Dry Lower Crust Research Articles

Structure and Evolution of Northern Juan de Fuca Crust and Uppermost Mantle Over the Last 8 Ma From an Active‐Source Seismic Tomography Study

AbstractWe present results from a two‐dimensional wide‐angle controlled source seismic transect designed to characterize the velocity structure of the oceanic crust and uppermost mantle spanning the northern Juan de Fuca (JdF) plate from near Endeavor ridge to the Cascadia margin. Reflection and refraction travel time inversion is used to derive a tomographic Vp model of sediments, crust, and upper mantle. Velocity model results are compared to baseline reference Vp values for unaltered crustal and upper mantle rocks at temperatures assuming plate cooling. Effective medium theory is used to infer the degree of hydration of the crust and mantle. Results indicate a somewhat fractured and hydrated upper crust (≤2.5 wt% water), a near dry lower crust (≤0.7 wt%) and upper mantle (≤0.5 wt%) west of the deformation front, and an ∼75 km wide region of modestly lower velocity in the mid‐plate. Comparison with prior results from a complementary transect offshore Oregon indicates significant differences in Vp of the upper crust, with lower Vp‐inferred porosity along most of the Washington transects that may reflect different extents of fault‐related alteration and sediment burial histories. Approaching the deformation front, Washington transect Vp structure indicates dryer conditions than offshore Oregon, consistent with differences in extent of subduction bend faulting found in reflection imaging studies. On both transects, quasi‐abrupt changes in plate properties at ages of 8, 6, and 3.4/4 Ma are found. Distinct crustal accretion modes are recognized, aligning with changes in JdF plate motion and recent history of Cobb‐Eickelberg hotspot influence on crustal accretion at the JdF ridge.

High Stress Deformation and Short‐Term Thermal Pulse Preserved in Pyroxene Microstructures From Exhumed Lower Crustal Seismogenic Faults (Lofoten, Norway)

AbstractEarthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal‐scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal‐plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short‐term evolution of high‐stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte‐bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high‐stress (at the GPa level) prerupture loading accommodated by low‐temperature plasticity, followed by coseismic pulverization‐style fragmentation and subsequent grain growth triggered by the short‐term heat pulse associated with frictional sliding. Thus, up to GPa‐level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure.

Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating

The rate and progress of regional-scale prograde metamorphism is commonly assumed to be controlled by conductive heat supply. In contrast, the rate of retrograde metamorphism is in most cases controlled by the rate of fluid supply. Crustal-scale heat conduction is a slow process, typically causing temperature changes at rates on the order of 10 °C/million years, but convective fluid fluxes can be substantial on much shorter time scales. Therefore, it is often assumed that prograde metamorphism usually takes place near thermodynamic equilibrium, whereas retrograde metamorphism may operate far from equilibrium. In many settings, including convergent plate margins, fluid-driven retrograde metamorphism is probably strongly coupled to fluid-producing prograde metamorphism. Here, we present new geochemical and geochronological results from the Lindås Nappe in the Bergen Arcs of Western Norway, which show that fluid-driven (retrograde) eclogite- and amphibolite-facies metamorphism of lower crustal granulites was synchronous with the emplacement of numerous pegmatites and dykes. These have geochemical characteristics suggesting an origin by fluid-present melting of the granulites themselves along with underlying schists. The morphology and chemical zoning of garnets from the exposed part of these schists suggest growth during rapid prograde metamorphism associated with emplacement of the Lindås Nappe. We present a simple model that indicates that rapid prograde metamorphism of the schists may have been driven by shear heating near the base of the Lindås Nappe, and that temperatures may have reached wet-melting conditions for both schists and granulites. The Lindås Nappe thus represents an example of how strong and dry lower crust may experience rapid retrograde metamorphic and structural reworking in direct response to shear-heating induced prograde metamorphism taking place near its base.

Pristine microstructures in pseudotachylytes formed in dry lower crust, Lofoten, Norway.

Feldspar-rich pseudotachylytes from the island of Moskenesøya, Lofoten, formed in dry granulites under lower crustal conditions during the Caledonian orogeny. The central parts of the pseudotachylytes, where the cooling rates were slowest, are characterized by microlites and spherulites of plagioclase and K-feldspar. K-feldspar surrounding plagioclase is consistent with crystallization from a melt during cooling instead of devitrification as the origin of the spherulites. Very thin (a few micrometres wide) injection veins, which experienced very rapid quenching, contain amorphous or cryptocrystalline material. The preservation of this material and of the fine-grained microstructures shows that, under fluid-absent conditions, recrystallization and reactions are slow and the original microstructures of the pseudotachylytes can be preserved.This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

A 2‐D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey

AbstractWe report results from a wide‐angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two‐dimensional velocity model of the crust and upper mantle is derived from a joint reflection‐refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages >1 Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5–2.1 wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8 wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (<0.3 wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.

Melting, fluid migration and fluid-rock interactions in the lower crust beneath the Bakony-Balaton Highland volcanic field: a silicate melt and fluid inclusion study

Plio-Pleistocene alkali basalt hosted mafic garnet granulite xenoliths were studied from the Bakony-Balaton Highland Volcanic Field (BBHVF) to trace fluid-melt-rock interactions in the lower crust. Two unique mafic garnet granulite samples were selected for analyses (optical microscopy, microthermometry, electron microprobe, Raman and IR spectroscopy), which contain a clinopyroxene-plagioclase vein and patches with primary silicate melt inclusions (SMI). The samples have non-equilibrium microtexture in contrast with the overwhelming majority of previously studied mafic garnet granulite xenoliths. Primary silicate-melt inclusions were observed in plagioclase, clinopyroxene and ilmenite in both xenoliths. The SMI-bearing minerals located randomly in Mi26 and in a clinopyroxene-plagioclase vein on the edge of Sab38 granulites. Petrography and the fluid and melt inclusion study suggest that at least three fluid events occurred in the deep crust represented by these xenoliths. 1. Primary CO2-dominated ± CO ± H2S fluid inclusions were observed in the wall-rock part of Sab38 xenolith. 2. The crystallization of new clinopyroxene from melt, with CO2 + H2O fluid. 3. The crystallization of new plagioclase occurred in a heterogeneous fluid-melt system with additional N2 and CH4 during crystallization. A local reaction was observed between sphene and acidic melt, which formed ilmenite + clinopyroxene + plagioclase ± orthopyroxene. The ‘water’ content of the rock forming minerals was determined by infrared spectroscopy. The calculated bulk ‘water’ content of the Mi26 xenolith is 171 ± 51 ppm wt. %. The bulk wall rock part of the Sab38 granulite contains 55 ± 17 ppm wt. % of ‘water’, whereas the bulk plagioclase-clinopyroxene vein contains 278 ± 83 ppm wt. %. These results imply a very dry lower crust, locally hydrated by percolating fluids and melts.

Crustal rheology of the Santorini–Amorgos zone: Implications for the nucleation depth and rupture extent of the 9 July 1956 Amorgos earthquake, southern Aegean

Abstract The 9 July 1956 Amorgos earthquake (Mw 7.6) was the largest event that hit Greece during the last century followed by a tsunami that inundated the coastal areas of the southern Aegean. This study investigates the rheological properties of the 1956 rupture zone between Amorgos and Santorini islands, in an effort to place some constraints on the nucleation depth and rupture extent of this large event. The seismic velocities inferred from tomographic and surface wave dispersion studies of the area are first correlated with laboratory determined velocities of known rock types. It is found that the lithosphere in the southern Aegean can be approximated by three layers representing the upper/lower crust and upper mantle consisting of quartzite, diabase and peridotite, respectively. Geotherms are calculated by using an analytical solution to the one-dimensional heat conduction equation, while Yield Strength Envelopes (YSEs) are produced after assuming laboratory estimated parameters of brittle and ductile deformation for each rock type. The depth frequency of earthquakes in the area, as well as other independent observations favour the YSE calculated for a wet upper crust/upper mantle, a dry lower crust and a geotherm corresponding to a surface heat flow of 62 mW m−2. In this YSE, the upper mantle exhibits maximum strength at 33 km becoming more ductile at greater depths. The lower crust retains significant strength and therefore cannot flow as it did during the early stages of extension, but it is relatively weaker than the upper mantle confirming the ‘jelly sandwich’ model previously proposed for the continental lithosphere. The downdip rupture width of the Amorgos event can be estimated from empirical relationships to be 26 km which means that its rupture may have extended from the depth of peak strength in the upper mantle (33 km) to 7 km upwards. Such a scenario agrees well with recent modelling results indicating that the Amorgos tsunami was probably caused by submarine landslides rather than coseismic rupture of the seafloor.

Crustal conductors along the SVEKA Profile in the Fennoscandian (Baltic) Shield, Finland

SUMMARY Electrical properties of the crust in the central Fennoscandian (Baltic) Shield are examined along the 435 km magnetotelluric profile SVEKA. A 2-D geoelectric model shows large variations in the crustal resistivity with the model resistivities ranging from 0.1 to 35 000 Ωm. The main geoelectrical features of the crust and upper mantle are: (1) a resistive Archaean lower crust (conductance ≤30 S). (2) A conductive Palaeoproterozoic lower crust beneath the Central Finland Granitoid Complex (170-900S). Free H2O-rich fluids trapped into the lower crust seem less plausible causes for enhanced conductivity as long retention times are required and no possible sources seem to be available to produce fluids into the dry lower crust. Most likely causes for permanent enhancement of lower crustal conductivity include the conductive fill of ductile shear zones, conductive graphite-bearing lamellae, conduction in mafic rocks perhaps via conduction in magnetite, or conductive solid remnants of an ancient shear zone between two crustal blocks. Thin graphitic grain-boundary films precipitated from CO2-rich fluids also seem less plausible. (3) A very resistive (10 000-35 000 Ωm) upper crust of the Palaeoproterozoic Central Finland Granitoid Complex and the Archaean Domain. (4) Narrow and elongated conductors (0.1-100 Ωm) beneath the Palaeoproterozoic Kainuu (KSB) and Tampere (TSB) Schist Belts are caused by graphite-bearing metasedimentary rocks buried deep into the crust in the collisions of crustal terranes during the formation of the Palaeoproterozoic Svecofennian crust. The 500 km long TSB conductor represents an internal Svecofennian terrane boundary whereas the KSB conductor is a part of a discontinuous Archaean-Proterozoic boundary conductor. (5) Resistive upper mantle. There are no indications of an upper mantle conducting layer within the uppermost 300 km in the central part of the Fennoscandian Shield.