John P. Craddock

Regional shortening fabrics in eastern North America: Far‐field stress transmission from the Appalachian‐Ouachita Orogenic Belt

Paleozoic carbonates of cratonic eastern North America comprise the footwall of the Appalachian-Ouachita fold-and-thrust belt and contain a layer-parallel shortening (lps) fabric that is preserved by mechanically twinned calcite. Shortening directions are generally parallel to the Appalachian-Ouachita thrust-transport direction in carbonates of the thrust belt proper (restored width ∼400 km) and within carbonates up to 1700 km into the foreland, giving a pre-thrusting sedimentary prism ∼2100 km wide through which compressive orogenic stresses were transmitted. The shortening strain magnitudes (<6%) and the inferred calcite twinning differential stress magnitudes (<90 MPa) decrease exponentially away from the orogenic front. Calcite twinning strain patterns in other adjacent tectonic provinces, such as the Grenville, Laramide, Keweenawan Rift, and Newark Basin, are distinct from the twinning strains preserved in the cratonic Paleozoic carbonates. (Appalachian orogen, far-field stresses, calcite microstructures).

Sevier-Laramide Deformation of the Continental Interior from Calcite Twinning Analysis, West-Central North America.

Abstract Paleozoic–Mesozoic carbonates that cover cratonic western North America contain a regional layer-parallel shortening (LPS) fabric that is preserved by mechanically twinned calcite. Shortening directions are generally parallel to the Sevier thrust-transport direction (E–W) in carbonates of the Idaho–Wyoming portion of the thrust belt and within carbonates as far as 2000 km into the plate interior. The inferred calcite twinning differential stress magnitudes generally decrease across the thrust belt, and decrease exponentially away from the orogenic front into the craton. Synorogenic calcite cements and veins preserve a distinct twinning deformation history: in the thrust belt, twinning strains commonly record local, out-of-transport piggyback strain events with high differential stresses (

The Rock Magnetic Fingerprint of Chemical Remagnetization in Midcontinental Paleozoic Carbonates.

Supported by NSF EAR 90-05075. Contribution number 9203 of the Institute for Rock Magnetism; the IRM is supported by grants from the Keck Foundation, the National Science Foundation, and the University of Minnesota

Calcite twinning strain constraints on the emplacement rate and kinematic pattern of the upper plate of the Heart Mountain Detachment

Two models that address the emplacement rate of the upper plate of the Heart Mountain Detachment (HMD) have been advanced. These are the catastrophic tectonic denudation model and the slow, incremental continuous allochthon model. We compared the two models by analyzing upper and lower plate rocks for a detachment-related overprint of the older, layer-parallel Laramide–Sevier calcite twinning strain fabric. Lower plate Pilgrim Limestone samples, even those collected a few meters below the detachment, show no evidence of a detachment-related twinning strain overprint (negative expected values, NEVs, avg. 8.3%) of the Laramide–Sevier layer-parallel fabric. The allochthonous upper plate carbonate rocks preserve the layer-parallel Laramide–Sevier shortening strain, but the shortening axis (e1) for each block occurs in different, non-EW and both subhorizontal and non-subhorizontal orientations. For the six upper plate blocks that preserve chaotic shortening axis orientations, no evidence of any detachment-related twinning strain overprint (NEVs avg. 7.5%) was found. In the absence of any HMD-related twinning overprint, the upper plate allochthon motion must have been rapid enough to be accommodated by fracturing of upper plate rocks without additionally twinning any calcite. We conclude that the emplacement of the upper plate of the HMD was not accompanied by an overprint of the Laramide–Sevier twinning fabric and that models that require catastrophic, rather than slow, incremental emplacement rates are best supported by these data.

Anhysteretic remanent magnetic anisotropy and calcite strains in Devonian carbonates from the Appalachian Plateau, New York

Jackson, M., Craddock, J.P., Ballard, M., Van der Voo, R. and McCabe, C., 1989. Anhysteretic remanent magnetic anisotropy and calcite strains in Devonian carbonates from the Appalachian Plateau, New York. Tectonophysrcs. 161: 43-53. Anisotropy of anhysteretic susceptibility (AAS) is a recently developed high-resolution method of measuring the magnetic fabric of rocks. In order to test the applicability and limitations of AAS for estimation of strain orientations in weakly-deformed and weakly magnetic rocks, we have used the method to examine the magnetic fabric of samples from a series of sites in limestones of the Helderberg and Onondaga formations along a 500-km E-W transect across New York State. Two distinct shortening directions have been previously identified and interpreted in terms of two separate phases of Alleghenian deformation. Over most of the transect, minimum anhysteretic susceptibility axes within the plane of bedding closely parallel the compression direction of the earlier (“Lackawanna”) phase. A few sites show minimum anhysteretic susceptibility parallel to the later (“Main”) phase. The threshold for resolution of the tectonic signal by AAS is at anhysteretic susceptibilities of about 2 X lo-’ (SI) and strain magnitudes of 0.5 to 1% as recorded by twinning in calcite. The central part of the transect exhibits minimum horizontal anhysteretic susceptibility perpendicular to the inferred tectonic compression, rather than parallel to it. We attribute this to either: (a) anisotropic transmission of stresses from the larger calcite matrix grains to the smaller magnetite grains during twinning; or (b) preferential recording of a late-stage non-coaxial stress direction in this area by late diagenetic magnetite.

Kinematic analysis of an en échelon—continuous vein complex

An array of sigmoidal tension gashes from the Idaho-Wyoming thrust belt changes laterally into a continuous vein. Detailed mechanical twin analysis was used to determine the strain variation in the optically and chemically homogeneous blocky calcite filling. In the continuous portion of the vein complex, the shortening axes are parallel to the vein boundary. However, the orientation of the shortening axes in the tip areas of the sigmoidal gashes are at an angle of approximately 35 ° to the vein boundary, and are parallel to the trend of the tips. Twinning patterns in the central portions of the gashes record two principal strain axes of shortening of nearly equal magnitude with the maximum perpendicular to the vein trend. Everywhere in the vein complex the orientation of the maximum extension axis is parallel to the twist axis of the gashes. The petrofabric strain results show that the vein filling has largely recorded local strains. The pattern of variation in orientation of the principal strains in the vein complex is in close agreement with the theoretically- determined stress distribution in similar structures. Our results show that the sigmoidal gashes were formed at the leading edge of a propagating vein and that the sigmoidal shape reflects changes in the local strain field rather than a remote shear. The orientation of these local strains closely corresponds to the orientation of the local stresses.

Detrital Zircon Provenance of the Mesoproterozoic Midcontinent Rift, Lake Superior Region, U.S.A.

We report the ages of detrital zircons (207Pb/206Pb LA-ICPMS; ) from 12 clastic units that are representative of the Mesoproterozoic Midcontinent Rift System (MRS) of North America. Magmatism of the MRS spanned 1115–1085 Ma. The Puckwunge, Nopeming, and Bessemer basal sandstones and the Oronto and Bayfield Group sandstones received a predominance of Keweenaw Rift–Grenville (1.3–1.0 Ga) zircons but also contributions of Wolf River (1.45 Ga), Yavapai (1.75 Ga), Penokean (1.8 Ga), and Archean (2.6–3.6 Ga) ages. Interflow sedimentary rocks in the North Shore Volcanic Group received only rift-aged zircons. The maximum depositional ages derived from detrital zircons also better constrain the age-stratigraphic relationship of the unfossiliferous rift section, in comparison to the overlying fossiliferous Franconia ( zircons), Mt. Simon (), and Munising () late Cambrian transgressive sands. We also report the date of a granite xenolith (∼1 km3) in the Beaver Bay Complex, which has a rift age (1091 Ma; ) and is therefore not a rafted inclusion of Archean basement as previously suspected.

Dynamics of the emplacement of the Heart Mountain allochthon at White Mountain: Constraints from calcite twinning strains, anisotropy of magnetic susceptibility, and thermodynamic calculations

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 K max 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 signifi cant amount of CO 2 by fl ash heating. This report is the fi rst to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters.

Calcite-twinning constraints on stress-strain fields along the Mid-Atlantic Ridge, Iceland

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 ridge-parallel 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.

Non-coaxial Horizontal Shortening Strains Preserved in Twinned Amygdule Calcite, DSDP Hole 433, Suiko Seamount, Northwest Pacific Plate.

Abstract Core sections from the lower, basaltic portions of DSDP Hole 433 (total depth: 550.5 m) in Suiko seamount contain amygdule and vein fillings of calcite which contain mechanical twins. Analysis of the calcite twins reveals the presence of two horizontal shortening strains that are nearly orthogonal to one another; the azimuthal orientation of these strains is only known with respect to stratigraphic top and bottom as the cores are not oriented in any other manner. Maximum shortening strain magnitudes for the best-developed, positive expected value (PEV) twin lamellae set is −1.7%. For the lesser-developed, negative expected value (NEV) split, the preserved horizontal strain magnitude is −4.3%. Inferred compressive paleostress magnitudes were on the order of 26 MPa. Horizontal differential stresses of this magnitude could be: (1) associated with hotspot plumes; and/or (2) are transmitted to the central portions of thin oceanic plates from distant plate boundaries. Suiko seamount is composed of Paleocene basalts overlain by lithified middle Paleocene limestones, and is part of the Emperor seamount chain on the Pacific plate. The absence of secondary vein calcite in the overlying sediments suggests that the underlying amygdule calcite is Paleocene in age which indicates that the twinned calcite preserves a stress field reorganization (the NEV split) after the middle Paleocene that is oriented 64° from the earlier Paleocene stress field (the PEV split).