Reconciling short- and long-term measures of extension in continental back arcs: heat flux, crustal structure and rotations within central North Island, New Zealand
Published:January 01, 2009
T. A. Stern, 2009. "Reconciling short- and long-term measures of extension in continental back arcs: heat flux, crustal structure and rotations within central North Island, New Zealand", Extending a Continent: Architecture, Rheology and Heat Budget, U. Ring, B. Wernicke
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Geodetic, palaeomagnetic and andesite-age data all point to a fan-like opening of central North Island, New Zealand, in the past 5 Ma. Palaeomagnetic rotation rates for the eastern North Island are c. 6° Ma−1 and this is accompanied by back-arc extension of up to 19 mm a−1 at the Bay of Plenty coast and lesser values further south. Although the geodetic observations only span a decade, they show a remarkable consistency with the palaeomagnetic and volcanic arc migration data that span 5 Ma. A fan-like pattern of extension in central North Island is implied, similar to that seen in other continental back-arcs. When rapid fan-like openings do occur, they are likely to be accompanied by extreme thermal events. Heat flux from the central North Island occurs at one of the highest continental rates recorded: c. 26 MW per km of strike of volcanic zone, or 4.3 GW in total. Prior to the Pliocene extension and rotation, central North Island had a c. 20 Ma history of compression and overthrusting. It is proposed that thickening, then subsequent detachment of mantle lithosphere during this phase had a role to play in the Pliocene back-arc opening and the consequent extreme heat flux.
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Extending a Continent: Architecture, Rheology and Heat Budget
Over the last three decades, there has been a growing appreciation of the role of extensional tectonics in convergent orogens. The opening contribution to this book, by Brian Wernicke, provides a flavour of how this ‘detachment era’ has changed our views on tectonometamorphic relationships in mountain belts. This introduction provides a historical account on how our views on large-scale tectonic contacts in mountain belts have changed over the years. Wernicke concludes that controversy still persists over the existence and mechanics of slip on shallowly dipping extensional detachments, although incontrovertible field evidence indicates that slip on shallowly dipping extensional faults occurs in nature. Other papers in the volume provide a mix of new, innovative and controversial ideas that may help to solve the mechanical paradox on slip on shallowly dipping extensional detachments and quantitative case studies from New Zealand, the Aegean extensional province, the Alps and Finland.