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The Hammett Grove Meta-igneous Suite, here named formally, is composed of altered ultramafite (soapstone and serpentinite), metapyroxenite, metagabbro, and metabasalt lithodemes. It is interpreted to represent a dismembered ophiolite that may have formed as fore-arc basement to the Carolina arc terrane. This amphibolite-facies and retrograde greenschist-facies suite of metamorphosed, comagmatic igneous rocks crops out in the Piedmont of northwestern South Carolina near the eastern edge of the Inner Piedmont belt. The northeasternmost part of the suite lies within the Kings Mountain shear zone, which constitutes the Inner Piedmont belt-Kings Mountain belt boundary. The suite is interpreted as a thrust slice or klippe derived from the Kings Mountain belt, implying that the boundary is—at least in part—an overthrust. As ophiolites occurring in ancient orogenic terranes are often related to fundamental boundary tectonics, it is proposed that the Inner Piedmont-Kings Mountain belt boundary represents a terrane suture and marks an accretionary event. Thrusting of the Hammett Grove Suite over rocks of the Inner Piedmont resulted either from orthogonal terrane accretion or from transpression related to wrench faulting.

Numerous northwest-trending (average, N40°W) diabase dikes of Mesozoic age intrude the Piedmont of northwestern South Carolina. Samples of the diabase were analyzed for major- and trace-element abundances and mineral chemistry. All samples are olivine tholeiite, containing 4 to 25% modal olivine. Plagioclase comprises 39 to 64% and pyroxene, dominantly augite, 13 to 45% of the diabases. Two contrasting pyroxene trends are represented: (1) augite → subcalcic augite (± low-Ca pyroxene), and (2) augite → ferroaugite (no low-Ca pyroxene). These are attributed to modest (about 2 wt%) differences in SiO 2 and iron oxide in the diabase magmas. The diabases also contain 1 to 2% titanomagnetite and up to 0.6% chromite. Textural relations suggest that chromite and olivine were first to crystallize, followed by plagioclase and then pyroxene and titanomagnetite. Major element variation trends cannot be satisfactorily accounted for by fractionation involving observed microphenocryst compositions. Wide ranges in REE patterns and in Th/Hf, Ti/K, and other incompatible element ratios further preclude derivation of the entire suite from a single parent magma by near surface fractional crystallization. As a group, the dikes span the gamut from light rare earth element (LREE) depleted (chondrite-normalized La/Sm <1), with chondrite-normalized Th/Hf <0.5 and very low K 2 O (<0.1 wt%), to LREE enriched (La/Sm >2) with Th/Hf >2 and K 2 O >0.35 wt%. The observed spread in geochemical values suggests significant heterogeneity in the mantle source.

Abstract The prehistoric Egiin Davaa earthquake rupture is well-preserved in late Quaternary deposits within the Hangay Mountains of central Mongolia. The rupture is expressed by a semi-continuous 80 km-long topographic scarp. Geomorphological reconstructions reveal a relatively constant scarp height of 4–4.5 m and a NW-directed slip vector. Previous researchers have suggested that the scarp’s exceptional geomorphological preservation indicates that it may correspond to an earthquake that occurred in the region c. 500 years ago. However, we constrain the last rupture to have been at least 4 ka ago from morphological dating and <7.4 ka ago based on radiocarbon dating from one of two palaeoseismic trenches. Our study shows that discrete earthquake ruptures, along with details such as the locations of partially infilled fissures, can be preserved for periods well in excess of 1000 years in the interior of Asia, providing an archive of fault movements that can be directly read from the Earth’s surface over a timescale appropriate for the study of slowly deforming continental interiors. The Egiin Davaa rupture involved c. 8 m of slip which, along with the observations that it is largely unsegmented along its length and that the ratio of cumulative slip ( c. 250 m) to fault length ( c. 80 km) is small, suggests relatively recent reactivation of a pre-existing geological structure. Supplementary material: All scarp profiles are available at http://www.geolsoc.org.uk/SUP18871

Salient features of the new aeromagnetic map of the Southeastern United States (Zietz and Gilbert, 1980) are: (1) the Charleston magnetic terrane that is generally high magnetically and has numerous distinct, very high magnetic anomalies within it; (2) the northern Florida magnetic terrane that is virtually identical magnetically to the Charleston magnetic terrane; (3) a linear series of magnetic low anomalies within a continuous magnetic low anomaly, collectively called the Altamaha magnetic anomaly, that is more than 1,150 km long and that arcs across the Continental Shelf, from about 33°30’N., 76°30’W., to the Georgia coastline at the mouth of the Altamaha River and trends inland across southern Georgia and Alabama; (4) the southern end of the East Coast anomaly and its shoreward branch, the Brunswick anomaly, which is part of the northern Florida terrane; (5) the characteristic magnetic terranes of the Piedmont and Valley and Ridge provinces; (6) the southern end of the New York-Alabama lineament and the magnetically high terrane west of it; and (7) the lineament formed by the nearly straight northwestern margin of the Charleston magnetic terrane in South Carolina and Georgia, the northwestern margin of the Altamaha anomaly, and the southern margin of the magnetically high terrane northwest of the New York-Alabama lineament in Alabama. A newly compiled gravity map of part of the Southeastern United States shows that the Charleston and northern Florida magnetic terranes have mixed gravity expression, with gravity high anomalies coinciding with the distinct, very high anomalies on the aeromagnetic map. In general, the trace of the Altamaha magnetic anomaly either coincides with or lies just south of gravity low anomalies that are linear and form a linear series along or closely parallel to the magnetic anomaly. Near its northeastern end, the Altamaha magnetic anomaly lies along a relatively steep, northwestward-sloping gravity gradient. The East Coast anomaly coincides with linear gravity high anomalies. The Charleston and northern Florida magnetic terranes are interpreted as being virtually the same geologic terrane, a predominantly mafic terrane, intruded by mafic plutons of batholithic proportions, that, until late Paleozoic time, was part of the African or African/South American plate. The northwestern margin of the Charleston terrane is interpreted as the Alleghanian suture between Africa and North America, and also as a major strike-slip fault, the Carolina-Mississippi fault, that extends to the southwest along the northwestern border of the Altamaha anomaly and the southern border of the magnetically high terrane northwest of the New York-Alabama lineament. The Altahama anomaly is interpreted as being caused by a fault-bounded, sediment-filled trough, locally as deep as 6 km, that divides the Charleston-northern Florida terrane. The Carolina-Mississippi fault truncates the Charleston terrane, the Piedmont and Valley and Ridge provinces, the New York-Alabama lineament, and the magnetically high terrane northwest of the New York-Alabama lineament, and the absence on the southeastern side of the fault of magnetic terranes found on the northwestern side suggests right-lateral displacement.

A region of migmatitic felsic paragneiss and pelitic schist containing concordant pods of serpentinite, talc schist, talc-amphibole schist, amphibolite, and metagabbro—here called the Burks Mountain complex—occurs in the southeastern limb of the Kiokee antiform, an Alleghanian D 3 structure in the eastern Piedmont of Georgia and South Carolina. A similar region of migmatitic felsic paragneiss containing small pods of metamorphosed mafic and ultramafic rocks, which occurs in the northwestern limb of the Kiokee antiform, is inferred to be a continuation of the Burks Mountain complex across the crest of the antiform. The composition of the felsic paragneiss and pelitic schist of the Burks Mountain complex suggests derivation from graywacke and shale, respectively. Relict textures preserved in mafic and ultramafic rocks of the complex, along with preliminary whole-rock geochemistry, indicate derivation from ultramafic tectonite (harzburgite), olivine-pyroxene cumulate (wehrlite, olivine pyroxenite), pyroxene-plagioclase cumulate (gabbro, anorthositic gabbro), and mafic volcanic rock. These protoliths suggest an origin as part of a cumulate mafic and ultramafic intrusive and extrusive complex with some associated mantle tectonite. The complex was disrupted prior to or during amphibolite facies regional metamorphism. The mechanism of disruption is not clear. Possible modes of origin include accumulation as an olistostrome, accumulation in a subduction-related accretionary complex, and formation as extreme boudinage during a regional deformation event. The crystalline rocks in the core of the Kiokee belt (including the Burks Mountain complex) are contained in the footwall of a major ductile shear zone (the Modoc zone) that experienced oblique, down to the north-northeast displacement during the early part of the Alleghanian orogeny. Prior to the Alleghanian orogeny, the Burks Mountain complex was located beneath rocks of the Carolina slate belt that are presently exposed north of the Modoc zone. The Burks Mountain complex may have been derived from a unit within the Carolina slate belt or from a Precambrian basement(?) unit beneath the Carolina slate belt. Alternatively, in view of a unique lithostratigraphy, the Burks Mountain complex may be included in a terrane, exotic with respect to both North America and the Carolina slate belt, which was tectonically incorporated into the Appalachians prior to the Alleghanian orogeny.