摘要:

The uppermost portion of oceanic crust, extrusive basalts underlain and fed by diabasic dikes, represents the chilled roof zone or lid to mid‐oceanic spreading center magma chambers. This rigid lid forms and evolves through a steady state process of flexure, subsidence, and internal rotation, all occurring within a couple of kilometers of the spreading axis. The lid accommodates lateral spreading with dikes emplaced along a unique axis. Dikes tap underlying magma to feed overlying flows. Basalts accumulating along the axis of the lid load the lid, which responds by flexing down and away from the intrusive axis. This subsidence affects lid structure in the following ways. Emplaced dikes are tilted away from, and basalt flows are tilted toward, the intrusive axis. Lid subsidence and section rotation extend the base of the lid more rapidly at the axis than elsewhere. Consequently, dikes preferentially intrude at the existing axis to maintain existing axis shape and relative position through continued spreading. Loading and flexure of the lid, plus faster accretion at the base of the lid as a result of lid rotation, cause the lid to be uplifted distal to the axis. This creates flanking marginal highs and generates extension structures (fissuring and faulting) flanking and paralleling the loading (extrusive) axis. The crustal structure and geometry produced by this process is compatible with structure and topography observed at present‐day mid‐oceanic spreading centers and with structure observed in ophiolites, particularly the Bay of Islands Ophi

ISSN:0278-7407    

DOI:10.1029/TC001i006p00471

年代:1982

数据来源: WILEY

摘要:

The November 29, 1975, Kalapana earthquake of magnitude 7.1 on the south flank of Kilauea Volcano, Hawaii, represents a major rift tectonic event. It is generally understood that the earthquake resulted from movement on a nearly horizontal fault plane, with the crustal block south of Kilauea's east rift zone moving south‐southeastward up to several meters. Well‐recorded earthquakes on the south flank of Kilauea from 1970 to 1979 were analyzed and focal mechanisms determined to obtain a better understanding of the details of south flank tectonics. Included are a large number of earthquakes from the aftershock sequence of the 1975 earthquake. Hypocenters are distributed in a subplanar zone that dips 2° to 3° to the west at a depth of about 8–9 km depth. We interpret this pattern to coincide with a portion of the slip plane of the Kalapana earthquake and to be consistent with the notion of the slip plane occurring in a weak layer at the top of the old oceanic crust. Focal mechanisms are remarkably constant throughout this time period, both before and after the 1975 event, and closely agree with the mainshock mechanism. P axis azimuths and slip vectors for the low‐angle fault planes are normal to the middle and lower east rift zones of Kilauea. There is suggestion of a change in the orientation of P axes from shallower to steeper plunge at the time of the Kalapana earthquake. Such a change is consistent with a general reduction in the horizontal compressional stress normal to the rift zone at the time of the large earthquake. No other evidence of precursory reorientation of the stress field was found and the precise predictability of this event remains in doubt. However, a general model of the tectonics of the volcanic rift system is now available that satisfies a number of geological and geophysical obs

ISSN:0278-7407    

DOI:10.1029/TC001i006p00495

年代:1982

数据来源: WILEY

摘要:

Focal mechanisms are investigated for shallow earthquakes (h ≤60 km) of central Japan by using telemetered seismic data of microearthquakes. Shallow earthquakes in central and southwestern Japan can be distinguished into three groups, upper crustal earthquakes, subcrustal earthquakes, and interplate thrust earthquakes, the first and the second ones being subjects of the present study. Most of the upper crustal earthquakes are of strike slip type or reverse faulting with E‐W to SE‐NW compression, whereas the subcrustal earthquakes are characterized by strike slip or normal faulting with E‐W to NE‐SW extension. The subcrustal seismic zone, subparallel to the Nankai trough, becomes progressively deeper landward to the north and progressively shallower to the east so that it emerges into a very shallow seismic activity in the Izu Peninsula which has been interpreted as a microcontinent of the Philippine Sea plate colliding against the Eurasian plate. On the basis of hypocentral distribution, three‐dimensional seismic velocity structure, and focal mechanisms, subcrustal earthquakes are considered to occur within the subducting Philippine Sea plate that extends down to a depth of at least 60 km. On the basis of this and other focal mechanism studies, stress trajectories are drawn for the subducting Philippine Sea plate and for the overriding Eurasian plate. The overriding plate is, on the whole, under the E‐W compression which coincides with the moving direction of the Pacific plate relative to the Eurasian plate. The subducting plate, on the other hand, is under lateral extension along its strike. We interpret that the latter is a stress regime unique to an incipient subduction zone, where the subducting plate at subcrustal depths is forced to warp downward, being constrained laterally at the arc‐arc junctions, and thereby comes under lateral stretching. The lateral constraints arise either from apparent increase in flexural strength of the plate at the arc‐arc junction or from buoyancy of a microcontinent that r

ISSN:0278-7407    

DOI:10.1029/TC001i006p00543

年代:1982

数据来源: WILEY