TANE 22, 1976

GEOLOGY OF THE WHITIANGA GROUP, GREAT MERCURY ISLAND PART II- STRUCTURES IN THE MINDEN RHYOLITE OF SOUTHERN

G R E A T MERCURY ISLAND

by Bruno Zutelija Department of Geology, University of Auckland, Private Bag, Auckland

(Present address: Dept of Geology and Geophysics, University of California, Berkeley, California 94720, U.S.A.)

S U M M A R Y The southern part of Great Mercury Island is interpreted as being a single

rhyolite dome emplaced in three closely-spaced intrusive pulses. Steep flow layering implies that the bulk of the dome is endogenous. There are several occurences of gently-dipping, in places folded, flow layering at a structurally high level. Minor structures in these flow units are used to determine flow vectors.

I N T R O D U C T I O N Great Mercury Island is a volcanic complex of Miocene-Pleistocene age,

subdivisible into a northern block of Miocene-Pleistocene andesites, basalts, and rhyolitic pyroclastics, joined by a tombolo to a southern part of pyroclastics and rhyolites o f Plio-Pleistocene age (Schofield 1967; Skinner in press). The island has been mapped by Brothers (unpublished notes); petrology and supplementary mapping, involving some photogeological work, has been carried out by Skinner (in press). This study is concerned exclusively with the rhyolite of the southern half o f the island, and specifically with the structures found therein. Field work was carried out over five days in May 1975 during the A . U . F . C . scientific trip to the island. In this time only the coastal exposures were mapped, those inland being deeply weathered and quite difficult of access.

S T R U C T U R E Overall, the southern half of Great Mercury consists of a single dissected

rhyolite dome with some associated flows in contact to the north and west, with a sequence of rhyolitic pyroclastics in one place intruded by a basalt body (Hayward 1976). Within the rhyolite dome and flows, the structure is defined by a prominent flow banding, ranging in thickness from one or two centimetres to a metre or more. It is this banding that is mapped in Fig. 1.

It can be seen that (except for the area west of Whaler Bay) the layering is more or less sub-parallel to the coastline. The layering is generally steeply-dipping, often subvertical, with the dips being mostly landward. Shallow dips are encountered only in the Peach Grove Bay section, and in two small outcrops of rhyolite flow within the pyroclastics, at Pukekoromiko Point and at Tokatihi (Fig. 1). The steep layering outlines a dome centred in the vicinity of Mohi Mt . ; photogeologic lineaments mapped by Skinner (in press) cross-cut such a structure, though there are several arcuate lineaments which appear congruent to

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Fig. 1. Geology of the Southern Part of Great Mercury Island.

it. West of Whaler Bay there is a rapid swing in strike from the previous subeasterly strikes to a very consistent 150-180 direction, subperpendicular to the coast. This part of the structure is probably not connected with a separate dome, but represents a separate phase of emplacement from the main dome. The notably elongate headlands and associated lines of stacks west of Whaler Bay and of Bumper Cove, that show prominently on aerial photographs, may be peripheral dykes associated with the main dome, and submeridional strikes in this block may be associated with the dyke emplacement.

Small-scale structures Flow-folds ranging in size from a few centimetres to several metres occur

frequently in the Peach Grove — Waihi section, and intermittently elsewhere. Such folds may be used directly to determine relative flow vectors. Although these may not be simply related to overall flow directions, an average of several flow vectors taken from one stretch of outcrop may indicate a general flow bearing (Christiansen & Lipman 1966). This type of analysis has been attempted on the Peach Grove section. Peach Grove Bay - Motutaika Point: A marked swing in strike occurs in this section, a NNE-plunging antiform being traceable in the shore platform. The foliation attitude then backs around to a southeasterly strike. The cliffs are of massive rhyolite some metres thick, with intercalations of thinly foliated rhyolite, in one place folded into an asymmetric antiform verging seawards, its axis plunging 15 to 130 (Fig. 2). Further in towards Peach Grove the situation is more complex; here a 2-3m thick unfoliated rhyolite — the unit occurring below the fold mentioned above — dips gently to the southwest and overlies a jumbled and complexly folded mass of thinly foliated rock (Fig. 3). The bottom of this massive unit passes into a great dextral fold pair plunging 40 to the SW-SSW. The folds are similar in style, and have associated congruent drag-folds in their cores (Fig. 4). Peach Grove Bay: The rhyolites at the head of Peach Grove Bay are again foliated, dipping at 30 to the NW. At the mouth of the main stream a large recumbent antiform-synform pair, probably asymmetric seawards, is exposed (Fig. 5). The fold couple plunges 15 to 060; minor drag folds are congruent to the main structure, and the folds, being in units less thinly foliated than those above, are relatively open. They indicate flow movements directed seawards. Peach Grove Bay west to Waihi: Small-scale structures here are very complex. The folds are tight and axes trend about 100 and have a shallow westerly plunge, with indefinite asymmetry. Seawards, the layering strikes 100 and is subvertical, whilst further round the dips again become gentle, to the NW. The closure of this larger structure is not exposed, but if the smaller-scale folds are congruent, the system is a sinistral couple, asymmetric to the south, indicating flow from the north.

The structures in the finely foliated rhyolites to the west of this are much more regular. Foliation dips gently to the NW. There are several convergence features reminiscent of flame structures in soft-sediment deformation. Gentle asymmetric folds trending 075 and overturned to the south indicate flow from about 345.

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Waihi: A series o f recumbent asymmetric folds verging to the SE is exposed in rhyolites foliated in intervals 30cm thick. The asymmetries are consistent with flow from the N N W . The folds are generally open, unlike the similar folds in the

Fig. 2. Recumbent flow-fold, Peach Grove - Motutaika. Looking East

Fig. 3. Massive rhyolite overlying disrupted foliated unit, Peach Grove - Motutaika.

f'8"^' ^Trf" 6 "' m i " 0 r s t r u c t u r e s m asymmetric flow-folds, Peach Grove - Motutaika.

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Fig. 5. Congruent minor structures in asymmetric flow-fold, Peach Grove. Looking 060.

more finely laminated rhyolite elsewhere (Fig. 6). The flow bearings in the Peach Grove section are generally southerly, or

down valley. On a finer scale, flow vectors in the Peach Grove Bay - Motutaika sector have a more westerly bearing than those in the Peach Grove Bay — Waihi sector; there appears to have been a regime of convergent flow (Fig. 7).

Other minor structures Whaler Cove: Very finely-foliated, subvertically-dipping rhyolite occurs in the shore platform. The most intriguing structure is a 'mushroom' some 10cm high, reminiscent of water-expulsion load structures in soft sediments. Above is a vertically-plunging sinistrally asymmetric fold, decaying rapidly upwards as the laminae drape over the fold nose. The differential fold rates must have been high here to produce such a fold. High differential flow may also have produced local pressure gradients, allowing perhaps local evolution of volatiles or escape of trapped gases and formation of the 'mushroom'. This would more readily occur in subhorizontal layering; perhaps a later broad flexuring took place during extrusion, giving the present steep orientations. Oruaki: The rhyolite foliae are 10cm thick trending NNW. A t the top of the cliff a set o f quite open recumbent folds, plunging gently NW-NNW and verging east, are exposed. Further down the cl iff the foliation dips steeply to seaward. The folds occur in a gently-dipping sequence probably towards the top of a dome structure.

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0 Fig 2 Fold o Peach Grove x Peach Grove-Wairere

Fig. 7. Flow directions inferred in Peach Grove Section.

Blowhole: A tight asymmetric fold verging landwards occurs in very thinly foliated N E dipping rhyolites (Fig. 8). The fold tightens and decays rapidly upwards, as i f the differential flow rates had been very high. The flow vector here cannot be directly related to the movement of the whole rhyolite mass as it is diametrically opposite to any reasonable flow direction (but cf Christiansen & Lipman 1966).

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Fig. 8. Flow fold at Blowhole.

C O N C L U S I O N The overall structure of the southern part of Great Mercury Island is that

of a single rhyolite dome. This is intrusive on the north and west into rhyolitic pyroclastics. As rhyolite flows are intercalated with these at two localities, it is apparent that rhyolite emplacement took place in several distinct pulses. The pulse forming the main dome was probably the earliest, and was mainly an intrusive event, becoming exogenous only at high structural levels. Slightly later, or perhaps as an exogenous phase of this event, the emplacement of the folded, gently-dipping layering of the Peach Grove Bay section occurred. The contact between this structure and that of the main phase may be abrupt (Fig. 3) but it is more commonly gradational. The convergent flow vectors obtained at Peach Grove Bay indicate a degree of topographic control for this phase as there is no evidence of this being due to an erosional feature (the present coincidence of Peach Grove Stream surely being fortuitous) so that it is probable that the main rhyolite emplacement produced two 'sub-domes' or at least welts in the main dome. The last pulse of rhyolite emplacement is probably that represented by the structures west of Momona Bay. The rapid but regular transition to 'main' trends at Momona Bay indicates that this intrusive phase occurred only a little after the main event, perhaps as a peripherally associated dyke emplacement.

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This steeply-dipping sequence probably becomes extrusive into the two high-level lobate flows at Pukekoromiko and Tokatihi Points. Rhyoli t ic pyroclastics were then laid down on these flows, and presumably on the whole structure.

A C K N O W L E D G E M E N T S I wish to thank Bruce Hayward and Don MacFarlan for advice and

criticism during the preparation o f this manuscript, and Dr R . F . Heming and Dr K . B . Sporli for critically reading and revising the manuscript.

REFERENCES Christiansen, R.L. & Lipman, P.W. 1966: Emplacement and thermal history of a rhyolite

lava flow near Forty mile Canyon, Southern Nevada. Bull. Geol. Soc. Am. 77: 671-684.

Hayward, B.W. 1976: Geology of the Whitianga Group, Great Mercury Island - Part I. Coroglen Subgroup Stratigraphy. Tane 22 (this issue).

Schofield, J.C. 1967: "The Geological Map of New Zealand, 1:250 000. Sheet 3, Auckland". Department of Scientific & Industrial Research, Wellington.

Skinner, D.N.B. in press: "Geological Map of New Zealand, 1:63 360". Part Sheets N35, N36 and N39, and Sheet N40, Northern Coromandel.

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