A new look at Gregory's Rift: The structural style of continental rifting

Bosworth, William; Lambiase, Joseph J.; Keisler, Ron

doi:10.1029/eo067i029p00577

Cited by 85 publications

(42 citation statements)

References 21 publications

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“…Polarity reversals are spaced -100 km apart, similar to the pattern observed in the Eastern African rift valley, where fault polarity also reverses at intervals of -100km (e.g. Bosworth, Lambiase & Keisler 1986;Rosendahl et al 1986;Dunkelman, Karson & Rosendahl 1988).…”

Section: Implications For the Active Tectonics Of Italysupporting

confidence: 56%

Seismological and field observations of the 1984 Lazio-Abruzzo earthquakes: implications for the active tectonics of Italy

Westaway

Gawthorpe

Tozzi

1989

Geophysical Journal International

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Seismological and field studies of the 1984 Lazio-Abruzzo earthquakes provide new constraints on the pattern of Quaternary extension in the Apennine mountains of central Italy. Waveform modelling of the mainshock ( M , = 5.8; M, = 0.59 X 10l8 N m) at 17:49 on May 7 indicates centroid strike 123", dip 48", rake -96" and depth 10 km. The largest aftershock (M, = 5.2; M, = 0.21 X 10l8 N m), at 10:41 on May 11, had strike 170", dip 46", rake -67" and centroid depth also 10 km. The mainshock fault plane was part of the Sangro fault, one of three major SW-dipping normal faults that take up much of the extension of this part of central Italy in Lazio and Abruzzo. A much larger earthquake occurred on 1915 January 13 on an in-line continuation of this fault -30 km further NW.Scarps -10 m high suggest -1 mm yr-' Holocene slip rate on the Sangro fault, and imply -2 mm yr-' average Holocene extension rate across central Italy in good agreement with rates calculated from the seismicity of the last few decades. Topography and present-day dips of active normal faults in Abruzzo suggest that extension has been occurring locally for no more than -2Myr. Slip vector azimuths for the Lazio-Abruzzo earthquakes were -45", approximately perpendicular to the overall strike of the actively-extending zone and major active normal faults inside it. They thus imply that extension in this zone is accompanied by negligible local rotation of fault-bounded blocks around vertical axes. On a larger scale, the instantaneous present-day extension occurring across Italy between northern Campania and Tuscany can be regarded as anticlockwise rotation of the Adriatic Sea region to the NE relative to the Tyrrhenian Sea region to the SW around a pole. We suggest this pole is at present near 44"30'N, 9'30'E. Our 2 mm yr-' extension rate in Abruzzo implies -0.3" Myr-' angular velocity around this pole, giving -4 mm yr-' extension rate in Campania.Moving further southeast, some major normal faults in Campania and Calabria show substantial left-lateral strike-slip, suggesting blocks there may be rotating around vertical axes. The southernmost block of Calabria has rotated 36" clockwise since -4Myr ago, approximately the time when extension began locally. The leading edge of the extending zone appears to have propagated northwestward during the course of extension at -200 mm yr-' from Calabria to its present-day position.

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Section: Implications For the Active Tectonics Of Italysupporting

confidence: 56%

Seismological and field observations of the 1984 Lazio-Abruzzo earthquakes: implications for the active tectonics of Italy

Westaway

Gawthorpe

Tozzi

1989

Geophysical Journal International

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“…Whilst the major faults can be visualized to extend deep into the crust, the small horizontal scale of the grid faults suggests a relatively small vertical scale; they are unlikely to reach depths greater than approximately 3 km (e.g. Bosworth, Lambiase & Keisler 1986). The structure seen here, with the young grid faulting in the centre and the older major faults defining the Rift shoulders, is typical of the Rift structure as a whole (Baker, Mohr & Williams 1972;Baker, Mitchell & Williams 1988).…”

Section: Despite Earth Scientists' Considerable Interest In the Kenyamentioning

confidence: 72%

Implications of the distribution of seismicity near Lake Bogoria in the Kenya Rift

Young

Maguire

Laffoley

et al. 1991

Geophysical Journal International

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Previous seismicity studies in the Kenya Rift have not been able to determine accurately depths of earthquakes, nor have most of them determined epicentres precisely enough to allow correlation of the seismicity with particular surface features. We operated a small dense seismic array for 3 months near Lake Bogoria in the Kenya Rift with the aim of locating microearthquakes in 3-D. 572 earthquakes, 81 per cent with ML< 1.0, have been located. The majority of the events are associated with the larger older faults on the Rift shoulder rather than the young 'grid' faults in the centre of the Rift. Seismic activity in the central trough cannot be related directly to the surface faulting; we infer that it indicates the presence of deep buried faults. This possibility has important implications for extension estimates and models of the Rift. Most of the activity occurs at depths less than 12 km, and no normal activity is deeper than 16 km. There is a peak in seismic activity at a depth of 9-10 km and the cut-off depth for brittle failure is taken at 12 km. The depth distribution of these earthquakes is similar to that found in other intracontinental areas with similar heat flow which suggests that the crust beneath the Kenya Rift is of normal rheology.

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“…He further states that minor faults are commonly synthetic to the major boundary faults, switching polarity where major faults switch polarity. The existence of large scale accommodation (shear) zones, as suggested by Bosworth et al (1986), have not been supported by geophysical data to date. For example, Simiyu and Keller (2001) show that the southern part of the Kenya rift consistently has the major fault along the western boundary of the rift valley.…”

Section: Geologic and Tectonic Settingmentioning

confidence: 91%