate belt. Both paleontological and geochronological controls indicate that the South Carolina slate belt is mostly younger than 570 Ma (Cambrian?), whereas the slate belt in North Carolina and Virginia is mostly of late Proterozoic age. A regionally significant mid-Paleozoic (ca. 340–360 Ma) thermal event is suggested by discordant 40Ar/39Ar whole-rock age spectra of slate/phyllite in the northwestern Carolina slate belt and from hornblende in the southeastern Charlotte belt. It is uncertain if this event was associated with deformation in the eastern Piedmont; however, mid-Paleozoic deformation has been previously documented elsewhere in the western Piedmont. A slightly disconcordant U-Pb zircon age of 317 ± 4 Ma is interpreted to closely date initial crystallization of the deformed Edge-field granite and confirms a record of late Paleozoic penetrative deformation in the Kiokee belt of South Carolina. U-Pb isotope data for the Lake Murray orthogneiss and Clouds Creek granite are discordant and suggest that the magmas of these plutons were derived by partial melting of a sialic Precam-brian source and then emplaced in the eastern Piedmont at ca. 315 Ma (prior to or during the early stages of the Alleghanian orogeny). The thermal maximum of Alleghanian regional metamorphism (amphibolite facies in the Kiokee belt; greenschist facies in the southeastern part of the Carolina slate belt) occurred during ca. 295–315 Ma. During the late Carboniferous and Early Permian, the eastern Piedmont experienced differential uplift, erosion, and relatively rapid postmeta-morphic cooling. Isothermal surfaces were folded into an antiform-synform-antiform configuration corresponding to the Kiokee, Carolina slate, and Charlotte belts, respectively. The geochronological data provide the following calibration for the late Paleozoic deformational chronology recorded in the Kiokee and Carolina slate belts: D2 (Lake Murray deformation), ca. 295–315 Ma; D3 (Clarks Hill deformation), ca. 285–295 Ma; and D4 (Irmo deformation), ca. 268–290 Ma.

Geology of the Spring Mountains, Nevada

The northwest-trending Spring Mountains, Nevada, contain a 45-mi-wide (75-km) cross section of the eastern part of the North American Cordilleran orogenic belt and geosyncline. This cross section is probably the most southerly exposed section which exhibits structure and stratigraphy “typical” of the eastern part of the Cordillera. Stradgraphically, the transition from Paleozoic craton to miogeosyncline is present from east to west across the Spring Mountains. The sedimentary succession through the middle Permian thickens from 8,800 ft (2,660 m) east of the Spring Mountains to approximately 30,000 ft (9,000 m) in the west. Thickening of individual formations accounts for 6,800 ft (2,070 m) of added section, addition of formations at unconformities accounts for 4,600 ft (1,400 m) of added section, and addition of a thick terrigenous late Precambrian sequence accounts for 9,800 ft (3,000 m) of added section. Three major thrust plates are exposed in the Spring Mountains, each structurally higher plate containing a thicker sequence of Paleozoic rocks. The easternmost thrust is the Keystone thrust, except where the earlier Red Spring thrust plate is present below the Keystone as isolated remnants. The Keystone thrust appears to be a decollement thrust, but complications at depth suggest that additional thrust slices may be present below the thrust or several thousand feet of late Precambrian terrigenous rocks may be present above the thrust. The structurally higher Lee Canyon thrust plate probably contains at least 4,000 ft (1,200 m) of these terrigenous rocks at its base, and the Wheeler Pass thrust plate contains at least 11,000 ft (3,300 m) of these rocks. Pregeosynclinal basement could be involved in some of the higher thrust plates, particularly the Wheeler Pass plate, but depths of exposure are inadequate to determine its role. Thrust faulting has produced a shortening of from 22 to 45 mi (36.6 to 75 km) in the geosynclinal rocks based on geometric constructions of cross sections at depth. This range probably represents a minimum figure. Some folding and thrusting occurred during the early Late Cretaceous, but data within the Spring Mountains only establish a much wider time bracket, post–Early Jurassic to pre–late Cenozoic for the easternmost thrust faults and post–Early Permian to pre–late Cenozoic for the westernmost thrusts.

Character of the Alleghanian orogeny in the southern Appalachians: Part I. Alleghanian deformation in the eastern Piedmont of South Carolina

The eastern Piedmont Province in South Carolina contains a sequence of Cambrian volcanic and sedimentary rocks that was penetratively deformed (D1) and regionally metamorphosed (M1) to the greenschist facies during the early and/or middle Paleozoic. The eastern Piedmont was subsequently affected by late Paleozoic (Alleghanian) polyphase deformation (D2–D4) and regional metamorphism. The earliest Alleghanian event (D2) is associated with amphibolite facies regional metamorphism and felsic plutonism in a mid-crustal infrastructure at ca. 295–315 Ma. The gradational interface between infrastructure and overlying suprastructure contained a steep M2 metamorphic gradient (between amphibolite facies below and greenschist facies above), numerous sheets of felsic orthogneiss, and a deformation front marking the upper limit of intense D2 penetrative deformation. D3 is represented by northwestward-vergent folding of S1 and S2 foliations and M2 isothermal surfaces. The Kiokee belt in South Carolina is interpreted as a D2 infrastructure exposed within the core of a D3 antiform. This interpretation suggests that an Alleghanian infrastructure may be present in the subsurface beneath much of the Piedmont and Coastal Plain. The polygenetic Modoc zone forms the northwest boundary of the Kiokee belt. It is interpreted to represent an interface between D2 infrastructure and suprastructure which was rotated into a steep, northwest-dipping attitude during development of the Kiokee belt antiform. Between ca. 267 and 290 Ma, portions of the Kiokee belt, Modoz zone, and Carolina slate belt were overprinted by ductile deformation (D4) along steeply dipping, northeast-trending dextral shear zones.

Kinematics of late paleozoic continental collision between laurentia and gondwana.

In the Appalachians, late Paleozoic Alleghanian orogenesis is widely regarded as resulting from dextral oblique collision between irregular margins of Gondwana and Laurentia. However, this relative plate motion cannot account for coeval convergence in the Ouachitas and Variscides and is incompatible with some tectonic transport indicators in the Appalachians. An alternative kinematic model is proposed in which early sinistral transpression in the Appalachians is followed by counterclockwise rotation of Gondwana and the development of a system of dextral strike-slip faults extending from southern Europe to Alabama.

On resolving shear direction in foliated rocks deformed by simple shear

We present a three-dimensional model for the formation of crenulations in ductile shear zones, based on compatibility conditions, infinitesimal-displacement equations, and our own field observations. In many shear zones, at least two and probably three mesoscopic slip systems are active. Crenulation slip is interpreted to compensate for the displacement component of foliation slip normal to the shear zone wall. In zones in which foliation slip is an important mode of deformation, foliation and crenulation slip vectors do not necessarily lie in the same plane, nor are they necessarily perpendicular to crenulation axes. Solutions for the orientation of the crenulation plane, crenulation slip vector, and magnitude of the crenulation slip shear strain belong to one of two possible solution sets, given the orientation of slipping foliation, the foliation slip vector, and the magnitude of slip on foliation. Hence, shear sense may be reliably interpreted from composite planar fabrics in ductile shear zones, but more specifically, shear direction cannot. These results have broad implications in the interpretation of the kinematic significance of mineral lineations oblique to crenulation axes, and in the deduction of shear strain from angular relations between planar fabric elements.