Decollement Plane Research Articles

The Decollement Depth of Active Thrust Faults in Italy: Implications on Potential Earthquake Magnitude

AbstractThrust fault ruptures during earthquakes do not often propagate down to the brittle‐ductile transition. Lithological variations control the behavior and depth of regional basal thrusts and decollement planes. Thrust fronts may be discontinuous along strike, limiting the dimension of single coseismic ruptures. These factors control the maximum expected magnitude in one region. This is the case of Italy where the convergence of few millimeter per year in the Apennines accretionary prism and along the retrobelt of the Alps generates compressional earthquakes with moderate to strong magnitudes. Here, using geological and geophysical data, we first compile a map of the undulated active basal thrust decollement for Italy that occurs from 1 to 17‐km depth. Then, we verify the relationship between the length of seismogenic ruptures in thrust faults (Lf) and the maximum depth of thrust faulting (zmax) of related earthquakes and find that their ratio (Lf/zmax) ranges between 2 and 4. Finally, we compute the potential seismogenic volume and estimate the maximum magnitude using an empirical relationship that multiplies the decollement depth and the Lf/zmaxratio. Maximum calculated magnitude is 6.7 ± 0.37 (depending on Lf/zmaxand fault dip angle), consistent with the largest magnitude of thrust‐related earthquakes recorded in Italy (6.5–7.0). Lower magnitudes are predicted in the Ionian Seas at the external front of the Apennines where smaller crustal volumes are involved, whereas higher magnitudes are expected in the southern Po Basin, the western Adriatic Sea, Sicily offshore, and the Southern Alps where the decollement is deeper and the brittle volumes are far greater.

Alps in the Apennines?

In the Southern Apennines and Calabria outcrop metamorphic slices of hercynian and/or alpine age. The reconstructions by Glauco Bonardi and co-authors of an alpine subduction-related origin for the emplacement of those rocks is still supported by a number of tectonic and geodynamic constraints. Alps and Apennines are two end members of opposite subduction zones (i.e., “E”-ward and “W”-ward), having different depth and evolution of the accretionary prism basal decollement. The PTt evolution of the metamorphic rocks of Calabria can be better reconciled with an earlier alpine-type evolution, where the continental and oceanic slices of crust have been subducted and re-exhumed by the deepening of the decollement planes into both the upper and lower plates and accreting the orogen. The early alpine emplacement of those metamorphic rocks is testified by their high structural elevation, the HP/LT assemblages and by stratigraphic constraints (e.g., Bonardi et alii, 2005). Therefore it can be interpreted that the earlier alpine subduction was gradually substituted by the opposite W-directed Apennines subduction, having in its hangingwall the boudinated relics of the alpine history.

Intrusion of eccentric dikes: The case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano

The 2001 eruption represents one of the most studied events both from volcanological and geophysical point of view on Mt. Etna. This eruption was a crucial event in the recent dynamics of the volcano, marking the passage from a period (March 1993–June 2001) of moderate stability with slow, continuous flank sliding and contemporaneous summit eruptions, to a period (July 2001 to present) of dramatically increased flank deformations and flank eruptions. We show new GPS data and high precision relocation of seismicity in order to demonstrate the role of the 2001 intrusive phase in this change of the dynamic regime of the volcano. GPS data consist of two kinematic surveys carried out on 12 July, a few hours before the beginning of the seismic swarm, and on 17 July, just after the onset of eruptive activity. A picture of the spatial distribution of the sin-eruptive seismicity has been obtained using the HypoDD relocation algorithm based on the double-difference (DD) technique. Modeling of GPS measurements reveals a southward motion of the upper southern part of the volcano, driven by a NNW–SSE structure showing mainly left-lateral kinematics. Precise hypocenter location evidences an aseismic zone at about sea level, where the magma upraise was characterized by a much higher velocity and an abrupt westward shift, revealing the existence of a weakened or ductile zone. These results reveal how an intrusion of a dike can severely modify the shallow stress field, triggering significant flank failure. In 2001, the intrusion was driven by a weakened surface, which might correspond to a decollement plane of the portion of the volcano affected by flank instability, inducing an additional stress testified by GPS measurements and seismic data, which led to an acceleration of the sliding flanks.

Subduction kinematics and dynamic constraints

The kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings. Two reference frames are used here to describe the simple 2D kinematics of subduction zones. In the first, the upper plate is assumed fixed, whereas in the second frame upper and lower plates move relative to the mantle. Relative to a fixed point in the upper plate U, the transient subduction hinge H can converge, diverge, or be stationary. Similarly, the lower plate L can converge, diverge or be stationary. The subduction rate V S is given by the velocity of the hinge H minus the velocity of the lower plate L ( V S = V H − V L). The subduction rate 1) increases when H diverges, and 2) decreases when H converges. Combining the different movements, at least 14 kinematic settings can be distinguished along the subduction zones. Variable settings can coexist even along a single subduction zone, as shown for the 5 different cases occurring along the Apennines subduction zone. Apart from few exceptions, the subduction hinge converges toward the upper plate more frequently along E- or NE-directed subduction zone, whereas it mainly diverges from the upper plate along W-directed subduction zones accompanying backarc extension. Before collision, orogen growth occurs mostly at the expenses of the upper plate shortening along E–NE-directed subduction zones, whereas the accretionary prism of W-directed subduction zones increases at the expenses of the shallow layers of the lower plate. The convergence/shortening ratio is > 1 along E- or NE-directed subduction zones, whereas it is < 1 along accretionary prisms of W-directed subduction zones. Backarc spreading forms in two settings: along the W-directed subduction zones it is determined by the hinge divergence relative to the upper plate, minus the volume of the accretionary prism, or, in case of scarce or no accretion, minus the volume of the asthenospheric intrusion at the subduction hinge. Since the volume of the accretionary prism is proportional to the depth of the decollement plane, the backarc rifting is inversely proportional to the depth of the decollement. On the other hand, along E- or NE-directed subduction zones, few backarc basins form (e.g., Aegean, Andaman) and can be explained by the velocity gradient within the hangingwall lithosphere, separated into two plates. When referring to the mantle, the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames. The subduction hinge is mostly stationary being the slab anchored to the mantle along W-directed subduction zones, whereas it moves W- or SW-ward along E- or NE-directed subduction zones. Surprisingly, along E- or NE-directed subduction zones, the slab moves “out” of the mantle, i.e., the slab slips relative to the mantle opposite to the subduction direction. Kinematically, this subduction occurs because the upper plate overrides the lower plate, pushing it down into the mantle. As an example, the Hellenic slab moves out relative to the mantle, i.e., SW-ward, opposite to its subduction direction, both in the deep and shallow hotspot reference frames. In the shallow hotspot reference frame, upper and lower plates move “westward” relative to the mantle along all subduction zones. This kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions. W-directed subduction zones rather provide about 2–3 times larger volumes of lithosphere re-entering into the mantle, and the slab is pushed down. This opposite behavior is consistent with the down-dip extension seismicity along E–NE-directed subduction zones, and the frequent down-dip compression along the W-directed subduction zones. Subduction kinematics shows that plate velocity is not dictated by the rate of subduction. Along the W-directed subduction zones, the rate of subduction is rather controlled i) by the hinge migration due to the slab interaction with the “easterly” trending horizontal mantle wind along the global tectonic mainstream, ii) by the far field plate velocities, and, iii) by the value of negative buoyancy of the slab relative to the country mantle. Alternatively, E–NE–NNE-directed subduction zones have rates of sinking chiefly determined i) by the far field velocity of plates, and ii) by the value of negative buoyancy of the slab relative to the country mantle. Along this type of subduction, the subduction hinge generally advances E–NE-ward toward the upper plate decreasing the subduction rate, but it moves W–SW-ward relative to the mantle. All this indicates that subduction zones have different origin as a function of their geographic polarity, and the subduction process is more a passive feature rather than being the driving mechanism of plate motions. A rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones.

Till deposition by glacier submarginal, incremental thickening

AbstractMacro‐ and micro‐scale sedimentological analyses of recently deposited tills and complex push/squeeze moraines on the forelands of Icelandic glaciers and in a stacked till sequence at the former Younger Dryas margin of the Loch Lomond glacier lobe in Scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment. Where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris‐rich basal ice by cumulative freeze‐on processes, glacier submarginal processes are dictated by seasonal cycles of refreezing and melt‐out of tills advected from up‐ice by a combination of lodgement, deformation and ice keel and clast ploughing. Although individual till layers may display typical A and B horizon deformation characteristics, the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record. At the macro‐scale, Icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings; very strong fabrics typify unequivocally lodged clasts. Despite previous interpretations of these tills as subglacial deforming layers, micro‐morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes, and water escape and sediment flowage features are widespread. A model of submarginal incremental thickening is presented as an explanation of these data, involving till slab emplacement over several seasonal cycles. Each cycle involves: (1) late summer subglacial lodgement, bedrock and sediment plucking, subglacial deformation and ice keel ploughing; (2) early winter freeze‐on of subglacial sediment to the thin outer snout; (3) late winter readvance and failure along a decollement plane within the till, resulting in the carriage of till onto the proximal side of the previous year's push moraine; (4) early summer melt‐out of the till slab, initiating porewater migration, water escape and sediment flow and extrusion. Repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Late Ordovician glacial pavements revisited: a reappraisal of the origin of striated surfaces

The recognition of ice-related unconformities is of prime importance when dealing with the sequential architecture of pre-Pleistocene glacial successions. Late Ordovician striated surfaces strikingly preserved in North Gondwana were long considered as abrasion surfaces at the sole of a grounded glacier overriding unlithified sediments, or having resulted from the scouring action of drifting icebergs. Field observations from Mauritania and Libya show that these striated surfaces require an alternative interpretation, which can be applied to similar surfaces described elsewhere, such as in Permo-Carboniferous or Neoproterozoic glacial sequences. A new model is proposed that involves superimposed concurrent decollement planes within a subglacial brittle shear zone in unlithified sand beds. Shear zones in subglacial sediments are well known in Quaternary deposits but, to date, it has not been demonstrated that they may form striated surfaces occurring at continent scale with orientations consistent with palaeo ice-flow reconstructions based on larger-scale indicators.

On the number and spacing of faults

AbstractOrogens and rift zones have a finite number of regional faults. The accretionary prisms analysed here have a number of thrusts &lt; 50, whereas extensional areas have a number of normal faults ranging between six and 44. The average spacing of thrusts is between 5 and 25 km; spacing of normal faults is more restricted into two peaks, at 25–29 km and 4–6 km, in which the latter is the most common. The number and spacing of faults appear to be mainly controlled by the depth of the decollement plane, which seems to be more variable in compressive settings with respect to rift zones. Basement‐involved orogens present fewer and more spaced thrusts; by contrast, a greater number of thrusts with shorter spacing characterize thin‐skinned thrust belts. The shallower the decollement is, the stronger it appears to control the palaeogeography, in the sense of rheological lateral variations in the sedimentary cover.

New insights into Kilauea's volcano dynamics brought by large‐scale relative relocation of microearthquakes

We investigated the microseismicity recorded in an active volcano to infer information concerning the volcano structure and long‐term dynamics, by using relative relocations and focal mechanisms of microearthquakes. There were 32,000 earthquakes of the Mauna Loa and Kilauea volcanoes recorded by more than eight stations of the Hawaiian Volcano Observatory seismic network between 1988 and 1999. We studied 17,000 of these events and relocated more than 70%, with an accuracy ranging from 10 to 500 m. About 75% of these relocated events are located in the vicinity of subhorizontal decollement planes, at a depth of 8–11 km. However, the striking features revealed by these relocation results are steep southeast dipping fault planes working as reverse faults, clearly located below the decollement plane and which intersect it. If this decollement plane coincides with the pre‐Mauna Loa seafloor, as hypothesized by numerous authors, such reverse faults rupture the pre‐Mauna Loa oceanic crust. The weight of the volcano and pressure in the magma storage system are possible causes of these ruptures, fully compatible with the local stress tensor computed by Gillard et al. [1996]. Reverse faults are suspected of producing scarps revealed by kilometer‐long horizontal slip‐perpendicular lineations along the decollement surface and therefore large‐scale roughness, asperities, and normal stress variations. These are capable of generating stick‐slip, large–magnitude earthquakes, the spatial microseismic pattern observed in the south flank of Kilauea volcano, and Hilina‐type instabilities. Rupture intersecting the decollement surface, causing its large‐scale roughness, may be an important parameter controlling the growth of Hawaiian volcanoes.

40Ar/39Ar Dating of Deformation Events and Reconstruction of Exhumation of Ultrahigh‐Pressure Metamorphic Rocks in Donghai, East China

Abstract Recent investigations reveal that the ultrahigh‐pressure metamorphic (UHPM) rocks in the Donghai region of East China underwent ductile and transitional ductile‐brittle structural events during their exhumation. The earlier ductile deformation took place under the condition of amphibolite facies and the later transitional ductile‐brittle deformation under the condition of greenschist facies. The hanging walls moved southeastward during both of these two events. The 40Ar/39Ar dating of muscovites from muscovite‐plagioclase schists in the Haizhou phosphorous mine, which are structurally overlain by UHPM rocks, yields a plateau age of 218.0±2.9 Ma and isochron age of 219.8Ma, indicating that the earlier event of the ampibolite‐facies deformation probably took place about 220 Ma ago. The 40Ar/39Ar dating of oriented amphiboles parallel to the movement direction of the hanging wall on a decollement plane yields a plateau age of 213.1 ± 0.3 Ma and isochron age of 213.4±4.1 Ma, probably representing the age of the later event. The dating of pegmatitic biotites and K‐feldspars near the decollement plane from the eastern Fangshan area yield plateau ages of 203.4±0.3 Ma, 203.6±0.4 Ma and 204.8±2.2 Ma, and isochron ages of 204.0±2.0 Ma, 200.6±3.1 Ma and 204.0±5.0 Ma, respectively, implying that the rocks in the studied area had not been cooled down to closing temperature of the dated biotites and K‐feldspars until the beginning of the Jurassic (about 204 Ma). The integration of these data with previous chronological ages on the ultrahigh‐pressure metamorphism lead to a new inference on the exhumation of the UHPM rocks. The UHPM rocks in the area were exhumed at the rate of 3–4 km/Ma from the mantle (about 80–100 km below the earth's surface at about 240 Ma) to the lower crust (at the depth of about 20‐30km at 220 Ma), and at the rate of 1–2 km/Ma to the middle crust (at the depth of about 15 km at 213 Ma), and then at the rate of less than 1 km/Ma to the upper crust about 10 km deep at about 204 Ma.

Seismic characterization of the structural front of southern Quebec Appalachians

Interpretation of recent industry seismic reflection data provides important insights into the structural style of the southern Quebec Appalachians structural front, south of Drummondville. Especially, the occurrence of thrusts propagating west of the mapped limit of the parautochthonous domain is widely documented. Extensive decollement planes dominate the structural style east of the Aston Fault and define the soles of tectonic slices stacked into hinterland-dipping duplexes. The Chambly-Fortierville Syncline and Logan’s Line are not imaged seismically. Growth faults were active since the deposition of the Potsdam Group and at least until the deposition of Trenton Group. Normal faults also extend into the overlying molasse and flysch units and antithetic faults are more common than previously documented. Normal faults are crosscut by Appalachian thrusts.

Anomalously high b-values in the South Flank of Kilauea volcano, Hawaii: evidence for the distribution of magma below Kilauea's East rift zone

The pattern of b-value of the frequency–magnitude relation, or mean magnitude, varies little in the Kaoiki-Hilea area of Hawaii, and the b-values are normal, with b=0.8 in the top 10 km and somewhat lower values below that depth. We interpret the Kaoiki-Hilea area as relatively stable, normal Hawaiian crust. In contrast, the b-values beneath Kilauea's South Flank are anomalously high ( b=1.3–1.7) at depths between 4 and 8 km, with the highest values near the East Rift zone, but extending 5–8 km away from the rift. Also, the anomalously high b-values vary along strike, parallel to the rift zone. The highest b-values are observed near Hiiaka and Pauahi craters at the bend in the rift, the next highest are near Makaopuhi and also near Puu Kaliu. The mildest anomalies occur adjacent to the central section of the rift. The locations of the three major and two minor b-value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity, geodetic data and differentiated lava chemistry. The existence of the magma reservoirs is also supported by magnetic anomalies, which may be areas of dike concentration, and self-potential anomalies, which are areas of thermal upwelling above a hot source. The simplest explanation of these anomalously high b-values is that they are due to the presence of active magma bodies beneath the East Rift zone at depths down to 8 km. In other volcanoes, anomalously high b-values correlate with volumes adjacent to active magma chambers. This supports a model of a magma body beneath the East Rift zone, which may widen and thin along strike, and which may reach 8 km depth and extend from Kilauea's summit to a distance of at least 40 km down rift. The anomalously high b-values at the center of the South Flank, several kilometers away from the rift, may be explained by unusually high pore pressure throughout the South Flank, or by anomalously strong heterogeneity due to extensive cracking, or by both phenomena. The major b-value anomalies are located SSE of their parent reservoirs, in the direction of motion of the flank, suggesting that magma reservoirs leave an imprint in the mobile flank. We hypothesize that the extensive cracking may have been acquired when the anomalous parts of the South Flank, now several kilometers distant from the rift zone, were generated at the rift zone near persistent reservoirs. Since their generation, these volumes may have moved seaward, away from the rift, but earthquakes occurring in them still use the preexisting complex crack distribution. Along the decollement plane at 10 km depth, the b-values are exceptionally low ( b=0.5), suggesting faulting in a more homogeneous medium.

Seismotectonics of the great 1905 Kangra earthquake meizoseismal region in Kangra–Chamba, NW Himalaya

Three great earthquakes with magnitude greater than 8 have struck the Himalayan belt within a span of 50 years, and the 1905 Kangra earthquake is one of them. The seismotectonics of the meizoseismal region of this earthquake in the Kangra–Chamba region are analyzed on the basis of the major structures and the present day seismic activity. The microseismicity recorded by a network of 12 stations for a period of 16 months spanning 4 years (1993–1996) shows the activity is mainly concentrated over the Main Boundary Thrust (MBT) and Panjal Thrust (PT) zones and the Chamba Nappe region. A NE–SW vertical section reveals that the majority of events are concentrated within a depth range of 5 to 18 km, and the majority of the foci lie between 12 and 18 km. The base of the microseismicity zone is interpreted as representing a contact or a decollement plane between the top of the Indian plate and the Himalayan rock formations. This decollement plane is interpreted as the northern continuation of the detachment observed in the sub-Himalaya in reflection profile. The southward propagation of the Chamba Nappe, as well as the PT and MBT zones, over the detachment causes accumulation of strain energy in this region.

Seismogenic Zone in the Nankai Accretionary Wedge: General Summary of Japan-U. S. Collaborative 3-D Seismic Investigation

A three-dimensional seismic survey was carried out at the western Nankai Trough accretionary wedge from June 18 to August 18, 1999. This experiment was Japan-U.S. collaborative investigation on seismogenic zones. The cruise imaged an 8 X 80 km area with 81, 80km-long, high quality, seismic reflection lines, all of which have nearly continuous coverage.The main objective of our experiment was to image the plate boundary fault at which major earthquakes and tsunamis are generated. Our primary goal is to image the thrust and identify the boundary between aseismic and seismic zones for a large inter-plate earthquake.On-board two-dimensional data processing has been carried out. This gives us a significantly clear image of the inner structure of the accretionary wedge at our survey area. One impressive image of the profile shows the large thrust slice zone that is classified by our interpretation. A number of out-of-sequence thrusts have developed and are concentrated in the area where the water depth is approximately 4, 000 to 3, 000m. A decollement plane touches the oceanic plate (layer II) down at there in first. We identified that the boundary between the stable sliding zone (ocean-ward) and the unstable stick slip zone (landward) is located there. We propose that the boundary is the up-dip limit of the seismogenic zone. Further data processing is ongoing. A true three-dimensional structure will reveal much more details and a clear image of seismogenic zone at the Nankai subduction margin.

Orogens and slabs vs. their direction of subduction

Subduction zones appear primarily controlled by the polarity of their direction, i.e., W-directed or E- to NNE-directed, probably due to the westward drift of the lithosphere relative to the asthenosphere. The decollement planes behave differently in the two end-members. In the W-directed subduction zone, the decollement of the plate to the east is warped and subducted, whereas in the E- to NNE-directed, it is ramping upward at the surface. There are W-directed subduction zones that work also in absence of active convergence like the Carpathians or the Apennines. W-directed subduction zones have shorter life (30–40 Ma) than E- or NE-directed subduction zones (even longer than 100 Ma). The different decollements in the two end-members of subduction should control different PTt paths and, therefore, generate variable metamorphic assemblages in the associated accretionary wedges and orogens. These asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast `eastward' migrating structural wave along W-directed subduction zones, whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones. The magmatic pair calc-alkaline and alkaline–tholeiitic volcanic products of the island arc and the back-arc basin characterise the W-directed subduction zones. Magmatic rocks associated with E- or NE-directed subduction zones have higher abundances of incompatible elements, and mainly consist of calc-alkaline–shoshonitic suites, with large volumes of batholithic intrusions and porphyry copper ore deposits. The subduction zones surrounding the Adriatic plate in the central Mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab. The western margin of the Adriatic plate contemporaneously overridden and underthrust Europe toward the `west' to generate, respectively, the Alps and the Apennines, while the eastern margin subducted under the Dinarides–Hellenides. These belts confirm the characters of the end-members of subduction zones as a function of their geographic polarity similarly to the Pacific subduction zones.

Strain partitioning in the footwall of the Somiedo Nappe: structural evolution of the Narcea Tectonic Window, NW Spain

The Somiedo Nappe is a major thrust unit in the Cantabrian Zone, the external foreland fold and thrust belt of the North Iberian Variscan orogen. Exposed at the Narcea Tectonic Window are Precambrian rocks below the basal decollement of the Somiedo Nappe, which exhibit a different deformation style than the overlying Paleozoic rocks above the basal decollement. During Variscan deformation, folding and widespread subhorizontal, bedding-parallel decollements were produced in the hanging wall within the Paleozoic rocks. Vertical folding, with related axial-planar cleavage at a high angle to the decollement planes, developed simultaneously in the upper Proterozoic Narcea Slates of the footwall, below the detachment. The relative magnitude of finite strain, measured in the footwall rocks, diminishes towards the foreland. These observations indicate that (1) significant deformation may occur in the footwall of foreland fold and thrust belts, (2) the shortening mechanism in the footwall may be different from that of the hanging wall, and (3) in this particular case, the partitioning of the deformation implies the existence of a deeper, blind decollement surface contemporaneous with the first stages of the foreland development, that does not crop out in the region. This implies a significant shortening in the footwall, which must be taken into account when restoration and balancing of cross-sections is attempted. A sequential diagram of the evolution of the Narcea Tectonic Window with a minimum shortening of 85 km is proposed, explaining the complete Variscan evolution of the foreland to hinterland transition in the North Iberian Variscan orogen.

Type of faulting and orientation of stress and strain as a function of space and time in Kilauea's south flank, Hawaii

Earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of Kilauea volcano, Hawaii, are used to infer the state of stress and strain as a function of time and space. We have determined 870 fault plane solutions from P wave first motion polarities for events with magnitudes ML ≥ 2.5 and depth ranging between 6 and 12 km. Faulting is characterized by a mixture of decollement, reverse, and normal faults. Most large earthquakes with magnitude M &lt; 7 slip on reverse faults striking NE at 40° and dipping SE between 60° and 70°. In Hawaii, the earthquakes with M &gt; 7 rupture the decollement plane, since it is the only surface large enough to generate magnitude 7 or larger earthquakes. The percentage of reverse faulting events is high compared to the decollement and normal faulting mechanisms for the period 1972–1983. The percentage of decollement type focal mechanisms becomes dominant after 1983. This pattern of faulting activity suggests that pressure was building up within Kilauea's rift zone prior to the 1983 Puu'Oo eruption. Overall, a single stress orientation with the maximum compressive stress oriented SE perpendicular to the rift and dipping at 45° is compatible with the coeval existence of decollement, reverse, and normal faults. However, in a crustal volume east of longitude 155°10′W, we find a change of the orientation of σ1 from nearly horizontal to plunging 45° SE occurring in 1979. This stress rotation suggests magma movements within the aseismic part of Kilauea's east rift zone. The strain and stress orientations are coaxial in the south flank except within the volume where the stress rotation is observed. We observe a change in the relationship between stress and strain directions caused either by the shifting of seismic activity from reverse faults to decollements, while stress stays constant, or by a rotation of stress, while strain remains constant. Assuming that the model of a noncohesive Coulomb wedge is appropriate for Kilauea's south flank, we find that high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0.85.

The genesis of Colombian emeralds: a restatement

A renewal of metallogenetical studies of Colombian emerald deposits produced new geological and geochemical data that favour a hydrothermal-sedimentary genetic model for these deposits. A comprehensive model is presented which integrates both chemical and structural aspects and invalidates some aspects of the model recently presented by Ottaway et al. The deposits result from a two-stage process in which shortening tectonics affect the two borders of the Eastern Cordillera of Colombia and provoke decollement planes, thrusting, and thrust-fault related folds in the Early Cretaceous black shale series. Three major descriptive and interpretative aspects of the deposits are presented: (1) time relationships between the two different sets of barren and mineralized extensional veins; (2) the association of graphite and emerald mineralization, and (3) the thermochemical sulfate reduction reaction acting at the site of the emerald deposits to reduce sulfate of evaporitic origin from the hydrothermal brines into hydrogen sulfide by interaction with organic-rich strata.

Structural history of ophiolite obduction, Bay of Islands, Newfoundland

Obduction-related structures in the ophiolitic Bay of Islands Complex, west Newfoundland, are classified into shearing dominated and accretion dominated. Mapping of these structures is combined with other geological constraints and provides the basis for a reconstruction of the obduction history of the ophiolite. The earliest detachment structures are shearing dominated and are preserved in the basal peridotites. They indicate north-directed thrusting. Detachment was facilitated by the shallow position of the lithosphere-asthenosphere boundary below the recently extinct Bay of Islands spreading ridge. Detachment involved lithosphere of the southern side and immediate northern side of the ridge. During the early stages of detachment, the Coastal Complex, an arc-related unit located to the west of the Bay of Islands Complex, acted largely as a lateral ramp to obduction. Early accretion occurred in two tectonic situations: (1) at the stage when the basal decollement plane intersected the boundary to the structurally and lithologically diverse Coastal Complex; this resulted in accretion of blocks up to several hundred meters thick with a lithological make-up not known from the Bay of Islands Complex; and (2) at the stage when the basal thrust plane intersected the oceanic crust. Deformation switched into the mafic and hydrated footwall lithologles and led to intermittent accretion of mafic oceanic crustal rocks now preserved in the metamorphic sole. Only the Lewis Hills massif, located close to the Coastal Complex during detachment, preserves evidence for both types of early accretion. At a later stage of obduction, localized removal of the basal sequence of the ophiolite was associated with accretion of a diverse assemblage of harzburgites, amphibolites, and greenschists.

A seismotectonic model for western Hawaii based on stress tensor inversion from fault plane solutions

For 57 fault plane solutions of earthquakes with magnitude ML≥3.0 located west of the southwest rift zone of Mauna Loa volcano in Hawaii, which occurred between 1972 and 1988, the dominant focal mechanisms (44 events) are decollement type with one nodal plane nearly horizontal (dip ≤ 30°). The average slip vector of the upper crust on the decollement plane points in an azimuth of 260° toward the ocean, away from Mauna Loa's southwest rift zone at an angle of about 150° with respect to the NE‐SW oriented rift. Two other types of focal mechanisms are present: normal faults (four events) and strike‐slip faults with normal component (nine events). The orientation of the principal stresses was derived by minimizing the sum of the misfits between the theoretical and observed fault geometry for each focal mechanism. After subdividing the data set into three different regions we found that the stress tensor is not homogeneous in west Hawaii. The orientation for the stress tensor is best resolved in the area located between latitudes 19.27°N and 19.4°N and longitudes 155.7° and 155.9° W where the greatest principal stress directions within the 95% confidence limits are nearly vertical with some spreading to the west. Their plunge varies between 53° and 85°. The intermediate and least principal stresses are mostly horizontal and have similar magnitudes. Their plunge varies between 1° and 36°. The area between latitudes 19.4° and 19.6°N and longitudes 155.7° and 156°W is characterized partly by near‐vertical and partly by east‐west oriented greatest principal stresses. The least principal stresses are approximately horizontal and have magnitudes similar to the intermediate principal stresses. In the two areas where the stress tensor was well resolved, the principal strain directions differed from the principal stress directions by 30° in the plunge, suggesting that the faulting in western Hawaii takes place on a weak plane, but this result could not be established at the 95% confidence level. The tectonic model proposed for west Hawaii is similar to the model for the Kalapana region. The strain is accumulated through magmatic intrusions in the southwest rift zone of Mauna Loa, and the earthquakes occur along a zone of weakness composed of oceanic sediment at about 10 km depth. The west flank of Mauna Loa slips in the direction away from the rift.

Comparison of orientations of stress and strain tensors based on fault plane solutions in Kaoiki, Hawaii

The stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly M = 3.5 ± 0.6 located in the 10‐km radius Kaoiki crustal volume. Separate inversions for subvolumes containing 20–50 events yielded the same results in several adjacent volumes, suggesting that the stress tensor is homogeneous in those parts of the Kaoiki area and that the inversion results are stable and meaningful. Five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 20°–80° and at confidence levels exceeding 95%. The volcano summits of Kilauea and Mauna Loa, and their rift systems, are identified as the source of stress in the Kaoiki crust, because the greatest principal stress points to Kilauea and Mauna Loa. In addition, the strain tensor due to energy released by these 238 earthquakes was computed for the Kaoiki area, and several subvolumes of it, by summing the moment tensors. The moment tensor of each earthquake was constructed from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment‐magnitude relationship. A comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike‐slip faulting. In these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes. These observations are interpreted to show that in the strike‐slip regime of the upper part of the crust, neither of the nodal planes is preferred for faulting. Rupture probably occurs along the NW and along the NH striking nodal planes in separate earthquakes. Subvolumes with more decollement faulting showed significant differences of 30°–40° between the principal strain and stress directions. In these volumes the near‐horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions. These facts are interpreted to indicate that the decollement plane is weak, allowing slip on it even if the principal stresses are inclined at a large angle to it. It is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength (no well developed fault plane) and regimes in which a fault with low frictional strength dominates. As a function of time, significant rotations of the strain tensor by approximately 45° can be observed, which seem to be related to the occurrences of Kaoiki mainshocks. During three background periods of about 7 years each, the average strain tensor showed an orientation typical for predominant decollement faulting, while two premainshock periods of 2.5 years each showed an orientation closer to strike‐slip faulting. It is proposed that this pattern may be repeated before the next Kaoiki mainshock. The strain released seismically is less than the geodetically observed strain by approximately an order of magnitude. (Stress, strain, fault plane solutions, Hawaii.)