Ductile Transition Zone Research Articles

Southward migration of volcanic activity in the central Mexican Volcanic Belt: asymmetric extension within a two-layer crustal stretching model

Magmatic activity in the Miocene to present Mexican Volcanic Belt (MVB) is migrating to the south. Volcanic activity is associated with extension, which is being accommodated along a major, 1000-km-long, E–W-oriented continental rift. In turn, the MVB is cross-cut by older, though active N–S- to NNW-oriented major extensional faults, along which the polygenetic volcanoes are aligned. A southward migration pattern is also observed for the monogenetic volcanism, which however appears to be related to E–W- and N60°E-oriented extensional faults. We suggest that the southward migration of volcanic activity can be explained in terms of a two-layer crustal stretching model (brittle and ductile domains). The layers would be separated at an upper crustal level by a zone of simple-shear decoupling, at the brittle–ductile transition zone (BDTZ). The overall movement above the BDTZ is southward-directed, the only direction to which the Central MVB can extend and grow. Our model suggests that the magmas that feed the volcanism become stored at the BDTZ. Evidence supporting this assumption is provided by the Al-in-hornblende thermobarometer and the Ol–Pl–SiOr pseudoternary diagram, which indicate average pressures between 2.5 and 4kbar. The magmas feeding the monogenetic volcanism ascend rapidly along active E–W and N60°E extensional faults (large strain rates), i.e. they do not have enough time to form large magma chambers. The magmas feeding the polygenetic volcanism are emplaced along N–S to NNW faults (lower strain rates). These magmas remain stored for longer periods at the BDTZ of the N–S to NNW faults, and therefore form large magmatic chambers, shaping vertical overshoots of several kilometers of height. The results from geobarometry indicating magma emplacement depth at around 8km for the polygenetic volcanism, and 12km for the monogenetic volcanism, are in good agreement with the rheological constraints of a BDTZ at about 10km of depth. We envisage a feedback mechanism regarding magma storage and shallowing of the BDTZ, i.e. magma emplacement shallows the BDTZ; in turn, this shallowing controls the new zones for magma emplacement, a southward directed process.

Detachments in high-pressure mountain belts, Tethyan examples

Using the case examples of the phyllite–quartzite nappe of Crete, the Bündnerschiefer nappe of the Engadine window in the Alps, and the infraophiolitic units of the Saih Hatat window in the Oman Mountains, we describe the tectono-metamorphic signature of large-scale extensional detachments in high-pressure belts. In these three cases retrograde P–T paths were accompanied by the formation of large-scale extensional shear zones that contributed to the exhumation and extreme thinning of the metamorphic pile. The direction of transport along later detachments was parallel to the direction of convergence, detachments being first localized within or near the brittle–ductile transition zone. Below the major shear zone, or between the shear zones in Oman, the deformation regime was more coaxial and involved a significant component of vertical shortening. Metamorphic data, mostly derived from the petrology of metapelites, show that domains characterized by coaxial strain have nearly isothermal decompression paths (type II), while those immediately below the detachments were subjected to more efficient cooling during exhumation (type I). The detachments thus affect both the deformation history and the P–T evolution of the lower plate. The existence of the two types of cooling paths is a good test of the extensional nature of a tectonic contact.

Coincident magnetotelluric,P-wave andS-wave images of the deep continental crust beneath the Weardale granite, NE England: seismic layering, low conductance and implications against the fluids paradigm

SUMMARY It has been postulated that deep crustal zones of both enhanced electrical conductivity and seismic layering may be ascribed a single physical explanation, and more specifically that both result from the presence of free, interconnected fluids around the brittle‐ ductile transition zone. We analyse three complementary geophysical data sets— broadband (10’2‐104 s) magnetotelluric (MT) and P-wave and S-wave deep seismic reflection—from coincident profiles traversing the Weardale granite in northeast England and demonstrate that the lowermost crust in this tectonically stable region contains insuY cient free fl uid to explain the highv P /v S ratios associated with very bright, subhorizontal lamellae seen within the lower crust (below 22 km depth) beneath the granite. Temperature‐depth profiles are presented, and the apparent global correlation between the depths to the tops of zones of enhanced conductivity, seismic reflectivity and inferred brittle‐ductile temperatures of 300‐450°C is also re-examined. The seismic layering demonstrably corresponds to higher temperatures than those usually attributed to the brittle‐ductile transition zone. A priori information of diVerent scalelengths is applied to the problem of static-shift correction of the MT data. Gravity models constrain the depth extent of the granite batholith, whilst pre-existing DC measurements and a borehole drilled into the granite provide information concerning the resistivity structure of the uppermost 500 m. A 3-D model study investigates the significance of possible 3-D induction eVects on the MT data, as implied following application of decomposition. Our results challenge the widespread and little-tested assumption that deep crustal conductors and seismic layering are physically interrelated and occur at the same depth. With static shift and 3-D eVects in the MT data accounted for, we demonstrate that below the Weardale granite the onset of any enhanced conductivity is shallower than the top of the seismic layering, and furthermore that the observed deep crustal conductance of less than 200 S is significantly less than that to be expected (>2000 S) if the high v P /v S ratios calculated from the seismic data are to be explained entirely by fluids. The total conductance of the mid to lower crustal region combined is too low to allow for the significant quantities of free fluids required to explain lower crustal seismic layering through this medium without challenging current hypotheses concerning pore geometries and interconnectivity.

Microseismicity, stress, and fracture in the Coso geothermal field, California

Microseismicity, stress, and fracture in the Coso geothermal field are investigated using seismicity, focal mechanisms and stress analysis. Comparison of hypocenters of microearthquakes with locations of development wells indicates that microseismic activity has increased since the commencement of fluid injection and circulation. Microearthquakes in the geothermal field are proposed as indicators of shear fracturing associated with fluid injection and circulation along major pre-existing fractures. High-seismicity zones are associated with the main fluid-flow paths within the geothermal system. Calculated stress patterns from focal mechanisms provide direct evidence for the boundary between significantly different stress regimes within the Coso geothermal field. Microseismicity in the Coso geothermal field is spatially but not temporally related to regional seismicity extending southeast of the field. The spatial distribution of these events defines a northwest-trending seismic-fracture zone, consistent with a previously defined northwest-striking zone. The abrupt decrease of seismicity below this fracture zone may provide seismic evidence for the existence of a brittle and ductile transition zone at shallow depth beneath the Coso geothermal field.

Faults and veins in the superdeep well KTB: constraints on the amount of Alpine intra-plate thrusting and stacking of Variscan basement (Bohemian Massif, Germany)

The crystalline rocks of the superdeep well KTB (9101 m depth) display abundant late- to post-Variscan brittle–ductile and brittle deformation structures. Clear depth-dependent variations in metamorphic conditions are obvious concerning Cretaceous subhorizontal tension gashes and reverse faults, whereas the Upper Carboniferous structures (subvertical tension gashes and reverse faults) show almost no variations with depth. The lack in depth-dependent gradients can be explained by strong Cretaceous reverse faulting that led to an intra-plate antiformal stack. The frontal ramp of this stack, a broad fault bundle referred to as the Franconian lineament, was drilled at 7300 m depth. It rises from a flat-lying detachment at approximately 9 km depth which corresponds to the brittle–ductile transition zone of quartz-bearing rocks. Supracrustal stacking and thickening along and above this ramp was particularly active during Alpine (Upper Cretaceous) foreland compression. Close to the Franconian lineament the Alpine impact strongly affected the Variscan crustal edifice disturbing the Variscan and late-Variscan depthdependent gradients in KTB.

Seismically active fold and thrust belt in the San Joaquin Valley, central California

Structural analysis of late Cenozoic folds along the western and southern margins of the San Joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics. The folds have formed by the processes of fault-bend folding and fault-propagation folding, which commonly occur in fold and thrust belts. Our structural interpretation attributes the seismically active Coalinga and Kettleman Hills North Dome anticlines to fault-bend folding above a thrust fault(s) that steps up from a detachment within the Franciscan Assemblage to a detachment at the base of the Great Valley Group. This thrust does not reach the surface (blind thrust), and its slip is consumed in backthrusting and formation of subsurface folds under the San Joaquin Valley. Movement on the postulated thrust(s) would explain the cause of the May 2, 1983, Coalinga earthquake and the August 15, 1985, Avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes. Well-documented examples of fault-bend and fault-propagation folding also occur at Wheeler Ridge, in the San Emigdio Mountains, and at Kettleman Hills South Dome. Deformation associated with fold and thrust belts can be extremely complicated. Considerable fault movement can occur without accompanying surface rupture or shallow-level faulting. Conversely, surface rupture can occur that has no direct relationship to fault slip at depth. An example would be flexural-slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth. Consequently, traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts. Our interpretation of the structural style that developed in central California during the late Cenozoic requires that the strain associated with the transpressive motion between the Pacific and North American plates be resolved into normal and tangential components: thrust faulting and folding normal to the plate boundary and strike-slip faulting parallel to the plate boundary (San Andreas fault). The thrust faults root in a deecollement at the brittle- ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction.

The origin and microstructures of metamorphic felsic dykes emplaced during brittle-ductile extension (southeast Sinai)

AbstractStructural, petrographic and chemical analyses of a set of northeast-trending quartzofeld-spathic dykes in Wadi Kid area (southeastern Sinai) show that they are the product of anatexis of pelitic rocks. Metamorphic P-T data indicate that anatexis could have occurred only 2 km below the present erosion level. The partial melts intruded into overlying rocks during regional metamorphism along a set of northeast-oriented extensional fractures, thereby recrystallizing and in some cases obtaining tectonite fabrics. The fractures were formed perpendicular to the regional mineral lineation and show that the area was within the deeper parts of the brittle–ductile transition zone. The original depth of the presently exposed crustal level corresponds well with the theoretical and experimentally derived depths of the brittle-ductile transition zone in high heat flow regimes. The emplacement of a granitoid pluton or gneiss dome, possibly of dioritic composition, could be responsible for both melting and tensile fracturing of the overlying rocks.

A Tertiary Thermal Event in South-Central British Columbia: Reply

Foliated and unfoliated rhomb-porphyry dikes, genetically related to the andesitic Marron Formation, cut some westernmost exposures of the Shuswap Complex in the southern Okanagan Valley of British Columbia. Emplacement of these dikes was associated with northerly trending fractures and flexural-slip folding, localized high heat flow, re-setting of country rock K–Ar systems and hydrothermal alteration. All these events are set at 42–48 m.y. B.P. based on K–Ar measurements on dikes and country rocks.Rhomb-porphyry dikes are part of an andesitic volcanic province in south-central British Columbia that coincides in time with ductile deformation within the Cascade Fold Belt to the west and latest thrust faulting in the Rocky Mountains to the east. The volcanic province is associated in space with thinning of the crust, whereas thickening of the crust is associated with ductile shortening to the west. It is likely that the volcanic province marks the position of the ductile–brittle transition zone in the crust...