Blanka Sperner

Fault-striae analysis: a Turbo Pascal program package for graphical presentation and reduced stress tensor calculation

We describe integrated computer programs for graphical presentation, on-screen selection, and reduced stress tensor calculation of fault-striae data. The input consists of dip azimuth and dip inclination of faults, bearing and plunge of associated striae, relative slip-sense of the hanging wall, and degree of confidence in the slip-sense determination. The data are checked for inaccurate measurements, if necessary corrected (program CHECK), and represented on the equal area, lower hemisphere stereonet either as fault poles (program F-SH) or as great circles (program F-S). In both situations, arrows point in the direction of relative hanging-wall slip; their heads express the confidence levels in slip-sense determination. Program ORIENT plots dip azimuth and dip inclination of faults, striae bearing and plunge, and rake as rose diagrams, enabling a quick recognition of the dominant fault types. Program TURNER determines the best-fitting contraction and extension axes (P-B-T axes method) for each fault-striae data set. Program INVERS calculates the principal stress directions and the stress ratio according to the direct inversion method. The angle between the calculated direction of maximum shear stress along the fault plane and the measured striae is determined for each data set, and the distribution of these angles is plotted in a histogram. The relative values of normal and shear stresses are plotted in a normalized Mohr circle diagram. Graphical output is in standard HPGL files, suitable for publication-quality plotting by commercial word-processing and graphics programs. Data handling by the program package is demonstrated with a test file. Finally, the output of our programs is compared with that provided by published routines implementing different techniques.

Tectonic stress in the Earth’s crust: advances in the World Stress Map project

Abstract Tectonic stress is one of the fundamental data sets in Earth sciences comparable with topography, gravity, heat flow and others. The importance of stress observations for both academic research (e.g. geodynamics, plate tectonics) and applied sciences (e.g. hydrocarbon production, civil engineering) proves the necessity of a project like the World Stress Map for compiling and making available stress data on a global scale. The World Stress Map project offers not only free access to this global database via the Internet, but also continues in its effort to expand and improve the database, to develop new quality criteria, and to initiate topical research projects. In this paper we present (a) the new release of the World Stress Map, (b) expanded quality ranking schemes for borehole breakouts and geological indicators, (c) new stress indicators (drilling-induced fractures, borehole slotter data) and their quality ranking schemes, and (d) examples for the application of tectonic stress data.

The North American-Caribbean Plate boundary in Mexico-Guatemala-Honduras

Abstract New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.

Seismotectonics of the Romanian Vrancea Area

The seismicity of the Romanian Vrancea area has peculiar features: (1) strong earthquakes occur at intermediate depths in a very narrow source volume; (2) the seismogenic zone is situated beneath continental crust, at the SE corner of the highly arcuate Carpathian arc; (3) no evidence for active ongoing subduction is found today. Several geophysical models were developed that tried to provide an explanation for the localization of seismicity at depth (Fuchs et al., 1979; Oncescu, 1984; Tavera, 1991). They contain ideas on interaction of a paleo-subduction zone with more recent subduction and include initial concepts of slab break-off. In recent years new facts and concepts came up that merit a re-evaluation of the tectonic scenarios related to Vrancea seismicity.

The Pieniny Klippen Belt in the Western Carpathians of northeastern Slovakia: Structural evidence for transpression

Abstract The Pieniny Klippen Belt represents a 600-km-long but only a few kilometers wide suture zone in the Carpathian orogenic belt. Based on a quantitative analysis along a part of its NW-trending segment in northeastern Slovakia, we present structural data supporting transpression, the continuous interaction of strike-slip shearing, horizontal shortening, and vertical lengthening, as a major deformation style in its polyphase deformation history. Dextral transpression is expressed in the map scale and outcrop fault pattern, the oblique orientation of fold axes to the faults bounding the Klippen Belt, and extension parallel to the fold axes. The transpression-related strain field is described and quantified by the analysis of: (1) orientation of rotated fold axes (displaying an acute angle to the margins of the Klippen Belt); (2) orientation and geometry of paleostress derived from mesoscale fault-striae analysis (E-trend of σ3-trajectories and flattening geometry); and (3) the deformation history indicated by extension veins (non-coaxial regime). Different techniques using fault-striae data quantify paleostress and subdivide heterogeneous data sets mathematically into homogeneous subsets. The observed deformation history is modelled as a homogeneous transpression deformation. The best-fitting model requires a NW-trending (present-day orientation) external contraction direction (e.g., plate-slip vector), and predicts 16% fold axes parallel extension and 23% axial plane normal shortening.

BUILD-UP AND DISMEMBERING OF THE EASTERN NORTHERN CALCAREOUS ALPS

The structural geometry and kinematics of a segment of the classical fold-and-thrust belt of the Northern Calcareous Alps (NCA) is analyzed by map, mesoscale fault-slip, and calcite-twin data. The most distinct feature of the eastern NCA is the Miocene SEMP (Salzachtal-Ennstal-Mariazell-Puchberg)-lineament, which extends 400 km WSW-ENE from the northern Tauern window in the west to the Vienna basin in the east. It forms an orogen-parallel strike-slip zone, constitutes the major left-lateral wrench corridor accommodating eastward lateral extrusion of crustal wedges along the central Eastern Alps, and crosses the NCA as an anastomosing zone of distributed shear. The SEMP-line and related structures dismember a fold-and-thrust belt formed during early transpressional and subsequent orthogonal contraction. Most Early Miocene structures are reversed in the Late Miocene accommodating little strain. Distinct strain fields are calculated based on fault-slip and calcite deformation-twin data. They are regionally consistent in orientation and relative age and correlate with the following deformation stages: (1) Late Cretaceous to early Tertiary top-to-NW nappe stacking and right-lateral wrenching along NW-trending tear/transfer faults as an expression of the build-up of the Austroalpine orogenic wedge (e3 = 317 ± 17°; e1 = 058 ± 28°; e1 ≥ e2 ≥ e3, principal strains). (2) Early Tertiary top-to-N stacking and conjugate strike-slip faulting reflecting the change from transpressional to frontal contraction within the NCA (e3 = 356 ± 12°; e1 = 088 ± 10°). (3) Early to Middle Miocene large-scale, left-lateral wrenching, including early transpression and late transtension, as expression of eastward displacement of the southern part of the NCA (e3 = 019 ± 22°; e1 = 109 ± 27°). (4) Post-Middle Miocene E-W contraction, reactivating strike-slip and normal faults (e3 = 092 ± 10°; e1 = 004 ± 12°). The distributed nature of the lateral extrusion deformation and the anastomosing faulting along the SEMP-line reflect the rheological heterogeneity inherited by the lithology and multiple deformations of the NCA. A combined extension and stress-deflection model possibly accounts best for the pattern of northward concave faults branching off the SEMP-line.

A new method for smoothing orientated data and its application to stress data

Abstract Smoothing algorithms provide a means of identifying significant patterns in sets of orientated data, eliminating local perturvations within the observations and predicting patterns of orientated data in places which lack observations. Here we present the smoothing of orientation data with a distance-related method of data weighting as an alternative to previous weighting algorithms. The data weighting and smoothing method presented here in theory and practice is developed on the basis of a statistical smoothing algorithm. The method can be applied to orientation data of 180° periodicity such as maximum horizontal tectonic stresses (SH) as compiled in the World Stress Map database. Our smoothing algorithm enables discrimination between local (<250 km of lateral extent) and regional (c. 250–5000 km of lateral extent) stress fields, and allows comparison of SH with other directional data such as fault trends or strain data. We present smoothed stress maps for northeastern America, the Himalayas and western Europe. By varying the scale and smoothing parameters we illustrate their influence on the accuracy and smoothness. We give recommendations for the appropriate choice of these parameters.

Building the Pamir‐Tibetan Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top-to- ~N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22–17 Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

High‐resolution 40Ar/39Ar dating using a mechanical sample transfer system combined with a high‐temperature cell for step heating experiments and a multicollector ARGUS noble gas mass spectrometer

40Ar/39Ar dating of young ( 5 × 10−16 mol 36Ar is better than 0.5‰–1.0‰ (1σ, n = 4–8). We illustrate the system performance by 40Ar/39Ar dating of whole-rock samples and mineral separates from the Oman ophiolite as well as from the Siebengebirge, Heldburg, and Rhon volcanic provinces in Central Germany.

Anomalously old biotite 40Ar/39Ar ages in the NW Himalaya

Biotite 40 Ar/ 39 Ar ages older than corresponding muscovite 40 Ar/ 39 Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40 Ar/ 39 Ar ages are commonly dismissed as “too old” on account of “excess Ar.” We present 32 step-heating 40 Ar/ 39 Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40 Ar/ 39 Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for “too-old” biotite is excess Ar, i.e., 40 Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40 Ar was released during Cenozoic prograde metamorphism; (2) 40 Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40 Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.