Frictional Melting in Hydrothermal Fluid-Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile).

Tectonic pseudotachylytes are thought to be unique to certain water-deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid-rich faults hosted in the continental crust. Pseudotachylytes were found in the >40 km-long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m-thick (ultra)cataclastic strands hosted in a damage-zone made of chlorite-epidote-rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite-, epidote- and calcite-bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s-1) resulted in the production of vesiculated pseudotachylytes both at dry and water-pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid-deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co-seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust.

Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone.

In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the physical processes responsible for the coseismic reduction in fault shear strength are poorly understood. Using a novel experimental setup, we measured pore fluid pressure during simulated seismic slip in clay-rich materials sampled from the deep oceanic drilling of the Papaku thrust (Hikurangi subduction zone, New Zealand). Here, we show that at seismic velocity, shear-induced dilatancy is followed by pressurisation of fluids. The thermal and mechanical pressurisation of fluids, enhanced by the low permeability of the fault, reduces the energy required to propagate earthquake rupture. We suggest that fluid-saturated clay-rich sediments, occurring at shallow depth in subduction zones, can promote earthquake rupture propagation and slip because of their low permeability and tendency to pressurise when sheared at seismic slip velocities.

Mechanical behaviour of fluid-lubricated faults.

Fluids are pervasive in fault zones cutting the Earth's crust; however, the effect of fluid viscosity on fault mechanics is mainly conjectured by theoretical models. We present friction experiments performed on both dry and fluid-permeated silicate and carbonate bearing-rocks, at normal effective stresses up to 20 MPa, with a slip-rate ranging between 10 µm/s and 1 m/s. Four different fluid viscosities were tested. We show that both static and dynamic friction coefficients decrease with viscosity and that dynamic friction depends on the dimensionless Sommerfeld number (S) as predicted by the elastohydrodynamic-lubrication theory (EHD).Under favourable conditions (depending on the fluid viscosity (η), co-seismic slip-rate (V), fault geometry (L/H02) and earthquake nucleation depth (∝σeff)), EHD might be an effective weakening mechanism during natural and induced earthquakes. However, at seismic slip-rate, the slip weakening distance (Dc) increases markedly for a range of fluid viscosities expected in the Earth, potentially favouring slow-slip rather than rupture propagation for small to moderate earthquakes.

Subseismic to Seismic Slip in Smectite Clay Nanofoliation.

Smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth (e.g., <1-km depth in the Japan Trench) and in the decollements of large landslides (e.g., 1963 landslide, Vajont, Italy). Therefore, deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides. Here, we use (1) laboratory experiments to investigate the mechanical behavior of partly water-saturated smectite-rich gouges sheared from subseismic to seismic slip rates V and (2) nanoscale microscopy to study the gouge fabric. At all slip rates, deformation localizes in volumes of the gouge layer that contain a "nanofoliation" consisting of anastomosing smectite crystals. "Seismic" nanofoliations produced at V = 0.01, 0.1, and 1.3 m/s are similar to "subseismic" nanofoliations obtained at V = 10-5 m/s. This similarity suggests that frictional slip along water-lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite-rich faults. Thus, if water is available along smectite grain boundaries and basal planes, nanofoliations can develop from slow to fast slip rates. Still, when nanofoliations are found highly localized in a volume, they can be diagnostic of slip that occurred at rates equal or larger than 0.01 m/s. In such a case, they could be markers of past seismic events when found in natural fault rocks.

Tratamiento quirúrgico de aneurismas cerebrales en pacientes en la 8va y 9na década de vida / Surgical treatment of cerebral aneurysms in patients in the 8th and 9th years of life

La hemorragia subarcnoidea por sangrado de aneurismas sigue siendo una enfermedad con una elevada mortalidad y morbilidad. Los avances en microcirugía y en terapia endovascular, no han logrado abatir los altos porcentajes de mala evolución. Por su parte, el tratamiento de los aneurismas no rotos se vuelve cada vez más rutinario, dados los buenos resultados. Un capítulo aparte es el tratamiento o no de aneurismas cerebrales con o sin hemorragia subaracnoidea en pacientes añosos (mayores de 70 años). Hay publicaciones al respecto. Los autores hacen una revisión retrospectiva de 179 pacientes mayores de 70 años portadores de aneurismas cerebrales, con o sin sangrado subaracnoideo. Todos fueron sometidos a cirugía. Se analizan los resultados, los que muestran que la morbimortalidad en este grupo etario no difiere sustancialmente de los que se dan en menores de 70 años.

Subarachnoid haemorrhage due to bleeding of cerebral aneurysms remains a disease with high mortality and morbidity. Advances in microsurgery and endovascular therapy have failed to reduce the high percentages of poor outcome. The treatment of unruptured aneurysms, on the other hand, becomes more and more routinary, given the good results. A separate chapter is the treatment or not of cerebral aneurysms with or without subarachnoid hemorrhage in elderly patients (older than 70 years). There are several publications about it. The authors make a retrospective review of 179 patients older than 70 years with cerebral aneurysms, with or without subarachnoid bleeding.All were submitted to surgery. We analyze the results, which show that morbidity and mortality in this age group do not differ substantially from those occurring under 70 years of age.

An empirically based steady state friction law and implications for fault stability.

Empirically based rate-and-state friction laws (RSFLs) have been proposed to model the dependence of friction forces with slip and time. The relevance of the RSFL for earthquake mechanics is that few constitutive parameters define critical conditions for fault stability (i.e., critical stiffness and frictional fault behavior). However, the RSFLs were determined from experiments conducted at subseismic slip rates (V < 1 cm/s), and their extrapolation to earthquake deformation conditions (V > 0.1 m/s) remains questionable on the basis of the experimental evidence of (1) large dynamic weakening and (2) activation of particular fault lubrication processes at seismic slip rates. Here we propose a modified RSFL (MFL) based on the review of a large published and unpublished data set of rock friction experiments performed with different testing machines. The MFL, valid at steady state conditions from subseismic to seismic slip rates (0.1 µm/s < V < 3 m/s), describes the initiation of a substantial velocity weakening in the 1-20 cm/s range resulting in a critical stiffness increase that creates a peak of potential instability in that velocity regime. The MFL leads to a new definition of fault frictional stability with implications for slip event styles and relevance for models of seismic rupture nucleation, propagation, and arrest.

G: Fracture energy, friction and dissipation in earthquakes.

Recent estimates of fracture energy G' in earthquakes show a power-law dependence with slip u which can be summarized as G' ∝ ua where a is a positive real slightly larger than one. For cracks with sliding friction, fracture energy can be equated to Gf : the post-failure integral of the dynamic weakening curve. If the dominant dissipative process in earthquakes is friction, G' and Gf should be comparable and show a similar scaling with slip. We test this hypothesis by analyzing experiments performed on various cohesive and non-cohesive rock types, under wet and dry conditions, with imposed deformation typical of seismic slip (normal stress of tens of MPa, target slip velocity > 1 m/s and fast accelerations ≈ 6.5 m/s2). The resulting fracture energy Gf is similar to the seismological estimates, with Gf and G' being comparable over most of the slip range. However, Gf appears to saturate after several meters of slip, while in most of the reported earthquake sequences, G' appears to increase further and surpasses Gf at large magnitudes. We analyze several possible causes of such discrepancy, in particular, additional off-fault damage in large natural earthquakes.

Tratamiento quirúrgico en malformación de chiari I en la edad pediátrica: estudio y resultados en serie de 57 casos / Surgical treatment in chiari I malformation in the pediatric age: study and results in a series of 57 cases

Introducción: Las malformaciones de Chiari constituyen un grupo heterogéneo de patologías que ocasionan la herniación descendente de estructuras de la fosa posterior. La malformación de Chiari tipo I (MCI) se define como la herniación amigdalina de al menos 3 a 5 mm por debajo del nivel del foramen magno. La resonancia magnética ha aumentado el número de casos diagnosticados, inclusive en la edad pediátrica. Este trabajo tiene por objetivo presentar los resultados en cuanto a hallazgos clínicos y evolución postoperatoria en una serie pediátrica y se realiza una revisión del tema. Pacientes y métodos: Se efectuó un análisis retrospectivo de cohorte única de 57 pacientes operados a lo largo de 20 años por el mismo equipo neuroquirúrgico. Se revisaron los registros de uno de los autores (ES) para obtener datos sobre presentación clínica, diagnóstico, tratamiento y evolución postoperatoria, con un seguimiento de al menos 6 meses en cada paciente. Resultados: Se operaron 57 pacientes de entre 3 a 13 años de edad. El 100% de las malformaciones de Chiari I operadas eran sintomáticas. El síntoma más frecuente fue la cefalea (90%). En todos los casos se efectuó descompresión de la fosa posterior con duroplastia. El 100% de los pacientes presentaron mejoría clínica postoperatoria que se mantuvo a los 6 meses de seguimiento. Se registraron 3 casos de pseudomeningocele y un solo caso de fístula externa de LCR (7%). Conclusiones: La descompresión de fosa posterior con duroplastia es una opción quirúrgica muy efectiva y con baja tasa de complicaciones.

Introduction: Chiari malformations constitute a heterogeneous group of disorders characterized by a maldevelopment of the hindbrain and posterior cranial fossa that determines downward herniation of the cerebellar tonsils. Chiari malformation type I is defined as a 3 to 5 mm tonsillar herniation below the level of the foramen magnum. MRI has increased the number of cases diagnosed even in the pediatric age. Hence, the aim of this article is to present our results concerning clinical findings and outcomes on an exclusively pediatric series and to perform a review of the subject.Patients and methods:this is a retrospective, single-cohort study involving 57 patients operated in a 20-year period by the same neurosurgical team. Clinical records of one of the authors (ES) were reviewed in order to obtain data about symptoms, diagnosis, treatment and postoperative outcome at 6-month follow up.Results: 57 patients between 3 and 13 years of age were operated on. All of the patients were symptomatic at the time of diagnosis. The most frequent symptom was headache (90. All of the patients underwent posterior fossa decompression with duroplasty. 100% of the patients experienced sustained clinical improvement even at 6 month-follow up. Three cases of pseudomeningocele and only one case of external CSF leak were observed (7%).Conclusions:according to our findings and international literature, posterior fossa decompression with duroplasty is an effective surgical treatment with a low rate of complications.

[Recomendation for the management of the brain arteriovenous malformations]. / Recomendaciones para el manejo de las malformaciones arteriovenosas cerebrales Actualización y modificaciones a las recomendaciones del Comité Vascular de la Federación Latinoamericana de Neurocirugía (FLANC) del año 2003

Intracranial arteriovenous malformations constitute an heterogeneous group of lesions that represent a real challenge to the neurosurgeon. Their variability in size, topography, angioarchitecture, angioarchitecture, etc., and also the multiple clinical presentations, lead to varied therapeutic approaches. There is no international consent regarding how to treat malformations of similar features. This fact is influenced by the different therapeutic options, and the personal experience in the management of these lesions. Based on the former considerations, the FLANC (Latin American Federation of Neurosurgery), through its Vascular Committee, published in 1993 the first guidelines in the management of arteriovenous malformations. The advances in the diagnosis and treatment verified in the past last years, has motivated the authors to review and actualize these first guidelines. The intention of these guidelines is to provide the neurosurgeon support when facing these lesions, and does not mean that they should be strictly followed.

Neurosarcoidosis: Presentación de un caso clínico / Neurosarcoidosis: Presentation of a clinical case

Se presenta el caso clínico de una paciente que ingresa por un cuadro neurológico proteiforme, pero que orientaba a un sufrimiento de tronco cerebral bajo y medular alto. La imagenología confirmó un descenso amigdalino, simulando un Chiari, sin aspecto malformativo, pero que mostró la presencia de lesiones fundamentalmente en la base de cráneo que orientaban a un proceso granulomatoso. La anatomía patológica del material obtenido durante la cirugía confirmó una neurosarcoidosis. La neurosarcoidosis es una enfermedad infrecuente. Forma parte de una de las formas de presentación de la sarcoidosis, afección multisistémica, de carácter inflamatorio, caracterizada por la formación de granulomas. Además de analizarse el caso clínico, se realiza una actualización de la enfermedad y se efectúa una revisión de la escasa bibliografía al respecto.

Aneurismas no rotos. Clasificación: análisis de resultados con cirugía en serie de 53 pacientes / Unrupted aneurysms. Classification: analysis of results with surgery in series of 53 patients

La hemorragia subaracnoidea (HSA) por rotura de aneurismas cerebrales sigue siendo una enfermedad con una elevada morbimortalidad a pesar de los continuos avances en diagnóstico y tratamiento. De ahí que cobra gran importancia el manejo de aquellos aneurismas no rotos. Sigue siendo controversial el manejo de estas malformaciones, ya que hay quienes sostienen que los porcentajes de sangrado de un aneurisma no roto son muy bajos. Por el contrario hayn quienes aseveran que dichos porcentajes pueden llegar al cuatro por ciento anual y acumulable, lo que redunda en una disminución de la expectativa de vida en los portadores de aneurismas no rotos. Los autores realizan una división de los aneurismas no rotos en tres grupos, con riesgos diferentes de complicación según el tipo en cuestión. Se hace además un análisis de los resultados quirúrgicos en una serie de cincuenta y tres pacientes con aneurismas no rotos, llegándose a la conclusión que los riesgos quirúrgicos son inferiores a los riesgos naturales de la enfermedad.

Hemorragia subaracnoidea grave (grados IV y V): mejoría del pronóstico con el tratamiento agresivo en agudo / Severe subarachnoid hemorrhage (IV and V degrees): improved prognosis with aggressive treatment in acute

La Hemorragia Subaracnoidea continua siendo una enfermedad muy grave, con elevada morbimortalidad a pesar de los continuos avances en el diagnóstico y tratamiento. Sea cual sea la opción terapéutica que se tome en consideración para cada caso en particular, lo que queda por definir y continua siendo motivo de discusión, es si tratar o no a los pacientes que ingresan con HSA muy graves, o sea los catalogados como grados IV y V. Muchos trabajos se han publicado en los últimos años al respecto, con resultados contradictorios. Los autores han puesto en práctica en los últimos 10 años un tratamiento agresivo de los pacientes con HSA graves, estableciéndose algunos criterios de selección de pacientes para cirugía y otros para terapia endovascular. Los resultados se exponen a continuación, así como también se hace una revisión de la bibliografía al respecto.

[Aneurysms of the distal anterior cerebral artery]. / Aneurismas del sector distal de la arteria cerebral anterior (arteria perocallosa)

The clinical, angiographic and surgical features of 13 (out of 18) patients harboring pericallosal aneurysms and operated on by the same group of neurosurgeons, are analyzed. The mortality rate of the whole series was 5.5% with good (GOS II) or excellent (GOS I) recovery in the 91.7% of cases. According to the experience of the authors pericallosal aneurysms should not be considered to have a worse prognosis than aneurysms in other locations of the anterior circulation.

[Descompresive craniectomy in head injury. Intractable I.C.P. ]. / Craniectomía descompresiva en el tratamiento de pacientes con un traumatismo creneoencefálico grave e hipeertensión intracraneal refractaria al tratamiento médico. Análisis de una serie de 4 casos.

Decompressive craniotomy is a neurosurical technique known since the origin of neurosurgery, but its use as a routine started at the end of the 19th century. In last decades, the use of decompressive craniotomy decreased, mainly because of poor results obtained and the advances in medical treatment of refractory intracranial hypertension in the Intensive Care Units. Nevertheless, in recent years there has been a renewed interest with the use of this surgical techique in young patients with head injuries and severe intracranial hypertensio'n, but no surgical mass lesion, as well as in patients with ischemic stroke causing life threatening mass effect. The authors present a series of four young patients with head injuries and "malignant" intracranial hypertension on whom wide craniotomies were performed with an excellent outcome. The literature is reviewed.

[Variants of the anterior circle of Willis. Anatomic and angiografic correlation and its implications in the surgery of intracranial aneurysms. (Acigos anterior cerebral artery, median artery of the corpus callosum and accessory middle cerebral artery)]. / Variaciones del sector anterior del polígono de Willis. Correlación anatomo-angiográfica y su implicancia en la cirugía de aneurismas intracraneanos. (Arterias: ácigos cerebral anterior, mediana del cuerpo calloso y cerebral media accesoria.

It is worlwide accepted that in almost 60% of cases, anatomical variants in the Circle of Willis can be found. Some of them are associated with vascular malformations such as aneurysms. The knowledge of these anatomical variants is of vital importance when facing surgery, being the aims to preserve arteries in unusual localisations, which when injured can determine invalidating sequelae. The authors have reviewed 192 cerebral hemispheres, finding three variants in the anterior Circle of Willis: Azigos anterior cerebral artery; Median artery of the corpus callosum and accessory middle cerebral artery. The authors make an anatomical description of the findings, their angiographical correlation, and describe the influence of these changes during surgery of aneurysms in the involved regions.

[Recurrent meningiomas with progressive aggressiveness and posterior extracranial extension]. / Meningiomas recidivantes con agresivisad progresiva y posterior extensión extracraneana. Análisis de 2 casos.

Meningiomas are in most cases benign tumors. They represent about 15% of primitive intracranial tumors. Complete surgical resection achieves healing in the majority of patients. Recurrences occur in 9 to 12% in spite of that kind of surgical resection in the initial approach. This percentage rises to 40% when removing was not complete. Recurrences sometimes show histological changes into aggressiveness, and even malignization. Two cases of originally benign meningiomas with several recurrences and progressive aggressive behaviour are presented. Both cases led to extracranial extension of the lesions, even passing through synthetic dural grafts (De Bakey's Dacron Fabric-Elastic) and the Titanium mesh covering the craniectomy.

Surgical treatment of intracranial hypertension in encephalic cryptococcosis.

The incidence of cryptococcosis has risen sharply together with the growing number of patients with Acquired Immunodeficiency Syndrome (AIDS). Cryptococcal meningitis is nowadays the most common intracranial non-viral infection in such cases. One of its most serious complications is intracranial hypertension (ICH), a situation that can lead either to early death, or disabling sequelae. The authors analyse a series of 10 cases of encephalic cryptococcosis with ICH, and describe the clinical course, diagnosis, medical and surgical treatment, and evolution. The physiopathology of ICH in these patients is discussed, proposing placement of a ventriculo-peritoneal shunt as the primary and emergency treatment, even when ventricular enlargement might be absent. Although the present series is certainly small, from the preceding discussion and according to an extensive bibliographical review, our conclusion is that patients with encephalic cryptococcosis and uncontrollable ICH should receive surgical treatment, consisting of an emergency diversion of the CSF, because serial lumbar punctures are not enough to improve the clinical course, that if left to its natural evolution would lead to a fatal outcome in a short time. In spite of the fact that CSF shunts were carried out on immunocompromised patients, no superinfections occurred.

Cirugía en agudo de los aneurismas intracraneanos: resultados en serie de 100 pacientes operados en el período 1998-2001 / Acute surgery of the intracraneal aneurysm: results in series of 100 operated patients in the period 1998-2001

La hemorragia subaracnoidea es una enfermedad muy grave, con una elevada morbimortalidad. Tiene una incidencia de 1 caso cada 100.000 habitantes, por lo que en Uruguay se dan unos 300 casos al año. La cirugia en agudo de los aneurismas intracraneanos, técnica de práctica habitual; se comenzó a utilizar en un promedio recien a mediados de los 90. Se presenta a continuación el análisis y los resultados de una serie de 100 aneurismas operados en agudo por un mismo equipo de neurocirujanos. Se efectua un estudio de los resultados y se los compara con los obtendios con la cirugía en diferido.

[Widespread endemic goiter and iodine deficiency in the province of Avellino]. / Diffusione del gozzo endemico e della carenza iodica in provincia di Avellino.

1352 schoolchildren between 6-14 years old (699 males and 653 females) and 943 adults (176 males and 767 females) from eight villages of the province of Avellino were studied. All subjects were examined for thyroid size by at least two expert examiners. In most of them urine samples were collected for iodine determinations. 387 schoolchildren and 161 adults from Flumeri and Villanova were evaluated by thyroid echography. The prevalence of goiter was from 23.5 to 52.2% and the median urinary iodine excretion was from 42.3 to 66.2 micrograms/l in schoolchildren. In adults the prevalence of goiter was from 41.2 to 86.7% and the median urinary iodine excretion was from 37.1 to 53.7 micrograms/l. Our data showed a degree of iodine deficiency from low to moderate. The echography permitted to point out a greater prevalence of nodules than the thyroid palpation.