Low-angle Detachment Research Articles

The effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low-angle normal faults

To investigate the formation of core complexes and low-angle normal faults, we devise thermo-mechanical simulations on a simplified wedge-like orogenic hinterland that has initial topography, Moho relief, and a preexisting mid-crustal shear zone that can accommodate shear at very low angles (<20°). We mainly vary the strength of the lower crust and the frictional strength of the preexisting mid-crustal shear zone. We find that the strength of the lower crust, and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes. With increasing lower crustal strength, we recognize varying extensional features with decreasing exhumation rate: these are characterized by bivergent metamorphic massifs, classic Cordilleran metamorphic core complexes, multiple consecutive core complexes (or boudinage structures), and a flexural core complex underlined by a large subsurface low-angle detachment fault with a small convex curvature. Topographic loading and mantle buoyancy forces, together with divergent boundaries, drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading. The detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust. Though the regional stress is largely Andersonian, we find non-Andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient. Our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve.

Structural controls on Fe mineralization at Thabazimbi Mine, South Africa

Chemical-sedimentary rocks of the Transvaal Supergroup, along the northern margin of the Transvaal Basin, were imbricated during formation of the Mohlapitsi Fold-and-Thrust Belt between ca. 2.55 and 1.93Ga. High-grade (>63% Fe), stratiform Fe-ore bodies occur at or close to the base of the banded ironstone-dominated Penge Formation, which forms a distinct E-W trending mountain range in the area. The Penge Formation is separated from underlying Malmani dolostone by a thin, laterally continuous, rheologically-weak, carbonaceous shale layer that has been intensely sheared. This horizon denotes the position of a regional, top-to-N, low-angle detachment that has been variably deformed during progressive development of the Mohlapitsi Belt. Several of Kumba's Thabazimbi iron ore deposits, including Kwaggashoek East, Donkerpoort West, Kumba-Donkerpoort West and Buffelshoek West, are confined to a zone directly above this detachment. Recent studies of ore petrography, fluid alteration, fluid composition and stable isotope geochemistry link economic iron ore mineralization to hydrothermal or hypothermal processes. Along with localized high-grade mineralization, a structural control on fluid flow has been suggested, although a lack of structural data has hampered more concise mineralization models thus far. In this study we present and analyse data from detailed structural mapping over a protracted period of mining, across four opencast pits. This mapping is combined with drillhole data to produce accurate, intergrated 3D models of these deposits, which reveal the true geometry of these ore bodies.Data and resultant models indicate that enhanced hydrothermal fluid flow associated with significant iron mineralization is linked to thrusting/shearing in response to: 1) fold amplification and flexural flow along the E-W trending detachment between the Malmani dolostone and banded iron formation; 2) subsidiary fold-accommodation structures such as conjugate shear joints, normal and reverse faults and thrusts, associated with inner- and outer-arc shortening and extension, respectively; 3) localization of Fe at the intersections of major reverse faults, the detachment and stratigraphically-higher, low-angle thrusts, which are preferentially developed at the contacts between diabase and banded iron formation. Collectively, these features both aided and compartmentalized regional, pervasive fluid flow along the detachment and in stratigraphically-higher sites. Subsequent ore preservation was enhanced by reactivation of NW- to NNW-trending, Mesozoic normal faults and concomitant downthrow of fault-bounded blocks. Structural relationships and timing of high-grade iron ore mineralization further suggest that associated top-to-N kinematic fold-and-thrust formation of the Mohlapitsi Belt and, subsequently, the main detachment between Malmani dolostone and banded iron formation of the Penge Formation, could have been active during and immediately after intrusion of the Bushveld Complex at ca. 2.05Ga.

Stratigraphy and Ar/Ar geochronology of the Miocene lignite-bearing Tunçbilek-Domaniç Basin, western Anatolia

The Tuncbilek-Domanic Basin is one of the Neogene basins containing economic coal deposits in western Anatolia, Turkey. The basin fill represents fluvial to lacustrine sedimentary units which are interlayered with volcanic rocks with bimodal composition. In order to reveal the stratigraphy and the exact ages of the basin fill and coal deposits, and to explore the tectonic evolution of the basin, we present new field data and Ar/Ar age data from the volcanic units. The field studies and the age data indicate that the whole basin fills were deposited between ~23 and ~19 Ma (Aquitanian–Early Burdigalian) without any unconformity. Taking into account the ages of the coal-bearing sedimentary units in the other Neogene basins in the region, it is concluded that most of the economic coal deposits in the western Anatolia were formed during Aquitanian. The field studies also show that the deposition of the sedimentary units in the basin was controlled by the NE–SW-trending strike- to oblique-slip normal faults. In a regional scale, tectonic evolution of the Tuncbilek-Domanic Basin is linked to the differential stretching in the hanging wall of the southerly located, a crustal-scale low-angle detachment fault (the Simav detachment fault) that controlled the Early Miocene exhumation of the Menderes Extensional Core Complex.

What do fault patterns reveal about the latest phase of extension within the Northern Snake Range metamorphic core complex, Nevada, USA?

The Northern Snake Range is a classic example of a metamorphic core complex, Basin-and-Range province, United States. It is composed of a plastically deformed footwall and a brittlely deformed hanging wall, separated by the Northern Snake Range low-angle detachment (NSRD). Brittle deformation, however, is not confined to the hanging wall.This paper focuses on exposures in Cove Canyon, located on the SE flank of the Northern Snake Range, where penetrative, homogeneous faults are well exposed throughout the hanging wall, footwall and NSRD, and overprint early plastic deformation. These late-stage fault sets assisted Eocene-Miocene extension. Detailed analysis of the faults reveals the following: (1) The shortening direction defined by faults is similar to the shortening direction defined by the stretching lineation in the footwall mylonites, indicating that the extensional kinematic history remained unchanged as the rocks were uplifted into the elastico-frictional regime. (2) After ∼17 Ma, extension may have continued entirely within elastic-frictional regime via cataclastic flow. (3) This latest deformation phase may have been accommodated by a single, continuous event. (3) Faults within NSRD boudins indicate that deformation within the detachment zone was non-coaxial during the latest phase of extension.

Myths about normal faulting

Abstract Analyses of normal faults in mechanically layered strata reveal that material properties of rock layers strongly influence fault nucleation points, fault extent (trace length), failure mode (shear v. hybrid), fault geometry (e.g. refraction through mechanical layers), displacement gradient (and potential for fault tip folding), displacement partitioning (e.g. synthetic dip, synthetic faulting, fault core displacement), fault core and damage zone width, and fault zone deformation processes. These detailed investigations are progressively dispelling some common myths about normal faulting held by industry geologists, for example: (i) that faults tend to be linear in dip profile; (ii) that imbricate normal faults initiate due to sliding on low-angle detachments; (iii) that friction causes fault-related folds (so-called normal drag); (iv) that self-similar fault zone widening is a direct function of fault displacement; and (v) that faults are not dilational features and/or important sources of permeability.

Did Oligocene crustal thickening precede basin development in northern Thailand? A geochronological reassessment of Doi Inthanon and Doi Suthep

The Doi Inthanon and Doi Suthep metamorphic core complexes in northern Thailand are comprised of amphibolite-grade migmatitic gneisses mantled by lower-grade mylonites and metasedimentary sequences, thought to represent Cordilleran-style core complexes exhumed through the mobilization of a low-angle detachment fault. Previous studies have interpreted two metamorphic events (Late Triassic and Late Cretaceous), followed by ductile extension between the late Eocene and late Oligocene, a model which infers movement on the detachment at ca. 40Ma, and which culminates in a rapid unroofing of the complexes in the early Miocene. The Chiang Mai Basin, the largest such Cenozoic Basin in the region, lies immediately to the east. Its development is related to the extension observed at Doi Inthanon and Doi Suthep, however it is not definitively dated, and models for its development have difficulty reconciling Miocene cooling ages with Eocene detachment movement. Here we present new in-situ LA-ICP-MS and SIMS U–Pb age data of zircon and monazite grains from gneiss and leucogranite samples taken from Doi Inthanon and Doi Suthep. Our new zircon data exhibit an older age range of 221–210Ma, with younger ages of ca. 72Ma, and 32–26Ma. Our monazite data imply an older age cluster at 83–67Ma, and a younger age cluster of 34–24Ma. While our data support the view of Indosinian basement being reworked in the Cretaceous, they also indicate a late Eocene–Oligocene tectonothermal event, resulting in prograde metamorphism and anatexis. We suggest that this later event is related to localized transpressional thickening associated with sinistral movement on the Mae Ping Fault, coupled with thickening at the restraining bend of the Mae Yuan Fault to the immediate west of Doi Inthanon. Further, this upper Oligocene age limit from our zircon and monazite data would imply a younger Miocene constraint on movement of the detachment, which, when combined with the previously recorded Miocene cooling ages, has implications for a model for the onset of extension and subsequent development of the Chiang Mai Basin in the early mid-Miocene.

Syn- to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

The Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure–temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9–13kbar) to lower pressure (M2, 6–9kbar) at a constant temperature of ca. 315°C. The basal unit has preserved a first set of HP/LT conditions (M1–2, 8–11kbar, 300°C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5–8.5kbar) associated with a slight but significant increase in temperature (∼350°C).These structural, petrologic and thermobarometric data document the tectonic evolution from construction to destruction of the Hellenic orogenic belt. The steep-dipping blueschist facies foliation, preferentially preserved at high structural level in the middle unit, is considered to reflect tectonic accretion marked by successive burial (D1M1) and syn-orogenic exhumation (D2M2) without thermal relaxation. The transposition-retrogression of the blueschist facies mineral assemblage into a shallow-dipping greenschist facies foliation associated with an increase in temperature corresponds to lateral flow of the thermally relaxed nappe stack (D3M3). The development of a low-angle detachment, accommodating post-orogenic exhumation of the orogenic root, is attributed to gravitational collapse of the Hellenic belt.

Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica 40Ar/39Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~700°C to ~60°C in less than 60Myr (136–80Ma) at variable rates ranging from ~50°C/Myr to ~13°C/Myr, (2) the Hengshan detachment fault accommodated ~8–12km of total slip at variable slip rates from 0.14 to 1mm/yr during tectonic exhumation, (3) the footwall has been tilted ~26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80Ma.

An overview on the main stratigraphic and structural features of a geothermal area: the case of Nazilli-Buharkent section of the Büyük Menderes Graben, SW Turkey

The study area is Nazilli-Buharkent section of the Büyük Menderes Graben. This is one of the most active intracontinental extensional structures shaping the south-western Anatolian graben–horst system, which comprises the southwestern frontal part of the Anatolian platelet. The Büyük Menderes Graben is about 3–30 km wide, 170 km long and approximately E–W-trending depression included in the back-arc section of the northerly-dipping south Aegean-Cyprian subduction zone. The Büyük Menderes Graben of episodic origin has two morphotectonic configurations: (1) a wider, uplifted, dissected and deformed initial configuration of late Early–late Middle Miocene age; and (2) the narrower, linear, undeformed and continuous recent configuration of Quaternary age. These two configurations are here termed as the Büyük Menderes palaeotectonic graben and the Büyük Menderes modern (neotectonic) graben, respectively. These two grabens are represented by two sedimentary packages separated by an intervening angular unconformity. The 1.1-km-thick older sedimentary package is deformed (steeply tilted to folded) and consists of, from bottom to top, unsorted boulder-block basal clastics and coal-bearing flood plain to lacustrine deposits of late Early–late Middle Miocene age. The younger sedimentary package is undeformed and consists of very thick (up to 0.9 km) debris flow and fluvial deposits of Quaternary age. The palaeotectonic configuration of the Büyük Menderes Graben is bounded by the Hasköy-Künüpe and İğdecik structural segments of the Büyük Menderes detachment fault. However, the modern configuration of the Büyük Menderes Graben is bounded by the Kuyucak and Menderes high-angle normal fault zones, composed of numerous fault segments, which cut and displace deeply the detachment fault. Hot water springs occur along the traces of the detachment faults. But they are not observed along the traces of modern graben-boundary faults. This reveals that the low-angle detachment fault is being still used by the geothermal fluids. Indeed, it is a fact that the geothermal fluids are shared by both the older detachment and younger high-angle normal faults at their intersection at depth, because the faults are the most suitable ways for the underground circulations of both cold meteoric waters and geothermal fluids, i.e. the geothermal system in south-western Turkey is fault-controlled. Consequently, the geothermal potential of this extensional domain is quite high based on the criteria such as the active tectonic and related faults, sources of high-heat, reservoir rocks with high porosity, reservoir cap rocks with no or low permeability and cold meteoric water supply enough. In order to obtain huge volume of geothermal fluids, first of all, whole steps of the geothermal exploration have to be completed, and then the site of borehole(s) has to be chosen so that it penetrates the intersection of low-angle detachment fault, high-angle normal fault(s) and the reservoir rocks overlain by the thick and impermeable cap rock(s).

Crustal architecture of the Borborema Province, NE Brazil, from receiver function CCP stacks: Implications for Mesozoic stretching and Cenozoic uplift

We have investigated the crustal architecture of the Borborema Province of NE Brazil by constructing common conversion point (CCP) receiver function stacks from teleseismic P-waveforms recorded at 64 seismic stations in the region. The Borborema Province represents the western portion of a larger Neoproterozoic mobile belt that experienced extension in the Mesozoic, leading to the formation of a number of intra-continental rift basins and, eventually, continental breakup. After continental breakup, episodes of uplift in the Province — perhaps related to coeval episodes of Cenozoic volcanism — helped shape the high topographies of the Borborema Plateau. Our receiver function CCP stacks image clear P-to-S conversions at the crust–mantle boundary and confirm independent evidence for a 36–38km thick crust under the southern portion of the Plateau and a thinner 30–32km thick crust in the surrounding regions, including the northern Plateau. The cross-sections also reveal the presence of an intra-crustal discontinuity at 9–18km depth under the regions of thin crust that fades away under the thick southern Plateau. We argue that the thin crust in the Borborema Province is the result of Mesozoic crustal stretching and that the intra-crustal discontinuity represents a low-angle detachment zone that helped accommodate extension in the crust. The thick crust under the southern Plateau would then represent a rheologically stronger portion of the Borborema Province that resisted deformation by Mesozoic extension, while the thin crust under the northern Plateau would be a portion of formerly depressed thin crust that was uplifted during the Cenozoic.

Evidence for Pleistocene Low-Angle Normal Faulting in the Annapurna-Dhaulagiri Region, Nepal

AbstractNorth-south-directed extension on the South Tibetan Fault System (STFS) played an important role in Himalayan tectonics of the Miocene Period, and it is generally assumed that orogen-perpendicular extension ceased in this orogenic system before the Pliocene. However, previous work in the Annapurna and Dhaulagiri Himalaya of central Nepal revealed evidence for local Pleistocene reactivation of the basal STFS structure in this area (the Annapurna Detachment). New structural mapping and (U-Th)/He apatite and zircon thermochronology in this region further document the significance of Pleistocene N-S extension in this sector of the Himalaya. Patterns of (U-Th)/He accessory-mineral ages are not disrupted across the reactivated segment of the STFS basal detachment, indicating that Pleistocene offset was limited. In contrast, the trace of a N-dipping, low-angle detachment in the hanging wall of the reactivated Annapurna Detachment—formerly linked to the STFS, but here named the Dhaulagiri Detachment—coinc...

Heterogeneous extension and the role of transfer faults in the development of the southeastern Betic basins (SE Spain)

Large strike-slip faults in the eastern Betics are interpreted to have developed in a transcurrent setting in response to 4–6 mm/yr of Africa-Iberia NW-SE convergence. However, here we show that some of these faults are transfer faults accommodating heterogeneous late Miocene extension. The North Cabrera dextral fault and other E-W to NE-SW strike-slip faults in the Sorbas basin were transfer faults produced under SW-NE extension. These faults together with related normal faults form the main boundaries of two sedimentary depocenters active between the Serravallian and the Tortonian. The older North Cabrera depocenter extended between the Serravallian and the early Tortonian (approximately 13.8 to 9 Ma), while the younger Gacia depocenter formed in response to late Tortonian extension (approximately 9 to 7.5 Ma). The latter formed to the west of the North Cabrera depocenter by a listric fan of normal faults with SW directed transport that are linked by dextral and sinistral transfer fault segments. These faults root on a low-angle detachment cutting into the exhumed high-pressure Nevado-Filabride complex rocks at ~0.8 km depth. The present work reveals that (1) this extension was partially coeval with and kinematically linked to sinistral displacement along the Carboneras fault farther south in the Nijar basin; (2) this westward directed extension produced elongated core complexes and tilted blocks to the north of the Carboneras fault and magmatic accretion upon thinned continental crust to the south, probably in response to slab tearing or detachment and associated edge delamination of the Iberian continental lithospheric mantle beneath the Betics.

The 3D interplay between folding and faulting in a syn-orogenic extensional system: the Simplon Fault Zone in the Central Alps (Switzerland and Italy)

Extensional low-angle detachments developed in convergent or post-collisional settings are often associated with upright folding of the exhumed footwall. The Simplon Fault Zone (SFZ) is a Miocene low-angle detachment that developed during convergence in the Central Alps (Switzerland and Italy), accommodating a large component of orogen-parallel extension. Its footwall shows complex structural relationships between large-scale backfolds, mylonites and a discrete brittle detachment and forms a 3D gneiss dome reflecting upright folding with fold axes oriented both parallel and perpendicular to the extension direction. We present a regional study that investigates the interplay between folding and faulting and its implications for the resulting exhumation pattern of the gneiss dome using 3D geometric modelling (computer software GeoModeller), together with a consideration of the chronological relationships from field relationships and 40Ar/39Ar dating. The early Simplon mylonitic fabric is clearly folded by both extension-parallel and extension-perpendicular folds, forming a doubly plunging antiform, whereas the later ductile-to-brittle fabric and the cataclastic detachment are only affected by wavy extension-parallel folds. This observation, together with the interpreted cooling pattern across the SFZ, suggests that updoming of the footwall initiated at the onset of faulting during ductile shearing around 18.5 Ma, due to coeval extension and perpendicular convergence. New 40Ar/39Ar dating on micas (biotite and muscovite) from a sample affected by a strong crenulation cleavage parallel to the axial plane of the Glishorn and Berisal parasitic folds establishes that these folds formed at ca. 10 Ma, broadly coeval with late movement along the more discrete detachment of the SFZ. These extension-parallel folds in the footwall of the SFZ developed due to continued convergence across the Alps, accelerating ongoing exhumation of the western Lepontine dome and promoting coeval uplift of the crystalline Aar and Gotthard massifs in the late Miocene.

Evolutionary process of Beppu Bay in central Kyushu, Japan: a quantitative study of the basin-forming process controlled by plate convergence modes

This integrated tectonic study reveals the basin-forming and deforming processes on an active margin. Southwest Japan (SWJ) is an island arc under the influence of oblique subduction of the Philippine Sea plate, which has provoked dextral slips on the arc-bisecting Median Tectonic Line (MTL). Plio-/Pleistocene sediments in Beppu Bay, a tectonic depression at the westernmost portion of the MTL, are categorized into lower (5 ~ 0.7 Ma), upper (0.7 ~ 0.3 Ma), and an auxiliary uppermost (0.3 Ma ~ present) unit in ascending order. Detailed seismic interpretation demonstrates that major structures in the deep interior of the basin are an older half-graben under a strong N-S extensional regime and a younger pull-apart sag that developed in a right-stepping part of the MTL as a result of late Quaternary-enhanced strike-slip rates on the fault. Sediments within the pull-apart have been deformed by later inversion events as a contraction phase arose. Conspicuous deformation of the hanging wall of the low-angle detachment of the basin was successfully reproduced by numerical modeling. Based on a discrete element method, this suggests that structural differences in the deformed sedimentary layers are caused by differences in the dip angles of the faults. Remarkable temporal changes in tectonic regimes around Beppu Bay and other areas of SWJ are probably related to transient modes of convergence, including the migration of the Euler pole, of the Philippine Sea plate since ca. 6 Ma.

The Connemara Antiform and its possible Silurian history

The Connemara Antiform has iconic status in the geology of western Ireland. With its eastto-west trend and eastward plunge and the disposition of Dalradian stratigraphy about its axis, it forms the dominant structure in the present-day landscape and is a focal feature of geological maps of the area. Its genesis has long been ascribed to D4, the last main major deformation phase of the Ordovician Grampian Orogeny that affected the Dalradian rocks of Connemara. But does the structure have such a straightforward and simple origin? This paper investigates the possible role of middle late Silurian deformation in the origin of the Connemara Antiform. Overview of the geology of Connemara Connemara was part of the south-eastern Laurentian margin Dalradian sequence that experienced a major short-lived early Ordovician orogenic episode, the Grampian Orogeny, caused by collision with, and obduction of, an arc that developed above oceanward-directed subduction (Chew 2009). The Connemara Terrane became detached from the Laurentian margin and now lies to the south of the obducted arc and the South Mayo Trough (Fig. 1). The translation of the Connemara Terrane from the Laurentian margin to its current position is indicated by an influx of coarse clastic rocks of Connemara provenance into the c. 464Ma Bunnacunneen Member of the Mweelrea Formation of the South Mayo Trough (McConnell et al. 2009). Dewey (2005) provides a succinct account of the local plate-tectonic, metamorphic and structural history of the Connemara Terrane and the South Mayo Trough during the Grampian Orogeny, and Graham et al. (1989), Leake and Tanner (1994), McConnell et al. (2002) and Long et al. (2005) provide a comprehensive overview of the detailed geology of the area. The Grampian Orogeny began with the subduction of rifted Laurentian margin sediments beneath a continent-facing oceanic arc c. 478Ma (Dewey 2005). The Connemara Dalradian sediments were buried in a thick nappe pile of D2 and D3 folds and experienced high-pressure, lowtemperature Barrovian metamorphism (Leake and Tanner 1994). Palaeomagnetic studies indicate that the D3 folds originally dipped c. 208 south (Robertson 1988). Between 474Ma and 470Ma (Friedrich et al. 1999) a suite of metagabbros and orthogneisses (the Metagabbro Orthogneiss Complex) were injected into the deforming nappe pile during D2 D3 in an east west belt that extends for more than 80km along the southern margin of the terrane (Fig. 1). The high-temperature metagabbros and orthogneisses drove sillimanite-grade, hightemperature, low-pressure Buchan metamorphism. Migmatites resulting from the metagabbro heat pulse formed at c. 6kbar but crystallised at much lower pressures of c. 3.5kbar, indicating rapid exhumation (Barber and Yardley 1985). Rapid exhumation may have been facilitated by extensional collapse. The Renvyle Bofin Slide, a lowangle post-D3 extensional detachment, has, on the Irish Journal of Earth Sciences 32 (2014), 71 78 doi: 10.3318/IJES.2014.32.71 # 2014 Royal Irish Academy

Tectonic development of the northeastern Tibetan Plateau as constrained by high-resolution deep seismic-reflection data

A 180-km-long, high-resolution seismic-reflection survey that imaged the entire crust and the uppermost mantle lithosphere was conducted across the northeastern Tibetan Plateau. This work had three aims: (1) to examine whether the left-slip Haiyuan and Tianjing faults defining the margin of NE Tibet are crustal- or lithospheric-scale structures, (2) to determine whether seismic fabrics are consistent with middle- and/or lower-crustal channel flow, and (3) to establish the minimum amount of Cenozoic shortening strain in the region. Analysis of our newly obtained seismic-reflection data suggests that the left-slip Haiyuan and Tianjing faults have multiple strands and cut through the upper and middle crust. The faults likely terminate at a low-angle detachment shear zone in the lower crust, because the flat Moho directly below the projected traces of the faults is continuous. The seismic image displays subvertical zones of highly reflective sequences containing parallel and subhorizontal reflectors that are truncated by seismically transparent regions with irregular shape. The transparent regions in the middle crust are traceable to the seismically transparent lower crust and are interpreted as early Paleozoic plutons emplaced during the construction of the Qilian arc in the region. The presence of the undisturbed subvertical contacts between zones of highly reflective and seismically transparent regions rules out the occurrence of channel flow in the middle crust, as this process would require through-going subhorizontal reflectors bounding the channel above and below. The lack of continuous reflectors longer than a few kilometers in the lower crust makes a laminar mode of channel flow unfavorable, but lateral lower-crustal flow could have occurred via small-scale ductile deformation involving folding (less than a few kilometers in wavelength and amplitude). Integrating surface geology and the seismic data, we find that the upper crust along a segment of the seismic surveying line experienced up to 46% crustal shortening postdating the Cretaceous and is thus interpreted as entirely accumulated in the Cenozoic. If the estimated shortening strain is representative across northeastern Tibet, its magnitude is sufficient to explain the current elevation of the region without an appeal for additional contributing factors such as channel flow and/or a thermal event in the upper mantle.

Fluid-assisted fracturing, cataclasis, and resulting plastic flow in mylonites from the Moresby Seamount detachment, Woodlark Basin

The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust.

Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback

Abstract. The island of Seram, which lies in the northern part of the 180°-curved Banda Arc, has previously been interpreted as a fold-and-thrust belt formed during arc-continent collision, which incorporates ophiolites intruded by granites thought to have been produced by anatexis within a metamorphic sole. However, new geological mapping and a re-examination of the field relations cause us to question this model. We instead propose that there is evidence for recent and rapid N–S extension that has caused the high-temperature exhumation of lherzolites beneath low-angle lithospheric detachment faults that induced high-temperature metamorphism and melting in overlying crustal rocks. These "Kobipoto Complex" migmatites include highly residual Al–Mg-rich garnet + cordierite + sillimanite + spinel + corundum granulites (exposed in the Kobipoto Mountains) which contain coexisting spinel + quartz, indicating that peak metamorphic temperatures likely approached 900 °C. Associated with these residual granulites are voluminous Mio-Pliocene granitic diatexites, or "cordierite granites", which crop out on Ambon, western Seram, and in the Kobipoto Mountains and incorporate abundant schlieren of spinel- and sillimanite-bearing residuum. Quaternary "ambonites" (cordierite + garnet dacites) emplaced on Ambon were also evidently sourced from the Kobipoto Complex migmatites as demonstrated by granulite-inherited xenoliths. Exhumation of the hot peridotites and granulite-facies Kobipoto Complex migmatites to shallower structural levels caused greenschist- to lower-amphibolite facies metapelites and amphibolites of the Tehoru Formation to be overprinted by sillimanite-grade metamorphism, migmatisation, and limited localised anatexis to form the Taunusa Complex. The extreme extension required to have driven Kobipoto Complex exhumation evidently occurred throughout Seram and along much of the northern Banda Arc. The lherzolites must have been juxtaposed against the crust at typical lithospheric mantle temperatures in order to account for such high-temperature metamorphism and therefore could not have been part of a cooled ophiolite. In central Seram, lenses of peridotites are incorporated with a major left-lateral strike-slip shear zone (the "Kawa Shear Zone"), demonstrating that strike-slip motions likely initiated shortly after the mantle had been partly exhumed by detachment faulting and that the main strike-slip faults may themselves be reactivated and steepened low-angle detachments. The geodynamic driver for mantle exhumation along the detachment faults and strike-slip faulting in central Seram is very likely the same; we interpret the extreme extension to be the result of eastward slab rollback into the Banda Embayment as outlined by the latest plate reconstructions for Banda Arc evolution.

Rheological evolution of a Mediterranean subduction complex

We use field and microstructural observations, coupled to previously published P-T-time histories, to track the rheological evolution of an intracontinental subduction complex exposed in the Betic Cordillera in the western Mediterranean region. The body of rock we focus on, known as the Nevado-Filabride Complex (NFC), was originally part of the upper crust of the Iberian margin. It was subducted into hot asthenospheric mantle, then exhumed back toward the surface in two stages: an early stage of fast exhumation along the top of the subducting slab in a subduction channel, and a late stage of slower exhumation resulting from capture by a low-angle detachment fault rooted at the brittle-ductile transition. Each stage of deformation in the NFC was punctuated by changes in the dominant deformation mechanism. Deformation during initial subduction of the complex was accommodated by pressure-solution creep in the presence of a fluid phase – the grain sizes, stress magnitudes, and estimated strain rates for this stage are most consistent with a thin-film model for pressure solution in which the diffusion length scale is controlled by the grain size. During the early stages of exhumation within the subduction channel, deformation transitioned from pressure solution to dislocation creep due to increases in temperature, which resulted in increases in both water fugacity and grain size, each of which favor the dislocation creep mechanism. Differential stress magnitudes for this stage were ∼10 MPa, and are consistent with simple models of buoyancy-driven channel flow. With continuing subduction-channel exhumation, deformation remained within the dislocation creep field because sequestration of free water into hydrous, retrogressive minerals suppressed the pressure-solution mechanism. Differential stresses progressively increased to ∼100 MPa near the mouth of the channel during cooling as the rocks moved into mid-crustal levels. During the final, core-complex stage of exhumation, deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid-crustal detachment fault. Localization was promoted by a transition from dislocation creep to dislocation-creep-accommodated grain boundary sliding at temperatures of ∼350–380 °C, grain sizes of ∼4 μm and differential stress magnitudes of ∼200 MPa. Peak differential stress magnitudes of ∼200 MPa recorded just below the brittle-ductile transition are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures. Overall, the distribution of stress with temperature, coupled to independent constraints on strain rate from field observations and geochronology, indicate that the naturally calibrated Hirth et al. (2001) flow law for wet quartzite accurately predicts the rheological behavior of mid-crustal rocks deforming by dislocation creep.

Seismic evidence of hyper-stretched crust and mantle exhumation offshore Vietnam

Abstract We study the evolution of the Eocene–Recent Phu Khanh Basin opened during the rifting of the South China Sea (SCS). This sub-basin formed when continental crust ruptured along the East-Vietnam Boundary Fault (EVBF) at the western edge of the SCS. Using high quality long-streamer seismic lines we interpret structures that highlight the different phases of the SCS rifting and processes related to crustal boudinage. Extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the Moho discontinuity mark the central part of the basin. The mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling. There, a low-angle detachment fault separates several crustal blocks from the Moho. The cylindrical axis of the Moho rise is roughly parallel to the trend of the South China Sea propagator. Above the mantle, the upper and lower crusts form large crustal boudins. The network of normal faults is dense in the upper crust and occasionally propagates into the lower crust. However, the lower crust is missing at some places. The seismic facies above the Moho rise is poorly stratified and might have been affected by a certain degree of metamorphism. At the apex of mantle uplift, there are frequent indications of fluid circulations, including volcanic edifices and gas escapes features. Three stages of extension are clearly identifiable, with ages of the two youngest constrained by well calibration: the first and oldest rift sequence is situated between the tilted pre-rift basement and the Oligocene horizons (32 Ma); the second is delimited by the Oligocene to the Mid Miocene (15.5 Ma) horizons, and the third is bound by the Mid Miocene and the Upper Miocene (before 10.5 Ma) horizons. These three rift episodes formed in at least two extension directions, the first N–S and the second NW–SE. The distinct Mid Miocene (15.5 Ma) horizon is tilted and the above layers show a diverging reflection. These are in turn sealed by an erosional unconformity before 10.5 Ma. Although tectonic activity appears diachronous from north to south, we suggest that cessation of rifting did not occur before 12–10.5 Ma. This differs from models derived from magnetic anomalies observed in the South China Sea (15.5 to 20 Ma).