The dating of shallow faults in the Earth's crust

Direct dating of ductile shear zones and calculation of uplift/exhumation rates can be done using various radiometric dating techniques. But radiometric dating of shallow crustal faulting, which occurs in the crust's brittle regime, has remained difficult(1-4) because the low temperatures typical of shallow crusted faults prevent the complete syntectonic mineral recrystallization that occurs in deeper faults. Both old (detrital) and newly grown (authigenic) fine-grained phyllosilicates are thus preserved in shallow fault zones and therefore their radiometric ages reflect a mixture of both mineral populations. Also, the loss of Ar-39 during neutron irradiation in dating of clay minerals can produce erroneously old ages. Here we present a method of characterizing the clay populations in fault gouge, using X-ray modelling, combined with sample encapsulation, and show how it can be used to date near-surface fault activity reliably. We examine fault gouge from the Lewis thrust of the southern Canadian Rockies, which we determine to be similar to 52 Myr old. This result requires the western North America stress regime to have changed from contraction to extension in only a few million years during the Eocene. We also estimate the uplift/exhumation age and sedimentary source of these rocks to be similar to 172 Myr.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62567/1/412172a0.pd

Deciphering the pathogenesis of tendinopathy: a three-stages process

Our understanding of the pathogenesis of "tendinopathy" is based on fragmented evidences like pieces of a jigsaw puzzle. We propose a "failed healing theory" to knit these fragments together, which can explain previous observations. We also propose that albeit "overuse injury" and other insidious "micro trauma" may well be primary triggers of the process, "tendinopathy" is not an "overuse injury" per se. The typical clinical, histological and biochemical presentation relates to a localized chronic pain condition which may lead to tendon rupture, the latter attributed to mechanical weakness. Characterization of pathological "tendinotic" tissues revealed coexistence of collagenolytic injuries and an active healing process, focal hypervascularity and tissue metaplasia. These observations suggest a failed healing process as response to a triggering injury. The pathogenesis of tendinopathy can be described as a three stage process: injury, failed healing and clinical presentation. It is likely that some of these "initial injuries" heal well and we speculate that predisposing intrinsic or extrinsic factors may be involved. The injury stage involves a progressive collagenolytic tendon injury. The failed healing stage mainly refers to prolonged activation and failed resolution of the normal healing process. Finally, the matrix disturbances, increased focal vascularity and abnormal cytokine profiles contribute to the clinical presentations of chronic tendon pain or rupture. With this integrative pathogenesis theory, we can relate the known manifestations of tendinopathy and point to the "missing links". This model may guide future research on tendinopathy, until we could ultimately decipher the complete pathogenesis process and provide better treatments

Tectonomagmatic evolution of bimodal plutons in the central Anatolian crystalline complex, Turkey

The NW-trending Agacoren Intrusive Suite (AIS) on the east side of the Salt Lake (Tuz Golu), Turkey, is part of a curvilinear volcanoplutonic complex along the western edge of the central Anatolian crystalline complex (CACC). Granitoids constitute the predominant lithological group within the AIS and range in composition from monzonite through granite to alkali feldspar granite. Gabbroic rocks occur as irregular intrusive bodies with sinusoidal, irregular contacts with the granitoid plutons and vary from dioritic compositions at the contacts to cumulate amphibole gabbros in their center. These gabbroid to granitoid rocks are all subalkaline and display tholeiitic to calcalkaline affinities, respectively. Modeling of the major element and trace element chemistry of the granitoids to gabbroids, together with their field relations, suggest that these rocks formed synchronously from a melt source that involved both mantle and crustal components. Mafic magmas were likely derived from a metasomatized upper mantle source above a subduction zone and were injected into crustal-level felsic magma chamber(s) in which incomplete mixing, commingling, and crystal fractionation processes produced the coeval granitoid to gabbroid plutons. New Ar-40/Ar-39 ages at 78.0+/-0.3 Ma to 78.8+/-1.0 Ma for the gabbros and 77.6+/-0.3 Ma for the granitoids support this petrogenetic interpretation. Aeromagnetic anomalies of the AIS also suggest that gabbroid to granitoid intrusions coalesce into a larger, single igneous mass at a depth of 1.55 km beneath the earth's surface. These findings collectively rule out an allochthonous, "ophiolitic" origin for the AIS gabbros. The AIS is interpreted as part of a Late Cretaceous volcanoplutonic complex, which evolved at the active margin of the western CACC (in the present coordinate system) as an Andean-style magmatic arc. This tectonic model implies that a major Tethyan seaway, the floor of which was consumed beneath the magmatic arc, separated the Tauride carbonate platform from the CACC during much of the Mesozoic

Early stages of Gondwana breakup: The Ar-40/Ar-39 geochronology of Jurassic basaltic rocks from western Dronning Maud Land, Antarctica, and implications for the timing of magmatic and hydrothermal events

[1] The timing of magmatic events forming Jurassic basaltic rocks in Dronning Maud Land (DML), Antarctica, and hydrothermal activity that affected them is addressed with detailed Ar-40/Ar-39 incremental heating dating of feldspars. Plagioclase from an Utpostane gabbro and from a Kirwanveggen dolerite dike yield indistinguishable plateau ages at 177 +/- 1.8 Ma. Because of geologic controls, this establishes the age of tholeiites in Vestfjella. These plateau ages demonstrate synchroneity of tholeiitic magmatism of both DML and the well-documented Ferrar Province of Antarctica, which together with the closely temporal Karoo constitute the Gondwana breakup magmatism. Vug-filling microclines from Vestfjella yield plateau ages of 150 and 139 Ma which are interpreted to give the ages of secondary mineralization events. These results suggest a lengthy period of extension, possibly accompanied by pervasive low-temperature hydrothermal activity, between flood basalt magmatism and inception of seafloor spreading (circa 160-165 Ma), with younger moderate temperature events recorded by K-feldspars and related to early stages of spreading and/or changes in plate motions. The majority of plagioclase samples yield discordant age spectra that reflect primarily overprinting by younger events and incorporation of excess Ar, and they illustrate the complexities that can be produced