Mark G. Steltenpohl

Exhumation of eclogitized continental basement during Variscan lithospheric delamination and gravitational collapse, Sudety Mountains, Poland

A Variscan,deep-crustal-level (eclogite-facies),continental basement massif in western Poland, the Snieznik complex, was tectonically exhumed. Crustal-penetrating mylonite zones record three main kinematic events: early top-to-the-north-directed thrusting, right-slip transpression-tension, and late top-to-the-south and -east normal faulting. Thrusting resulted in extreme crustal thickening and associated eclogite-facies metamorphism. Right-slip movements produced retrogressive crystal-plastic simple-shear zones. Normal faults flank Carboniferous to Early Permian terrigenous sedimentary basins, documenting tectonic and erosional denudation of the Snieznik complex during lithospheric extension. Sm/Nd isotopic dates previously reported for the in situ eclogite-facies metamorphic mineral assemblages are 341, 337, and 329 Ma (Brueckner et al., 1991). 40 Ar/ 39 Ar isotopic dates for metamorphic hornblende (338, 333, and 332 Ma), muscovite (329 and 329 Ma), and biotite (328 Ma) reflect times of cooling through the ∼500, 350, and 300 °C isotherms, respectively. These nearly concordant mineral dates document rapid cooling from ∼850 °C (eclogue-facies temperatures) to ∼300 °C. Rapid denudation of these deep-crustal rocks (∼19-22 kbar pressures, >70 km depth) is attributed to processes, similar to those of metamorphic-core complexes, that operated during lithospheric delamination and gravitational collapse. The sequence of late Paleozoic (Alleghanian) crustal thickening followed by right-slip transpression-tension followed by normal faulting recognized in the U.S. Appalachians implies that this tectonic pattern may exist throughout the Afleghanian-Variscan belt.

An 40Ar/39Ar thermochronology of the Ofoten‐Troms region: Implications for terrane amalgamation and extensional collapse of the northern Scandinavian Caledonides

Fifteen 40Ar/39Ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the Ofoten nappe stack of northern Norway. Eight cooling ages on hornblende range from 425 to 394 Ma and seven muscovite ages, from the same or nearby outcrops as the hornblendes, range from 400 to 373 Ma. These data are compared with 40Ar/39Ar ages from over a large part of the northern Caledonides to evaluate regional mineral cooling patterns. Results indicate that (1) Scandian (Silurian-Devonian) metamorphism was predominant; (2) most of the nappes investigated contain some vestige of pre-Scandian tectonism and/or metamorphism; (3) hornblende and muscovite cooling ages are progressively younger to the west and south, which suggests a hinged-to-the-east mineral cooling pattern; and (4) a late, out-of-sequence thrust is the only disruption of this cooling pattern. Synmetamorphic amalgamation of the nappes resulted from Scandian A type subduction. The hinged-to-the-east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping, crustal-scale extensional fault, located somewhere west of the present Norwegian coast, during late synorogenic gravitational collapse. The late out-of-sequence fault formed contemporaneously with uplift in the hinterland, implying a kinematic and temporal connection with east directed contractional faulting in the foreland.

Evidence for ophiolite obduction, terrane accretion and polyorogenic evolution of the north Scandinavian Caledonides

Abstract Recent work on the thrust sheets of the Ofoten-Troms region, north Norwegian Caledonides, has resulted in revision of the tectonostratigraphic correlations providing new constraints on models for terrane accretion and poly-orogenic Cambro-Ordovician and Siluro-Devonian development. Previous studies on Late Ordovician-Early Silurian fossils in metasediments unconformably overlying the Lyngen Ophiolite complex within one of the Troms nappes documented a period of ophiolite obduction prior to the major Siluro-Devonian Scandian orogenic event. Correlation of these units and underlying and overlying thrusts sheets southward into the Ofoten area, 150 km to the south, is demonstrated. This implies that an extensive, completely transported, Upper Ordovician-Lower Silurian basin, developed on top of the obducted ophiolite, composes the bedrock of much of northern Norway. Metamorphism and deformation within these basinal rocks resulted from the Scandian phase of the Caledonian orogeny. The relatively weak metamorphic and deformational imprint on the ophiolitic basement provides a rare opportunity to evaluate the pre-Scandian tectonic evolution of northern Scandinavia. Poly-orogenic evolution, a controversial subject in northern Norway, is documented. The character and relative extent of Cambro-Ordovician and Siluro-Devonian orogenic zones in northern Norway are elucidated. The Balsfjord/Ofoten unconformity formed approximately concomitant with other post-ophiolite obduction unconformities in central and southern Norway, implying an orogen-wide Late Cambrian-Early Ordovician event.

New Caledonian eclogite province in Norway and potential Laurentian (Taconic) and Baltic links

Field observations and 4 0 Ar/ 3 9 Ar isotopic dating indicate that eclogites exposed in the Lofoten Islands, north Norway, formerly presumed to be Proterozoic features, most likely formed as a result of early to middle Paleozoic, i.e., Caledonian, metamorphism. The Lofoten eclogites occur in shear zones that cut Baltic Precambrian continental basement. This unusual style of occurrence is shared only with Caledonian shear-zone eclogites of the allochthonous Bergen arcs of western Norway. Our findings help to link Scandinavian eclogites with those on the Laurentian side of this collisional zone in East Greenland. Ordovician to Silurian eclogites also are found locally throughout the southern, Appalachian continuation of the orogen in eastern North America. We compare the pressures, ages, and tectonic and structural settings of the eclogites along the ∼10,000 km length of the Appalachian-Caledonian system. Our synthesis supports the idea that Laurentian Taconic elements may be preserved in high-level nappes in Norway. The rare, deep-crustal metamorphic relicts also appear to be shared between Baltica and Laurentia, offering a new perspective in which to view the geodynamic evolution of this once-Earth-spanning orogenic system.

Thermobarometric profile through the Caledonian nappe stack of Western Ofoten, North Norway

P-T estimates from pelite thermobarometry along a transect through the Caledonian nappe stack in Ofoten, north Norway (68° 30° N) plot as a trend in P-T space (ranging from 443° C, 6.9 kb, to 688° C, 10.6 kb). The P-T range for each nappe overlaps the others, except for those from the lowest unit (Narvik Group) which has significantly higher values. Within the P-T ranges for each nappe, the position on the trend of a given sample is independent of its tectonic position. Results support our earlier conclusion, based on structural and petrographic grounds, that the regional metamorphic peak was reached during or after stacking of the nappes in the Siluro-Devonian Scandian orogeny.The P-T trend closely resembles a trend reported from the Narvik Group in a nearby area which was interpreted to record varying amounts of retrograde reequilibration during uplift and cooling and thus define a portion of the areas cooling trajectory. However, evidence for an earlier, higher P-T event recorded only in the Narvik Group includes: 1) the Narvik Group gives the only P-T values exceeding 630° C, 9kb; 2) it preserves the only evidence suggesting polymetamorphism; and 3) it underlies an unconformity which has been interpreted on regional grounds to record the Cambro-Ordovician Finnmarkian orogeny. Therefore, the Narvik Group P-T trend probably does not quantitatively constrain the cooling history during uplift but reflects superposition of metamorphism in the Finnmarkian and Scandian orogenies. A Monte Carlo analysis of error propagation applied to the thermobarometric calculations indicates that the trend at least partly resulted due to an artifact of errors introduced by analytical uncertainties in the microprobe analyses.

Kinematics of the Towaliga, Bartletts Ferry, and Goat Rock fault in the southernmost Appalachians

Subhorizontal shear sense along subvertical mylonite zones marking the southeast and northwest flanks of the Pine Mountain belt in Alabama, i.e., the Towaliga, Bartletts Ferry, and Goat Rock fault zones, has been deduced from S-C composite planar fabrics, extensional shear bands, displaced broken grains, asymmetric folds, and porphyroclast systems. Quartz and feldspar elongation lineations are generally subhorizontal and closely correspond to estimated sliplines. Each of the fault zones records dominantly dextral shear; the Towaliga has an apparent minor oblique, down-to-the-north normal component, and the Goat Rock has a minor down-to-the-south normal component. The mylonite zones postdate the early to middle Paleozoic schistosity in rocks outside the shear zones and thus are considered to be late Paleozoic in age. Results imply persistence of the late Paleozoic (Alleghanian?) dextral shear system into the southernmost exposures of the Appalachian orogen.

Retrograded eclogite-facies pseudotachylytes as deep-crustal paleoseismic faults within continental basement of Lofoten, north Norway

Field observations and electron microprobe analyses indicate that pseudotachylytes discovered on the Lofoten island of Flakstadoy, north Norway, represent rare examples of deep-crustal paleoseismic faults. The pseudotachylyte occurrences are restricted to the margins of eclogite-facies shear zones that sharply cut pristine granulite-facies continental basement rocks. Generally, pseudotachylyte veins are sharply truncated by the eclogite shears, but some have been sheared and folded into them, documenting prekinematic to synkinematic injection. Textures preserved in the pseudotachylyte matrix document crystallization directly from the frictional melt; for example, dendritic garnets, similar in appearance, size, and composition to those from eclogite pseudotachylytes of the Bergen Arcs and Alesund (Austrheim and Boundy, 1994; Lund and Austrheim, 2003), refl ect rapid (likely in terms of tens of seconds) crystallization, and distinct fi ning of grains toward the margins of the pseudotachylyte veins indicates quenching textures. Electron microprobe analysis and backscattered-electron imaging document that the pseudotachylyte matrix is composed of microlites of garnet (Gr 25–30 , Py 15–19 , and Al 54–58 ), orthopyroxene (En 61–64 ), low-Na clinopyroxene (Jd 6 ), amphibole (ferroan pargasite), with or without K-feldspar, quartz, biotite, various Fe opaques and FeTi opaques, kyanite, dolomite, and calcite. The cogenetic eclogite-facies shear zones and pseudotachylytes were variably retrograded during Caledonian amphibolite-facies metamorphism. Omphacite is replaced by clusters or symplectites of low-Na clinopyroxene (Jd 6 ) and oligoclase/andesine (An 20–36 ); kyanite, orthopyroxene, Na-Ca clinopyroxene, amphibole, and dolomite occur as inclusions in garnet. The Flakstadoy pseudotachylytes indicate that the rocks exposed in Lofoten were rigid and resilient parts of the lower crust of an ancient continent from ca. 1.8 Ga until the Middle Ordovician. Subduction to deepercrustal levels (depths >~45 km) caused the stiff, nonreacted granulite to accommodate aseismic, steady-state fl ow in fl eclogite shear zones by concomitant, brittle, seismogenic failure and pseudotachylyte formation. Later in the Middle Ordovician, these deep-crustal rocks were exhumed to middle-crustal levels, where they were retrograded under amphibolite-facies conditions. Our results help to explain how deep-crustal earthquakes form in modern continent-continent collisional zones like the Himalayas.

Cross folds and back folds in the Ofoten-Tysfjord area, north Norway, and their significance for Caledonian tectonics

The outcrop pattern of Caledonian allochthons in northern Norway is controlled by two late fold sets: cross folds (F 3 ), which have axes at a high angle to the structural trend of the orogen, and back folds (F 4 ), which have axes parallel to the structural trend but verging in the opposite direction from that of nappe emplacement. Interference of cross folds and back folds produces Ramsay type I (dome and basin) and type II (boomerang or canoe) interference patterns on all scales. The Ofoten area contains the closure of a regionally extensive, northeast-trending back fold, the Ofoten synform. Within its core are klippen of the structurally highest nappes preserved in this portion of the Caledonides. At Tysfjord, 15 km south of Ofoten, some of the deepest structural levels within the orogen are exposed in the core of the Tysfjord cross-fold culmination. The Norwegian Caledonides contain two strike-parallel belts of gneiss domes underlain by Precambrian basement rocks flanking an elongate structural basin to which the Ofoten synform in part corresponds. In Ofoten- Tysfjord, these domes and basins reflect a large-scale cross-fold-back-fold interference pattern. Structural characteristics of the cross folds and back folds indicate that they formed in response to layer-parallel shortening, implying that gneiss domes in these areas reflect interference of folds formed in crustal compression rather than diapirism. Because this dome-and-basin pattern is present along virtually the entire length of the Norwegian Caledonides, we suggest that other gneiss domes in Norway may have a similar origin. The orientation of the cross folds in Ofoten-Tysfjord suggests a component of sinistral shear affecting the orogen after the Early Silurian and before the Late Devonian. Paleomagnetic and structural data from other parts of the Caledonides suggest large-scale mid-Paleozoic (post-collisional) lateral shear, commonly with a sinistral sense. Although Paleozoic strike-slip faults have not been identified in the Caledonides at this latitude, we suggest that the cross folds may be an alternative expression of large-scale sinistral shear.

New York–Alabama lineament: A buried right-slip fault bordering the Appalachians and mid-continent North America

The New York–Alabama (NY-AL) lineament, recognized in 1978, is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern North America that is deeply buried beneath the Appalachian basin. Data not in the original aeromagnetic data set, particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of Tennessee, left the source of the lineament open to conjecture. We use modern digital aeromagnetic maps to fill in these data gaps and, for the first time, constrain the southern termination of the NY-AL lineament. Our analysis indicates that the lineament reflects a crustal-scale, right-lateral strike-slip fault that has displaced anomalies attributed to Grenville orogenesis by ∼220 km. Palinspastic restoration of this displacement rearranges the trace of the Grenville belt in southern Rodinia and implies only passive influence on later-formed Appalachian structures. The precise timing of dextral movement on the NY-AL structure is not resolvable from the existing data set, but it must have occurred during one of, or combinations of, the following events: (1) a late, postcontractional (post-Ottawan) stage of the Grenville orogeny; (2) late Neoproterozoic to Cambrian rifting of Laurentia; or (3) right-slip reactivation during the late Neoproterozoic–Cambrian rifting of Laurentia, or during Appalachian movements. Our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the Eastern Tennessee Seismic Zone are metasedimentary gneisses, and it provides an explanation for the spatial location and size of the seismic zone.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe–reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40 Ar/ 39 Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate “Appalachian” dates (e.g., Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may be related to Carolina zone terranes that accreted during the Alleghanian orogeny.