Extension and gold mineralisation in the hanging walls of active convergent continental shear zones

Accepted Manuscript Extension and gold mineralisation in the hanging walls of active convergent continental shear zones Phaedra Upton, Dave Craw PII:

S0191-8141(13)00146-6

DOI:

10.1016/j.jsg.2013.08.004

Reference:

SG 2948

To appear in:

Journal of Structural Geology

Received Date: 14 February 2013 Revised Date:

7 August 2013

Accepted Date: 8 August 2013

Please cite this article as: Upton, P., Craw, D., Extension and gold mineralisation in the hanging walls of active convergent continental shear zones, Journal of Structural Geology (2013), doi: 10.1016/ j.jsg.2013.08.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Extension and gold mineralisation in the hanging walls of active convergent

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continental shear zones

3 Phaedra Uptona,* and Dave Crawb

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5704198

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[email protected]

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GNS Science, PO Box 30368, Lower Hutt, New Zealand, [email protected], +64 4

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Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand,

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Abstract: Orogenic gold-bearing quartz veins form in mountain belts adjacent to

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convergent tectonic boundaries. The vein systems, hosted in extensional structures

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within compressively deformed rocks, are a widespread feature of these orogens. In

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many cases the extensional structures that host gold-bearing veins have been

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superimposed on, and locally controlled by, compressional structures formed within

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the convergent orogen. Exploring these observations within the context of a three-

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dimensional mechanical model allows prediction of mechanisms and locations of

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extensional zones within convergent orogens.

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convergence angle and mid-crustal strength on stress states and compare them to the

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Southern Alps and Taiwan. The dilatation zones coincide with the highest mountains,

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in the hanging walls of major plate boundary faults, and can extend as deep as the

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brittle-ductile transition. Extensional deformation is favoured in the topographic

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divide region of oblique orogens with mid-lower crustal rheology that promotes

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localisation rather than diffuse deformation.

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influences the stress state to a depth approximately equal to the topographic relief,

Our models explore the effect of

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In the near surface, topography

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bringing the rock closer to failure and rotating

to near vertical. The distribution of

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gold-bearing extensional veins may indicate the general position of the topographic

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divide within exhumed ancient orogens.

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29 Keywords: gold; structure; extension; tectonics; stress field; convergent orogen

31 1. Introduction

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Orogenic gold deposits have formed in convergent orogens throughout geological

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time, and are still forming in modern orogens (Bierlein and Crowe, 2000; Craw et al.,

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2002; Goldfarb et al., 2005; Groves et al., 1998; Groves et al., 2003). These gold

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deposits are hosted in a wide range of lithologies in primarily greenschist facies

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metamorphic rocks that have undergone compressional deformation (Bierlein and

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Crowe, 2000; Goldfarb et al., 2005; Groves et al., 1998; Groves et al., 2003). Most of

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these orogenic gold deposits are dominated by quartz veins that have filled dilational

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structural sites in compressively-deformed host rocks (Allibone et al., 2002; Cox et

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al., 1995; Kontak and Horne, 2010; MacKenzie et al., 2008; Sibson et al., 1988; Witt

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and Vanderhor, 1998). Many of these extensional structures formed locally during

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compressional deformation (Cox et al., 1995; Goldfarb et al., 2005; Witt and

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Vanderhor, 1998). In addition, structures that form in a regional extensional stress

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regime, within regionally compressively-deformed rocks, are widespread features in

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orogenic gold deposits (Bierlein and Crowe, 2000; Goldfarb et al., 2005; Vielreicher

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et al., 2010; Witt and Vanderhor, 1998). These structures include normal faults and

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steeply dipping extensional vein arrays, with dilational openings on the centimetre to

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metre scale, and the structures commonly post-date, and overprint, compressional

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structures.

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This common occurrence of regional-scale superimposition of structures

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formed in an extensional stress regime on compressionally-deformed rocks in

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orogenic gold systems does not require changes in tectonic plate vectors, although

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plate tectonic vectors have apparently changed in some ancient settings (Allibone et

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al., 2002; Goldfarb et al., 1991). Instead, on-going transfer of rocks from a

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compressional regime to an extensional regime within the same orogen (Craw et al.,

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2002; Koons, 1995; Koons et al., 1998; Phillips and Powell, 2009; Upton et al.,

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2009a) can achieve the same end result. This process is different from the more

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localised changes in stress orientation at the local scale (metres to hundreds of metres)

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including variations in the interactions between different lithologies and structures

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(Cox et al., 1995; Upton et al., 2008; Witt and Vanderhor, 1998), or from variations in

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fluid pressures (Ridley, 1993; Sibson et al., 1988).

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Transfer of rocks from a compressional stress regime to an extensional stress

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regime without a change in far-field plate vector direction can be difficult to infer

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from ancient orogens, as evidence for the plate boundary geometry and motions are

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typically poorly recorded. However, direct observation of active orogens can provide

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information on the structural evolution of mineralised rocks in the context of directly-

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observable plate vectors and stress regimes. In this paper, we present observations

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from two active convergent orogens, Taiwan and New Zealand (Fig. 1), where gold

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deposits are forming in extensional sites in the hanging walls of major compressional

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plate boundary structures. We present these observations in the context of a numerical

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model that reflects the same types of deformation that are occurring in our examples,

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to show where and how extensional zones form to facilitate fluid flow and gold

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mineralisation, and the geometry of rock motion that results in transfer of rocks from

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one stress regime to another.

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2. General settings Both New Zealand and Taiwan (Fig. 1) are oblique convergent tectonic zones on the

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margin of the Pacific Ocean (Wu et al., 2007). The Southern Alps of southern New

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Zealand are forming in the hanging wall of the Alpine Fault, a major transcurrent

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structure that forms the plate boundary through pre-existing Paleozoic-Mesozoic

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continental crust (Fig. 1A,B,C) (Norris et al., 1990; Walcott, 1986). Taiwan is a

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young orogen constructed from Cenozoic marine sediments that lay on the Chinese

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continental margin before the current collision with the Luzon volcanic arc, with a

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component of pre-existing metamorphic crust at depth (Byrne et al., 2013; Fisher et

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al., 2002; Ho, 1986; Suppe, 1981). The main mountain ranges of Taiwan are in the

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hanging wall of the Longitudinal Valley Fault that forms the on-land plate boundary

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(Fig. 1D,E,F).

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Rapid uplift of mountains in both New Zealand and Taiwan has resulted in

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regional-scale conductive thermal anomalies (Koons, 1989) that are driving meteoric

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hot-spring systems in regions of elevated thermal gradients (Allis, 1981; Upton et al.,

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2011). Deep-sourced fluids, derived from progressive dehydration of metamorphic

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rocks beneath the orogens, rise beneath the mountains to contribute to the tectonic-

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hydrothermal systems, and mix with circulating meteoric waters (Bertrand et al.,

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2009; Upton et al., 2003; Wannamaker et al., 2002). The tectonic-hydrothermal

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system in Taiwan is distributed right across the island (Chen, 1985; Lee and Cheng,

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1986), whereas that in the Southern Alps occurs primarily beneath, and to the west of,

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the topographic divide (Allis, 1981; Barnes et al., 1978; Upton et al., 2011).

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2.1.

Extensional veins and gold deposits

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Gold deposits associated with active transpressional tectonism are small in both New

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Zealand and Taiwan, and most deposits have no known economic significance. Minor

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historic gold mining has occurred in one of these deposits in Taiwan (Tan et al., 1991)

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and in several in New Zealand (Becker et al., 2000; Craw et al., 2010; Craw et al.,

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2006b). Placer gold derived by erosion of these small but numerous vein deposits

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forms the most significant resource, and beach placers in particular have been mined

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extensively in New Zealand and locally in Taiwan (Craw et al., 1999; Tan et al.,

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1991; Williams, 1974). The large volumes of exhumed and eroded rock containing

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gold-bearing veins in the hanging wall of the plate boundary fault zones continue to

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contribute to those placer deposits (Craw et al., 1999).

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The hydrothermal vein systems that host gold deposits in the Southern Alps of

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New Zealand fill extensional fractures and faults that cut across variably folded

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metamorphic fabrics (Fig. 2A,B,C). The metamorphic fabrics are mostly Mesozoic in

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origin, with some Cenozoic overprint, and these metamorphic fabrics have been

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folded during Cenozoic deformation related to the rise of the Southern Alps (Craw et

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al., 2002). Folds are typically upright and shallow-plunging, with rounded hinges.

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Folds at the outcrop scale occur in swarms that are part of larger scale (1-3 km) fold

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zones that trend parallel or subparallel to the mountain chain axis. Extensional

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fractures, many of which host veins, occur in swarms with a range of orientations but

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fractures parallel to Cenozoic fold axial surfaces are particularly common (Fig. 2A).

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Most gold-bearing veins in extensional fractures strike north to northeast (Craw,

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2006). Normal faults that cut and deform the metamorphic fabric also host gold

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deposits, in well-defined veins (cm to m scale; Fig. 2C). Veins form at a range of

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depths, from near-surface to >6 km (Craw et al., 2002; Craw et al., 2009). Most of the

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Southern Alps gold deposits are characterised by distinctive ankeritic alteration of

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metamorphic chlorite in host rocks, and coarse grained ankerite in veins, with stable

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isotopic signatures implying variable mixtures of metamorphic and meteoric fluids

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(Fig. 2B,C; Craw et al., 2009). Gold-bearing veins in Taiwan occur immediately east of the topographic

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divide in Miocene sediments that have been metamorphosed and recrystallised during

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the collisional deformation that characterises the modern orogen (Craw et al., 2010;

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Upton et al., 2011). These auriferous veins were emplaced between 4 and 10 km

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depth, with the deeper veins forming under late metamorphic (greenschist facies)

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conditions (Craw et al., 2010). Extensional veins in and near gold-bearing systems in

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Taiwan are dominated by ankeritic quartz veins that cut greenschist facies

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metamorphic foliation and post-metamorphic folds of that foliation (Fig. 2D,E). Gold-

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bearing quartz-ankerite veins fill fold axial surface fractures of upright NE and ESE

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trending post-metamorphic kink folds, and shallow-dipping fractures that crosscut the

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metamorphic foliation. Historic mining focussed on these extensional features,

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especially where they intersected each other (Craw et al., 2010). Earlier generations of

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veins, emplaced during compressional deformation in the latter stages of

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metamorphism, are subparallel to foliation, and some of these veins contain gold as

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well (Craw et al., 2010).

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2.2.

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2.2.1. Tectonic extension zones

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Extension in the collisional orogens

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An examination of the 3D stress tensor and strain rates in Taiwan has previously

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been outlined in the context of the permeability structure at depth (Upton et al., 2011).

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In summary, Mouthereau et al. (2009) use earthquakes combined with geodetic

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measurements to invert for the 3D strain rate field and find that surface strain in the

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topographic divide region is extensional. They determine a maximum extension

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direction in central Taiwan ranging from NNE-SSW to NE-SW. Extension beneath

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the Central Range persists to depths of more than 15 km. Evidence for tectonic extension in the central Southern Alps is largely geological

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as a high resolution inversion of earthquake events similar to that for Taiwan does not

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exist at present. A small number of earthquakes with a normal fault solution are

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recorded in the central Southern Alps (Anderson et al., 1993; Leitner et al., 2001).

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Models of elastic deformation in the central Southern Alps based on geodetic

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measurements imply a NW-SE extension direction in the topographic divide region

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north of Lakes Pukaki and Tekapo (Beavan et al., 1999). The principal geological

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evidence for extension in the Southern Alps is the widespread occurrence of small-

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displacement normal faults and extensional quartz and carbonate filled extensional

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veins, in addition to the extensional gold-bearing veins, that cut the basement rocks

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throughout the mountains (Fig. 2A-C) (Cox et al., 1997; Craw et al., 2002; Craw et

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al., 2009).

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2.2.2. Topographic extension zones

In addition to the tectonically driven extensional processes described in the previous

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section, there is commonly an overprint of shallow extensional fractures associated

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with the steep topography of active mountains. The steep slopes facilitate mass

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movement of surficial zones, in variably-coherent landslides that are typically tens or

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even hundreds of metres thick (Beck, 1968). The headward parts of these landslides

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are commonly marked by abundant ridge rents (sackungen), or normal faults, with

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scarps 1-10 m high. The landslides can extend for kilometres downslope to adjacent

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axial valleys, with total vertical extent of ~2000 m. These landslides dominate most

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mountain hillsides in Taiwan, up to 3000 m above sea level (Upton et al., 2011).

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Landslides are also common on New Zealand mountains, especially those that were

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deglaciated in the early Holocene. The surficial landslides reflect overall extensional

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strain and topographic collapse of the steep mountain terrain (Koons and Kirby,

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2007). 3. Numerical Methods

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We use three-dimensional numerical models to explore the stress state and

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deformation within generic oblique orogens. Our approach is to use simple models to

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test the influence of various parameters on the dynamics of the model. We then

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compare our model results to structural data, including vein orientations, stress

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inversions and gold deposits, from the Southern Alps and Taiwan. Our models are a

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generic version of a previously published geometry and rheological description of the

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Southern Alps (Koons et al., 1998; Koons et al., 2012; Upton and Koons, 2007; Upton

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et al., 2009a) (Fig. 3A). Although the model geometry is based on the Southern Alps,

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the models are generic enough, representing the collision of a rigid block with a

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readily deformed crustal wedge, that we can also apply it them to Taiwan by changing

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the convergence direction (Suppe, 1980, 1981).

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Models were developed using the numerical code FLAC3D, a three-dimensional

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finite difference code (Version 4.0, Itasca, 2009) which we have modified to

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accommodate local erosion.

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geological problems ranging from plate-scale tectonics to consideration of material

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dynamics at the thin-section scale (Johnson et al., 2004; Koons et al., 1998; Oliver et

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al., 2006; Upton et al., 2009b). Our large scale model domain extends 500 km normal

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to the plate boundary (x) by 1000 km parallel to the plate boundary (y) by 50 km

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depth (z). We focus our analysis on a region in the centre of the model, at a distance

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from the boundaries, to ensure that the magnitudes and orientations we observe are

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not influenced by these boundary conditions. We use a two-layered crust overlying

FLAC3D has been used to model a wide range of

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an elastic mantle (Fig. 3). The western edge of the model consists of an elastic block

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which deforms minimally and represents a strong indentor (corresponding to the

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Australian plate in the case of the Southern Alps or the Luzon Arc in the case of

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Taiwan). The erosional regime of an orographic mountain range, similar to the

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Southern Alps, is simulated by maintaining the western slope at a constant elevation

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(Koons et al., 1998; Koons et al., 2012; Upton and Koons, 2007; Upton et al., 2009a).

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The models are run to represent snapshots in time.

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We vary the obliquity of collision and the mid-crustal strength to explore how

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these parameters influence the degree of extension predicted to occur during

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convergence. All model runs are listed in Table 2. Three model velocity conditions

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are used: (1) orthogonal collision, (2) oblique collision where vx = vy and (3) highly

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oblique collision where vy = 4vx. As the Mohr-Coulomb rheology is rate independent,

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the absolute magnitudes of the velocity boundary conditions do not influence the

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results. The boundary conditions are imposed on the edges of the model and the base

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(Fig. 3A). The velocity of the mantle simulates a subduction style boundary with

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crustal material being dragged down by the crust-mantle coupling at the base of the

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orogeny (Ellis et al., 2006; Upton and Koons, 2007). We use material properties

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identical to previous modelling efforts where we compared the different inherited

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rheological properties of the Canterbury and Otago regions of the Southern Alps

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regions (Table 2) (Koons et al., 2012; Upton and Koons, 2007; Upton et al., 2009a).

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Here we generalise those models to a strong or weak thermally activated mid-lower

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crust beneath a frictional upper crust (Fig. 3). The strength profiles are static initial

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conditions and are based on thermal equilibration of Mesozoic thinning (Canterbury)

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and thickening (Otago) of quartzofeldspathic crust (Upton et al., 2009a).

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In addition to generic models of oblique orogens, we build a topographic model of

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the central Southern Alps by placing a DEM smoothed to 500 m onto a numerical

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elasto/Mohr-Coulomb model crust. The material is modelled using a friction angle of

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30° and a cohesion of 2e7 Pa (Clark, 1966). Using this model, we are able to explore

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the influence of fine-scale (ridges and valleys) topography on the stress state.

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4. Model results

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4.1.

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Orthogonal convergence (Model 1)

The strong two-dimensional case, where

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conjugate shears uplifting a block of material between them (Koons, 1990). The

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horizontal component of velocity decreases across the shear zones which are regions

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of contractional deformation as shown by negative

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crust, between the conjugate shear zones, a small zone of extensional (positive)

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(Fig. 4A). In the upper

A weaker mid-crust produces more diffuse deformation and

negligible extension in the topographic divide region.

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occurs.

, produces a two-sided wedge with

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The stress state through most of the model domain is very simple with

parallel

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to the x-axis, with

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small region of extension does exist close to the surface the stress tensor will have

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rotated (Fig. 4A). The relative magnitudes of

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sectional area of the model domain, producing a relatively low differential stress for

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the strong model (Fig. 6A).

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parallel to the y-axis, with

4.2.

near vertical (Fig. 5A). Where the

and

are similar through the cross-

Oblique (1:1) convergence (Model 2)

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In this model convergence is oblique such that

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wedge is similar to the 2D model except that a higher velocity zone has developed

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beneath the topographic divide at depths of ~5-20 km (Fig. 4B), leaving a rock packet

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at the topographic divide that is moving to the left more slowly than the packets either

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. The resulting two-sided

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side of it. A weaker mid-crust again produces more diffuse deformation, no localised

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high velocity zone, and negligible topographic divide extension.

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The stress tensor has rotated relative to the 2D model and varies through the model domain.

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horizontal and

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deforming zone),

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However, beneath the developing topography, where the obliquity has increase the

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horizontal shear stress

,

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relative magnitude of

is similar to the 2D case but

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shear stress producing higher differential stress in the mid-crust (Fig. 6B). 4.3.

is sub-

is subvertical. In the outboard region (distal from the boundary and

is near vertical (crosses on Fig 5B).

becomes steeper as

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is near horizontal and

rotates away from vertical. The increases with increasing

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is still horizontal but has rotated ~25° north of east.

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Highly oblique (4:1) convergence (Model 3)

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In this model

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beneath the topographic divide region is more pronounced in the strong model and

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there is more significant extension developed beneath the topographic divide (Fig.

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4C). Extension in the weaker case is not as pronounced but it is more significant than

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it was in the corresponding weak orthogonal and oblique models (Fig. 4D,E,F).

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The stress tensor varies significantly though the model domain. In the outboard

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region,

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has rotated ~40° north of east from the 2D model,

is near horizontal and

is near vertical. Beneath the topographic divide, the stress tensor rotates such that

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, producing a highly oblique system. The higher velocity zone

is now sub-vertical and

is sub-horizontal (Fig. 5C). Differential stress in this

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model is much higher than in the previous models, a combination of

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magnitude and

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4.4.

reducing in

increasing (Fig. 6C).

Model of finer-scale topographic variations (Model 4)

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Koons and Kirby (2007) show that the surface state (topography) can influence crustal

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deformation, especially in regions of high relief. Topographic stresses, arising from

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changes in vertical loading (σzz) and changes in both vertical (τxz and τyz) and

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horizontal (τxy) shear stresses, are greatest near the free surface for the Southern Alps

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example (Koons and Kirby, 2007). We calculate the contribution of topographic stress to the present stress state.

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The topographic stress state is represented by a ratio of strength to stress which is an

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indication of the proximity of the rock mass to failure. The ratio varies from >10, far

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from failure, to 1 where the stresses match the strength and the rock mass is at failure.

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If failure occurs, this reduces the topographic stresses and the strength/stress ratio

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increases toward 10. Topography influences the stress state in the model to a depth

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extent approximately equal to the relief (Fig. 7A).

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through the topographic divide of the Southern Alps, material down to ~3 km b.s.l.

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has been brought closer to failure by the topographic stresses. The stress tensor is

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also rotated by the topography. In the near surface

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horizontal and their orientation varies significantly (Fig 7A insert).

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5. Discussion

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5.1.

is vertical,

and

are

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On the cross-section shown

Development of extensional zones

That extensional deformation is occurring in both Taiwan and the central Southern

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Alps is evident from geodetic measurements (Beavan et al., 1999; Hsu et al., 2009;

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Lin et al., 2010; Mouthereau et al., 2009; Wu et al., 2010), a small number of

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earthquakes with normal focal mechanisms (Anderson et al., 1993; Crespi et al., 1996;

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Leitner et al., 2001; Upton et al., 2011) and the presence of extensional fractures and

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dilational veins (Craw et al., 2010). Our models suggest that extension beneath the

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topographic divide, to depths below the brittle-ductile transition in some cases, is a

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predictable result of oblique convergence, especially when the mid-crust is strong

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enough to focus deformation into discrete shear zones rather than distributing strain

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throughout the deforming region (Fig. 4). A weaker mid-crust results in more diffuse

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deformation and negligible extension in the topographic divide region. Other factors may enhance the degree of extensional deformation that occurs.

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Upton and Koons (2007) show in models based specifically on the Southern Alps, that

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upper crustal NW-SE oriented dilatation is enhanced by the presence of elevated pore

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pressure at depth beneath the topographic divide (Wannamaker et al., 2002). The

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isotopic signature of Plio-Pleistocene mineralised veins in the topographic divide of

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the Southern Alps is dominated by a rock-exchanged signature, reflecting rock-

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buffered mineralising fluids (Becker et al., 2000; Cox et al., 1997; Koons et al., 1998).

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In those models, elevated pore pressure in the mid-lower crust acts to weaken the

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mid-crust and enhances the development of a higher velocity zone leading to material

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in the mid-crust moving toward the Australian plate more rapidly than the upper

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crustal material overlying it (Upton and Koons, 2007). The net result is extension in

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the x-direction between the nearly stationary material and the material to the west

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(Upton and Koons, 2007).

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Variation of the stress tensor through the oblique orogen

Our models allow us to track the orientation of the stress tensor through the orogeny

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as a function of obliquity (Fig. 8). As a rock moves through an orogen, the rock will

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move through one or more stress regimes depending on the trajectory of the rock and

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the degree of stress variability within the orogeny. Thus it should be expected to see

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exhumed rocks with evidence for superimposition of structures from different stress

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regimes through which they had passed.

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represent changing stress states they do not necessarily represent a change in the far

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field tectonic plate vectors (Platt and Rubie, 1987).

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Whilst these overprinting relationships

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In the simplest case, an orthogonal collision zone, where the transport direction is

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perpendicular to the boundary, the stress tensor is invariant (Fig. 5A) and exhumed

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rocks would be dominated by compressional structures with little evidence of

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extensional structures overprinting that compression. With increasing obliquity, the

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stress tensor rotates.

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deforming zones the two minor principal stresses rotate away from horizontal and

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vertical (Fig. 5B, E). In the highly oblique case, similar to that of the Southern Alps, and

rotates away from the transport direction and in the

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have rotated so much that they have flipped with

now associated with the

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horizontal axis and

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mechanics for a strike slip fault (Anderson, 1905). Comparison of these results to

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stresss inversions (Boese et al., 2012) and vein orientations from the Southern Alps

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(highly oblique) shows good agreement with the models (Fig. 8A, C). Comparison of

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model results to stress inversions (Hsu et al., 2009; Mouthereau et al., 2009; Wu et al.,

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2010) and vein orientations (Craw et al., 2010) from Taiwan (oblique) is more

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variable (Fig. 8B, D). Hsu et al. (2009) determine that σ1 has a plunge close to

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horizontal and trend which is consistent with the compressive stress regime of the

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collision zone. They find that σ2 and σ3 vary a lot across most of Taiwan (Fig. 8B)

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and that the stress regime of the Central Range is transitional to either thrust or

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normal faulting (Hsu et al., 2009; Fig. 7). Wu et al. (2010)also predict variable σ1 and

341

σ2 in the Central Range. The complexity in Taiwan is likely due to two factors.

342

Firstly, as an oblique orogen, the regionally controlled stress state is transitional

343

between convergent and strike slip, and is thus more likely to be influenced by local

344

variability. This means it is difficult to distinguish between region or first order

345

controls verses topographic effects which will extend to ~3 km b.s.l. as discussed

346

below.

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sub-vertical (Fig. 5C, F), consistent with Andersonian

Secondly, the stress state in the Central Ranges experienced significant

14

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perturbation as a result of the 1999 Chichi earthquake (Hsu et al., 2009; Lin et al.,

348

2010). Temporal variability in stress state is beyond the scope of the study presented

349

here but it likely to have contributed to the complexity of observations (Hsu et al.,

350

2009; Lin et al., 2010).

351

5.3.

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347

Superposition of topographic effects

In detail, vein orientations in both orogens suggest more complex stress orientations.

353

In Taiwan, many of the veins emplaced at shallow levels have steep dips and most

354

strike approximately NW-SE (Fig. 8D), implying that

355

(Upton et al., 2011). In the Southern Alps, gold bearing veins and partly-coeval

356

lamprophyre dykes show a range of orientations, but three stand out: EW, NW-SE

357

and NE-SW (Fig. 8D) (Cooper et al., 1987; Craw et al., 2006a). The regional stress

358

tensor, as measured by Boese et al. (2012) and calculated from our models predicts

359

relatively steep extension fractures and extension shears striking NW-SE,

360

perpendicular to

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is horizontal not vertical

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(Fig. 8B).

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352

In both cases, we suggest that the variability in the stress state we see is the near

362

surface influence of the considerable topography in these regions. Our model of the

363

topographic stresses in the central Southern Alps shows that topography can rotate the

364

stress tensor so that

365

determined by the regional topographic stresses. In general, given a vertical

366

orientation of

367

perpendicular to the strike of the mountain ranges.

368

topographic stresses produce steep extension fractures with a strike of NE-SW,

369

parallel to the main mountain chain. This extension orientation is consistent with the

370

orientations of some of the Haast Pass lamprophyre dikes and dipping sills and many

371

extensional veins in the central Southern Alps (Fig. 8D) that formed as the mountains

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361

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becomes vertical and the orientation of the minor stresses is

will tend to be along the strike of the mountains and

15

, the will be

In the Southern Alps case,

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developed progressively towards the NE since the Miocene (Cooper et al., 1987;

373

Craw and Campbell, 2004). Finer scale ridge and valley topography will be more

374

variable resulting in a complex array of orientations of near surface fractures and

375

shears. Near surface fractures and shears in Taiwan strike approximately parallel to

376

the relative plate-vector, implying

377

(Craw et al., 2010; Upton et al., 2011).

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372

oriented along the strike of mountain ranges

This topographically controlled extension controls the shallowest mineralised

379

zones. The rock mass close to the surface hosts hydrothermal fluid flow in fractures

380

with numerous open cavities (Fig. 7B,C). Large (cm scale) prismatic crystals of

381

quartz and adularia, and bladed calcite crystals, are common in these cavities (Fig.

382

7B,C), some of which have grown on pre-existing hydrothermal deposits (Fig. 7C).

383

Vein formation typically occurred between 200 and 300°C from meteoric water that

384

has had variable amounts of isotopic exchange with host rocks (Craw et al., 2010;

385

Jenkin et al., 1994). Co-existing liquid and vapour fluid inclusions in many of the

386

New Zealand examples attest to boiling during entrapment (Craw, 1997; Craw et al.,

387

2009). Likewise, the mineral assemblage quartz-bladed calcite-adularia is typical of

388

geothermal boiling zones (Simmons and Christenson, 1994). Boiling of these fluids

389

occurs at depths of <2 km, and possibly only a few hundred metres, below the

390

hydrological surface, where fluid temperature exceeded rock temperature (Craw,

391

1997). This hydrological surface may be near the top of a mountain ridge, but could

392

also be at or near valley level (Fig. 7D). Hence, the location of the boiling zone is

393

poorly defined but typically lies beneath the highest mountains (Craw, 1997; Craw et

394

al., 2009). These shallow extensional vein systems generally do not contain gold, but

395

some very shallow level (near-surface) gold deposition has occurred locally (Craw et

396

al., 2002).

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397

5.4.

Significance for ancient orogens

The extensional zones described in the Taiwan and Southern Alps convergent orogens

399

are being actively uplifted and eroded, and the gold deposited in them contributes to

400

nearby placers (Craw et al., 2010; Craw et al., 1999). Hence, individual veins have a

401

limited life within the basement rocks of the orogen, and are replaced by more of the

402

same as the convergence continues. When convergence ceases, and the locus of

403

tectonism changes because of plate boundary evolution, the last-formed vein systems

404

are preserved in the quiescent orogen until slower uplift and erosion over longer time

405

periods exposes veins in the deeper parts of the orogens (Craw et al., 2013). At this

406

stage, almost all evidence of the topography and tectonic processes of the active

407

orogens have been removed. Nevertheless, the distribution of zones of any remaining

408

gold-bearing extensional veins may give some clues to the general position of the

409

topographic divide within the orogen, as seen in the active orogens described above.

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398

At the outcrop or mine scale, extensional veins that fill fractures in

411

compressional structural elements, particularly fold axial surface fractures, are

412

widespread in metamorphic belts within ancient collisional orogens, and many of

413

these veins are gold-bearing (Bierlein and Crowe, 2000; Cox et al., 1995; Goldfarb et

414

al., 2005; Witt and Vanderhor, 1998) For example, numerous swarms of small

415

auriferous extensional veins hosted in Jurassic fold axial surfaces have provided

416

sources for the rich Yukon placers of Canada (MacKenzie et al., 2008) (Fig. 2F), and

417

gold-bearing veins were emplacement in fold axial surfaces in the late Mesozoic

418

collisional Zagros Orogen in Iran (Niroomand et al., 2011).

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410

419

On a larger scale, there is a strong association between gold mineralisation and

420

regional faults or shear zones, with the gold emplaced in minor structures adjacent to,

421

but not in, the regional structures (Goldfarb et al., 2005). Many of these gold-bearing

17

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structures are extensional, and have been superimposed on compressionally-deformed

423

host rocks (Bierlein and Crowe, 2000; Cox et al., 1995; Goldfarb et al., 2005; Witt

424

and Vanderhor, 1998). Rather than reflecting regional changes in tectonic regimes,

425

this structural superimposition may reflect ancient translation of host rocks into the

426

root zones of the extensional portions of the collisional orogens, similar to the

427

processes described in the active orogens in this study. However, this late extension-

428

hosted mineralisation is distinctly different from mineralisation that occurs in local

429

extensional sites during compressional deformation (Cox et al., 1995) or localised

430

extensional mineralisation associated with earthquake events (Cox and Ruming, 2004;

431

Micklethwaite et al., 2010).

432

Conclusions

433

The common occurrence of regional-scale superimposition of extensional structures

434

on compressionally-deformed rocks in ancient orogenic gold systems is often cited as

435

requiring a change in the tectonic plate vectors. While this has apparently occurred in

436

some ancient settings (Allibone et al., 2002; Goldfarb et al., 1991), it is not required.

437

We use two active oblique orogens, the Southern Alps of New Zealand, and Taiwan,

438

as examples of the on-going transfer of rocks from a compressional regime to an

439

extensional regime within the same orogeny.

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422

Extensional deformation within a convergent orogen develops beneath the

441

region of the topographic divide. Extension is favoured by a high degree of obliquity

442

and by a mid-lower crustal rheology that promotes localisation of shearing rather than

443

diffuse deformation.

444

observed in both Taiwan and the Southern Alps, the stress tensor must rotate from the

445

standard convergent situation where σ3 is vertical and σ3 must be small or

446

extensional.

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440

In order to produce the sub-vertical extensional structures

Increasing obliquity rotates the stress tensor and, by increasing the

18

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447

horizontal shear stress τxz, reduces the magnitude of σ3, thereby increasing the

448

differential stress. In the near surface, topographic stresses are superimposed onto the tectonic

450

stresses and in regions of high relief such as the Southern Alps and Taiwan, can have

451

a significant effect. In the central Southern Alps, material to depths of ~3 km b.s.l. is

452

brought closer to failure by the topographic stresses. The stress tensor is rotated so

453

that σ1 is vertical and the orientation of the vertical extensional shears that develop is

454

largely controlled by the local topography.

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449

With cessation of the convergence, the last-formed vein systems are preserved

456

in the quiescent orogeny while longer term erosion exposes veins from deeper in the

457

orogeny. By then almost all evidence of topography and tectonic processes will have

458

gone. Nevertheless, the distribution of any remaining extensional vein swarms may

459

point to the general position of the topographic divide. Similarly, igneous rocks may

460

be preferentially emplaced in the extensional zone, and their eroded roots form linear

461

intrusive belts in ancient orogens.

462

Acknowledgements

463

This research was funded by NZ Ministry for Business, Innovation and Employment,

464

and University of Otago. The authors contributions are as follows: PU and DC:

465

conceptual development of the study; PU: did the numerical modelling; PU and DC

466

wrote the manuscript. Discussions with Peter O Koons, Doug MacKenzie and Grant

467

Caldwell and reviews by Shaun Barker and one anonymous referee have helped to

468

develop the ideas expressed here.

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470 References

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705 Figure captions

707

Figure 1: A: Plate tectonic setting of New Zealand. B: Hillshade map of the South

708

Island of New Zealand showing the Southern Alps. Elevations range from 0 to

709

3754 m. C: Map of the South Island showing major active faults, relative plate

710

motion from De Mets et al. (1994), Mesozoic and late Cenozoic gold bearing

711

veins, hot springs, and the region of the Southern Alps where extensional

712

structures are found. D: Hillshade map of Taiwan. Elevations range from 0 to

713

3952 m. E: Map of Taiwan showing late Cenozoic gold bearing veins, hot

714

springs, and the region of the Central Ranges where extensional structures are

715

found. F: Plate tectonic setting of Taiwan.

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Figure 2: Photographs of extensional veins (Ex. vein), with summary sketches, in

717

folded rocks. FAS = fold axial surface, commonly with parallel joints or

718

fractures that host veins. A-C: Southern Alps, New Zealand. The FAS fractures

719

in the cliff face in A host extensional veins. D-E: Slate Range, Taiwan; veins in

720

E are gold-bearing and are exposed in an underground mine face cut

721

perpendicular to fold axial surface fractures. F: Jurassic extensional gold-

722

bearing veins formed in fold axial surface fractures, Klondike goldfield, Yukon,

723

Canada.

AC C

EP

TE D

716

724

Figure 3: A: Geometry and boundary conditions for the three-dimensional

725

mechanical model. Model is of a two layered crust which is subjected to three

726

different velocity boundary conditions as shown by the three arrows. Material

727

is pushed from the right and dragged along the base.

728

approximately horizontal except for an elevated brittle-ductile transition close to

729

the model plate boundary; the upper crust is represented by a pressure-

25

Crustal rheology is

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dependent rheology and the lower crust and mantle by a temperature-dependent

731

rheology. Approximately steady state elevations are maintained adjacent to the

732

plate boundary. Axes define reference frame used where y is parallel to the

733

plate boundary and x is normal to the plate boundary. B: Cross-section through

734

the centre of the model showing locations of stress tensor plots in figures 5 and

735

8.

Figure 4: Horizontal velocity component ( ) and its spatial derivative (

) for

SC

736

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730

the series of models described in Table 2. A: Strong crust with an orthogonal

738

boundary condition. B: Strong crust with an oblique boundary condition. C:

739

Strong crust with a highly oblique boundary condition. D: Weak crust with an

740

orthogonal boundary condition.

741

condition. F: Weak crust with a highly oblique boundary condition. See text

742

for more details.

E: Weak crust with an oblique boundary

Figure 5: A-C: Stereonets showing principal stress orientations at 14 points within

TE D

743

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737

the three models, all with strong rheology.

745

outboard region and circles are values from beneath the deforming region (Fig.

746

3B). See text for more details. D-F: Block diagrams showing model geometry,

747

boundary conditions and resultant rock trajectories.

748

principal stress directions. On E and F the plane containing

AC C

749

sectional view orientated perpendicular to

750

Figure 6: Stress magnitudes,

751

Orthogonal convergence.

752

Crosses are values from the

EP

744

,

and

Also shown are the and

is a cross-

. for the three strong models.

B: Oblique convergence.

A:

C: Highly oblique

convergence.

753

Figure 7: A: Model of topographic stresses in the central Southern Alps. Map view

754

contours are elevation in metres. Cross-section contours are the strength/stress

26

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755

ratio resulting from topography which is a measure of how close the rock mass

756

is to failure. Areas in blue are close to failure, areas in red are far from failure.

757

Purple band along the top shows regions that have failed in tension. Inset shows

758

sample model stress tensors close to the surface.

759

orientations of σ2 and σ3 depend on the nearly topography.

760

adularia (Ad) and quartz (Q) from a meteoric water boiling zone in the Southern

761

Alps. C: Prismatic quartz (Q) and bladed calcite (Ca) from a meteoric water

762

boiling zone in Taiwan Central Range. D: Sketch cross section through a

763

typical mountain range in an active collisional orogen (based on Southern Alps

764

and Taiwan Central Range). Meteoric and deep-sourced fluids mix beneath the

765

mountains via an extensional fracture network that is partly formed by

766

topographic collapse. Rising hot fluids boil in these fractures at depths (fluid

767

pressures) controlled by fracture connections to the surface at various

768

topographic levels, depositing mineral assemblages such as those shown in B

769

and C.

RI PT

σ1 is vertical while the

TE D

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SC

B: Euhedral

Figure 8: A: Model stress tensors for the highly oblique model rotated into a

771

Southern Alps reference frame for comparison with structural data from that

772

orogeny. Large open circles are stress tensors from the topographic model. The

773

maximum horizontal stress (MHS) determined by Boese et al. (2012) is also

775 776

AC C

774

EP

770

shown. B: Model stress tensors for the oblique model rotated into a Taiwan reference frame for comparison with structural data from that orogeny. C: Example stress tensor from Boese et al. (2012) from the central Southern Alps,

777

and rose diagrams showing vein and dyke orientations from the length of the

778

Southern Alps (after Cooper et al., 1987; Craw et al., 2006a).D: Extent of

779

regional extension in Taiwan as determined from geodetic measurements and

27

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780

strain directions from earthquakes (Mouthereau et al 2009).

781

tensors from Hsu et al. 2009.

Sample stress

782

AC C

EP

TE D

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783

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Table 1. Tectonic and geological settings of extensional zones and gold-bearing vein systems in the hanging wall of major crustal structures, Taiwan and South Island, New Zealand.

Movement sense Lithologies

Principal mountains Maximum altitude Exhumation rate Extensional zone

Location

Extension direction Gold-bearing veins

Distribution

TE D

Late metamorphic veins Extensional ankeritic veins

Fluid composition

Distribution Geological control Temperature Water source Heat source

AC C

Warm/hot springs

EP

Fluid source

New Zealand SI Pacific-Australia Oblique collision 70° from perpendicular 37 mm/year Miocene Alpine Fault

RI PT

Geology

Taiwan Eurasia-Phillipine Oblique collision 20° from perpendicular 88 mm/year Pliocene Longitudinal Valley Fault Sinistral reverse Cenozoic sediments, minor crystalline basement; outboard Pliocene- Recent molasse Central Range, including Slate Belt 3952 m >5 mm/year in mountains Immediately east (inboard) of topographic divide NE, high angle to convergence vector Focussed zones near topographic divide Shallow dip, subparallel to folded foliation Cut steeply across folded foliation, fill fold axial surface fractures

Dextral reverse Mesozoic basement; outboard Cenozoic cover & PlioceneRecent molasse

SC

Principal on-land structure

Plate boundary Motion Obliquity Rate Initiation Surface feature

Southern Alps

3754 m Up to 8 mm/year at plate boundary Centred on present topographic divide and paleo-divide NW, high angle to convergence vector Widely dispersed near topographic divide Irregular veins, abundant host rock alteration Cut steeply across folded foliation, fill fractures & normal faults Low-salinity rockexchanged fluid Metamorphic devolatilisation beneath mountains Near plate boundary Extensional fractures Max. 56°C Meteoric Rapid rock uplift

M AN U

Tectonics

Brines; rock-exchanged fluid Metamorphic devolatilisation beneath mountains Throughout mountains Extensional fractures Up to 99°C Meteoric Rapid rock uplift

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Table 2: Parameters, and material properties used in the numerical models Model name Orthogonal

Boundary condition vy = 0

Rheology

Results: extension

10

K = 1x10 Pa 9 G = 3x10 Pa ρ = 2800 kg m-3 φ = 30° coh = 2e7 Pa 1

6

1a

kσ = 1 x 10 – 1 x 10

1b 2

kσ = 1 x 10 – 6 x 10

5

6

M AN U 5

7

kσ = 1 x 10 – 6 x 10

yv = 4×vx

3a

K = 1x10 Pa 9 G = 3x10 Pa ρ = 2800 kg m-3 φ = 30° coh = 2e7 Pa 6

8

5

7

TE D

kσ = 1 x 10 – 6 x 10

10

Topographic stresses

K = 1x10 Pa 9 G = 3x10 Pa ρ = 2800 kg m-3 φ = 30° coh = 2e7 Pa

AC C

EP

Southern Alps topography

Minor extension to ~10km depth beneath topographic divide Negligible extension

10

kσ = 1 x 10 – 1 x 10

3b

4

8

kσ = 1 x 10 – 1 x 10

Highly oblique

Minor extension to ~9km depth beneath topographic divide. Negligible extension

SC

K = 1x10 Pa G = 3x109 Pa -3 ρ = 2800 kg m φ = 30° coh = 2e7 Pa

2a

2b 3

7

10

vx = vy

Oblique

8

RI PT

Model number 1

Significant extension to ~19 km depth beneath the topographic divide Minor extension to ~10 km depth beneath topographic divide Depends on local topography. σ1 near vertical, σ2 subparallel to mountain range, σ3 perpendicular to mountain range

K = bulk modulus, G = shear modulus, ρ = density, φ = friction angle, coh = cohesion.

Creep flow law is εɺ = Aσ e A is a pre-exponential factor, σ is the differential stress (MPa), n is the power-law exponent, Q is the activation energy, R the universal gas constant and T is temperature (K). A, Q and n are determined from extrapolation of laboratory creep experiments on wet synthetic quartzite (Paterson and Luan, 1990). A = 6.5x10-8 MPa-ns-1; Q = 135 KJ mol-1; n=3.1. In the lower crust the rheology is that of diabase, A = 2 x 10-4 MPa-ns-1; Q = 260 KJ mol-1; n = 3.4 (Shelton and Tullis, 1981). kσ is determined from the calculated differential stress, kσ = 0.5σ. 1

n

( − Q / RT )

1

AC C

EP

TE D

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A B

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D

FAS, veins

Foliation

Foliation

Ex. veins

Reverse faulted quartz veins

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10 cm

SC

C

Normal fault zone

TE D

Ex. veins

EP

E

AC C

Ex. vein

Foliation

Foliation

FAS

FAS

10 cm

FAS

Veins

n

tio

lia Fo

Foliation

Altered rock

Ex. vein

RI PT

Folded metamorphic veins

F

AC C

EP

TE D

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AC C

EP

TE D

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SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

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SC

RI PT

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AC C

EP

TE D

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AC C

EP

TE D

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Topographic divide extension is a predicable feature of oblique convergence

•

Folded rocks are laterally translated into the extensional regime

•

Extensional structures are overprinted on compressional structures

•

Gold is emplaced in extensional sites beneath the mountains

•

Vein orientations in the upper 3 km reflect both tectonic and topographic stresses

AC C

EP

TE D

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•