Bull Volcanol (!993) 55 : 155-165 Vol6i ology �9 Springer-Verlag 1993

Large-scale rheomorphic shear deformation in Miocene peralkaline ignimbrite E, Gran Canaria Philip T Leat 1. and Hans-Ulrich Schmincke 2

1 Institut fiir Mineralogie, Ruhr-Universit~t Bochum, W-4630 Bochum 1, F.R.G. 2 Abteilung Vulkanologie und Petrologie, GEOMAR, WischhofstraBe 1-3, W-2300 Kiel 14, F.R.G.

Received November 17, 1991/Accepted September 24, 1992

Abstract. The single ignimbrite cooling unit E (average thickness, 28 m; volume, ca. 30 km 3) forms the upper- most member of the Miocene Upper Mogfin Formation on Gran Canaria. It is strongly chemically zoned from basal, first-erupted comendite (peralkaline rhyolite) to late-erupted trachyte, and, apart from an upper trachy- tic zone, it is densely welded. E was emplaced onto a surface inclined ca. 2-5 ~ from the source caldera. De- tailed mapping of key Sections, up to 300 m long, ex- posed in barranco walls, ca. 10 km from the caldera margin, reveals structures that are interpreted to have been produced by rheomorphic deformation of the ig- nimbrite along shear zones. The shear zones formed within the lower-viscosity comenditic tuff. Extensional structures include mega-boudinage and 'decapitated se- quences' and compression resulted in sequence repeti- tion by overthrusting. Mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs (compaction, lateral creep, folding, vertical density-driven diapirism) cannot account for these fea- tures, which reflect lateral (post-compactional) rheo- morphic movement locally in excess of 800 m. We sug- gest the following sequence of events: emplacement of the several flow units; compactiol~, with little lateral movement; rheomorphic deformation. During and after compaction, layers of secondary porosity developed within the comenditic tuff, possibly where upward es- cape of gas was prevented by overlying, relatively im- permeable layers of densely compacted ignimbrite. These structurally weak layers of high porosity subse- quently acted as shear zones.

Key words: Ignimbrite - Canary Islands - Mog~in For- mation - Comendite - Trachyte - Rheomorphism - Shear zone

* Present address: British Antarctic Survey, High Cross, Mading- ley Road, Cambridge, CB3 0ET, U.K.

Introduction

Many densely welded ignimbrites contain features such as stretched pumices, folded foliations and ramp struc- tures formed during transport and deformation as slow- moving, non-particulate, plastic (rheomorphic) masses, before finally coming to rest (e.g. Schmincke and Swan- son 1967; Noble 1968; Chapin and Lowell 1979; Wright 1980; Wolff and Wright 1981). Apart from one pub- lished description of large scale intracaldera rheomor- phic folds (Hargrove and Sheridan 1984), the way rheo- morphic ignimbrites deform over a scale of hundreds of metres is poorly understood. This paper documents such large scale rheomorphic structures in a highly welded Miocene ignimbrite on Gran Canaria, Canary Islands. We argue that a major deformation process was the sli- ding of thick plugs of the welded tuff along detachment or shear zones, a process analogous to the development of gravitational nappes.

Geological set t ing

Ignimbrite E is the uppermost of about 15 widespread comenditic and pantelleritic ignimbrites and lavas that comprise the Miocene Mog~in Group on the volcanic ocean island of Gran Canaria (Schmincke 1969a, 1969b, 1976, 1990). Its eruption has been dated at 13.42+0.09 Ma by single grain 4~ analysis (vd Bogaard et al. 1988). The Mog~m ignimbrites were erupted from the ca. 20 km diameter Tejeda caldera at the approximate cen- tre of the island (Schmincke 1967, 1990). The Mogfin Group comprises a lower sequence of mostly subalkaline ignimbrites and lavas (members of the Lower and Mid- dle Mog~n formations), and an upper sequence of dom- inantly peralkaline ignimbrites (the Upper Mog~n For- mation), of which ignimbrite E is the youngest. All of the ignimbrites are strongly chemically zoned (Schmincke 1969a, 1969b, 1990; Freundt and Schmincke 1992). The extracaldera ignimbrites dip gen- erally 5 ~ away from the caldera margin. This inclination is interpreted to have been that of the emplacement sur-

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face. The only evidence of significant tectonic deforma- tion of the Mog~in Group is that of ring faulting during several stages of caldera collapse (Schmincke 1968). Erosion between successive ignimbrite emplacements was minimal. The Mog~in Group is conformably over- lain by up to 700 m of mildly silica-undersaturated tra- chyphonolitic ignimbrites and lavas of the Fataga Group.

Previous work

Schmincke and Swanson (1967) described several types of deformational flow structures within the Mog~in Group ignimbrites, and argued that they formed during the last stages of initial emplacement, when deflation and welding caused a large viscosity increase in the still- flowing pyroclastic suspension. Ragan and Sheridan (1972) suggested that the structures could have origi- nated during post-emplacement compaction, possibly in association with downslope creep. A similar view was expressed by Wolff and Wright (1981), who stressed the importance of post-compactional lateral flow. They pointed out the similarity of structures in the ignimbrites with those in deposits then considered to be rheomor- phic welded air-fall tufts, and, as a result, implied a sim- ilar process in their development, viz response to em- placement on sloping ground. Schmincke (1969a) recog- nised two distinct elements within ignimbrite E" a com- enditic part, which deformed plastically, and a later, trachytic part (ET), which was more brittle. He inter- preted the discontinuous distribution of ET and local enclosure of large tongues of ET in the comenditic part of the ignimbrite as the result of lateral burrowing of ET into the soft, unwelded comendite, combined with the downsagging o f ET and diapiric rise of comendite. Crisp (1984) suggested that large density-driven diapirs formed by gravitational sagging of the trachytic tuff ET into the welded, but still-fluid, comenditic tuff. Schmincke (1974) argued that thorough degassing of ig- nimbrite E was inhibited because dense welding of layers of the ignimbrite formed an impermeable barrier.

Ignimbrite E

Ignimbrite E (Fig. 1) is one of the most widespread ig- nimbrites on Gran Canaria. It is colour-banded into white, blue and red-brown zones and thus an important stratigraphic marker. Fiamme (flattened pumices) are present almost throughout, and it is poor in accidental lithic material. It is generally 15-35 m thick (average ca. 28 m). Its main outcrop in the south of Gran Canaria is ca. 185km 2, but erosional remnants at M. de las Carboneras and lower Barranco de Tirajana (Fig. 2) suggest that it may have originally covered most of the southern part of the island (825 km2), with an estimated volume of ca. 23 km 3. From its current outcrop it is rea- sonable to assume that a proportion of the ignimbrite flowed into the sea, and that its volume could have been possibly up to 50 km 3. Ignimbrite E forms an almost continuous apron radiating from the caldera margin,

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Fig. 1. A Generalized stratigraphy of ignimbrite E. It is divided into three compositionally and lithologically distinct flow unit groups: the lower comendite (EC), the middle mixed flow units (EM), and the upper, dominantly trachytic flow units (ET). Typ- ical spacing of flow units within each group shown by horizontal lines. B Proportions of pumices of different compositions within each flow unit group

and coming to rest on slopes of 2-5 ~ . It has no known associated fall-out tephra' layer.

The key to understanding the rheomorphic behaviour of ignimbrite E is that it is strongly chemically zoned (Fig. 3). The different chemical zones produced con- trasting rheologies which behaved differently during rheomorphic flow. The chemical zones can be recog- nized in outcrop, and their relationships distinguished in abundant exposures in barranco (steep-sided valley) walls, revealing large-scale structures. We distinguished outcrops of E showing either complex or simple defor- mation. Examples of complex deformation are de- scribed under 'complex rheomorphic structures', where- as those of simple deformation are described in the next section, as they bear on the eruptive stratigraphy. Ig- nimbrite E typically consists of three main zones, which can be traced with.varying thicknesses throughout the main outcrop area (Figs. 1 and 2). This stratigraphic succession is sufficiently widespread that there is little doubt that it records the gross eruptive sequence.

Lithofacies zonation and chemical composition

This geochemical summary is based on over 80 new ana- lyses of pumice clasts from ignimbrite E (Schmincke 1990; Leat and Schmincke 1990 and unpublished data). We use the term ignimbrite E to describe the whole coo- ling unit, and the terms EC, EM and ET to describe the lower comenditic, middle mixed and upper trachytic zones respectively. This terminology differs slightly from that of Schmincke (1969a, 1976) and Crisp (1984).

The lowermost, first-erupted zone 0EC) consists of > 3 flows of comendite, is everywhere densely welded,

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1 Fig. 2. a Map of SW Gran Canaria with locations of outcrops dis- cussed in the text (Mt. C, Montafia de las Carboneras). b Thick- nesses of flow unit groups in relatively non-deformed sequences of ignimbrite E (locations o f sections show in map; ornamentation of EC, EM and ET as in Fig. la). Differences in flow unit group thicknesses relate largely to topographic irregularities

even where only 4 m thick, and consequently, internal flow unit boundaries marked by abrupt changes in lithic clast abundance and fine-grained, non-foliated basal layers can only rarely be distinguished. Its base always consists of a densely welded ca. 15 cm thick vitrophyre commonly altered to yellow clay. Above, the ignimbrite is crystalline and fiamme-rich. It is mildly peralkaline (agpaitic index, A.I. = 1.09-+0.2) and analyses of pu- mices show that it is substantially homogeneous though with slight chemical zonafion (SIO2, 67.8-69.0 wt.O/0;

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Fig. 3. Summary geochemical plot of pumice compositions in ig- nimbrite E. The comendite is weakly chemically zoned, shown by variations in Zr abundances. The trachyte is strongly chemically zoned, shown by a large increase in Sr abundances in late-erupted trachytic pumices. Note the large chemical gap between comendite and trachyte compositions. L a b e l s o n a r r o w s represent agpaitic in- dex and MgO wt.%, respectively, at indicated Sr and Zr abun- dances

TiO2, 0.74-0.65 wt. %; Fe203 (T), 3.85-3.65 wt.%). Two contrasting lithofacies can be distinguished, one blue (grading to grey-green) with low porosity, and the other white (grading to pink and buff) and very porous. This porosity of the white lithofacies is considered to be sec- ondary due to interconnected pore spaces unrelated to original spaces between shards and within pumices. It post-dates welding and deformation (Schmincke 1974). The blue and white lithofacies normally form inter- leaved horizontal bands 2 cm-6 m (rarely up to 16 m) thick, which nearly everywhere parallel the foliation de- fined by flattened pumices. White bands commonly form the base and top of EC, with blue bands most common in the core. The different lithofacies reflect dif- ferent degassing during compaction and cooling of the ignimbrite, which resulted in different vapour-phase and groundmass crystallization; the blue lithofacies is char- acterized by fine grain size and groundmass alkali am- phibole; the white lithofacies by coarse grained high- temperature groundmass and vapour phase crystals, with aegirine as the dominant groundmass mafic phase (Schmincke 1974). No bulk chemical differences have been found between the two lithofacies (Schmincke 1990).

The trachytic zones (EM and ET) of ignimbrite E overly EC with a sharp flow boundary marking a signif- icant change in magmatic clast content. Flow unit boun- daries within EM and ET are marked by abrupt changes in clast size anti.relative proportions of comenditic and trachytic pumice clasts. The middle, mixed zone (EM) commonly comprises two flows (EM1 and EM2 in Fig. 1). They consist of approximately equal amounts of light-coloured comenditic and dark grey-black trachytic

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pumice. The comendite is identical in composition to that of EC. The trachyte has a relatively evolved, margi- nally peralkaline, low-Sr, low-Mg composition (Fig. 3). EM is everywhere welded or densely welded, with low porosities.

The uppermost zone (ET) consists of many flows dominated by black pumices of a Mg- and Sr-rich, sub- alkaline trachyte, with few comendite and low-Sr tra- chyte pumice clasts (Figs. 1 and 3). It ranges from non- welded to densely welded, the non-welded tuff normally being restricted to the uppermost 7 m of the zone. The non-welded lapilli are cemented by vapour phase miner- als.

The three zones within the flow form a single cooling unit. A large compositional gap exists between the com- endite and the trachyte (Fig. 3). The overall progressive eruption of more Mg- and Fe-rich and more Si-poor magma, while simultaneously the most Si- (and Zr-) rich magma was available even in the youngest flow units (Fig. 1), is a predictable result of tapping a zoned mag- ma chamber from the top (Blake 1981).

The phenocryst assemblage of ignimbrite E was do- cumented in detail by Schmincke (1969a), Crisp (1984) and Crisp and Spera (1987). The ignimbrite is moderate- ly porphyritic, with platy anorthoclase feldspar up to 4 mm long as the dominant phenocryst phase. The com- endite (up to 18% phenocrysts) is more porphyritic than the trachyte. Crisp and Spera (1987) estimated Fe-Ti oxide phenocryst temperatures of 836 ~ C for the comen- dite, and 899~ for a trachytic composition.

we are right in our interpretation that these lithofacies formed during the compaction, degassing and welding of EC, rheomorphic folding postdated compaction. Very rare folds in EC, defined by fiamme foliation, are overprinted by the lithofacies zonation, and therefore predate at least the final stages of compaction. These re- lationships suggest the following sequence of events for EC: (1) emplacement of several pyroclastic flow depos- its; (2) deflation and welding of these deposits during which some local lateral movement may have generated open folds in EC overprinted by colour and porosity zonation, but such movement was insufficient to gener- ate a lineation in the basal vitrophyre before it chilled; (3) down-slope rheomorphic flow of the compacted and welded viscous mass. Unanswered questions include: was all of EC compacted and welded before EM and ET were emplaced ontop? How much was EC rheomor- phosed prior to EM and ET emplacement? Did some of the stretching and flattening of pumices take place with- in the moving pyroclastic flows, in response to aggluti- nation of the viscous pumice clasts, as thought by Schmincke and Swanson (1967)? If EC had not com- pletely welded when ET was emplaced, it is possible that ET locally ploughed into the non-welded top of EC, as proposed by Schmincke (1969a). However, some bodies of ET incorporated within EC contain several flow boundaries, locally cross-cutting welding foliation of ET. Such relationships imply that ET was deposited on the surface before incorporation into EC, and that de- position of ET flows was punctuated by time intervals.

Emplacement

The component flows of ignimbrite E must have been emplaced rapidly. Apart from the uppermost part of ET, it is densely welded throughout. The bases of EM flows are locally weakly chilled against underlying welded EC, and, where they were brought into contact during rheomorphism, ET is commonly chilled against EC (Fig. 4b). Trachytic matrix within EM and ET com- monly has chilled against pumices of comendite. These relationships suggest that the trachytic tuff was hotter than the comenditic tuff on emplacement, consistent with the differences in magmatic temperatures. How- ever, EC is also locally chilled against blocks of ET, the reverse relationship (Fig. 4c); so heat loss from EC be- fore emplacement of ET must have been small.

The basal vitrophyre has a foliation defined by fiamme, but no lineation. It is interpreted to have formed from material which was deposited, then com- pacted and chilled, with no phase of laminar rheomor- phic flow. The dominant fabric of the bulk of EC and EM is a foliation defined by fiamme (Fig. 4c), which locally increases in intensity, progressively upwards through the basal 40 cm. A weak-to-strong lineation, in- dicating laminar flow (Schmincke and Swanson 1967), and defined by stretching of pumices, is present throughout EC and EM. All folds within EC deform the foliation defined by fiamme; nearly all deform, and therefore postdate, the blue/white colour lithofacies. If

Complex rheomorphic structures

In many outcrops of ignimbrite E, the simple internal stratigraphy (Fig. 1) is absent and the ignimbrite has un- dergone complex deformation. Common structures in- clude: blocks of variably rotated ET and EM enclosed within deformed EC (Fig. 4b, d); EC at the top of the ignimbrite and overlying ET (Fig. 4a); diapir-like intru- sions and dykes of EC within ET (Fig. 4c); EM or ET at the base of the ignimbrite (Fig. 4a). Examples of these structures have been previously illustrated and described (Schmincke and Swanson 1967; Schmincke 1969a, 1990). The larger-scale structures described below re- quire a different explanation.

Competence contrasts between EC, EM and ET during rheomorphism

Contrasts between the rheologies of EC, EM and ET during rheomorphism were large. We should make it clear that the rheologies of concern are those of the welded tuffs, at the temperatures at which rheomorphic deformation took place, and with their appropriate po- rosities. These rheologies cannot be predicted from those of magmas of the same composition as postulated by Crisp (1984). The relative ease of plastic deformation of EC, EM and ET is reflected in the amount of strain shown by fiamme in the different zones, and in their

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contacts: we cannot quantify this ease of deformat ion in terms of viscosity.

EC clearly could deform plastically with the greatest ease. Enclosed f iamme are elongate discs, with maxi- m u m / m i n i m u m axis length ratios generally between 30 and 80. Alignment of long axes of f iamme normally de- fines a lineation. Where blocks of ET are enclosed with- in EC (Fig. 4b, d), the foliation of EC is contact-paral- lel, whereas that of ET is cut by the contact. Dykes of EC, possessing contact-parallel foliation, 0.3-1 m thick, commonly cut virtually non-deformed ET. These rela- tionships indicate that EC was much less competent than ET. Boudinage of bands of blue lithofacies of EC within white suggest that the latter was less viscous (Fig. 4d). The ease of plastic deformat ion of EM was interme- diate between that o f EC and ET; lineation defined by

elongated f iamme is poorly developed, and weakly de- formed blocks of EM locally occur within highly de- formed EC, However, in contrast, EM is locally de- formed around brittle blocks of ET, and locally intrudes ET as diapir-like bodies (Fig. 4d), ET, in contact with EC or EM (Fig. 4b), always deformed in a relatively brittle fashion.

Structures formed by lateral rheomorphic deformation in shear zones

Mapping revealed large scale structures in ignimbrite E whose origin cannot be explained by processes such as laminar viscous flowage (Schmincke and Swanson 1967), static compact ion (Ragan and Sheridan 1972),

Fig. 5. Field sketch of a near-vertical road section illustrating an example of compressive lateral displacement in ignimbrite E. Lo- cation: Barranco de Perchel de Mogfin, 2.5 km NNW of Puerto de Mog~in, Stop 54a in Schmincke (1990). An upper (hanging wall) of

welded ET was emplaced over a lower (foot wall) of non-welded ET along a shear zone within EC and having lateral displacement of at least 20 m. Short lines indicate foliation defined by flattened fiamme

Transport Direction

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Ignimbrite F

Fig. 6. Map of near-vertical barranco wall showing an example of compressive lateral displacement in ignimbrite E. Location: 100 m below Presa de Chamoriscan, 7 km NNW of Maspalomas, Stop 27 in Schrnincke (f990). The gross horizontal layering of EC, EM and ET is not the original depositional stratigraphy, but a result of

rheomorphic deformation. The band of welded heterogeneous rock (WHI) is interpreted to have acted as a thrust plane, carrying an Upper unit of blocks of ET enclosed in welded EC over a lower unit of EM. Short lines indicate welding foliation in ET and EM. Minimum lateral displacement, 60 m

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Transport Direct ion

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Fig. 7. Map of near-vertical barranco wall illustrating lateral compressive displacement in ignimbrite E. Location: east side of Barranco de Chamoriscan, 1 km south of Presa de Chamoriscan. EM is a minor component and is included in the outcrop of ET. We explain the 'double decker' repeating sequence by lateral era-

placement of an upper layer of boudinaged ET within EC over a lower one, the two layers being separated by a detachment hori- zon. If. this interpretation is correct, the amount of lateral dis- p[acemem exceeds 120 m. The fault is one of a set of normal faults which roughly parallel the caldera margin

A OBSERVATION Recent ~ Transport Direction Ignimbrite F valley . . . .

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Fig. 8. Map of near-vertical barranco wall illustrating large-scale structures which may have been a result of extensional rheomor- phic movement along detachment surfaces. Location: west side of Barranco de Chamoriscan, facing that shown in Fig. 7. The gross

'multi-decker' structures dip downslope (in the transport direc- tion), and are interpreted, to be separated by detachment surfaces situated within EC. Ornamentation as in Fig. 6

folding (Wolff and Wright 1981) or diapirism (Crisp 1984). Figures 5-8 show these structures which could have been formed either by gravitational loading of ET into welded or non-welded EC (cf. soft-sediment defor- mation load casts, Allen 1982) or by lateral movement of the welded deposit (cf. soft-sediment deformation slumps, Allen 1982). It is unlikely that gravitational loading produced the structures because: (1) the length of rafts of ET are locally much greater than the deposit thickness (in load casts, fold wavelength is similar to bed thickness, e.g. Collinson and Thompson 1982 p. 138); (2) rafts o f ET locally have strong preferred orientations (Figs. 5, 7 and 8), whereas loading structure limbs are randomly orientated. Lateral movement explains the

length and preferred orientation of rafts of ET and is examined below.

Figure 5 shows a structure we interpret as a shear zone that has accommodated thrusting. Above this shear zone the normal stratigraphy of ignimbrite E is present; below it is nan-welded ET. The fact that welded ET in the hanging wall is now at a higher structural level than the non-welded ET in the foot wall suggests that thrusting took place after welding of ET. The thickness of EM in the hanging wall is close to its thickness in non-deformed successions in the area. It is moderately deformed, with max imum/min imum length ratios of f iamme about 10, and may be close to its deposited thickness. The thickness of EC does not represent the

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inferred deposited thickness as it contains very stretched fiamme, which have maximum/minimum length ratios of about 100. It follows that the shear zone developed within EC, just below EM. EC acted as a lubricant dur- ing thrusting. The shear zone dips toward the caldera, in the opposite direction to the topography. This is con- sistent with relative downslope movement of the hang- ing wall.

Figure 6 shows another structure we interpret as a shear zone, but which is more complex than that shown in Fig. 5: (1) the normal stratigraphic sequence is revers- ed; (2) in nearby outcrops, EC most commonly occurs at the base, implying that it was removed from the base of the sheet at the place shown in Fig. 6; (3) ET occurs as blocks enclosed within EC; (4) an additional compo- nent, a distinctive welded, heterogeneous ignimbrite (WHI) forms a 1-2 m thick layer between EC and EM. All parts of ignimbrite E (Fig. 6) are densely welded. The WHI consists of highly stretched and locally folded EC enclosing angular fragments (1-20 cm) of EM (main- ly clasts of welded tuff, but with some individual pu- mices). The main fabric in the WHi is an extreme con- tact-parallel foliation, locally with small folds, and indi- vidual fiamme are obscured. We interpret this rock as a shear zone along which EC, with enclosed blocks of ET, was thrusted over EM. Most of the deformation was ac- commodated by shearing at the base of EC, but frag- ments torn from underlying EM were incorporated into the shear zone. Above the shear zone, EC contains large blocks of ET whose three-dimensional shape is uncer- tain. They appear to have formed from an original con- tinuous layer, overlying EC, that was boudinaged. We do not know why EM forms the base of the ignimbrite in the outcrop shown in Fig. 6, but perhaps the base of EM is also a detachment surface, of the type illustrated in Fig. 8.

Figure 7 illustrates structures we interpret to result from stretching and compression. Blocks of ET form two layers within EC. The upper layer consists of blocks mostly 15-45 m long, which probably represent a boudi- naged layer. Locally vertically inclined welding foliation in ET in the upper layer indicates that the welding pre- dated the latest rheomorphic deformation. Our interpre- tation of the two layers (Fig. 7) is that the upper layer was emplaced over the lower one by overthrusting al- though a sub-horizontai shear zone, located within the central part of EC, cannot be distinguished owing to the highly stretched nature of fiamme. It is possible that no discrete shear zone exists and that deformation occurred more or less evenly through EC. If our interpretation is correct, the amount of lateral movement exceeded 120 m assuming transport was parallel to the outcrop.

In places relationships are extremely complex (Figs. 8, 4). Here, four layers of ET can be discriminated with- in EC, some discontinuous. The layers dip in the same direction as, but more steeply than, the regional gra- dient. One of the layers of ET forms the top of the ig- nimbrite in the right-hand side of the figure, but the base of the ignimbrite in the left-'hand side. Exposure of this layer is almost continuous along the outcrop. Our interpretation of this outcrop is that the layers of ET are

separated by downslope-dipping shear zones situated within layers of EC. Discrete shear zones have not been identified because EC contains very stretched fiamme to the point that strain ratios of fiamme cannot be mea- sured. Probably, shear was distributed throughout EC. A detachment surface must lie at the base of the ignim- brite where ET forms its base (Fig. 4a). The different layers show different amounts of brittle extension and boudinage of ET. This suggests that layers were stretched before being stacked. We cannot reconstruct the deformation history of this outcrop in detail, but suggest that the upper layers originated as a multiple stack of layers (similar to that in Fig. 7), which was then emplaced to its present position along the lowest detach- ment. However these features developed, they require lateral movement and cannot be the result of vertical density-driven diapirism of EC into ET. Rough balanc- ing of the section implies at least 800 m of lateral move- ment along shear zones, which represents about 10% of the distance to the caldera margin. This suggests that rheomorphic movement was a significant part of the to- tal distance travelled by the ignimbrite from its source, at least in this sector of the ignimbrite apron.

The possible mechanisms to produce the relationships described above are synthesized in Fig. 9. An essential point is that movement of a part of the ignimbrite along a detachment surface causes stretching at its uphill end and, at the same time, compression at its downhill end. Shear is locally accommodated on a surface where ET is emplaced to the base of E, but is probably normally dis- tributed through EC. The structures are similar to soft- sediment deformation slumps in which strain is locally accommodated along shear zones (Allen 1982).

Discussion

Hot ash deposits undergo considerable degassing, as manifested by the numerous fumaroles on top of the ash flow erupted in 1912 from Novarupta (Griggs 1922; Curtis 1968). These fumaroles, and the chimneys and fumarolic moulds on thick ignimbrites such as the Bishop Tuff (Sheridan 1970), indicate that gas can es- cape from thick, rather poorly welded deposits through discrete vertical channels. Nevertheless, observations of recent pyroclastic flow deposits, such as those at Mount Lamington and Mount St. Helens, indicate that hot, un- compacted ash remains for weeks after the flows come to rest (Taylor 1958; Wilson and Head 1981). As gas es- capes from pyroclastic deposits, the particle matrix be- comes more closely packed (Ragan and Sheridan 1972; Riehle 1973); in highest temperature deposits glass shards are distorted and weld into a contiguous glass matrix. Such compaction of pyroclastic deposits is thus an example of two phase flow (McKenzie 1987; Miller 1990) in that compaction and welding involves flow of the glassy shards as a high-viscosity phase. Efficient compaction of the tuff occurs if the gas can escape free- ly through the entire thickness of the deposit. Such es- cape may be inhibited if layers of the deposit weld suffi- ciently to achieve very low permeabilities, while underly-

A COMPACTION STRATIGRAPHY

163

B 'MEGA-BOUDINAGE' C DETACHMENT

deformation

)formation

D THRUSTING

'decapitated E sequence' double layers

ET emplaced to base

Fig. 9. Summary diagram, showing the main types of large-scale rheomorphic structures in ignimbrite E, and how compressive and extensional features may be linked

ing layers are still degassing (e.g. Schmincke 1974; Branney et al. 1992). The alternation of porous with non-porous layers in ignimbrite E, and other strongly welded ignimbrites, indicates gas was entrapped as the deposits compacted and cooled. If vertical migration of gas is inhibited by overlying welded layers, it is likely (1) that gas would move laterally in the compacting tuff in order to escape and (2) secondary porosity would devel- op. Evidence for lateral escape of gas during compac- tion has not been recorded in any welded tuff, but sec- ondary porosity in welded tufts is well-documented.

There are several types of secondary porosity in welded turfs (Schmincke 1974): 1. Large gas voids having sizes >ca. 1 cm. The gas voids are extended within the compaction foliation of

: the tuff. They are common in peralkaline ignimbrites, such as those in Gran Canaria (Schmincke 1974), where the voids commonly occur in pressure shadows around lithic clasts (Schmincke and Swanson 1967). 2. Spherical gas vesicles formed by revesiculation of formerly compacted glassy shards and lapilli. The phe- nomenon may be restricted to tuffs of peralkaline com- position. The growth of spherical vesicles indicates that the gas pressure within the compacting tuff was equal to or slightly greater than lithostatic, which would not have been the case if the overlying and surrounding tuff was permeable. 3. Porosity due to high-temperature devitrification of glassy material. In some Gran Canaria ignimbrites, in-

cluding ignimbrite E, pumices have a coarser grain size and better developed pore spaces than the matrix, sug- gesting that the pumices retained more dissolved vola- tiles up to devitrification.

Noble (1968) noted that the strong vertical variations in porosity in welded ruffs probably impart vertical var- iations in ease of plastic deformation (the porous zones being more easily deformed than the non-porous). We speculate that this may have been the case for EC. When the tuff began to move downhill rheomorphically, de- formation was probably strongly constrained to take place preferentially along the horizontally arranged zones of greater porosity (white lithofacies). Noble (1968) further suggested that layers of entrapped gas in welded ignimbrite might act similarly to fluids in thrusts, but provided no field evidence for such lateral movement in rheomorphic tuffs. Other authors have made similar suggestions. Leat (1985) suggested that structures in a Kenyan welded peralkaline ignimbrite de- veloped by local shearing at its base. Reedman et al. (1987) described the welded Pitt's Head Tuff which ex- perienced lateral sliding of large rafts of rheomorphic ignimbrite over distances of several tens of meters. Hen- ry et al. (1989) described the Barrel Springs ignimbrite, Texas, which consists of an upper zone of brecciated welded tuff, overlying an intensely stretched zone simi- lar to E C , which clearly accommodated deformation and may be considered as a shear zone. Kobberger and Schmincke (1990) argued that the development of a

164

shear zone near the base of ignimbrite D on Gran Canaria was an important phase in its deformational history. They calculated minimum shear of 5 m in the shear zone, using data from rotated clasts, but suggesfed that the true displacement was much greater, because of the effects of grain boundary sliding. None of these studies, however, anticipated the scale of rheomorphic movement on shear zones - up to 800 m - that we have described from ignimbrite E. The Pitt 's Head Tuff probably provides the closest analogy to ignimbrite E, in that parts of it are thought to have moved large dis- tances laterally during (and after) rheomorphic defor- mation, a n d also that it locally contains a well-develop- ed nodular facies, which probably represents silicifica- tion in zones of secondary porosity. In their description of this tuff, Reedman et al. (1987) state that the nodules (silica-filled lithophysae) must have developed during compaction of the tuff, as welding foliation partially passes into, and partially deflects around the nodules. In addition they indicate that the zone of largest nodules occurs at the base of the most densely welded zone of the ignimbrite, as would be expected if they represent trapped gases. The Wall Mountain Tuff, Colorado (Chapin and Lowell 1979) may also have had a similar deformational history to ignimbrite E. This densely welded ignimbrite has internal unconformities, defined by cross-cutting foliations, thought by Chapin and Low- ell to be flow unit boundaries, which could be reinter- preted as shear zones. It also contains highly porous zones, thought by Chapin and Lowell to be gas pockets trapped during primary deposition, but which could be interpreted as zones of secondary porosity, which may have localized shear.

We suggest that large-scale lateral rheomorphic movement along shear zones is likely to occur in densely welded pyroclastic deposits that degas inefficiently dur- ing compaction. Welding of pyroclastic deposits during compaction can produce zones of well-degassed, com- pact tuff which prevent upward escape of gas from still- compacting zones below, and zones of secondary poros- ity are likely to develop. Subsequent rheomorphic defor- mation may be concentrated along these porous zones of structural weakness, leading to thrust-like deforma- tion.

Acknowledgements. This work was done while PTL held a Royal Society European Exchange Fellowship at Ruhr-Universit/~t Bo- chum, which is gratefully acknowledged. We thank members of the Arbeitsgruppe especially H Ferriz and G Kobberger for their support and enthusiasm. We are grateful to M Branney, H Ferriz, M Howells, G Kobberger and JA Wolff for making detailed crit- ical comments on the manuscript, part of the work was financially supported by a Leibniz grant from the Deutsche Forschungsge- meinschaft.

References

Allen JRL (1982) Sedimentary Structures, their Character and Physical Basis II. Elsevier, Amsterdam, pp 1-163

Blake S (1981) EruPtions from zoned magma chambers. J Geol Soc London 138:281-287

Bogaard vd P, Schmincke HU, Freundt A, Hall CM, York D (1988) Eruption ages and magma supply rates during the Mio- cene evolution of Gran Canaria. Naturwissenschaften 75:616- 617

Branney MJ, Kokelaar BP, McConnell BJ (1992) The Bad Step Tuff: a lava-like ignimbrite in a calc-alkaline piecemeal cal- dera, English Lake District. Bull Volcanol 54:187-199

Chapin CE, Lowell GR (1979) Primary and secondary flow struc- tures in ash-flow tuffs of the Gribbles Run palaeovalley, cen- tral Colorado. Geol Soc Am Spec Pap 180:137-154

Collinson JD, Thompson DB (1982) Sedimentary Structures. George Allen & Unwin, London, pp 1-207

Crisp JA (1984) The Mogan and Fataga Formations of Gran Canaria (Canary Islands): geochemistry, petrology, and com- positional zonation of the pyroclastic and lava flows; intensive thermodynamic variables within the magma chamber; and the depositional history of pyroclastic flow E/ET. Ph.D. thesis, Princeton Univ, pp 1-304

Crisp JA, Spera FJ (1987) Pyroclastic flows and lavas of the Mo- gan and Fataga formations, Tejeda Volcano, Gran Canaria, Canary Islands: mineral chemistry, intensive parameters, and magma chamber evolution. Contrib Mineral Petrol 96:503- 518

Curtis GH (1968) The stratigraphy of the ejecta from the 1912 eruption of Mount Katmai and Novarupta, Alaska. Geol Soc Am Mere 116:153-210

Freundt A, Schmincke H-U (1992) Mixing of rhyolite, trachyte and basalt magma erupted from a vertically and laterally zoned reservoir, composite flow P1, Gran Canaria. Contrib Mineral Petrol 112:1-19

Griggs RF (1922) The Valley of Ten Thousand Smokes (Alaska). Nat Geog Soc, Washington DC, pp 1-340

Hargrove HR, Sheridan MF (1984) Welded tufts deformed into megarheomorphic folds during collapse of the McDermitt cal- dera, Nevada-Oregon. J Geophys Res 89:8629-8638

Henry CD, Price JG, Parker DF, Wolff JA (1989) Mid-Tertiary silicic alkalic magmatism of Trans-Pecos Texas: rheomorphic tufts and extensive silicic lavas. New Mexico Bur Mines Min Res Mem 46: 231-274

Kobberger G, Schmincke HU (1990) Deformation history of rheo- morphic ignimbrite D (Gran Canaria): the role of shear zones. Abstracts, IAVCEI International Volcanological Congress, Mainz

Lear PT (1985) Interaction of a rheomorphic peralkaline ash-flow tuff and underlying deposits, Menengai volcano, Kenya. J Vol- canol Geothermal Res 26:131-145

Lear PT, Schmincke HU (1990) Emplacement of strongly welded ignimbrite E, Mogan Formation, Gran Canaria. Abstracts, IAVCEI International Volcanological Congress, Mainz

McKenzie DP (1987) The compaction of igneous and sedimentary rocks. J Geol Soc London 144:299-307

Miller TF (1990) A numerical model of volatile behavior in non- welded cooling pyroclastic deposits. J Geophys Res 95 : 19349- 19364

Noble DC (1968) Laminar viscous flowage structures in ash-flow tuffs from Gran Canaria, Canary Islands: a discussion. J Geol 76: 721-723

Ragan DM, Sheridan MF (1972) Compaction of the Bishop Tuff, California. Geol Soc Am Bull 83:95-106

Reedman AJ, Howells MF, Orton G, Campbell SDG (1987) The Pitts Head Tuff Formation: a subaerial to submarine welded ash-flow tuff of Ordovician age, North Wales. Geol Mag 124: 427-439

Riehle JR (1973) Calculated compaction profiles of rhyolitic ash- flow tuffs. Geol Soc Am Bull 84:2193-2216

Schmincke HU (1967) Cone sheet swarm, resurgence of Tejeda caldera, and the early geologic history of Gran Canaria. Bull Volcanol 31 : 153-162

Schmincke HU (1968) Faulting versus erosion and the reconstruc- tion of the mid-Miocene shield volcano of Gran Canaria. Geol Mitt 8 : 23-50

165

Schmincke HU (1969a) Petrologie der phonolithischen bis rhyoli- thischen Vulkanite auf Gran Canaria, Kanarische Inseln. Habilitationsschrift, Ruprecht-Karl-Universitfit, Heidelberg, pp 1-151

Schmincke HU (1969b) Ignimbrite sequence on Gran Canaria. Bull Volcanol 33 : 1199-1219

Schmincke HU (1974) Volcanological aspects of peralkaline silicic welded ash-flow tufts. Bull Volcanol 38 : 594-636

Schmincke HU (1976) The geology of the Canary Islands. In: Kunkel G (ed) Biogeography and Ecology of the Canary Is- lands. Dr W Junk BV Publishers, The Hague, pp 67-184

Schmincke HU (1990) Geological field guide, Gran Canaria, 4th edition. Pluto Press, Witten, pp 1-210

Schmincke HU, Swanson DA (1967) Laminar viscous flowage structures in ash-flow tufts from Gran Canaria, Canary Is- lands. J Geol 75 : 641-664

Sheridan MF (1970) Fumarolic mounds and ridges of the Bishop Tuff, California. Geo! Soc Am Bull 81:851-868

Taylor GA (1958) The 1951 eruption of Mount Lamington, Pa- pua. Aust Bur Miner Resour Geol Geophys Bull 38:1-117

Wilson L, Head JW (1981) Morphology and theology of pyroclas- tic flows and their deposits, and guidelines for future observa- tions. In Lipman PW, Mullineaux DR (eds) The 1980 erup- tions of Mount St. Helens, Washington. US Geol Surv Prof Pap 1250:513-524

Wolff JA, Wright JV (1981) Rheomorphism of welded tufts. J Volcanol Geotherm Res 10: 13-34

Wright JV (1980) Stratigraphy and geology of the welded air-fall tufts of Pantelleria, Italy. Geol Rundschau 69: 263-291

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