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A traverse across the Stone Church syncline in the Ordovician Martinsburg turbidites reveals an axial planar cleavage (N40°E, SE dips) in regional thrust-related folds (N40°E, shallow plunges) and five phases of sparry calcite. Calcite fillings are bedding-parallel, cleavage-parallel, and one vein set cross-cuts both earlier phases; the youngest calcite filling is a bedding-parallel fault gouge that crosscuts the cleavage and preserves top-down-to-the-southeast normal fault kinematics. Calcite veins unique to disharmonically-folded calcareous siltstones (Maxwell, 1962) were also analyzed. Stable isotopic analysis (O, C) of all of the calcite phases indicates a uniform fluid source (δ13C - 2.0, δ18O - 13.3 VPDB) and, potentially, a similar precipitation and mechanical twinning age. The twinning strains (n = 1341; average Δσ = -32 MPa; average ε1 = -2.9%) in the calcite suite are consistent with SE-NW thrust shortening, and sub-horizontal shortening perpendicular to evolving axial planar cleavage planes in the Stone Church syncline. Calcareous siltstone layers within the Martinsburg Fm. turbidites share concordant bedding planes and are unique, chemically (XRF), but folded and cleaved differently than the surrounding clay-rich Martinsburg turbidites. Neither sediment type yielded detrital zircons. Electron backscatter X-ray diffraction (EBSD) and calcite twinning results in a folded calcareous siltstone layer preserving a layer-normal SE-NW shortening strain and Lattice Preferred Orientation (LPO). Shortening axes for the five-phase calcite suite trends ~N40°W, consistent with tectonic transport associated with crystalline nappe emplacement of the Reading Prong within the Piedmont province. [ABSTRACT FROM AUTHOR]

The Archean nucleus of Laurentia, the Superior province, formed between 2.7 and 2.6 Ga. There is little record of sediment deposition on the craton for 100 m.yr. until deposition of the Huronian clastic sedimentary rocks (4-12 km thick) began along the south-central margin of the Superior province between 2.5 and 2.2 Ga. Huronian detrital zircon ages ( zircon SHRIMP 207Pbx/206Pbx np266 ages from six samples) are dominantly 2.6-2.7 Ga, with a small population aged 3.0-3.5 Ga in the middle of the Huron section. The mainly younger, and western, Animikie basin (6-12 km thick; 2.21-0.8 Ga) contains a wide range of nearshore and deep-water clastic rocks, including banded iron formations and the 1.85 Ga Sudbury "impactite" deposits with overlying turbidites. Animikie detrital zircon ages (np724 laser ablation ICP-MS 207Pbx/206Pbx zircon ages from 10 samples) contain a complex spectrum, with ages ranging from 1.8 to 3.8 Ga: the basal Chocolay (Denham, East Branch Arkose) samples contain zircons ranging in age from 2.1 to 3.7 Ga, with a dominant population at ~2.6 Ga; the basal Animikie (Menominee) quartzites contain zircons with ages between 2.1 and 3.8 Ga, with peaks variably at 2.1, 2.3, 2.6, and 3.0 Ga; the upper Animikie (Baraga) turbidites contain zircons with ages between 1.8 and 3.6 Ga, with a dominant peak at 1.85 Ga. The Huron and Animikie basins were intensely deformed by Penokean orogenic (1.85 Ga) accretion from the south. Deposition into the Huron basin was from the north and west for 200 m.yr.; then the depocenter shifted to the west at ~2.2 Ga, with the Animikie basin receiving clastic debris from all directions, including from terranes (e.g., Marshfield and the Wisconsin Magmatic Terrane) approaching from the south. Combined Huron and Animikie detrital zircon results record a migrating eastto- west depocenter, perhaps in conjunction with the dynamics of the central Marathon large igneous province mantle plume and dike swarm (~2.07 Ga), over a period of 500 m.yr. [ABSTRACT FROM AUTHOR]

The authors comment on the arguments of geologists Ed Beutner and Tom Hauge on the Heart Mountain and South Fork fault systems. They stress that the Aharonov and Ander model and the phreatomagmatic-hydraulic model of geologists W. T. Straw and C. J. Schmidt is only similar in the assumption that there is pressurized water in the regional fracture system. They also contradict the former model's statement that Cathedral Cliffs and White Mountain have many more dikes in the upper plate because it did not calculated the dike density.

White Mountain is centrally located in the bedding-plane portion of the Eocene Heart Mountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126-340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where Kmax is parallel to the detachment slip direction and records a westward and down paleopole (287° and 27°), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0° and 45°). This brief and catastrophic detachment event produced a significant amount of CO[[2 by flash heating. This report is the first to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters. [ABSTRACT FROM AUTHOR]

Abstract: Two perpendicular ultramafic lamprophyre dikes on Little Presque Isle near Marquette, Michigan were studied to compare their intrusive magmatic fabric using anisotropy of magnetic susceptibility (AMS) with the strains preserved by mechanical twins in dike magmatic calcite (δ 18O=−21.93, δ 13C=−4.53) and dike-margin hydrothermal calcite (δ 18O=−9.72, δ 13C=−3.76) veins. Phlogopite from the lamprophyres yield an 40Ar–39Ar plateau age of 1877Ma, and the lamprophyres intrude metadiabase dikes and the Compeau Creek gneiss (2691±25 Ma; LA-ICPMS U–Pb on zircon), all of which is cross-cut by Keweenaw basaltic dikes and unconformably overlain by the undeformed Jacobsville Sandstone (Meso- to Neoproterozoic). The lamprophyre dikes strike east–west and north–south in this small (3km2), complex outcrop. Analysis of mechanically twinned calcite in the E–W dike (n =89 grains, 26.97% NEVs) reveals a sub-horizontal dike-acute shortening strain axis (N38°E, 15°). Strain analysis of the N–S dike (n =107 grains, 22.43% NEVs) reveals a sub-horizontal and dike-parallel shortening strain axis (N17°E, 14°). Calcite veins from the margins of both lamprophyre dikes reveal a NE–SW sub-horizontal shortening, and both the lamprophyres and vein calcite preserve a vertical shortening strain overprint. Differential stress magnitudes for twinning the lamprophyre calcite and adjacent veins was −35MPa. AMS analyses, used as a proxy for the primary magmatic flow direction, reveal a sub-horizontal and dike-parallel K max, with a dike-normal K min for the E–W dike. The N–S dike preserves a vertical K max and a dike-normal K min. Hysteresis studies on both dikes show that this anisotropy is carried by multi-domain ferromagnetic minerals. These lamprophyres preserve a complex, local intrusive fabric (AMS) that was unaltered by Penokean sub-horizontal shortening (∼SW–NE), or the younger, sub-vertical shortening strain overprint ∼200km inboard of the unrestored Penokean margin. The twinning strain results are consistent with other Penokean finite strains but not with the strain fabrics of younger orogens (e.g., Yavapai, Mazatzal, Keweenaw-Grenville). [Copyright &y& Elsevier]

Abstract: Lithologies of two formations in the frontal ranges of the Cape Fold Belt, South Africa, host synorogenic calcite precipitated at low metamorphic grade during the end-Permian deformation of this Gondwana-wide orogen. The calcite crystals are mechanically twinned and this deformation is linked to both folding and the formation of an axial planar cleavage with associated extensional veins. Twinning strain analysis reveals a complex, rotational synfolding history in the upper, thin-layered Prince Albert Formation. Twinned calcite within the fold preserves two unique strain events. The primary shortening strain fabric is layer-parallel and transport-parallel (∼north–south). The shortening strain overprint (−5.82%) is layer-normal and plunges steeply to the northeast. These results are inconsistent with a flexural-slip folding mechanism. By contrast, a homogeneous syn-cleavage stress–strain field is recorded in the underlying, massive thick-bedded Dwyka Tillite. Analyses of calcite twins in clast-hosted, syn-cleavage fibrous calcite and rare limestone clasts define a sub-horizontal N–S shortening, and sub-horizontal, E–W extension. The intermediate axis (ε 2) is vertical and preserves shortening (−5.3%). The extension axis (ε 3) is horizontal and parallel to the clast ‘tension gash’ (fracture) plane. These deformed syn-cleavage calcite materials provide an independent constraint to the debate about contemporaneous stress–strain fields associated with folding and formation of an axial planar cleavage. [Copyright &y& Elsevier]

Calcite veins and amygdule fillings within basalts (older than 0.7 Ma) are mechanically twinned and preserve a subhorizontal shortening strain that resulted from compression and shortening normal to the Mid-Atlantic Ridge on both sides of the plate boundary. Our sample suite includes 19 specimens, 7 from the North American plate (4 veins, 3 amygdule fillings) and 12 from the European plate (9 veins, 3 amygdule fillings), 18 of which record ridge-normal subhorizontal shortening. Five of the strain analyses, two from the North American plate and three from the European plate, have a high percentage of negative expected values, and these secondary strain results record a ridgeparallel shortening strain with plunges that vary parallel to the ridge axis. Averaged shortening strain magnitudes for the twinned calcite (-2.5%, European plate; -6.02%, North American plate) and inferred differential stresses (-48 MPa) that caused the twinning are modest and are thought to represent regional tectonic conditions (i.e., ridge push), not local (e.g., hotspot or glacial loading) phenomena. Keywords: divergent plate boundary; calcite twinning; stress-strain field; ridge push. [ABSTRACT FROM AUTHOR]

The Cretaceous oceanic crust of the Caribbean plate has a long and complex tectonic history as an indentor plate bounded by transcurrent faults (N and S) and subduction margins (W [east slab dip] and E [west slab dip]) with no strain data. Miocene forearc sediments and accreted ophiolites are exposed along the central Pacific coast of Costa Rica and preserve a variety of folding and faulting styles. These clastic sediments, deposited in the forearc as the Middle America trench evolved, contain calcite cements and are cross cut by calcite veins. Analysis of the mechanical twins in these two calcite groups (8 samples, n = 267 twins) at Punta Dominical and the Nicoya Peninsula ophiolite preserve a consistent sub-horizontal shortening strain that is oriented parallel to the current trench axis, as are most of the accretion-related fold axes. Differential stress magnitudes for the calcite twinning strain average −450 bars, and there is little evidence of a twinning strain overprint (low NEVs). Obduction along the Middle America trench involves, in part, a component of trench-parallel horizontal shortening. The Motagua fault originates in the Middle America trench and strikes east as the northern boundary of the Caribbean plate as a sinistral margin. The Roatan Islands are a series of islands composed of south-dipping rocks along the southern side of the Motagua fault. Recent, deformed clastic sediments are drapped over gneisses, schists (40Ar-39Ar biotite age of 14 Ma), conodont-free marbles and ophiolitic fragments of presumed Cretaceous age. Twinning studies (n = 6 samples, n = 425 twins) of these marbles preserve a N–S, horizontal shortening strain, normal to the sinistral plate boundary. Differential stress magnitudes for the calcite twinning strain average −36 MPa. Only in a combined sample (n = 189 grains) is a strain overprint (high NEVs; n = 47) present, which records horizontal shortening strain at 132°, 5°. Jamaica is also along the eastern Motagua fault (Cayman trough) and twinning strain studies in Eocene limestones and younger calcite veins (n = 4 samples; n = 136 twins) preserve a horizontal shortening strain normal to the sinistral plate boundary. Twinning strain results are compared to regional GPS, focal mechanism and SKS anisotropy results around the Caribbean plate, especially along the Middle America trench on the western margin of Costa Rica with some new insights into subduction dynamics. • First study of calcite twinning strains around the Caribbean plate. • Costa Rican (Middle America trench) has a complex arrangement of focal mechanism solutions parallel to the trench, and twinned calcite preserves horizontal shortening parallel to the trench • We integrate strain data with focal mechanism solutions, geodetic data, seismicity and SKS fabrics to better understand the dynamics of the Caribbean plate. [ABSTRACT FROM AUTHOR]