A Bedrock Transect Across the Champlain and Hinesburg Thrusts in West-Central Vermont: Integration of Tectonics with Hydrogeology and Groundwater Chemistry
Guidebook to field trip: New England Intercollegiate Geologica Conference
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Download (.pdf) Alpine metamorphic and tectonic evolution of the Inzecca-Ghisoni area (southern Alpine Corsica, France)
Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Gianfranco Principi
In the Inzecca-Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units,... more In the Inzecca-Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units, affected by Alpine deformations and metamorphism, crops out. Among them, there are some tectonic units (Parautochthonous Units, i. e. parautochtone of previous studies), that represent fragments of the continental Corse basement (Palaeozoic granitoids and associated volcanic and metamorphic pre-Carboniferous rocks) and of its Mesozoic to Tertiary sedimentary cover, that are tectonically sliced between the allochthonous Ligurian-Piedmontese Units (Schistes Lustrés) and the autochthonous basement (Variscan Corsica). The reconstructed polyphase deformation and metamorphic evolution of such units and the finding of high-pressure/low-temperature mineral assemblages in the continental-derived tectonic slices, points to the involvement of the south-eastern border of the European basement of Corsica in the tectonic processes linked to the Alpine subduction
STRATIGRAPHY AND TECTONIC AND METAMORPHIC EVOLUTION OF THE PORTO AZZURRO UNIT IN THE MONTE CALAMITA PROMONTORY (SOUTHEASTERN ELBA ISLAND, TUSCANY)
Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Gianfranco Principi
The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic... more The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and of the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the SE part of the Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piedmontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dikes. Moreover, in this part of the Island, the relationships between the uplift of the plutonic bodies and the final deformations of the tectonic stack are well exposed. To improve the geological knowledge of SE Elba, the authors carried out a 1:10.000 geological survey of the Calamita Promontory (mostly made up of the Porto Azzurro Unit) and performed petrographic and meso-/micro-structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, likely pre-Carboniferous basement (Mt. Calamita Fm.), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt.Calamita Fm., five main lithofacies were recognized and mapped. In particular, garnet-bearing, albite micaschist (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); Porphyroids-like rocks (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of the lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello Geothermal Field, whereas the other lithofacies can be probably correletable with the ?Ordovician formations of the Tuscan Metamorphic Units. The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (Dx), recognized in the Mt.Calamita Fm., which is defined by pre- Alpine schistosity and mineralogic relics (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the Greenschists facies, which deformed also the Mesozoic covers; c) a following folding event (D3) which probably occurred during or immediately after the strong thermometamorphic imprint (including the magnetite-rich skarn bodies), due to the Neogene magmatic intrusions; d) Subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt.Calamita Fm. and the Mesozoic cover) and between the latter and the overlying tectonic Units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch). A final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and N-S trending high –angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphic, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that defined for the Larderello geothermal region. Thus, the Mt.Calamita area can be considered as a little older, but similar geological model for all the future interpretations of the deep structure of southern Tuscany crossed by the Crop 18 profile.
The Porto Azzurro unit (Mt. Calamita promontory, south-eastern Elba Island, Tuscany) : stratigraphic, tectonic and metamorphic evolution
Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Principi Gianfranco
Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution... more Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the south eastern part of Elba Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piemontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dykes. Moreover, in this part of the island, the relationships between the emplacement of the plutonic bodies and the final deformations of the tectonic stack are easily detectable. To improve our geological knowledge of south eastern Elba, the authors carried out 1:10,000 geological mapping of the Calamita promontory (mostly composed of the Porto Azzurro Unit) and performed petrographical and structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, probably pre-Carboniferous basement (Mt. Calamita Metamorphic Complex), which is unconformably overlain by the PTriassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt. Calamita Metamorphic Complex, five main lithofacies were recognized and mapped. In particular, garnet-bearing-, albite mica-schists (lithofacies a) geometrically underlie a phyllitic-qurtzitic unit (lithofacies b); porphyroids (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello geothermal field, whereas the other lithofacies can probably be correlated with the ?Ordovician formations of the Tuscan Metamorphic Succession (e.g. Apuan Alps). The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (D,), recognized in the Mt. Calamita Metamorphic Complex, which is defined by a pre-Alpine foliation and mineral relicts (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the greenschist facies, which also deformed the Mesozoic cover; c) a later folding event (D3) which probably occurred during or immediately after the thermometamorphic imprint (including the magnetite-rich skarn bodies), caused by Neogene magmatic intrusions; d) subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Ml. Calamita Metamorphic Complex and the Mesozoic cover) and between the latter and the overlying tectonic units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch Unit); e) a final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and NE-SW trending high-angle normal fault systems, locally sealed by hydrothermal, sometimes Ferich mineralizations. The lithostratigraphical, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that detected for the Larderello geothermal region. Thus, the Mt. Calamita area can be considered as an older, but similar geological model for all future interpretations of the deep structure of southern Tuscany crossed by the CROP 18 profile.
Salt diapirism during basin inversion: inferences from analogue modelling
CHIARA DEL VENTISETTE, DOMENICO MONTANARI, MARCO BONINI, FEDERICO SANI
The development of diapirs is generally correlated with
the activity of normal faults developed in extensional
the activity of normal faults developed in extensional
regime, which is considered the most efficient mechanism
triggering diapirism (e.g. JACKSON & VENDEVILLE, 1994).
The influence of contractional settings on diapir growth is
doubtless considered a less efficacious mechanism (or an
opposing mechanism to diapirism growth; e.g., VENDEVILLE,
1991). As far as we are concerned, in spite of numerous
works on analogue modelling exploring salt diapirism
mechanisms (i.e. COTTON & KOYI, 2000; COSTA & VENDEVILLE,
2001; SANS & KOYI, 2001; BONINI, 2003; BRUN &
FORT, 2004), analogue models have never explored the
influence of positive fault inversion on diapirism.
In this work we present the results of the modelling
built to understand the relationships between fault reactivation
and their orientation in respect to the maximum
horizontal stress axis (DEL VENTISETTE et alii, 2004). The
models is performed at the Tectonic Modelling Lab of the
CNR-IGG and of the Department of Earth Sciences of
Florence using a pure-shear/simple-shear deformational
apparatus.
Scaling of analogue models to the natural prototype
requires the geometrical, rheological, kinematical and
dynamical similarities to be satisfied (e.g., HUBBERT,
1937; RAMBERG, 1981; WEIJERMARS & SCHMELING, 1986;
WEIJERMARS et alii, 1993).
The models had initial dimensions of 45.5cm × 42cm
× 7cm and consisted of a pure brittle system representing
the crystalline basement of a natural prototype.
The models were extended at a constant velocity of 10
mm/h for seven hours up to the bulk extension (BE) of
about 16.5%. After 1 hour of deformation (approximately
2.3% BE), when the graben was 1 cm deep, a mixture of
silicone and oleic acid (Newtonian behaviour, ρ=1060 kg
m-3, η=103 Pa s) was placed into the graben to simulate
the ductile behaviour of salt layers that are generally
associated whit this geodynamic context. The successive
syn-tectonic sedimentation consisted of dry quartz sand
layers (Fontainebleau sand with grain dimension <250
mm) with different colours sieved at regular time intervals
(1 mm every 30 minutes).
The following deformation phase simulated a compressive
stress field (σ1≅σh) that was applied at the constant
velocity of 10 mm/h for seven hours.
The deformation of these models was mostly accommodated
by the dip-slip reactivation of pre-existing extensional
structures (fig. 1a), both in the basement and in the
basin fill, without the development of important newlyformed
structures away from these reactivated normal
faults.
The results of analogue models reported here suggest
a new triggering mechanism for diapiric rise during
basin inversion. This mechanism relates the localization
of ductile diapirs in correspondences of early normal
faults inverted during shortening. In this case, diapiric
growth is related to the strong dip-slip reactivation component
along the fault extruding upwards the siliconesimulating
salt.
In many places around the world, diapiric rise locally
developed along the reactivated extensional fault system.
For example, in the Nzala des Oudayas area (Saiss Basin,
Morocco), which is characterized by a Tertiary and Quaternary
shortening phases reactivating the Mesozoic
extensional structures (e.g. AIT BRAHIM et alii, 2002 and
references therein), the Triassic salt diapirs are directly
located above the inverted normal faults. The comparison
between the overall features of Nzala des Oudayas diapirs
(fig. 1b) with the model geometries (fig. 1a) allow us to
suggest that the triggering mechanism for this diapiric
structure could be most likely related to the syn-shortening
fault reactivation.
Kinematic evolution of the Vena del Gesso (Romagna, Italy), analogue modelling of the interaction between erosion and tectonics
DOMENICO MONTANARI, CHIARA DEL VENTISETTE, MARCO BONINI & FEDERICO SANI
INFLUENCE OF HETEROGENEOUS LITHOSPHERIC STRUCTURE ON CONTINENTAL RIFTING: POSSIBLE IMPLICATIONS FOR THE LIGURE-PIEMONTESE OCEAN
Marco Bonini, Giacomo Corti, Chiara Del Ventisette, Gianfranco Principi, Federico Sani
Continental rifting represents the thermo-mechanical process by which continents break. This process is characterised... more
Continental rifting represents the thermo-mechanical process by which continents break. This process is characterised by an high variability in terms of duration, volcanicity, width and symmetry. Models formulated to explain such a structural variability consider differences in (e.g., Davison, 1997; Ziegler and Cloetingh, 2004): initial rheological or thermal structure of the lithosphere, rift kinematics (e.g., oblique vs. orthogonal) and mechanics (e.g., pure vs. simple shear), strain rate, melt volumes, presence of pre-existing weakness zones. Among these processes, the lithospheric structure inherited from previous deformation phases and the presence of weakness zones have been shown to play a major role in controlling continental extension (e.g., Dunbar and Sawyer, 1989; Ziegler and Cloetingh, 2004). As a consequence, rifting processes often affect continental lithospheres characterized by important lateral variations in rheology and these variations are able to influence parameters as the width and symmetry of the rift, the patterns of uplift and subsidence, the location and amount of volcanic products.
Extension in the Ligure-Piemontese ocean has been shown to be superimposed onto a previous ercinic chain, thus configuring a lithospheric weakness zone. In this frame, lateral rheological heterogeneities is expected to have played a major role in controlling the structure of the passive margins of the Adria/Europe system. In this work, this influence is investigated through scaled analogue modelling.
Small-scale experiments were performed in an artificial gravity field of 31g, by using the Large Capacity Centrifuge of the Tectonic Modelling Lab of the CNR-IGG, settled at the Earth-Science Department of Florence University. The models, with dimensions of 10cm x 16cm x 4cm, were built with suitable analogue materials (sand, silicone, oleic acid) able to reproduce the brittle/ductile stratification of the continental lithosphere and were properly scaled to be directly comparable to natural prototypes.
The experimental series investigated the occurrence of three different continental lithospheres: 1) a weak orogenic lithosphere (WL; Thickness: 80-100km; Thermo-mechanic Age: 50 Ma; Effective Elastic Thickness: 20km); 2) a normal 4-layer lithosphere (NL; Th: 120km; TA: 250 Ma; EET: 40km); 3) a strong cratonic lithosphere (CL; Th: 140km; TA: 400 Ma; EET: 70km).
Results show that in the simple case of WL, continental extension is taken up by regularly-spaced faulting in the upper brittle layer coupled to flow of the ductile lower crust and uppermost lithospheric mantle. No strain localisation configures with this initial rheological stratification; instead, the deformation style can be ascribed to a wide-rifting mode (e.g., Buck, 1991; Brun, 1999). The base of the lithosphere in these models was rather flat, suggesting that no major asthenospheric upraising took place during deformation.
In case of lateral variation between WL and CL, deformation was mostly localised at the craton border and within the weak domain; no deformation developed within the strong lithosphere. The largest normal fault developed at the contact between WL and CL, whereas regularly-spaced faults and lower crustal flow (with development of core complex structures) characterised the weakest portion of the models. Extension was accommodated by downward flexure of the WL and elastic rebound with strong shoulder uplift of CL; limited asthenospheric upraising was mainly localised within the WL.
In models with NL and CL separated by a central WL, extensional deformation was strongly localised by the weakness zone. This rheological configuration leaded indeed to a rapid necking of the lithosphere and asthenospheric upraising leading to continental break-up in the central part of model.
The modelling results support an important role played by the initial lithospheric structure on rift structure and duration and suggest that lateral rheological variations may have an important influence in controlling the local pattern of uplift/subsidence and faulting in continental rift settings.
These results may have also relevance for the opening of the Ligure-Pemontese ocean. Preliminarily, the structural conditions of continental extension between the European and Adriatic margin may be correlated to the experiment reproducing the strength contrast between weak and cratonic lithospheres. In particular, the downward flexure associated with low subsidence rates of the WL may well account for the development of the Calcare Massiccio fm. on the Adriatic margin, whereas the areas characterised by high subsidence at the WL-CL contact may be correlated with the coeval development of the Briançonnaise domain in the European margin.
The role of stress orientation on fault reactivation: an analogue modelling approach
Del Ventisette C., Montanari D., Sani F., Bonini M.
A series of analogue models have been performed to get insights into the relationships between
reactivation of... more
A series of analogue models have been performed to get insights into the relationships between
reactivation of pre-existing extensional structures and the orientation of a successive compressive
stress field. Influence of rheological behaviour of syntectonic sedimentation (pure brittle or brittleductile)
during the extensional phase has been also taken into account. The models consisted of two
deformational phases: 1) a first extensional phase (σh=σ3) during which a graben delimited by major
faults developed, and 2) a successive compressive phase with the orientation of maximum stress axis
σ1 (σh=σ1) variable between 0° and 90° (obliquity angle α to the trend of faults bounding the firstphase
graben. The experimental results suggest that the development of compressive structures is
strongly controlled by the pre-existing extensional structures and by the obliquity angle α. All the
experiments have shown that the extensional structures were invariably reactivated during shortening.
Notably, the shortening was firstly accommodated by the reactivation of the pre-existing structures
and localized preferentially in correspondence of lateral rheological discontinuity, whereas new faults
developed only successively. In all models, the dip rotation of major fault planes took place and
increased with angle α. Mean horizontal displacement vectors illustrate the role of basin fill ductile
layers and obliquity angle (α) on strain partitioning and brittle-ductile decoupling.
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.
Stratigraphy and structure of the Kapp Lyell diamictites (upper Proterozoic), Spitsbergen
Kowallis, B.J. and Craddock, C., 1984, Stratigraphy and structure of the Kapp Lyell diamictites (Upper Proterozoic), Spitsbergen: Geological Society of America Bulletin, v. 95, p. 1293-1302.
![](http://photos.academia.edu/1202873/432132/535606/tiny_zori.dotzeva.jpg?1329145687 "<strong>Zornitza Dotzeva</strong><br/>Sofia University "St.Kliment Ohridsky""){kind=small}
Crust–mantle boundaries in the Taiwan–Luzon arc-continent collision system determined from local earthquake tomography and 1D models: Implications for the mode of subduction polarity reversal
Kamil Ustaszewski, Yih-Min Wu, John Suppe, Hsin-Hua Huang, Chien-Hsin Chang & Sara Carena. Tectonophysics (2012), doi:10.1016/j.tecto.2011.12.029
In order to better understand the mode of subduction polarity reversal in the Taiwan–Luzon arc-continent collision... more
In order to better understand the mode of subduction polarity reversal in the Taiwan–Luzon arc-continent collision zone, we mapped its crust–mantle boundaries using local earthquake tomography. By contouring surfaces of constant Vp = 7.5 km s− 1, we identified three Moho discontinuities and the plate interface that juxtaposes Eurasian lower crust against mantle lithosphere of the Philippine Sea plate. The plate interface
dips to the east under southeastern Taiwan and steepens progressively towards north until it becomes vertical at 23.7°N. From there it continues northward in a vertical orientation, until the limit of the tomographic model inhibited further mapping. For the Moho, additional depth constraints were derived from 1D models using P-wave arrivals of local earthquakes. The Mohos of the Eurasian and Philippine Sea plates are disconnected across the plate interface. Beneath southern Taiwan, the Eurasian Moho dips to the east at 50–60°, following the orientation of the plate boundary and continuous with the Benioff zone. Towards north, the Eurasian Moho steepens into subvertical, again together with the plate boundary. The Philippine Sea plate Moho exhibits a synform-like crustal root, interpreted as the base of the magmatic Luzon arc. Towards the
north, this root deepens from 30 to 70 km underneath the Ryukyu trench. In northernmost Taiwan, the hinge of the vertically subducting Eurasian slab steps westward out of the thrust belt, leaving the deformation front to the east inactive and giving way to the north-dipping Philippine Sea plate. A subhorizontal Moho at 30–35 km depth overlies the north-dipping Philippine Sea slab and is interpreted as a newly formed
Moho, established after westward rollback and delamination of the subducting Eurasian slab. In combination, these data support a model of progressive subduction polarity reversal, in which a tear within the Eurasian lithosphere propagated southwestward, deactivating the deformation front.
The Late Triassic and Late Jurassic stress fields and tectonic transmission of North China craton
by Hari K R
For reprint request, please send a mail to krharigeology@gmail.com
Structural geometry and evolution of the Toyuk thrust zone, Brooks Range, Alaska
This was my Master's thesis, completed in 1998 at the University of Alaska, Fairbanks
The Toyuk thrust zone is a regional tectonic boundary in the Brooks Range that has been interpreted previously as a... more The Toyuk thrust zone is a regional tectonic boundary in the Brooks Range that has been interpreted previously as a large-displacement fault. Just west of the Dalton Highway the thrust zone is defined by the intersection of the erosional surface with several imbricate thrust faults. These faults have cut detachment folds formed between incompetent shale units and define a duplex limited to three units. Apatite fission-track dating indicates cooling in the area at approximately 60 Ma as well as “mixed ages” that reflect both the 60 Ma cooling and earlier cooling at approximately 100 Ma. These ages have been documented previously in the region, but these samples are the first to record the 60 Ma event so far south of the range front. This suggests that the formation and growth of the Toyuk duplex accommodated structural thickening with the orogenic wedge coeval with deformation at the range front.
Duplex structure and Paleocene displacement of the Toyuk thrust zone near the Dalton Highway, north-central Brooks Range
Chmielowski, R.M., Wallace, W.K., and O'Sullivan, P.B., 2000, in Pinney, D.S., and Davis, P.K., eds., Short Notes on Alaska Geology 1999: Alaska Division of Geological & Geophysical Surveys Professional Report 119B, p. 11-31.
The Toyuk thrust zone is a regional tectonic boundary in the central Brooks Range of northern Alaska that has been... more
The Toyuk thrust zone is a regional tectonic boundary in the central Brooks Range of northern Alaska that has been interpreted previously as a single large-displacement fault. Just west of the Dalton Highway, the thrust zone is defined by several north-vergent imbricate thrust faults. These faults have cut detachment folds formed in the competent Kanayut Conglomerate between detachments in the underlying Hunt Fork Shale and overlying Kayak Shale, thereby forming a duplex. The exposed structural geometry suggests to end-member models of duplex geometry. In model one, each linking thrust has roughtl the same small displacement and the roof thrust is overlain by a normal stratigraphic succession. In model two, the uppermost and hindmost fault has greater displacement than the other faults and so forms a duplex roof above which the stratigraphic section is duplicated. Discrimination between these models is not possible because the duplex roof has been eroded in the field area.
Apatite fission-track analyses of samples collected with the Toyuk thrust zone record two distinct episodes of cooling interpreted to represent unroofing in response to tectonic deformation. The initial episode occurred at ~100 Ma, while the later episode occurred at ~60 Ma. These two cooling/denudation events have been documented previously in the region, but the samples from the thrust zone are the first to document displacement on a specific fault during the ~60 Ma event so far south of the front of the central Brooks Range. The results suggest that at least part of the growth of the Toyuk thrust zone accommodated structural thickening within the orogenic wedge coeval with deformation at the range front.
Modeling of seismic guided waves at the Dead Sea Transform
C. Haberland, A. Agnon, R. El-Kelani, N. Maercklin, I. Qabbani, G. Rümpker, T. Ryberg, F. Scherbaum, M. Weber (2003). Journal of Geophysical Research, 108(B7), 2342, doi:10.1029/2002JB002309
On several recordings of linear seismometer arrays crossing the Arava Fault (AF) in the Middle East, we see prominent... more On several recordings of linear seismometer arrays crossing the Arava Fault (AF) in the Middle East, we see prominent wave trains emerging from in-fault explosions which we interpret as waves being guided by a fault zone related low-velocity layer. The AF is located in the Arava Valley and is considered the principal active fault of the mainly N-S striking Dead Sea Transform System in this section. Observations of these wave trains are confined to certain segments of the receiver lines and occur only for particular shot locations. They exhibit large amplitudes and are almost monochromatic. We model them by a two-dimensional (2-D) analytical solution for the scalar wave field in models with a vertical waveguide embedded in two quarter spaces. A hybrid search scheme combining genetic algorithm and a local random search is employed to explore the multimodal parameter space. Resolution is investigated by synthetic tests. The observations are adequately fit by models with a narrow, only 3-12 m wide waveguide with S wave velocity reduced by 10-60% of the surrounding rock. We relate this vertical low-velocity layer with the damage zone of the AF since the location of receivers observing and of shots generating the guided waves, respectively, match with the surface trace of the fault. The thickness of the damage zone of the AF, at least at shallow depths, seems to be much smaller than in other major fault zones. This could be due to less total slip on this fault.
Seismic structure of the Arava Fault, Dead Sea Transform
N. Maercklin (2004). Dissertation (doctoral thesis), University of Potsdam, Germany, urn:nbn:de:kobv:517-0001469
Also: Scientific Technical Report, 04/12, GeoForschungsZentrum Potsdam, Germany, ISSN 1610-0956, urn:nbn:de:kobv:b103-041283
The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the... more
The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in the north. Formed in the Miocene 17 Ma ago and related to the breakup of the Afro-Arabian continent, the DST accommodates the left-lateral movement between the two plates. The study area is located in the Arava Valley between the Dead Sea and the Red Sea, centered across the Arava Fault (AF), which constitutes the major branch of the transform in this region.
A set of seismic experiments comprising controlled sources, linear profiles across the fault, and specifically designed receiver arrays reveals the subsurface structure in the vicinity of the AF and of the fault zone itself down to about 3-4 km depth. A tomographically determined seismic P velocity model shows a pronounced velocity contrast near the fault with lower velocities on the western side than east of it. Additionally, S waves from local earthquakes provide an average P-to-S velocity ratio in the study area, and there are indications for a variations across the fault. High-resolution tomographic velocity sections and seismic reflection profiles confirm the surface trace of the AF, and observed features correlate well with fault-related geological observations.
Coincident electrical resistivity sections from magnetotelluric measurements across the AF show a conductive layer west of the fault, resistive regions east of it, and a marked contrast near the trace of the AF, which seems to act as an impermeable barrier for fluid flow. The correlation of seismic velocities and electrical resistivities lead to a characterisation of subsurface lithologies from their physical properties. Whereas the western side of the fault is characterised by a layered structure, the eastern side is rather uniform. The vertical boundary between the western and the eastern units seems to be offset to the east of the AF surface trace.
A modelling of fault-zone reflected waves indicates that the boundary between low and high velocities is possibly rather sharp but exhibits a rough surface on the length scale a few hundreds of metres. This gives rise to scattering of seismic waves at this boundary. The imaging (migration) method used is based on array beamforming and coherency analysis of P-to-P scattered seismic phases. Careful assessment of the resolution ensures reliable imaging results.
The western low velocities correspond to the young sedimentary fill in the Arava Valley, and the high velocities in the east reflect mainly Precambrian igneous rocks. A 7 km long subvertical scattering zone (reflector) is offset about 1 km east of the AF surface trace and can be imaged from 1 km to about 4 km depth. The reflector marks the boundary between two lithological blocks juxtaposed most probably by displacement along the DST. This interpretation as a lithological boundary is supported by the combined seismic and magnetotelluric analysis. The boundary may be a strand of the AF, which is offset from the current, recently active surface trace. The total slip of the DST may be distributed spatially and in time over these two strands and possibly other faults in the area.
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Shallow architecture of the Wadi Araba fault (Dead Sea Transform) from high-resolution seismic investigations
C. Haberland, N. Maercklin, D. Kesten, T. Ryberg, C. Janssen, A. Agnon, M. Weber, A. Schulze, I. Qabbani, R. El-Kelani (2007). Tectonophysics, 432(1-4), 37-50, doi:10.1016/j.tecto.2006.12.006
In a high-resolution small scale seismic experiment we investigated the shallow structure of the Wadi Araba Fault... more In a high-resolution small scale seismic experiment we investigated the shallow structure of the Wadi Araba Fault (WAF), the principal fault strand of the Dead Sea Transform System between the Gulf of Aqaba/Eilat and the Dead Sea. The experiment consisted of 8 sub-parallel 1 km long seismic lines crossing the WAF. The recording station spacing was 5 meters and the source point distance was 20 m. The first break tomography yields insight into the fault structure down to a depth of about 200 m. The velocity structure varies from one section to the other which were 1 to 2 km apart, but distinct velocity variations along the fault are visible between several profiles. The reflection seismic images show positive flower structures and indications for different sedimentary layers at the two sides of the main fault. Often the superficial sedimentary layers are bent upward close to the WAF. Our results indicate that this section of the fault (at shallow depths) is characterized by a transpressional regime. We detected a 100 to 300 m wide heterogeneous zone of deformed and displaced material which, however, is not characterized by low seismic velocities at a larger scale. At greater depth the geophysical images indicate a blocked cross-fault structure. The structure revealed, fault cores not wider than 10 m, are consistent with scaling from wear mechanics and with the low loading to healing ratio anticipated for the fault.