Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

Abstract

The Blue Lake Fault Zone occurs at a major crustal boundary between Otago Schist and Torlesse greywacke. This zone has been active since the middle Cretaceous (c. 112 Ma), when extensional exhumation of the Otago Schist belt was initiated. The Blue Lake Fault Zone is dominated by a set of normal faults that have caused juxtaposition of rocks of different metamorphic grade, from prehnite-pumpellyite facies turbidites to pervasively recrystallised greenschist facies schists. This metamorphic transition has been thinned from c. 15 km to c. 2 km by normal fault motion on the scale of kilometres. This condensed section is the narrowest such section on the Otago Schist margin. The faults in this condensed section are defined by gouge zones that are hundreds of metres wide and constitute about 30% of the section. Gouge zones were locally reactivated as thin Late Cenozoic reverse faults on the centimetre to metre scale.

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“…2A;Mortimer 1993;Turnbull 2000;Forsyth 2001;Henne et al 2011). These normal faults are most prominent on the north-eastern flanks ( Fig.…”

“…These normal faults are most prominent on the north-eastern flanks ( Fig. 2A), where they have been responsible for the sharp decrease in crustal thickness towards Canterbury (Deckert et al 2002;Upton et al 2009;Henne et al 2011). This faulted zone is at least 50 km wide from the Waihemo Fault System to the Waitaki Fault System, with their inland extensions ( Fig.…”

“…2A). Most of these faults have been reactivated by subsequent events and some are still active, although movement senses and amounts of offset are variable (Forsyth 2001;Cox & Barrell 2007;Henne et al 2011). Some of the faults have been instrumental in the rise of late Cenozoic mountain ranges with 1Á2 km of offset, whereas others have less than 10 m of reactivation motion (Youngson et al 1998;Forsyth 2001;Cox & Barrell 2007;Henne et al 2011).…”

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“…1) that have no metamorphic fabric. The higher grade rocks of the schist belt were at least partially exhumed by Cretaceous normal faults that resulted in the juxtaposition of rocks of different metamorphic grades in a tectonically thinned metamorphic transition (Figs 1 and 2A, 2B;Deckert et al 2002;Gray & Foster 2004;Henne et al 2011).…”

“…The low-grade rocks are in the hanging wall of a major Cretaceous fault zone, the Blue Lake Fault Zone ( Fig. 2A; Forsyth 2001;Henne et al 2011). This fault zone has caused extreme structural thinning of the Otago Schist margin, and has juxtaposed blocks of differing metamorphic grade up to lower greenschist facies ( Fig.…”