Václav Hanuš

Earthquake distribution and volcanism in Kamchatka, Kurile Islands, and Hokkaido Part 1: Kamchatka and northern Kuriles

SummaryThe morphology of the Wadati-Benioff zone in the region of Kamchatka and Northern Kuriles, based on the distribution of 1102 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A system of deep seismically active fracture zones, genetically connected with the process of subduction, was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic belts of Kamchatka.

Seismically active fracture zones and distribution of large accumulations of metals in the central part of Andean South America

Abstract The analysis of the geometry of distribution of earthquake foci in the central part of Andean South America between 18° and 34°S made the delineation of several seismically active fracture zones in the continental wedge overlying the subducting Nazca plate possible. Correlation of their position with the distribution of hypogene accumulations of metals revealed that the majority of large mineral deposits and mining districts are situated in the outcrops of these fracture zones. We present geometrical documentation (map of epicentres, vertical and longitudinal cross sections) of the most important fracture zones and data on mineralogical composition, genetic type and available radiometric ages of mineral deposits. Sixteen mining districts in Chile, and 24 in Argentina, were attributed to the seismically active fracture zones. Major mining districts and individual large mineral deposits occur in six seismically active fracture zones roughly parallel to the axis of the Peru-Chile trench (Carachas-Portillo, Choquelimpie, Iquique, Domeyko, Río Blanco-Los Bayos and Farellones F.Z.), in six fracture zones roughly perpendicular to the trench (El Salvador, Maricunga, Jaroma, Ujina, Tumbaya and Incahuasi-León Muerto F.Z.) and in two fracture zones oriented at an angle of about 45° in relation to the direction of the presently active Andean subduction (Aconcagua and Sierra del Volcán F.Z.). The occurrences of large mineral deposits of different ages show that these fracture zones were also active in the geological past and represent sites of permanent re-opening of paths allowing ore-bearing solutions and long-term accumulation of large amounts of metals to occur in relatively restricted domains of the Earths crust. The mining districts with dated mineral deposits are arranged into four periods of hypogene mineralization: Upper Miocene-Pliocene, Upper Oligocene-Middle Miocene, Upper Eocene-Middle Oligocene, Lower Paleocene-Upper Eocene. These periods of metallogenic activity correlate well with four supposed Andean subduction cycles active in the Tertiary. The occurrence of mineral deposits of different ages in recently active fracture zones can be used as an important evidence in favour of long-term spatial permanence and activity of these zones and as a guide for the discovery of further mineral deposits hidden under young sedimentary and volcanic cover in the fracture zones.

Seismic Activity Around and Under Krakatau Volcano, Sunda Arc: Constraints to the Source Region of Island Arc Volcanics

There is general agreement that calc-alkaline volcanic rocks at convergent plate margins are genetically related to the process of subduction (Ringwood, 1974; Maaloe and Petersen, 1981; Hawkesworth et al., 1997). However, opinions on the mode and site of generation of primary magma for island arc volcanism differ substantially. The site of generation of calc-alkaline magma is thought to be either in the mantle wedge (Plank and Langmuir, 1988; McCulloch and Gamble, 1991) or in the subducting slab (White and Dupré, 1986; Defant and Drummond, 1990; Edwards et al., 1993; Ryan and Langmuir, 1993). We present seismological evidence in favour of the latter concept. A distinctive seismicity pattern around and under the Krakatau volcano was identified during systematic studies of the SE Asian convergent plate margins by means of global seismological data. A column-like cluster of events, probably associated with the dynamics of the volcano, is clearly separated from the events in the Wadati-Benioff zone. The accuracy of hypocentral determinations of the events of the cluster does not differ from the accuracy of the events belonging to the subducting slab. The depths of the cluster events vary from very shallow to about 100 km without any apparent discontinuity. On the other hand, there is a pronounced aseismic gap in the Wadati-Benioff zone directly beneath the volcano at depths between 100-150 km. The Krakatau cluster connects this aseismic gap to the volcano at the surface. The pervasive occurrence of earthquakes in the continental wedge between the subducting slab and the Earth surface bears witness to the brittle character of the continental lithosphere and casts doubt on the existence of large-scale melting of mantle material. The aseismic gap (Hanuš and Vaněk, 1985), interpreted by us as a partially melted domain occurring in subducted slabs in practically all active subduction zones that reach depths greater than 100 km, is here used as evidence for the location of the primary source region of island arc volcanics in the subducting plate.

Deep seismically active fracture zones in ecuador and northern peru

РезюмеСuсmемa 8 сеŭсмuческu aкmuвных рaзломных зон былa обнaруженa нa основaнuu рaсnре¶rt;еленuя очaгов землеmрясенuŭ в конmuненmaльноŭ лumосфере Эквa¶rt;орa. Былu оnре¶rt;елены nоложенuе u шuрuнa нa nоверхносmu, mольщuнa, уклон u мaксuмaльнaяглубuнa оm¶rt;ельных рaзломных зон. Сущесmвовaнuе эmuх зон незaвuсuмо nо¶rt;mвер¶rt;aюm нaблю¶rt;енuя uсmорuческuх рaзрушumельных землеmрясенuŭ u соnосmaвленuе зон с nоверхносmноŭгеологuеŭ, mекmонuкоŭ, вулкaнuзмом uгu¶rt;роmермaльнымu nроявленuямu.SummaryA system of 8 seismically active fracture zones was delineated on the basis of the distribution of earthquake foci in the continental lithosphere of Ecuador. The position and width of the outcrop, thickness, dip and maximum depth of the individual fracture zones were estimated and correlated with surface geological and tectonic phenomena, volcanism and hydrothermal manifestations. The existence and strike of the fracture zones was independently confirmed by the occurrence of historical disastrous earthquakes.

Earthquake distribution and volcanism in Kamchatka, Kurile Islands, and Hokkaido Part 3: Southern Kuriles and Hokkaido

РезюмеМорфологuя зоны Wadati-Benioff в облaсmu Юужных Курuльскuх о-вов u Хоккaŭ¶rt;о, основaннaя нa рaсnре¶rt;еленuu 4015 очaгов землеmрясенuŭ, nоmвер¶rt;uлa сущесmвовaнuе nерехо¶rt;ноŭ aсеŭсмuческоŭ зоны u ее связь с aкmuвным aн¶rt;езumовым вулкaнuзмом. Былa нaŭ¶rt;енa u оnuсaнa зонa naлеосуб¶rt;укцuu, aкmuвuзuровaннaя nромежуmочнымглубuнным сmолкновенuем с aкmuвноŭ зоноŭ суб¶rt;укцuu.SummaryThe morphology of the Wadati-Benioff zone in the region of Southern Kuriles and Hokkaido, based on the distribution of 4015 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A paleosubduction zone activated by an intermediate depth collision with the active subduction zone was found and described.

Geodynamic interpretation of the earthquake distribution in the kermadec subduction zone

SummaryThe morphology of the Wadati-Benioff zone in the Kermadec region, based on the distribution of 1100 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Pacific plate. Two cycles of the recently active subduction in the Tonga-Kermadec island arc were found.

Position of the Disastrous 1999 Puebla Earthquake in the Seismotectonic Pattern of Mexico

The location of the 1999 Puebla earthquake shows that this earthquake belongs to the class of disastrous intraplate earthquakes generated by fracturing of the continental lithosphere caused by the process of subduction. This is obvious from the vertical section across the Middle American Wadati-Benioff zone and from its position in the revised seismically active Los Azufres-Rio Salado fracture zone delineated in the continental lithosphere of the North American plate.

Discontinuous Nature of the Lower Part of the South American Wadati-Benioff Zone in The Arica Elbow Region

The Arica Elbow region represents that part of Andean South America where the azimuth of the strike of the Peru-Chile trench changes from 150° to 190°. The area under study is roughly bounded by latitudes 17 °S and 23 °S, The shape of the Wadati-Benioff zone was studied in terms of the distribution of ISC hypocentres dated between 1964 and 1993. A system of 22 vertical cross-sections, perpendicular to the trench axis, and a map of epicentres was used to derive the detailed shape of the Wadati-Benioff zone of the presently descending slab. The distribution of earthquake foci indicates a fingerlike shape of the lower part of the Wadati-Benioff zone beneath the aseismic gap. The slab length shows small changes around 350 km in the northern sections, pronounced length oscillations between 350 and 750 km in the neighbouring central sections and a constant value of 650 km in the southern sections. The dip and thickness of the Wadati-Benioff zone are practically constant in all sections. Fault plane solutions, separated spatially into three zones, were used to estimate the state of stress in the slab.

Seismically active fracture zones related to the Eastern segment of the Hellenic subduction

A system of 12 seismically active fracture zones was delineated on the basis of the distribution of earthquake foci in the continental lithosphere in the western domain of the Hellenic area. The position and width of the outcrop, thickness, dip and maximum depth of the individual fracture zones were estimated and complemented by tectonic and geological evidence. The existence of the fracture zones was confirmed by the occurrence of historical disastrous earthquakes.

Recent geodynamic evolution of convergent plate margins and the reconstruction of fossil plate boundaries

SummaryThe discovery of paleoplates buried in the upper mantle leads to an interpretation of the subduction as a discontinuous process running in cycles and shifting the place of its operation in or against the direction of ocean floor spreading. This mechanism explains the distribution of calc-alkaline volcanism of different age in fossil convergent plate boundaries. The establishment of regular spatial correlation of the aseismic gap in the Wadati-Benioff zones with the distribution of calc-alkaline volcanism enables to reconstruct fossil plate boundaries and to define allochtonous terranes in apparently homogeneous continental plates. The hampering effect of the ocean floor morphology and of the fragments of continental plates approaching the trench, which substantially influences the rates of subduction and the geodynamic history of active continental margins in different domains along the trench, allows us to understand the complicated geological development of continental wedges in fossil convergent plate margins. The establishment of the segmented nature of active subduction zones and the dramatic morphology of the lower limit of the active subducted slab along the trench help us to interpret extensive lateral gaps in volcanic chains overlying active as well as fossil subduction zones.