Faulting (Structural Geology)

Fault array evolution in extensional basins: insights from statistical analysis of gravel deposits in the Cecina River (Tuscany, Italy)

by Ilaria Consoloni

ANDREA CIAMPALINI, ILARIA CONSOLONI, GIOVANNI SARTI. Sedimentolgy, 58, 1895-1913

Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany,... more Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany, Italy) have been combined to unravel changes in the palaeo-drainage system. Data from 16 outcrops were collected and 6400 clasts described. Facies analysis, micro-palaeontology and macro-palaeontology and petrographic characteristics of the gravel deposits have highlighted the presence of three allostratigraphic units. Clast lithology is the main discriminator among these units. Cluster and principal component analyses of the 6400 clasts have improved understanding of the stratigraphy of the Lower Pleistocene deposits and constrain the re-routing of the lower palaeo-Cecina River from a supposedly south-east to north-west direction to the present east to west direction. Short rivers feeding small fan deltas represented by the oldest allostratigraphic units were abandoned in the Lower Pleistocene, when the re-routing of the Cecina River caused the capture of these streams. This evolution suggests a change in the tectonic regime of the area. The fan deltas developed on the hanging wall of normal faults sub-parallel to the coast; a change to a transtensile tectonic regime caused the deviation of the main river channel toward the present coast and the formation of a pull-apart basin, which is now exploited by the Cecina River. This study illustrates the value of lithological analyses of gravel deposits for understanding the tectonic evolution of an area.

Tectonic Evolution of the Ohrid Basin (Macedonia/Albania): preliminary results for a future ICDP deep drilling site

by Tomas Fernandez-Steeger

Hoffmann, N., Reicherter, K., Fernández-Steeger, T., Arndt, M. 2009. Tectonic Evolution of the Ohrid Basin (Macedonia/Albania): preliminary results for a future ICDP deep drilling site Geophysical Research Abstracts, 11, (EGU2009-7835).

Active Tectonics in the Ohrid Basin (Macedonia/Albania)

by Tomas Fernandez-Steeger

Reicherter, K., Hoffmann, N., Fernandez-Steeger, T.M. (2009): Active Tectonics in the Ohrid Basin (Macedonia/Albania). Geophysical Research Abstracts, 11 (EGU2009-11385).

Fault array evolution in extensional basins: insights from statistical analysis of gravel deposits in the Cecina River (Tuscany, Italy)

by Andrea Ciampalini

A. CIAMPALINI, I. CONSOLONI, G. SARTI, Sedimentology 58, Issue 7, pages 1895–1913, 2011

Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany,... more Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany, Italy) have been combined to unravel changes in the palaeo-drainage system. Data from 16 outcrops were collected and 6400 clasts described. Facies analysis, micro-palaeontology and macro-palaeontology and petrographic characteristics of the gravel deposits have highlighted the presence of three allostratigraphic units. Clast lithology is the main discriminator among these units. Cluster and principal component analyses of the 6400 clasts have improved understanding of the stratigraphy of the Lower Pleistocene deposits and constrain the re-routing of the lower palaeo-Cecina River from a supposedly south-east to north-west direction to the present east to west direction. Short rivers feeding small fan deltas represented by the oldest allostratigraphic units were abandoned in the Lower Pleistocene, when the re-routing of the Cecina River caused the capture of these streams. This evolution suggests a change in the tectonic regime of the area. The fan deltas developed on the hanging wall of normal faults sub-parallel to the coast; a change to a transtensile tectonic regime caused the deviation of the main river channel toward the present coast and the formation of a pull-apart basin, which is now exploited by the Cecina River. This study illustrates the value of lithological analyses of gravel deposits for understanding the tectonic evolution of an area.

Slip in the 2010-2011 Canterbury Earthquakes, New Zealand

by John Elliott

Fluid transfer into the wedge controlled by high-pressure hydrofracturing in the cold top-slab mantle

by José Alberto Padrón-Navarta

Published in Earth and Planetary Science Letters, 2010

Before attaining the mantle wedge, where they trigger partial melting, volatiles released from dehydration reactions... more Before attaining the mantle wedge, where they trigger partial melting, volatiles released from dehydration reactions in the slab have to migrate across a relatively cold (<750 °C), peridotite-layer above the incoming slab. In order to unravel the mechanisms allowing for this initial stage of fluid transport, we performed a detailed field and microstructural study of metamorphic prograde peridotites in the Cerro del Almirez ultramafic massif (Betic Cordillera, Spain), where evidences of one of the most important dehydration reactions in subduction zones, the high-pressure antigorite breakdown (P=1.6–1.9 GPa and T≈680 °C), can be mapped in the field. This reaction led to arborescent growth of centimeter-size olivine and orthopyroxene, producing a chlorite–harzburgite with a spinifex-like texture. Microstructural observations and crystal preferred orientations (CPO) mapping show no evidences of solid-state deformation during the prograde growth of olivine and orthopyroxene at the expenses of antigorite. However, a few tens to a hundred meters away from the reaction front, the metamorphic texture is partially obliterated by grain-size reduction in roughly planar conjugate zones, a few mm to meters wide. Grain size reduction zones (GSRZ) are characterized by (1) sharp contacts with undeformed spinifex-like texture domains, (2) important reduction of the olivine grain size (60–250 μm), (3) olivine color change from brownish to colorless, (4) decrease in the modal amount of orthopyroxene, and (5) at the mm- to cm scale, irregular shapes and abrupt terminations. Field and microstructural observations exclude that relative displacement took place across these GSRZ. Changes in modal composition imply reactions with fluids undersaturated in silica. Analysis of olivine crystal-preferred orientations (CPO) in GSRZ shows patterns similar, but more dispersed, than those in neighboring spinifex-like domains. It also reveals mm- to cm-scale discrete domains with rather homogeneous crystallographic orientations suggesting inheritance from the preexisting spinifex-like olivines in the host peridotite. Misorientation angles between neighboring grains in the GSRZ show peaks at 5–10° and 20°, but rotations are not crystallographically controlled. Based on these observations, we rule out the formation of the GSRZ by dynamic recrystallization during dislocation creep and propose that they record brittle deformation (microcraking) of the spinifex-like chlorite–harzburgite, probably induced by hydrofracturing at high pressure and relative low temperature conditions (680–710 °C). High-pressure hydrofracturing can, thus, be invoked as an efficient mechanism for fluid flow across the cold top-slab mantle layer, hence allowing the slab-derived fluids to ingress in the wedge.

The long-term burial and exhumation history of basement blocks in the footwall of the Wasatch fault, Utah

by Bart Kowallis

Nelson, S.T., Harris, R.A., Kowallis, B.J., Dorais, M., Constenius, K.N., Heizler, M., and Barnett, D., 2009, The long-term burial and exhumation history of basement blocks in the footwall of the Wasatch fault, Utah: Rocky Mountain Geology, v. 44, p. 103-119.

Kinematic analysis of northeast-trending faults of the Allen's Ranch 7.5' quadrangle, Utah County, Utah

by Bart Kowallis

McKean, A.P., and Kowallis, B.J., and Christiansen, E.H., 2011, Kinematic analysis of northeast-trending faults of Allens Ranch 7.5’ quadrangle, Utah County, Utah, in Sprinkel, D.A., Yonkee, W.A., and Chidsey, T.C., Jr., editors, Sevier thrust belt: northern and central Utah and adjacent areas: Utah Geological Association Publication 40, p. 89-116.

Mapping and kinematic analysis of the Deep Creek fault zone, south flank of the Uinta Mountains, Utah

by Bart Kowallis

Haddox, D.A., Kowallis, B.J., and Shakespear, J.D., 2005, Mapping and Kinematic Analysis of the Deep Creek Fault Zone, South Flank of the Uinta Mountains, Utah, in Dehler, C., Pederson, J., Sprinkel, D. and Kowallis, B. (eds.), Geology of the Uinta Mountains: Utah Geological Association Publication 33, p. 321-333.

InSAR slip rate determination on the Altyn Tagh Fault, northern Tibet, in the presence of topographically correlated atmospheric delays

by John Elliott

Crustal-lithospheric structure and continental extrusion of Tibet

by John Elliott

The 2009 L'Aquila earthquake (central Italy): A source mechanism and implications for seismic hazard

by John Elliott

The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt, Iran

by John Elliott

Extension on the Tibetan plateau: recent normal faulting measured by InSAR and body wave seismology

by John Elliott

The 2010 MW 6.8 Yushu (Qinghai, China) earthquake: Constraints provided by InSAR and body wave seismology

by John Elliott

Depth segmentation of the seismogenic continental crust: The 2008 and 2009 Qaidam earthquakes

by John Elliott

Interseismic strain accumulation across the Manyi fault (Tibet) prior to the 1997 Mw 7.6 earthquake

by John Elliott

Lateral versus vertical emplacement in shallow-level intrusions? The Slieve Gullion Ring-complex revisited.

by Fiona Meade

Emeleus C.H., Troll V.R., Chew D.M. and Meade F.C. (in press) Lateral versus vertical emplacement in shallow-level intrusions? The Slieve Gullion Ring-complex revisited. Journal of the Geological Society London.

Recent studies on shallow-level arcuate intrusions have identified numerous examples of horizontal mineral fabrics.... more Recent studies on shallow-level arcuate intrusions have identified numerous examples of horizontal mineral fabrics. These are commonly interpreted as reflecting considerable lateral flow during magma emplacement, thus querying established ‘semi-vertical’ ring-dyke models. We question the recent lateral emplacement model proposed for the Paleocene Slieve Gullion Ring-complex, NE Ireland, where the absence of steep fabrics in parts of the ring-complex has been used to support a shallow, semi-horizontal sheet intrusion mechanism. We argue that such simple flow models cannot be applied to explosive ring-fissure eruptions and that fabric data alone do not warrant rejection of the ring-dyke model. Moreover, the apparent ‘absence of steep intrusive contacts’ along the intrusion’s perimeter is re-addressed and we present numerous examples of outcrops (27) with steep-sided geometries. The Camlough Breccias are reinterpreted as the product of gas-driven tuffisites injected along the active ring fault (rather than of purely tectonic origin). Crucially, the porphyritic microgranite and porphyritic rhyolite ring-dyke rocks exhibit geochemical and petrographic signatures of contamination by the geographically restricted Palaeozoic Newry granodiorite and are best explained through crustal interaction vertically beneath the ring-complex. Subsequently, these silicic magmas rose into ignimbrite feeders along a caldera ring-fault system that was emplaced into near-surface vent-filling breccias.

Timing of deformation phases within the South Finland shear zone, SW Finland.

by Taija Torvela

Torvela, T., Mänttäri, I., Hermansson, T., 2008. Precambrian Research 160, 277-298

The Palaeoproterozoic Svecofennian crust in southern and central Fennoscandia was established about 1.8 Ga ago after a... more The Palaeoproterozoic Svecofennian crust in southern and central Fennoscandia was established about 1.8 Ga ago after a prolonged history of accretion and intrusion. During late stages of the Svecofennian orogeny, deformation was partitioned into several crustal-scale shear zones in present-day Finland, Sweden and Estonia. One such major ductile deformation zone, ‘the South Finland shear zone’ (SFSZ) extends for almost 200 km through the Aland archipelago in southwestern Finland, and further along the southern and southwestern coast of Finland. This more than a kilometer wide transpressional zone appears to have been repeatedly reactivated. The deformation started with a period of regional, ductile dextral shearing of igneous rocks, producing striped granodioritic and tonalitic gneisses. The ductile phases are locally overprinted and followed by ductile to semi-ductile deformation evidenced by mylonite zones of variable width. The last stage of tectonic activity along the shear zone is recorded by pseudotachylites.

Within this study, we dated zircons (SIMS U–Pb) and titanites (ID-TIMS U–Pb) from eight rock samples, and two pseudotachylite whole-rock samples (40Ar/39Ar) in order to reconstruct the deformation and (re)activation history of the shear zone. The results suggest that the medium-grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation. The ductile deformation within the study area initiated at ∼1.85 Ga. A second, more intensive deformation phase existed around 1.83 Ga, by which the shear zone was already well developed. Finally, the last ductile event is recorded by ∼1.79 Ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures, suggesting the initiation of large-scale mylonitisation around or soon after this time. The age of a pseudotachylite sample and, hence, the brittle deformation is bracketed between 1.78 and 1.58 Ga based on the age of pegmatites cut by pseudotachylites as well as 40Ar/39Ar minimum ages for the pseudotachylite, respectively. The data imply that the rocks within the study area entered the ductile–brittle transition zone due to rapid cooling and exhumation of the crust after ∼1.79 Ga.

Do experts use idealised structural models? Insights from a deepwater fold-thrust belt.

by Taija Torvela

Torvela, T. & Bond, C. E. (2011). Journal of Structural Geology, Volume 33, 51-58.

Theoretical models are often used to aid interpretation of geological data. For fold-thrust belts, structural and... more Theoretical models are often used to aid interpretation of geological data. For fold-thrust belts, structural and kinematic models have existed for over a century. While greatly contributing to our understanding of thrust systems, the usage of models can result in oversimplification and false kinematic interpretations. This paper investigates how experts use structural models in the interpretation of a seismic image from a deepwater fold-thrust belt. The results show that in the majority of cases experts produced interpretations that were compliant with key features in existing structural models. Those experts who returned interpretations that were not compliant, to existing models, better accounted for features present in natural and experimental analogues. This has implications for the general applicability of structural models in interpretation.