Earthquake nucleation in the lower crust by local stress amplification

Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep

Calculation of CO 2 activities using scapolite equilibria: constraints on the presence and composition of a fluid phase during high grade metamorphism

Thermodynamic and phase equilibrium data for scapolite have been used to calculate CO 2 activities ( a CO 2 ) and to evaluate the presence or absence of a fluid phase in high-grade scapolite bearing meta-anorthosite, granulites, calc-silicates, and mafix xenoliths. The assemblage scapolite-plagioclase-garnet±quartz may be used to calculate or limit a CO 2 by the reaction Meionite+Quartz = Grossular+Anorthite+CO 2 . Granulites from four high-grade terranes (Grenville Province, Canada; Sargut Belt, India; Furua Complex, Tanzania; Bergen Arcs, Norway) yield a CO 2 =0.4-1, with most >0.7. For scapolite-bearing granulites from the Furua Complex, in which a CO 2 ≥0.9, calculated H 2 O activities ( a H 2 O) based on phlogopite dehydration equilibria are uniformly low (0.1–0.2). The a CO 2 calculated for meta-anorthosite from the Grenville Province, Ontario, ranges from 0.2 to 0.8. For Grenville meta-anorthosite also containing epidote, the a H 2 O calculated from clinozoisite dehydration ranges from 0.2 to 0.6. Calc-silicates from the Grenville, Sargur, and Furua terranes mostly yield a CO 2 1). The calculated fluid activities are consistent with metamorphism (1) in the presence of a mixed CO 2 −H 2 O fluid phase in which CO 2 is the dominant fluid species but other C−O−H−S species are minor, (2) in the absence of a bulk fluid phase (“fluid-absent metamorphism”), or (3) in the presence of a fluid-bearing melt phase. The results for many granulites and Grenville meta-anorthosite are consistent with the presence of a CO 2 -rich, mixed CO 2 −H 2 O fluid phase. In contrast the relatively restricted and low values of a CO 2 for calc-silicates require an H 2 O-rich fluid or absence of a fluid phase during metamorphism. The range of values for xenoliths are most consistent with absence of a fluid phase. The primary implication of these results is that a CO 2 -rich fluid accounts for the reduced a H2 O in scapolite-bearing granulites. However, scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase, and the presence of scapolite is not a priori evidence of a CO 2 -rich fluid phase. In addition, close association of scapolite-free mafic granulites with scapolite-bearing granulites having identical mineral compositions in the Furua Complex, and the absence of scapolite from most granulite terranes implies that a CO 2 -rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47295/1/410_2004_Article_BF00307340.pd

Combining compositional zoning and foliation intersection axes (FIAs) in garnet to quantitatively determine early P-T-t paths in multiply deformed and metamorphosed schists: north central Massachusetts, USA

Quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite–staurolite–garnet grade rocks from the northeastern flank of the Pelham dome, north central Massachusetts, distinguishes the effects of Acadian deformation and metamorphism from extensive overprinting Alleghanian shearing. The P–T conditions and the metamorphic path during the Acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts (FIAs). This Acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an Alleghanian overprint. For each sample where garnet first nucleated during one of these stages in the tectonic history, the PT of core growth was determined by plotting the intersection of the Mn, Fe and Ca isopleths calculated for the core composition on a P–T pseudosection for that sample using THERMOCALC. Combining the PT data from all these samples indicates that the temperature and pressure increased throughout Acadian orogenesis, causing episodic garnet growth. During the Alleghanian, locally intense shearing, especially against the margin of the Pelham dome, formed the dominant schistosity, which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of Acadian deformation and metamorphism in the rock matrix. Shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700°C in those rocks lying directly adjacent to the Pelham dome, and resulted from the rocks of the Northfield syncline being thrust a large distance southwards over the gneisses in the dome

SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia

U–Pb SHRIMP ages obtained in zircons from the Sotosalbos and Toledo anatectic complexes in Central Spain give new constraints to the evolution of the inner part of the Hercynian Iberian belt. Pre- Hercynian ages in zircons from the Sotosalbos complex (~464 Ma) are well preserved and reveal that an age diversity of the Lower Paleozoic magmatism in the area exists, as previous data on westernmost orthogneisses yield significant older ages. Zircon ages in the pelite-derived granites from the Toledo complex also show an important Neoproterozoic age component which points to a metasedimentary protolith deposited maximally 560 Ma ago. Younger zircon populations in both complexes at ~330 Ma in the Sotosalbos region and ~317 Ma in the Toledo complex indicate an important diachronism between the anatectic processes in both areas but also that these processes are mainly unrelated to the generation of the later Hercynian granite batholith of Central Spain, which could be of deeper crustal derivation. In addition, as migmatization occurred late in the metamorphic cycle, after peak conditions were attained, the age of anatexis is younger than the age of the main Hercynian metamorphic event, which still is not well constrained

Calculation and application of clinopyroxene-garnet-plagioclase-quartz geobarometers

Recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibriaPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47284/1/410_2004_Article_BF00399441.pd