Transtension in the Brittle Field

Speaker- John Dewey, UC Davis and University College Oxford

Entry to all lectures is free to all, but by ticket only. To obtain a ticket please contact Alys Hilbourne (see contact details below).

The meeting format for each lecture will be as follows:

17.30 Lecture begins
18.30 Short reception
19.30 Depart

Abstract

A major problem of structural geology is how biaxial, uniaxial, and triaxial strains in the brittle upper crust are effected by fault systems. Transtension generates noncoaxial constriction in an evolving sedimentary basin and poses difficult problems of wrench/normal fault-combination, block rotation, and strain. Normal faults rotate around vertical and horizontal axes to give oblique slip, and wrench faults rotate around vertical axes. Kinematic problems arise because blocks of varying size and shape bounded by normal and wrench faults rotate at different rates about vertical and horizontal axes while shortening or lengthening. Compatibility problems may be solved by block margin deformation, by bulk block strain, by buckling of faults, by volume increase with holes opening at block intersections, by alternating periods of normal and wrench faulting, and/or by discrete arrays of normal and wrench faults. The Coso transtensional region, in southern California is active, the horizontal strain rates are high, and there is a huge amount of seismic, heatflow, fluid flow, and borehole and surface geologic data. Between the Sierra Nevada and the Argus Range, the transport direction (from GPS) is roughly NNW at about 10 mmyr. This generates triaxial constriction with an instantaneous stretching direction roughly WNW and a horizontal strain rate of about 10-14 sec-1. Constriction is modeled by a combination of NNE normal faults, NE sinistral wrench faults, and WNW folds and thrusts, which rotate clockwise with vorticity, and NNW wrench faults that rotate counterclockwise against vorticity, a pattern of faulting, folding, and bulk strain recorded closely by fault slip data from earthquakes and field observations. In deformation zones, GPS and moment tensor sum data indicate very smooth velocity fields, which in turn means that the commonly accepted view of the rotation of large rigid blocks cannot be correct; instead, the upper crust behaves as a "continuum rubble" of very small "blocks". This generates a very variable fracture permeability, which constantly but unpredictably changes Where transtension occurs between non-parallel boundaries, strain and strain rates vary along and across the basin, yielding complex variations in subsidence rate and the ratio of coaxial/mon-coaxial strain.

Speaker Biography

John Frederick Dewey is widely regarded as one of the world’s great authorities on the development and evolution of mountain ranges and the application of plate tectonic theory to elucidating their origin and fate.

Dewey was educated at Bancroft's School and Queen Mary and Imperial College at the University of London, where he was awarded a BSc and PhD in geology. Following a period as lecturer at the University of Manchester, the University of Cambridge and the Memorial University of Newfoundland, Dewey was appointed Professor of Geology at the University at Albany, New York. It was during this period that he produced a series of classic papers centred on the history of the Newfoundland Appalachians and the Scottish and Irish Caledonides. More recently, his research has concentrated on producing a model to describe the development and orogenic history of the Himalayas.

Dewey returned to the UK in 1982 as Professor of Geology at Durham, a position he held for four years. Like L. R. Wager before him, Dewey was then appointed Professor of Geology at Oxford (and Fellow of University College) in 1986 - a position he held until his resignation in 2000. Since then he has returned to the US as Professor of Geology at the University of California at Davis, although he maintains a position as Senior Research Fellow at University College, Oxford.

John Dewey was elected a Fellow of the Royal Society of London (FRS) in 1985 and has received numerous medals and awards, notably the 1999 Wollaston Medal of the Geological Society of London (that society's senior award, first given to William Smith) and the 1992 Penrose Medal of the Geological Society of America. In 1998 he delivered the Geological Society of London’s Distinguished Lecture Series at the universities of Belfast, Leeds and Edinburgh, under the title The Growth and Decay of Mountains.