Structural Or Climatic Control in Granite Landforms? the Development of Sheet Structure, Foliation, Boudinage, and Related Features

Cadernos Lab. Xeolóxico de Laxe Coruña. 2010. Vol. 35, pp. 189 - 208 ISSN: 0213-4497

Structural or climatic control in granite landforms? The development of sheet structure, foliation, boudinage, and related features

VIDAL ROMANÍ, J.R.1 and TWIDALE, C.R.2

(1) Instituto Universitario de Geología (Universidad de A Coruña). Campus Universitario de Elviña. 15071- A Coruña. Email: [email protected] (2) School of Earth and Environmental Sciences, Geology and Geophysics, University of Adelaide, Adelaide, South Australia, 5005.

Abstract Granite landforms have been interpreted in terms of climatic geomorphology, or morphogenetic regions, but the field evidence overwhelmingly points to structural control. Some features are developed after the exposure of the granitic bodies, for joggling of the brittle crust continues and external agencies also achieve change but the origin of some forms can be traced to the emplacement of the granite bodies, and to strains and stresses developed in magmatic bodies during their intrusion. Various mineral, magmatic and magnetic fabrics are produced. The consolidation of the magma begins at the contact between the emplaced body and the host rock. At this stage, the marginal zone is already crystalline and brittle. Arguably, shearing consequent on continued emplacement causes deformation and the development of planar fractures, some aligned roughly parallel with the cooling and crystallisation surface developed in the uppermost zone of the intrusive body, others imposed by lateral stresses. Differential movements between the sheets produced by shear causes the development of stretching and/ or shortening movements. Extension produces a structural fabric that is later exploited by weathering and thus may contribute to the generation of such forms as pseudobedding, foliation, polygonal cracking, boudinage, and where deformation is pronounced, the formation of spheroidal cores within cubic or quadrangular blocks. The second type of fabric, due to shortening, generates folding or buckling of the previously defined planar structures and the formation of sheet structures. Both deformational signals, though of opposite sign, are a continuum in a close spatial relationship. This indicates a simultaneous or at least sequential development of the two types of planar fabrics at the end of the emplacement stage. Once the rock is at the land surface, it is affected by external processes, and the structural fabric determines the planes of ready water access in the rocky massif thus determining the progress of weathering and significantly influencing the evolution of granitic landscapes. Key words: climatic effects, structural control, sheet fracture, boudinage, polygonal cracking, imbrication. 190 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

BACKGROUND unusual pillars or pitons like those associated with the exploitation of prominent steep- Granite is a term that embraces plutonic ly-dipping and open fractures in the Organ holocrystalline rocks containing significant Mountains of New Mexico (e.g. SEAGER, amounts of quartz and potash feldspar but 1981), the Cathedral Rocks of the Yosemite which display variations in mineralogy and region in the Sierra Nevada of California hence chemistry, texture, fabric, and fracture (HUBER, 1987), The Needles of South Da- density and pattern. Granitic bodies vary kota (TWIDALE, 1971), and similar acicu- also in size and shape and in mode of em- lar forms reported from the Sierra Guadar- placement. Some authors (see WILHELMY, rama of central Spain, granite hills fall into 1958; MIGON, 2006) consider that granite three categories (figure 1). The basic form landscapes develop distinct characteristics is the domical bornhardt (BORNHARDT, according to climate and process, and that 1900; WILLIS, 1934), with its geometrical anomalous features reflect climatic change variations and regional names. It has been and landform inheritance. argued (TWIDALE, 1981) that some born- Alternatively, the morphological similarity of granitic landscapes can be explained in terms of structure, with the course and rate of weathering and erosion determined by the structural characteristics of the rock developed during the emplacement of the magma (VIDAL ROMANÍ, 2008) or as a result of later and continued brittle deformation, the latter including neotectonic features (e.g. TWIDALE and BOURNE, 2000, 2003; TWIDALE and VIDAL ROMANI, 2005). First, then, what is the status of morphogenetic theory as applied generally and outlined by such workers as PELTIER (1950), TRICART (1957), TRICART and CAILLEUX (1958), and BÜDEL (1977), and, in the context of granitic terrains, by WILHELMY (1958); and second, how do various structural factors affect granitic landform development?

CLIMATIC AND GRANITIC TERRAINS

Several morphological varieties of gra- Fig. 1. Cathedral Rocks, a pair of spires, or pitons, nitic hill have been recognised in the field and in monzonite, Yosemite National Park, California recorded in the literature. Leaving aside the (C. Wahrhaftig). CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 191

(a) (b)

Fig. 2. (a) Bornhardt, Pilbara Craton, Western Australia. (b) Castle koppie, Zimbabwe. (c) Kestor, Dart- moor, southwestern England. (d) Nubbin, northwest Queensland. hardts, many, perhaps most, of which are result of the disintegration into blocks and initiated by differential fracture-controlled boulders, partly in the subsurface, partly af- weathering in the subsurface at the weather- ter exposure, of the outer one or two shells ing front (FALCONER, 1911; MABBUTT, of these sheet structures. Evidence from 1961) while still located underground, are quarry exposures suggests that this process attacked by moisture-related processes in is initiated in the subsurface, though it con- the shallow subsurface to produce either tinues after exposure and most bornhardts castle koppies (the tors of Britain and parts display some surficial blocks and boulders. of Europe) or nubbins (figure 2). Imagine a domical mass of granite the Bornhardts have been reported from the crest of which is exposed as a low large-radi- midlatitude deserts and the tropical rain for- us dome or rock platform. It sheds water to ests, from Mediterranean lands and the cold the immediately adjacent plains underlain by higher latitudes. They are structural azonal regolith. The near-surface (the top 4–10 m?) forms. Their plan shape is determined by or- regolith is rich in biota and chemicals so that thogonal fracture systems. They are charac- the granite of the dome is altered. In cool teristically subdivided by arcuate fractures climates the water shed from the platform is into massive slabs known a sheet structures subject to freeze–thaw. Again, the weathering (DALE, 1923). Nubbins are formed as a front migrates laterally as well as vertically, 192 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

(a)

(b)

Fig. 3. (a) Stepped northwestern slope of Yarwondutta Rock, northwestern Eyre Peninsula, South Australia. (b) Multiple flared zones on the flanks of Castle Rock, a koppie located east of Albany southwestern Western Australia. and the areal extent of the residual is reduced southern Africa where the regolith has been and koppies are markedly smaller than the stripped by gullies and streams, or a tor in bornhardts from which they are derived. For southwestern England, exposed by nival, or this reason the French name for such koppies periglacial, processes. This is an example of is inselberg de poche. The erstwhile sloping convergent development with a similar form flank of the dome is converted into a steep, emerging as a result of the activity of differ- even vertical, wall. Like several bornhardts, ent processes. some are flared as a result of multiple epi- Nubbins or knolls (USA) are block- and sodes of weathering in the shallow subsurface boulder-strewn hills, but their derivation (figure 3). With further lowering of baselevel from bornhardts is suggested by the massive and landscape revival, the regolith is stripped domes that are in some instances visible be- and the bedrock mass is revealed as a steep- neath the blocky veneer. Nubbins are typical sided castellated hill, ‘about as big as a house’ of monsoon Australia (north and northwest (LINTON, 1953, p. 354): a castle koppie in Queensland, Darwin area, the Pilbara of CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 193

Western Australia, and of monsoon lands evidence in the Sahara, and in the Mojave elsewhere e.g. southern China and India. It (e.g. OBERLANDER, 1972). But granite can be argued that in the humid tropics the nubbins are found also outside the humid regolith charged with water, chemicals and tropics, in local wet sites. For instance, they biota is capable of attacking and breaking occur in arid central Australia just north of down even massive granite (e.g. CAMP- Alice Springs, and within the Macdonnell BELL and TWIDALE, 1995). Commonly, Ranges, and also at the Devils Marbles (fig- two or three shells are disintegrated, but in ure 4), located some 50 km south of Ten- north Queensland, for example, some quite nant Creek, a granite exposure intrusive large nubbins such as Black Mountain, near into a plunging anticline and flanked by the Cooktown, appear to consist entirely of quartzite ridges of the Davenport Range blocks and boulders. (TWIDALE, 1980). The ridges are bevelled Nubbins are found outside the humid and a ferricrete crust is preserved on one tropics in the Sahara, the Mojave Desert granite rise. The nubbins were formed as of the southwestern USA, and central Aus- result of differential weathering beneath a tralia. They are found also in semi-arid planation surface of possible Cretaceous Hausaland, or northern Nigeria (FAL- age (TWIDALE, 2007). A humid subtropi- CONER, 1911; BAIN, 1923). Some can cal climate prevailed during the Middle be explained as inherited from former pe- Tertiary (WOODBURNE, 1967) but in riods of humid climate of which there is any event both the Devils Marbles and the

Fig. 4. Devils Marbles, Northern Territory, a degraded nubbin with the inner domical core exposed, but with a few remnant boulders intact. 194 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

Fig. 5. Corestones in grus, Snowy Mountains, southeastern Australia.

Macdonnell Ranges occurrences are local are nevertheless widely distributed. They are wet sites produced by the ridge and valley also found developed in other lithologies. topography and the concentration of run- On the other hand, the structural off and groundwaters beneath valley floors. characteristics of granite find universal Such sites sustain the suggestion that nub- expression. bins are developed in humid environments and are modified bornhardts. Nubbins are DEVELOPMENT OF THE the only major granite form that is essen- STRUCTURAL FABRIC IN GRANITE tially restricted to a specific climatic zone. BODIES Of minor forms commonly found in granitic landscapes, tafoni are found in deserts, Granitic magma intrudes the crust by both hot and cold, and on some coasts, that exploiting weaknesses in the lithosphere is, wherever haloclasty prevails. But most or by stoping into the host rock to produce minor granite forms such as rock basins, stocks and batholiths, laccoliths, lapoliths gutters or Rillen, flared slopes, and pitting and phacoliths, dikes and diapirs. Here the are found in various and contrasted climatic consequences of intrusion are considered. environments – more common and better The initial movement of the magma is developed in some than in others, and of responsible for the development of vari- course rates of development vary, but they ous mineral, magmatic and magnetic fab- CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 195

rics (TRUBACˇ et al., 2009). However, weathering of compartments comprising cooling and crystallisation progressively numerous orthogonal blocks. change the physical state of the magma from its initial plastic state to a more and ARCUATE PARTINGS more rigid condition (ARZI, 1978; PET- FORD, 2003). The development of the Three types of arcuate parting, dis- structural fabric is essentially due to syn- posed parallel to fresh rock surfaces, occur intrusive deformation in the fragile field. in granite. Very thin scales, flakes or lami- Some authors favour the formation of the nae are developed both on exposed rock magma by fusion of the protolite in situ surfaces and on the interior walls of tafo- by shear heating without considering a ni. They occur seemingly wrapped around later mobilization of the melting (CHEN corestones (figure 6). Their formation can and GRAPES, 2007). Possibly because of be attributed to preferential weathering of lithostatic loading at depth, they do not the corners and edges of joint blocks in the refer to any discontinuities formed at this shallow subsurface (e.g. LARSEN, 1948; stage, but they imply the generation of a HUTTON et al., 1977; TWIDALE, 1986). structural fabric by shear deformation. Water infiltrating the rock reacts with mica Thus, it is likely that when crystallisation and feldspar to produce clays, which ex- is complete, traces of this deformation pand on taking in water causing lamina- episode remain and as the land surface is tion (figure 7). The laminae break down eroded lithostatic pressure decreases and to a granular mass of quartz feldspar and the zones of shear become fractures. clay. Most of the feldspar is converted to Most authors referring to CLOOS’s clay and eventually even the quartz is dis- ideas (1923, 1931) simplify the prob- solved so that the corestones are set in a lem and identify systems of orthogonal matrix of grus (figure 5). Thus, granite is fractures and sets of sheet partings as- converted to a clayey regolith as a result sociated with crustal stress (TWIDALE, of water attack in the shallow subsurface. 1982; THOMAS, 1994; VIDAL ROM- Water continues to play a significant role ANI and TWIDALE, 1998; MIGON, in alteration after exposure but in addition 2006; TWIDALE and VIDAL ROMANI, physical processes intrude in particular cir- 2005). Orthogonal systems are exploited cumstances – gelifraction in cold climates by weathering in the shallow subsurface and including haloclasty in arid lands and to produce corestone boulders (figure 5; on the coast, where tafoni appear (WIN- HASSENFRATZ, 1791; MacCULLOCH, KLER and SINGER, 1972; BRADLEY et 1814; BECHE, 1839, p. 450; SCRIVEN- al., 1978). Pressure release at the granular OR, 1913, 1931, pp. 364-365). In similar scale also possibly contributes to disinte- fashion the fractures defining very large gration (e.g. BAIN, 1931). Other, larger (ca one km diameter) orthogonal blocks scale partings are referred to as pseudobed- have been exploited to form bornhardts ding, pseudostratification, or flaggy joints (e.g. LISTER, 1987; CAMPBELL and that subdivide the rock into slabs a few cen- TWIDALE, 1991). Such forms have also timetres or a few tens of centimetres thick emerged as a result of the differential (figure 8). Most are discontinuous and all 196 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

Fig. 6. Lamination around weathered block in granite nubbin, Señor de la Peña, Anillaco, La Rioja, Argentina.

Fig. 7. Corestones with lamination, exposed in quarry on Karimun Is- land, western Indonesia. occur within a few metres of, and parallel commonly known as offloading joints. They to, the surface. They can be attributed to are well and widely developed in granitic the exploitation by various shallow subsur- rocks, including gneiss and migmatite but face weathering agencies, particularly the also in other massive rocks such as dacite, freeze–thaw action, of strain lines within rhyolite (cryptovolcanic facies: GONNER- the rock mass. MANN and MANGA, 2003; BACH- MANN et al., 2007), and sedimentary rocks SHEET FRACTURES AND like sandstone, conglomerate and limestone STRUCTURES (BRADLEY, 1963; TWIDALE, 1978, 2009; BOURNE and TWIDALE, 2003). Sheeting The third partings are the sheet fractures planes cut across other bedrock structures that define the thick slabs known as sheet including orthogonal systems, cleavage and structures (figure 9) that are, however, still foliation, crystal boundaries, rift and grain, CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 197

(a)

(b)

Fig. 8. Pseudobedding in granite (a) on Wattern Tor, Dartmoor, southwestern England, and (b) exposed in road cut- ting in the Adirondack Mountains, northern New York State flow structures and bedding. Many are in- being essentially horizontal on hill crests but truded by pegmatite, quartz veins or by dipping steeply on hillsides, and together aplite sills (figure 10). forming basinal structures. However, at a Sheet fractures are essentially continu- few but significant sites the arcuate fracture ous and of curvilinear geometry. They sub- sets display a synformal geometry beneath divide the rock into slabs from 50 cm to 10 hills and domes (VIDAL ROMANI et al., m or more thickness and have been observed 1995; TWIDALE et al, 1996). at depths of 100 m or more in quarries and The idea that the sheet structure is of en- tunnels. It is frequently claimed that the dogenous origin dates from well before the thickness of sheet structures increases sys- end of the Twentieth Century. MERRILL tematically with depth, but there are many (1897, p. 245) stated: “... with many geolo- exceptions. Most sheet partings run roughly gists these joints, in themselves, would be ac- parallel to the land surface (or vice versa), cepted as due to atmospheric action. In the 198 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

(a)

(b)

Fig. 9. Sheet structure (a) in granite, Pearson Island, Great Australian Bight, and (b) in sandstone Colorado Plateau (W.C. Bradley). CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 199

writer’s opinion they are, however, the result ture is common to different types of rocks of torsional stress and once existing are lines because all have suffered the same type of of weakness which become more and more compressive deformation. pronounced as weathering progresses. Some Thus, contrary to common belief encap- granitic residuals as bornhardts are invari- sulated in the term “offloading joint” (GIL- ably associated with sheet structure and that BERT, 1904), sheet fractures are of endog- give rise to the domed shape which is com- enous origin having developed at the end of monly accepted. According to MERRILL the intrusive magmatic stage when the mag- (1897, p. 245), the boss or dome-like form ma has already lost mobility as its crystal- of the bornhardts is “incidental and conse- lisation has finished (see VIDAL ROMANI quent” on internal structure. Several writers, et al., 1995). Like all fractures they are an including some of the earliest, consider that expression of diminished lithostatic pres- sheet jointing and related sheet structure are sure (CHAPMAN, 1956), but they are basi- due to the stresses imposed on magmas dur- cally tectonic. ing injection or emplacement and, hence, Other authors also have referred to the correspond to the shape of the original plu- endogenous origin of the fracturing in the ton. As mentioned, in granite some planar granite bodies but invoked and synintrusive fractures may be initiated during intrusion processes (see WATERS and KRAUSKO- but most result from horizontal stress (e.g. PF, 1941). They postulated that during em- DALE, 1923; VIDAL ROMANI et al., placement there are generated structural 1995; TWIDALE et al., 1996). Sheet struc- features due to brittle deformation (pro-

Fig. 10. Inverted sheeting in the aplitic sill, and both fracture and sill displaced by faulting, Joshua Tree National Monument, southern California 200 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

toclastic failure) of rock in the peripheral zones that will be affected by shear-induced fragmentation expressed as brittle fractures (ARZI, 1978; VIDAL ROMANÍ, 1990; GONNERMANN and MANGA, 2003). Obviously, however, such mechanisms can- not apply, for instance, to sheeting in sedimentary settings.

OTHER RELATED STRUCTURAL FEATURES

Planar structural fabrics developed in a magmatic body correspond to a continuum of deformation developed at the end of the emplacement stage and associated with the intrusive contact with the host rock (VIDAL ROMANÍ, 2008). In this stage sheeting is associated spatially and genetically to other types of planar structures as pseudobedding, polygonal cracking, boudinage, imbrication or overthrusting, and folding or buckling (TWIDALE, 1982; TWIDALE and VIDAL Fig. 11. Fault exposed in quarry west of Palmer, ROMANÍ, 2005; MIGON, 2006). All are Mount Lofty Ranges, with barrel-shaped corestones related to the deformation of the pluton by and tetrahedral corners exposed in adjacent granite blocks. planar shearing at the nearest contact with the host rock and exploitation by exogenous agencies (water–related weathering). quences regardless of any deformation. Though detailed analyses of the gen- The most characteristic landform of gran- esis of these structures in plutonic rocks is ite landscapes is the boulder or well-rounded uncommon (VIDAL ROMANÍ, 1990), the groups of boulders (compayrés: TWIDALE, deformation of jointed sedimentary rocks 1982). The morphology of such boulders can has been the subject of many treatises that be attributed to the more rapid weathering suggest analogies with intrusive fabric in (lamination, then granular disintegration of plutonic rocks (e.g. RAMSAY and HU- the corners of a joint block rather than its BER, 1987). From the structural point of edges and even more than the weathering on view the main difference between the two is plane faces: as MacCULLOCH (1814, p. 76) that in plutonic rocks the “bedding” (sheet put it: Nature mutat quadrata – they are ren- structure) must be developed before the oth- dered spherical by decomposition (see also er minor features noted can evolve (see e.g. BECHE, 1839, p. 450). But some corestones VIDAL ROMANÍ, 1990), whereas bedding are set in blocks no part of which has been is usually well developed in sedimentary se- weathered, for tetrahedral corners of fresh CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 201

(a) Fig. 12. (a) Corestones associated with mineral banding near Tooma Dam Snowy Mountains southeastern Australia, and (b) detail of Figure 12a.

rock abut the internal spherical mass (figure (b) 11). The three-dimensional geometry of the orthogonal systems determined the size and shape of the blocks attacked by waters pen- etrating along the partings. This pattern suggests the shearing of preexisting orthogonal blocks (TWIDALE, 1968, pp 96-101, 1982, pp. 114-116). Elsewhere, concentric patterns of minerals suggest that weathering may have been determined by patterns of circulation developed while the magma was still fluid or plastic (figure 12). The development of boudinage in granite as well as its relationship with the most advanced stage of deformation implies a spheroidal disjunction (VIDAL ROMANÍ, 2008). This type of structure has been interpreted as an alteration form when in fact it is a deformative structure. The three types of structures: sheet structure, boudinage, and spheroidal disjunction, are closely related forming a gradually increasing sequence of deformation developed in the fragile (or brittle) field. Boudinage in sedimentary or even metamorphic rocks, like the fabric of 202 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

an augen gneiss, determines weathering pat- all drawn boudins, the layers are deflected terns in detail, is explained in terms of the into the boudin neck in a characteristic ge- deformation of a layered structure formed ometry (GOSCOMBE et al., 2004). by the alternation of materials with differ- In some cases, the surfaces of sheets de- ent competence and where the layers of less velop stretching features (EYAL et al., 2006) competent lithology are disrupted into elon- that appear as sets of patterned cracks (see gated fragments. In some magmatic, pluton- RAMSAY and HUBER, 1987; VIDAL RO- ic, and cryptovolcanic rocks a striped tex- MANÍ, 1990; PLOTNIKOV, 1994). They ture is attributed to original compositional may be repeated in the surfaces of super- differences or to fluid structures defined in posed sheets. Similar features have been de- an early stage of the magmatic intrusion. It scribed in deformed sedimentary beds and later influences the course of weathering. called polygonal cracking or chocolate tab- In granitic rocks, boudinage follows the let structure (RAMSAY and HUBER, 1987, development of sheet structure. Thus, the using the same nomenclature for their equiv- differences in rheology between a relatively alent features in plutonic rocks). Polygonal rigid layer and a solid but highly strained cracking consists of a pattern of cracks, matrix are exclusively due to the different some as much as 5 cm wide, developed in a deformation grade. The rigid layer ruptures superficial shell or shells of rock (figure 13): or thins (‘necks’) normal to the stretching they extend to no more than a few centime- direction in the rock (RAMSAY and HU- tres beneath the surface of boulders, blocks BER, 1987). Because of this similarity and or sheets. They form orthogonal, rhomboi- the apparent link to foliated rocks, this type dal or polygonal patterns (though some are of structure is known as “foliation boudi- irregular or crazy, as in crazy-paving) defin- nage” (HAMBREY and MILNES, 1975; ing thin plates. The polygonal plates range in PLATT and VISSERS, 1980). diameter from 2 cm to some 24 cm, with the The examples of boudinage develop- average and mode both near the upper end ment in granite rocks do not appear in of the range. They are clearly of endogenous broad zones but in very limited layers with origin (LEONARD, 1929) because the later discrete development of sheeting. Consid- pegmatite or aplite dikes have frequently ering the intensive development of the fo- been intruded in them. In polygonal crack- liation, the deformation of a massif could ing they may separate the tiles of the mosaic have been extreme giving the rock a foli- (TWIDALE and VIDAL ROMANÍ, 2005; ated aspect. In the granite rock the boudins VIDAL ROMANÍ, 2008). are individualised in layers between 40 cm Such patterns have been observed on and 2 m thick and some metres long. They corestones recently exposed in a road-cut- develop by planar fracturing into parallel- ting in the Snowy Mountains of southeast- epipedic (rectangular in section) fragments ern Australia. They are variously attrib- (‘torn boudins’) or in the instance cited uted to insolation or chemical weathering case by necking and tapering into elongate (JOHNSON, 1927), freeze–thaw action depressions and swells known as ‘drawn involving soil moisture (TWIDALE, 1982), boudins’. The boudins are separated by cracking due to expansion of the outer fracture zones known as ‘boudin necks’. In layer of rock (SOSMAN, 1916; SCHUL- CAD. LAB. XEOL. LAXE 35 (2010) Structural or climatic control in granite landforms? 203

Fig. 13. Polygonal cracking and tafoni in granite boulder, The Granites, near Mt Magnet, Western Australia.

KE, 1973) and weathering of exposed associated with the generation of sheeting joint planes or desiccation and cracking of planes. This is comparable to the stress of duricrusts as a result of insolation (ROB- opposite contrasted stresses developed dur- INSON and WILLIAMS, 1989). ing the shearing of a cubic block (e.g. WEIS- SENBERG, 1947). SHORTENING STRUCTURE These different types of structural fabric can be taken to suggest a deformation se- Pseudoripples, described in deformed quence of increasing grade and different im- sedimentary rocks (RAMSAY and HUBER, pacts (stretching or shortening): pseudobed- 1987), also are found, though less frequently, ding, sheet fractures and structures, shear- in plutonic rocks (VIDAL ROMANÍ, 1990). ing with foliation and maximum stretch- It is a structure related to shortening planes. ing corresponding to polygonal cracking, The imbrication and triangular and laminar boudinage, and spheroidal disjunctions. wedges associated with the slippage of one Buckling, sheet structure, wedges, and over- sheet over another in a compressive (antifor- thrusting are associated with compression. mal) structure is similar (cf striae developed The formation of planar structures, caused on bedding planes as a result of slippage in- either by stretching or shortening, leads to duced by folding). another set of discontinuities one disposed All these effects indicate that in the final normal to the first, together forming radial stage of magmatic emplacement the rock patterns (CLOOS, 1923, 1931; BALK, 1937; undergoes both stretching or shortening, MIGON, 2006). Hence, the disintegration 204 ��Vidal Romaní ��and Twidale CAD. LAB. XEOL. LAXE 35 (2010)

of sheet structures into blocks is a result of between the final magmatic stage and the weathering, either in the shallow subsurface beginning of the post-magmatic stage, and or after exposure along fractures disposed hence the explanation of structures by ex- normal to the arcuate surfaces of the slabs. ternal agencies.

EPIGENE STAGE CONCLUSIONS

The development of forms in granitic The relief developed on granite rocks rocks has been understood as a two-stage may be of exogenous or endogenous origin. process: first, one of etching at the weath- Exogenous processes are directly related to ering front and a second stage of exposure the climate and contribute to the exposure due to the stripping of the regolithic cover of the granite rock. However, most granitic (TWIDALE, 2002). And this is broadly forms, major and minor, reflect the struc- correct, but the first etching stage may have ture of the rock developed over eons of time involved the exploitation of an earlier tec- from the stage of intrusion onwards. tonic, thermal, magmatic event (GAGNY ACKNOWLEDGEMENTS and COTTARD, 1980) which defined the structure of the rocks subjected to etching We thank the Autonomous Government (TWIDALE and VIDAL ROMANÍ, 1994, of Galicia and the University of Adelaide 2005). Ancient sedimentological events also for supporting the research on which this influence contemporary landform develop- paper is based. We also thank Professor M. ments (TWIDALE, 2005). The exposure Thomas (Stirling University, U.K.) for com- mechanism – by gravity, rivers, glaciers, the menting on the paper in early draft stage wind, waves – may be triggered by eustatic, and Professor Carlos Costa (San Luis Uni- isostatic-epeirogenic or tectonic processes versity, República Argentina) for his par- (either altogether or in isolation). The ex- ticular comments on the protoclastic stage posure of the erstwhile weathering front of magma evolution. We also thank Ana as a bedrock surface reflects the structural Martelli for the initial English translation of differences imposed on the rocky massif parts of the paper.

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