Dike modes of the western continental margin of South America
ABSTRACT
This summary presents a regional compilation and mapping of dikes for the western margin of South America, covering an NS-distance of 4,000 kilometers and accounting for >22,000 features. The dikes are classified for estimated composition and placed in a structural framework regarding the magmatic arc axis. Mafic arc-parallel dikes, mostly from the Mesozoic, account for half of the dataset and indicate pronounced periods of tension along the continental border. Intermediate to dacite compositions define variable orientations, including newly identified centers marked by radial dikes, which suggest mainly neutral stress conditions during Cenozoic arc magmatism. Comparison of paleostress markers around world-class copper porphyry deposits, and other magmatic related deposits with associated dikes and veins, to a broader regional understanding of dike modes is important for placing these localized deposits into an appropriate tectonic setting. Dikes have largely remained under mapped and sampled, described, or dated by government agencies and in the geological literature addressing South America. The dikes examined here span from before the formation of the Andes, developed across varying tectonic conditions of alternating extension and contraction, and provide key markers in understanding structural patterns within a prolonged ocean-continent convergent plate boundary.
Keywords: dikes swarms, geometric classification, Andes, South America western margin.
INTRODUCTION
Dikes are a common manifestation during magmatism and yet remain extremely understudied and reported in the literature on Andean geology. Dikes are rarely indicated on government geological quadrangle maps at a scale of 1:100,000 or larger. On the other extreme, in certain examples, published maps indicate overly long sparse dikes that simply do not exist. Very few topical studies have directly addressed dike formation in the Andes, and most of these are limited in their study area size (e.g., Mégard et al., 1984; Heki et al., 1985; Bussell, 1988; Skarmeta, J. 1993; Lefort et al., 1999; Creixell et al., 2011). Fundamental questions addressed by regional mapping of dikes are how abundant are they in the Andes, what are their typical orientations, what are the main dike compositions, where do dike swarms occur, and what is the characteristic lengths of dikes? What do dikes indicate about the state of stress in the margin? What role did dikes play during regional transverse faulting and localization of major porphyry deposits? Developing a regional-scale dike database is the first step to addressing these questions.
It took 15 years of part-time work to conduct the financially unsupported regional mapping of 22,000 dikes; at first recording locations seen in the field, compiling the limited examples from mapping and publications, and then to a much greater extent mapped from aerial photographs using Google Earth. Dikes captured from previous studies account for 500 features in the database. Dike locations were mapped either directly from .kml markers placed in Google Earth or on georeferenced images. Google Earth image quality and resolution placed additional restrictions, with areas of poor coverage in the Andes still hindering the analysis, but they have made in the last several years great advancement with the availability of interpretable images. The dikes were all hand-drawn as polyline features in ESRI ArcMap shapefile format using the WGS84 decimal degree projection.
The present dataset covers from 5.1°S to 38.17°S, running from Piura, Perú to Los Angeles, Chile, and corresponds to about 4,000 km of the plate margin (Fig. 1). The latitudes examined had the least restrictions imposed by vegetation cover. Additional dike locations are certainly present farther south, but large ice sheets and fjords along with cloud cover generally disrupt the continuity of mappable geology by aerial photographs. A cluster of dikes are found at 51°S around the Torres de Paine Miocene laccolith, expressed in sets of EW, WNW, and NE strikes with subvertical to vertical dips. Abundant dikes are likely around the Fitz Roy intrusive bodies and clearly someday additional mapping will bring a great number of dikes from the southernmost portion of the magmatic arc into the database. There is a good correlation with the number of exposed dikes to the sectors of modern flat oceanic plate subduction where the most dikes are exposed between the northern, central, and southern volcanic zones. Glaciation and steep topography south of Santiago permit locating dikes in the regions between active volcanoes at higher elevations free of vegetation. Estimates of the number of dikes in the highland areas are also obscured by talus, moraines, and other young sedimentary debris lining canyons, which can account for upwards of 50% of the map area in some regions despite the rugged relief. Along the coastal region of Perú and certain expanses of the Atacama Desert major dune fields or regional Miocene pediments and ash-flow sheets cover dike bearing formations. The largest area without mappable or exposed dikes is covered by the Central Volcanic zone, encompassing much of western Bolivia, parts of southern Perú and northern Chile. From northernmost Peru up through Colombia, rainforest cover limits mapping dikes from aerial photographs. Likewise, sparse published data in Ecuador and Colombia have not reported much information on dikes.
Figure 1. Map showing areas of extent of studied dikes along the western margin of South America and figure locations of major areas discussed in this paper. The dike thicknesses are exaggerated or not to scale so that the features can be more easily seen at this map size.
The most directly observable dikes are those with high color contrast with their surrounding host rocks, thus mafic dikes in light-colored plutonic rock or felsic dikes in dark aspect volcanic rocks or sedimentary formations were the more noticeably tracked features. Significant time was spent examining imagery in the andesitic volcanic sections, finding similar composition dikes via their weathering characteristic and textural breaks where they intruded across bedding. Andesite dikes cutting andesite flows and layered breccia are located only in areas of high-resolution imagery where the textural breaks across bedding can be observed, including cooling joints in the dikes that otherwise have zero or very faint color contrast with the wall rock. Granitic dikes in similar composition plutons likewise remain greatly under-reported in this study. On other occasions new dike locations were observed in the field, and then once “calibrated” to their contrast and texture, their extent was then mapped using Google Earth.
In general rock compositions lend to the overall character of the dike, such as color and thickness, with more mafic, and therefore lower viscosity, dikes being thin whereas felsic dikes form thicker bodies (Wada, 1994). Dacitic dikes tend to weather out with positive relief and in addition have marked blocky cooling joints, which typically distinguish them texturally in areas where little color contrast is present. Dike thickness, along with color, was used in estimating the rock composition. Mafic, intermediate, and felsic terms were applied where there was no other composition indication. More specific rock compositions were used where information was available through field visits or in the published literature and government maps.
Dike compositions are highly dominated by the exposures of swarms along the Pacific flank of the Andes where there is the least amount of cover and greater depth of erosion than along the Neogene volcanic arc where relatively recent volcanic rocks provide covering deposits. The more abundant estimated rock types have the following counts: 10,702 mafic dikes, 2,675 basaltic andesite, 2,199 andesite, 1,147 intermediate dikes, 1,825 felsic dikes, 1,031 granitic, 507 dacitic, 343 rhyolite, and 1,045 unassigned. The majority of the dike swarms are characterized by a single intrusion type. Bimodal dike swarms found in highly extended terrains have not been found. Some segments of the continental margin do have mafic and felsic dikes following the same structural corridor, occurring in groups nearby one another, but probably represents very different times of emplacement. About 62 percent of the dikes studied are classified as being mafic. The viscosity of magmas may also have a relationship with the abundance of dikes for a particular composition. Mafic dikes tend to be very thin and abundant whereas dacite develops thick dikes and generally make clusters that are low in the number of dikes. Neogene dikes at the latitude of Santiago tend to be dominated by andesitic compositions whereas the Peruvian Cordillera favors dacite dikes and more commonly dacite plugs and domes fed by pipes instead of major dike swarms.
MODES OF DIKE EMPLACEMENT
Structural measurements reporting the strike of planar features are useful for relative comparison between varying orientations in small domains, however, what is of greater importance along active margins is the examined structure’s relative angle with respect to the principal stress fields instead of magnetic north. Geometric patterns along both subduction and rift margins tend towards margin-parallel oriented features, and make repeated orthogonal structural elements following the classical Anderson fault model (Anderson, 1951). Nakamura et al. (1977) adapted Anderson’s structural scheme, oriented the principal stress directions in orthogonal sense regarding magmatic arcs to explain the high-level stress-strain context of volcanoes. In the broadest sense the paleo-trench direction and continental margin geometry can be approximated by the axis of magmatism (or best-fit center line running parallel through the magmatic arc), although plate excursions do cause transitory variations, nonetheless, overall magmatism parallels the plate margin and trench (Dickinson, 1973). Dike sets in this study have been classified according to their respective orientation to this primary stress-strain controlling configuration compared to the direction of the magmatic arc, as diagrammed in Fig. 2. Most of the mapped dikes likely crop out at a deeper level of magmatism than conceptualized by Nakamura (1977), which may have included some additional variations from his model. The arc geometry is generally taken as the center line of batholiths and the median line through regional exposures of similar-aged volcanic units. The main dike classification categories used are arc-parallel, arc-perpendicular, arc-oblique, conjugate sets, and radial patterns (Fig. 3). Additional explanation of each of these categories is provided in the following sections. While more complex structure settings involving transpression/transtension are likely locally important, the data type from regional mapping cannot definitively interpret this condition due to the absence of outcrop-scale offset markers. Within the five–fold first pass classification system used in this study the arc-parallel dikes are the most abundant type, with the majority of these dikes falling into a mafic to intermediate compositions.
Figure 2. Geometric classification used in this study. The lower inset sketch follows an interpreted variation of dike elongation with respect to the principal stress axis from Nakamora (1977).
Figure 3. Chart showing the relative percentages of dike modes along the western margin of South America.
Arc-parallel dikes
Arc-parallel dikes align with the long axis of the magmatic arc, volcanic arc, or axis of the Coastal batholith of Chile and Peru. Arc-parallel oriented dikes account for 50 percent of the mapped dikes. In general, these dikes are dominantly older than the formation of the Andes, representing Mesozoic continental borderland processes during plate subduction forming marginal basins (Atherton and Webb, 1989; Hanson and Wilson, 1991; Vergara et al., 1995; Aguirre et al., 1999). Tensional stress fields may be driven by slab-hinge roll back (Uyeda and Kanamori, 1979; Royden and Husson, 2006), forming backarc tectonism and extension, and to a certain degree margin tension formed by contrasts of the continental leading-edge plate thickness and density (Whittaker et al., 1992).
Perhaps one of the oldest examples of dike swarms in the western South American continent comes from mixed composition intrusions cutting a Permian granodiorite to granite batholith in the San Juan province of Argentina (29.4°S). These dikes have not been directly dated. Here a near vertical dike swarm 30-km wide by 126-km long averages with an NNE strike (Fig. 4) and consists of >1,204 dikes crosscutting the Colanguil batholith. NS-striking subvertical dikes are granitic to rhyolite porphyries, whereas arc-perpendicular and radial dikes are andesitic (Llambias and Soto, 1990). The early arc-parallel diking here represents regional rifting and arc magmatism that accompanied the first major subduction along the margin (López-Garmundi et al., 1994) that essentially lies parallel to the Mesozoic and Cenozoic magmatic arcs.
Figure 4. Map showing the Colanguil dike swarm, Argentina. The black lines in this figure and all subsequent maps represent the locations of dikes. The Colanguil dike swarm is mainly NNE-striking which may be parallel to the overall Permian plutonic axis, however, the northernmost end has similar mafic dikes that define a radial pattern, most likely indicating two separate events. Many NS-striking dikes are either rhyolitic or granitic, but also include mafic dikes. The radial dikes are mainly andesitic.
The northernmost area in Fig. 4 shows a radial dike swarm cutting the Tabaquito pluton (Llambia and Soto, 1990). The southern part of the map in Figure 4 is dominated by an NNE-striking dike set. Similar striking dikes to those at Colanguil lie to the west in Chile, hosted in Permian plutonic rock with the swarm intersected by the La Serena-Paso Negra road (Route 41) at 70.249°W, 29.989°S. Between 29.78°S and 30.23°S a total of 582 NNE-striking mafic dikes have been mapped by aerial photograph interpretation. Furthermore, potentially genetically related dikes are present far south of Colanguil, to the west of Calingasta, Argentina at 69.705°W, 31.38°S. If all three areas of Permian-hosted NNE-striking dikes are related then the swarm would cover an area 90-km by 300-km in size and include a grand total of 1,998 dikes. These areas of NNE-striking dikes are discontinuous and correlated by strike and age of host rocks.
In Peru, the classic Santa Rosa dike swarm (11.25°S), first reported on in detail by Cobbing and Pitcher (1972), includes 2,410 basaltic andesite dikes in this present study, which is a number far greater than any previous published map of the swarm (Fig. 5). The main body of the Santa Rosa swarm measures 25-km at its widest and runs about 94-km long. The mean dike length is 463 meters, the longest dike in the swarm extends for 3.14 km. Dike widths are variable, averaging 1 to 2 meter in thickness, but some dikes can approach 7 to 9 meters in thickness. Typically, the better exposed dikes will pinch out along strike and comprise en echelon arrays or include complex relay structures with adjoining dikes. The swarm continues along much of the length of the Western Peruvian trough/Coastal batholith in Perú for a strike length of at least 725-km. The dikes are hosted in ~73 Ma Santa Rosa tonalite pluton, and along the northern end of the swarm they are cut out by a ~68 Ma pluton of the Huaral Centered complex near the town of Sayan (Pitcher and Bussell, 1985). K-Ar ages on dikes in the region by Cobbing et al. (1981) gave the following constraints on intrusion: 75 Ma (whole rock), 73 Ma (hornblende), 73.9 Ma (biotite) and 61.1 Ma (hornblende). Wilson (1975) reported a K-Ar age from a dike from the swarm at 72.5 Ma. Pitcher and Bussell (1985) illustrated dike deviations in the Santa Rosa swarm to represent perhaps point source perturbations to the stress field, however, in this compilation the dikes are more nearly parallel and maintain a pattern dominated by regional arc-perpendicular tension such that the dikes crosscut numerous units and remain in the arc-parallel pattern. Mapping the Santa Rosa dike swarm in detail has documented greater intensity of diking along the western side of the swarm. A structural explanation for the asymmetry of dike density across the swarm remains to be determined, as does an explanation for the swarm at Santa Rosa to obtain the greatest width as compared to the swarms in the Casma and Lima segments. Pitcher and Bussell (1985) emphasized local macro-petrographic relations, indicating syn-plutonic intrusion of some early dikes in this swarm. The boudinage of the dikes results in discontinuous features. In contrast, this current study finds the majority of the dikes in the swarm are rather laterally continuous (mean length of 463-m, with numerous segments approaching 1-km in length), indicating the swarm mostly was emplaced during brittle conditions when the host plutonic rock was solid.
Figure 5. Map of the Santa Rosa Dike swarm, Perú. Light gray areas are comprised of the Coastal batholith and dikes are indicated by thin black lines (the same pattern is used for the rest of figures throughout this paper). The dike swarm has asymmetrical intensity with the greatest dike density occurring along the western preserved margin of the swarm; this dense swarm is located NNW of Huaral.
Farther north in Peru, the Casma dike swarms define arc-parallel clusters that have apparent spaced-out distribution that in part are a function of broad alluvial valleys covering the rocks (Fig. 6). Between 7.6°S and 10.6°S 1,660 dikes were mapped. These dikes define a zone more or less 6-11 km wide, running discontinuously 150-km in length, that in terms of character appear related to the Santa Rosa dike swarm to the south. A major mafic dike swarm was reported in the region between Chimbote and Casma (Sanchez et al., 1995; their figures 4.6 and 4.7) with compositions described as being microdiorites to diabase that intruded into gabbro. North of Casma the dikes crosscut the Chimbote pluton dated by U-Pb method at 70.6 Ma (Mukasa, 1986). In a conflicting age estimate, Sanchez et al. (1995) placed the dike swarm age at about 95 to 102 Ma. In the Casma segment of the batholith age data are very sparse, but are generally thirty million years and older that the age reported by Mukasa (1986). The general character and composition of the dikes suggest an age of 70 Ma similar to that of the Santa Rosa dike swarm is the best estimate. The geochronology along much of the Peruvian dike swarms needs updating by modern methods.
Figure 6. Map showing dikes (thin black lines) in the segment near Casma, Perú. These dikes are mainly parallel the elongation direction of the batholith (light-gray shaded areas) which in turn lies parallel to the modern coastline and the offshore trench.
The Río Mala mafic dike swarm lies 82-km south of Lima (12.52°S) and comprises one of the densest dike swarms in South America (Fig. 7). This Lima segment of arc-parallel dike swarms has not been previously documented, and essentially is the continuation of the Casma-Santa Rosa dike swarms. At Río Mala, locally up to 90% of the rock volume is occupied by dikes, forming a sheeted complex (Figs. 8A and 8B). The pyroxene-bearing aphanitic basalt dikes are intruding an undated strongly foliated granitoid. Likewise, the dikes have never been directly dated. Nearly all the dikes align parallel with the arc axis, forming a zone that is 3-km wide and at least 10-km long. The greater Lima segment of arc-parallel dikes is perhaps less preserved than to the north, having younger cross-cutting plutons and the western margin covered by the coastal plain. The age of the youngest pluton crosscut by the swarm is <100 Ma and older than 67 Ma; the dates in the batholith are sparse between Lima and Río Pisco with the few reported numbers being from old Rb-Sr determinations that are not considered reliable. Geochronology of the dikes in Ríos Mala and Omas canyons require substantial new work.
Figure 7. Map showing the mixed composition dikes in the south Lima segment and the location of the Río Mala dike swarm. Most of the dikes are mafic, except south of Tupe where felsic dikes comprise half of the population.
Figure 8A. Outcrop photograph of part of the Río Mala dike swarm. View looking northwest.
Figure 8B. Another view, looking southeast at the Río Mala dike swarm.
Conjugate dike sets
Conjugate dike sets are a special subset of arc-oblique dikes, and in this study the term is used for areas of crosscutting dikes that are not arc-parallel. Because of the lack of geochronology, the formality of coeval formation for conjugate structures cannot be verified. Portions of the Casma dike swarm have crosscutting dikes that would fit this definition, but overall, the arc-parallel classification better describes the entire swarm. Conjugate patterns are prevalent in the dikes of Chile and account for about 19% of the mapped dikes. The conjugate dike sets represent arc-perpendicular compression, sigma 1 principal stress axis remains horizontal and in the direction of plate convergence while sigma 2 axis is vertical, which generally relates to the margin being under transpression (Taylor et al., 1998; Wilson and Grocott, 1999; Cembrano et al., 2005).
In northern coastal Chile, the Cerro Vetado dike swarm defines one of the most dramatic areas of dike exposures in South America. The dikes are located in the Atacama Desert at the latitude of Chañaral, right along the Pan-American Highway where for decades geologists have halted driving to stand along the road to take pictures (Figs. 9 and 10). These dikes form a related grouping that is west of the Atacama fault zone; they cut older rocks of the Coastal batholith and generally are mafic. The corridor south of Taltal to 29.8°S latitude has 4,705 dikes mapped to date. This area is about 33-km wide by 382-km long. Note that a similar total number of dikes in Perú can be arrived at by grouping the Casma and Santa Rosa dike swarms. The middle segment of this dike set is shown in Figure 10, covering the Cerro Vetado area where details of the crosscutting, conjugate dikes can be appreciated. Mutual cross-cutting relationships are complex, with some of the intersecting dikes showing no offset, and in others the older dikes are jogged in small displacements. Some of these offsets are from slip along younger minor faults. Given the great number of dikes, the detailed work of tracking which dike terminations are from en echelon step overs versus hard displacements was not attempted. Dallmeyer et al. (1996) used the 40Ar/39Ar method to date two dikes from near Cerro Vetado. The first was at the type location yielded 153.5±0.7 Ma whole rock age. The second age came from near Puerto Flamenco and gave a whole rock age of 155.5±0.06 Ma. Similar patterned dikes south of Taltal are hosted in a Permian pluton, and have a comparable K-Ar age on hornblende of 155±5 Ma (Scheuber and Gonzalez, 1999).
Figure 9. Photograph of mafic dikes at the classic locality of Cerro Vetado, northern Chile. Note the near orthogonal crosscutting relationship.
Figure 10. Map showing a pattern of dikes in the Cerro Vetado region, northern Chile. Thin lines represent dikes, heavy lines are faults, and shaded areas mark plutonic rocks.
A set of 46 granitic composition NE-striking dikes to the north of the Río Blanco-Los Bronces porphyry copper district crosscut the early Miocene Farellones and Abanico Formations (32.9°S), which defines only one side of the conjugate pair. These dikes do not have any publically available age determinations, but they could be important in showing the changes in the regional stress field surrounding the world class late Miocene copper deposits. The Los Bronces breccia pipes have a map pattern that is highly NNW-elongate, formed in the late Miocene from clearly a different stress field. The transition between the granitic dikes to the north and intrusion of the breccia pipes must be between about 20 and 5 Ma and remains to be better constrained. For many years the Google Earth imagery at Los Bronces, and along portions of the cordillera both north and south of Santiago, has displayed scenes with a high amount of snow coverage resulting in incomplete mapping of dikes around the major copper deposits. Published detailed geology at Los Bronces is wanting. Clearly additional age determinations and mapping is required to better understand this region. The recent study of the district by Piquer et al. (2015) interprets conjugate faulting to be an important aspect of the structural geology, calling for ongoing EW-directed compression.
Arc-perpendicular dikes
Arc-perpendicular dikes, developed in the mode 1 orientation in the plane defined by the maximum and intermediate principal stress axes (Pollard and Aydin, 1988), represent dilation during compression. Several studies incorrectly place sigma 1 direction as either tensional or compressional and oriented parallel to the arc (e.g., McKinnon and Garrido de la Barra 2003; Masterman et al., 2005; Chelle-Michou et al., 2015, their Fig. 12), a configuration that is structurally nearly impossible in the active convergent margin. A total of 1935 dikes (10.3%) have been identified in the arc-perpendicular category.
At the coastal range in the Illescas reserve (6°S), south of Bayovar on the northernmost Peruvian coast, is a set of thick ENE-striking mafic dikes that have left-stepping en echelon segments extending for a distance of 19-km that may be arc-perpendicular dikes. No age determinations have been made in the area. Because of the uncertainty of the dike ages, the orientation of the magmatic axis has questions, if the dikes are relatively young, then these dikes would like fit the arc perpendicular category, if they are very old, then direction of the magmatic axis become problematic. Several small plutons of gabbro cut the Precambrian and metamorphosed Paleozoic sedimentary rocks (Caldas et al., 1980). This area is in the Amotope-Tahuin accreted terrane, and regional exposure of the pluton trends is not great enough to determine the arc axis direction. Nonetheless, if the dikes formed after docking of the terrane, then perhaps they would mark the northernmost continuation of the mafic swarms that are last seen in the Casma segment 260-km to the south, but with the state of stress no longer favoring arc-parallel intrusion.
In central Perú, along the southern side of Río Rímac (11.7°S), a 35-km long discontinuous set of arc-perpendicular ENE-striking rhyolitic dikes intruded at about 5.8 Ma (Mégard et al., 1985). The main arc axis then was situated close to the modern continental divide and had porphyry intrusions forming the major polymetallic base-metal vein mining district of Morococha. The Río Rímac dikes lie west of the arc axis and point towards the mining camp. Within Morococha, several WNW-striking arc- dacite dikes have been dated at 8 Ma (Kouzmanov et al., 2008) that lie subperpendicular to the late Miocene magmatic axis. Arc transverse linears and structures have been argued to control the locations of important mineral districts in the Andes. The relationship, if any, of the felsic Río Rímac dikes following the magmatic activity at Morococha remains unknown.
The Potrerillos porphyry district of northern Chile (Fig. 11; 26.5°S) provides a fine Eocene example of arc-perpendicular dikes formed in the mode 1 fracture orientation and similarly oriented pebble dikes that imply emplacement of the porphyry systems during arc-normal EW-directed compression (Olson, 1989). Age constraints place these paleo-stress markers at about 35-37 Ma. In contrast, the 41-43 Ma El Salvador porphyry complex to the north has evidence for radial pebble dikes (Gustafson and Hunt, 1975) implying the local point-induced stress field overcame the regional stress field. This particular district provides an example where the regional stress field went from Eocene near neutral conditions followed by marked compression (Tomlinson, 1994; Mpodozis et al., 1994; Cornejo et al., 1997) to arc-perpendicular extension during emplacement of Oligocene NS-striking rhyolite dikes. Collectively, these relations demonstrate that deformation is not a continuous process in the Andes.
Figure 11. Map showing dikes around the El Salvador and Potrerillos porphyry clusters, Chile. The NS-striking dike swarm to the west of El Salvador is rhyolitic (blue lines) and dated at about 29 Ma. The less abundant and shorter length dikes at Potrerillos are Eocene, and are accompanied by paralleling EW-striking pebble dikes (Olson, 1989). Green lines show intermediate composition dikes.
At El Teniente copper district (34°S), central Chile, is a handful of EW-striking, arc-perpendicular, likely Mode 1 fracture type, young lamprophyric dikes (Stern et al., 2011). The structural evolution of the district is complex, with a local stress field developed around the breccia-pipe that formed inward-dipping concentric veins and fractures (Cannell et al., 2005). The Miocene arc to the south has numerous areas of arc-perpendicular, likely andesitic composition, widely spaced dikes that define swarms. About 265 andesite dikes have been identified from 34.4°S to 36.21°S, with a high percentage of them aligning EW; if these dikes are interpreted as forming as Mode 1 cracks, it indicates regional Miocene to Pliocene compression of the arc during volcanism.
Oblique dike sets
Oblique dike sets apparently develop outside of the predicted fracture-fault directions from the Anderson fault model. Assignment of dikes to this class may be equivocal if the arc axis is poorly constrained. Dikes formed in this structurally non-predicted orientation may be responding to pre-existing weaknesses, such as dikes intruding along older joint sets or along faults, or localized stress perturbations that are exceptions to the regional pattern. This classification accounts for 11.4% of the mapped dikes.
Perhaps one of the better examples of an oblique dike set is provided for by Pliocene dacite dikes near Huancavelica, Perú (12.7°S). Here the presently NS-striking dikes, occurring discontinuously from the Huachocolpa base-metal vein camp to the Huancavelica mercury district, follow older, likely Incaic compressional event related faults that align about 35 degrees from the arc axis. Assignment of dikes to this category has been conservative during this study. For example, in certain dike sets deviations are present, in part reflecting anastomosing patterns, which may be from fracture refraction across contacts or stress interactions during formation, but when a set of dikes is seen to move into the non-ideal direction, or an orientation that does not fall within the Anderson fault model, commonly only to return to the average trend, these dikes are grouped under the main category for the entire swarm. Apparent oblique sets of dikes may also develop following post-intrusion vertical axis rotations to re-orient the dikes into a misleading pattern, however, no documented examples are presently known for this hypothetical scenario. Perhaps some of the en echelon appearing dikes found adjacent to the regional Atacama fault would fit this category, where earlier formed dikes have been sheared into an apparent oblique pattern. Such may be the case for the set of dikes immediately SSW from El Salado, Chile. For a more detailed structural analysis of this region, see the study by Scheuber and Gonzalez (1999).
Radial dikes
Radial dikes account for 8.9 percent of the database and generally are more common in moderately eroded volcanic centers where they have little color contrast with the surrounding host rock or in exposures around mines on high-level stocks. Radial dikes form in clusters, and the number found in historic reports plus newly mapped localities from this study in South America remain very low. In Perú are 14 defined radial dike sets (only 1 was previously published), in Argentina 6 clusters, (three have been published on- see Fig. 4 on the Tabaquito radial dikes; Halter et al., 2004; González and Aragón, 2000), one location in Bolivia at the late Miocene Porco mining camp (Cunningham et al., 1994), and in Chile 13 clusters (all from this study). Occurrences of radial dikes in Ecuador and Colombia are very likely, but I have not found any published references.
Radial dikes form in response to a local point or cylinder-like stress field, generally induced by a high-level stock, diatreme or breccia pipe, or volcanic neck, but may also be present in dome complexes and distributed about the flanks of major stratovolcanoes and make spoke-like pattern around the magma source (Nakamura, 1977). Outstanding examples of radial dike localities of North America, such as Shiprock in New Mexico or Spanish Peaks in Colorado (Ode, 1957), have no widely cited equivalents in the Andes despite the abundance of volcanoes and protracted Cenozoic volcanism. One of the better radial dike patterns documented in a mineral district comes from the Miocene San Francisco volcanic complex at the Bajo de la Alumbrera porphyry mine (Halter et al., 2004). Numerous radial dikes are likely present on the modern volcanos, yet the flanks lack erosion to expose them. Slightly older volcanos should have these features well exposed, but little work has been done to document them in the existing published geological maps. Radial dikes provide important information regarding the stress field and warrant comprehensive studies.
In Perú, 24.5-km south of the city of Ayacucho (74.002°W, 13.392°S), an asymmetric radial late Miocene Orcos dacite dike set cuts the late Miocene Ayacucho Formation (Heki et al., 1985), defining a pattern suggestive of emplacement during EW-directed compressional stress. Four additional radial dike swarms were located to the south. Two of these are dacite complexes with center points located at 74.14°W, 14.018°S (NW-elongated) and 74.263°W, 14.249°S (true radial pattern). Another two-andesite composition NW-elongate radial dike center were found at 74.462°W, 13.964°S and 74.197°W, 14.452°S. These dikes are likely late Miocene in age, but they have not been dated.
Several andesite radial dike sets were also mapped in central Perú in the Cordillera Occidental near the town of Castrovirreyna. To the south of Castrovirreyna radial dike centers are at 75.367°W, 13.481°S and 75.133°S, 13.382°S. To the north of Castrovirreyna four centers were found at 75.311°W, 13.216°S, 75.385°W, 13.069°S, 75.354°W, 13.016°S, and 75.358°W, 12.878°S. These dikes cut, folded early Miocene andesite flows and tuffs, but do not show any clear evidence that the dikes have been tilted along with the host rocks, suggesting that their emplacement came after the ca. 17.5 Ma Quechua 1 contraction event (Benavides, 1999). The dikes have not been dated, although they are likely middle Miocene and related to the volcanic arc that erupted the andesite flows at the Castrovirreyna district.
An important series of late Miocene to Pliocene andesite dikes are exposed to the southeast of Santiago, Chile and define radial swarms that align parallel to the modern volcanic arc (Fig. 12). These have not been directly dated, nor previously mapped, but they cut mainly early Miocene volcanic rocks of the Farellones Formation and younger units. While four clusters are shown in Figure 12, another three clusters are nearby but outside of the figure area.
Figure 12. Map showing andesitic radial dike centers in a likely Pliocene arc hosted in Miocene volcanic rocks east and southeast of Santiago, Chile. Four centers are present with the center point marked by a star. Cenozoic stocks are shown by the patterned polygons. A best-fit line through these radial dike centers would mark a magmatic arc axis parallel to that of the modern volcanoes located to the east.
In northern Chile, a pair of felsic radial dike centers lie at 69.43°W, 25.855°S and 69.511°W, 25.831°S likely coming from Eocene stocks in the Sierra Jardin porphyry prospect. Some of these dikes extend up to 3-km in length. Rivera et al. (2004) dated sericite alteration in the porphyry as ranging from 41.1 to 43.3 Ma. If the radial dikes are of similar age, and because these dikes show no obvious preferred direction or elongation, it suggests neutral stress at this time.
Radial fracture patterns, defined by dikes, veins, and joints, are common around deposits formed by stocks. Radial pebble dikes have been described at the Miocene dacite domes of Julcani, Perú (Shelnutt and Noble, 1985), the Eocene El Salvador deposit, Chile (Gustafson and Hunt, 1975), and hydrothermal quartz veins at Verde West, Refugio district, Chile (Muntean and Einaudi, 2001) make a radial set. Base-metal veins surrounding the early Paleocene Relincho porphyry in Chile have a ringing pattern that appears radial. NW-elongate possible radial dike pattern is found around the Cretaceous Andacollo deposit in Chile. Intrusions in areas experiencing neutral stress or having local point stress overwhelming the far-field stress field should be relatively common phenomena in the Andes. The sparse number of localities to discuss represents a lack of reporting on these features.
A special case local stress field related type of intrusion called ring dikes have been reported in the Coastal batholith north of Lima, Peru (Knox, 1974; Myers, 1975; Bussell, 1988). They have been related to magma chamber formation and caldera development. While being rather well exposed in four centers clustered from Rio Forteleza to Rio Chancay, these ring dikes are in fact extremely rare features and here considered atypical for dike emplacement mode. The only other possible ring-dike documented in Peru, and at a higher level relative to diatreme formation, has incomplete encirclement by dacite porphyry and latite dikes at the Toquepala porphyry deposit in southern Peru (Simmons et al., 2013). Ring dikes may be possible within the edifices or lower in the foundation of the present day stratovolcanoes but remain undescribed.
DISCUSSION
Scalar relations
A question during the beginning and right to the end of this study has been where is the longest dike along the Andes? What composition would it have and what geologic process would be behind its intrusion? These questions are not definitively answered here, but from what has been recorded we can say the following. The longest reported dikes, actually numbers 1 and 3, come from the Neuquén province in Argentina. These ~101 Ma old mafic dikes are EW-oriented and have lengths at 18.3-km and 9.6-km (Zamora Valcarce et al., 2006). These dikes are nearly continuous and singular, but do have slight left-steps that suggest they are linked through en echelon emplacement of a single feeder; each dike segment may correspond to the fringe zone or hackle offsets found bordering the more through going fracture (Hodgson, 1961; Ramsay and Lisle, 2000, their Fig. 36.5). The second longest dike is 10.8-km in length, also located in Argentina, is part of the Permian mafic dike complex north of San Juan. It forms an NS-strike, but is part of more variable oriented dikes, which include a set that is radial. The 4th longest dike comes from Chile, an 8.4-km long mafic dike striking NW that lies south of Chañaral and the dike complex at Cerro Vetado. In contrast, the mean dike length in the database is at 572-m. Dikes, like faults and veins, share a fracture process that have a lengthy distribution where shorter fractures are most abundant and longer fractures decrease in abundance following an asymptotic decay (see Segall and Pollard, 1983). None of the South America western margin dikes come even close in length to the older dike swarms in the craton areas of North America that extend for tens to hundreds of kilometers (Leech, 1966; Gibson et al., 1987). It is possible that dike lengths in a terrane scale to the size of source magma chambers (Buck et al., 2006).
The greatest density dike swarms of the Andes
Dike swarms have been touched upon in the above-described field relations. Areas of higher density of dike emplacement represent extreme tension zones. Most examples come from the mid-Jurassic and late Cretaceous magmatic arcs formed during a time of general intra-arc spreading following batholith formation as considered in the detail at the Concón and Cartegena dike swarms by Creixall et al. (2011). Having the greater number of dikes in the Mesozoic time probably pertains to the amount of erosion to expose the roots of the magmatic arc. Particularly impressive is the late Cretaceous Río Mala dike swarm in central Perú for the percent dikes occupying the rock mass. This is probably the most intense dike swarm of South American’s western margin. The Santa Rosa dike swarm, perhaps kinematically linked to the Río Mala swarm, has one of the largest areas of high dike density, but it does not obtain the same nearly complete replacement or sheeted character as at Río Mala. The dikes at the Santa Rosa swarm have asymmetric localization across the swarm, and the pattern of diking pertains to regional arc-perpendicular tension that extends well to the north and south of Santa Rosa and has little to do with local perturbations from point sources as suggested by Pitcher and Bussell (1985; their figure 10.5).
In Chile, another area of closely spaced mafic dikes lies 8-km west of El Salado (70.3999°W, 26.41°S). These NNW-striking arc-parallel dikes intrude Jurassic plutonic rock and formed at nearly the same time. The diking along the south canyon wall of Río Salado is intense, but the zone is only 700-m wide and of limited strike length. The Salado dike swarm is situated 7.5-km west of the Atacama fault zone. A bit farther south, at 70.462°W, 26.576°S, an unnamed mafic arc-parallel dike swarm is about 960-m wide and positioned 11.8-km west of the Atacama fault zone. Two more swarm centers to the south align with this last location and are located at 70.533°W, 26.876°S and 70.543°W, 27.026°S. If all of the above zones are mechanically related it may be showing a crustal weakness that lies parallel and offset to the west from the Atacama fault.
Large Neogene and younger volcanic formations and fields are likely dike fed, but have not had sufficient erosion to expose their plumbing system. Although, having the Neogene continental borderland under a regime of ongoing subduction lent a different configuration as compared to during the Mesozoic, and therefore the amount of diking beneath the buried arcs is probably not as intense.
Timing constraints
Singular age assumptions on the major dike swarms require a word of caution in light of the demonstrated varying ages within the nearly compositionally identical Independence Dike Swarm (IDS) in the Sierra Nevada batholith of California (Chen and Moore, 1987). The IDS is perhaps the most similar dike swarm in North America to the batholith-hosted dikes in South America. The main phase of the IDS intruded at 149 Ma, a time fairly close to the dikes in northern Chile at about 154 Ma, perhaps indicating a global plate tectonic event covering the western margins of both Peru-Chile and California. The IDS dikes formed during two periods, with the majority perhaps during the Jurassic, and then younger Cretaceous dikes following the same general orientation and pattern (Coleman et al., 2000). In an older example, bimodal appearing Late Permian to Triassic dikes cutting batholithic rock in Australia can been classified as being arc-parallel swarms and yet have two widely spaced times for intrusion (Allen, 2000). Dikes in the Santa Rosa swarm have been for the last 30 years constrained by a set of K-Ar ages to about 70 Ma. However, new geochronology may reveal additional complications or refine the timing.
Age constraints on dikes of the South America remain remarkably sparse. Of a total of 6,848 compiled ages for the study area, only 118 are described as being from dikes, and of these two-thirds of the samples are from Cenozoic dikes. The two most abundant phases of dike intrusion into the crust was during the ca. 70 Ma event in Perú and the ca. 153-155 Ma event in north Chile. These older Peruvian diking events apparently lack any preserved associated erupted material, whereas the Chilean mafic dikes are broadly coeval with the andesitic lava flows of the Jurassic La Negra Formation. Cenozoic dike sets can loosely be bracketed for timing given the fairly abundant amount of ages on the host rocks. Finally, commonly the dated dikes have results from old K-Ar ages, either on whole rock or hornblende, both of which may have serious errors that go undetected by this methodology. Dating of systematic dike sets within a geological study area, particularly in government survey mapped quadrangles, should become standard practice.
Marker of paleostress during the formation of economic mineral deposits and paleo-volcanic arcs
Several economic geology studies of porphyries make paleo-stress interpretation from locally mapped geology, and infer changes in the principal stress directions without having the regional context documented to see if the claims are compatible with dikes outside of the mineral district (e.g., Love et al., 2004; Cannell et al., 2005; Masterman et al., 2005). Furthermore, potential tectonic framework of dikes and regional stress character may have the following important patterns; late Eocene compressive with arc-perpendicular dikes formed (Potrerillos, Chile; Olson, 1989; Tomlinson, 1994), early to late Miocene compressive with conjugate sets formed (Julcani, Huachocolpa [Wise, 2010], and Morococha districts, Perú, and Los Bronces, Chile [Piquer et al., 2015]). Detailed geochronological studies on dikes mainly come from the major ore deposits where pre-, syn- and post-mineral dikes contribute valuable brackets on the timing of mineralization. Paleo-stress at the time of mineralization may likewise be evaluated from dikes in the major deposits, however, comparison to the regional pattern of dikes remains hampered due to the lack of mapping and dating. Some porphyry deposits have a particularly high density of dikes, which if they are post-mineral cause dilution problems, lowering the grade, such as at Bajo de Alumbrera, Argentina and Cotabambas, Perú. Other porphyry systems overall have the deposits centered on a dike like geometry, such as Haquira and Los Calatos in Perú.
Charrier et al., (2002; 2009) places much of the early Miocene volcanic arc in a remarkable long arc-parallel graben/extensional setting. The studies by Charier et al. do not mention the presence of dikes in their field relations. Andesitic dikes mapped during this study are rather few and inconclusive to evaluate the graben tectonic setting. One would expect, given post-eruption tight folding with hinge directions mostly parallel to the graben axis being locally present, that arc-parallel feeder dikes may be difficult to detect, whereas as arc-perpendicular dikes should remain distinguishable despite bedding rotations. In addition, dikes may also intrude parallel to fold hinge radial fractures and cleavage, making the classification from the former case even more difficult to evaluate. In the region considered by Charrier et al. (2002), this current study mapped between latitudes 34.08°S and 35.09°S mainly andesitic arc-perpendicular dikes, and one set of dacitic radial dikes centered at 34.08°W, 70.51°S. Without geochronology, some of these dikes may have intruded significantly after the host rock formation, and perhaps even after the folding that tilted the early Miocene rocks. Detailed mapping and examination of feeder dikes to these volcanic units may provide a test for this hypothesized tectono-stratigraphic setting.
The state of stress in the late Miocene arc extending from Los Bronces to south of El Teniente can be inferred from the collective interpretation of dikes and veins. Pre-mineral, 8.9 Ma, arc-perpendicular lamprophyre dike at the El Teniente mine (Stern et al. 2011) seem compatible with the similar oriented intermediate dikes that are distributed well south of El Teniente. These dikes are considered as forming in the mode 1 orientation, which is parallel to sigma 1 principal stress axis, and internally consistent with the known direction of ocean-continent plate convergence. This observation is in sharp contrast to the study by McKinnon and Garrido de la Barra (2003) that called for NS-oriented compression after the formation of El Teniente, if anything, the geometry the Braden circular breccia body with a smaller localized NNW-striking breccia extension or apothoses, if interpreted as a viscous dike, suggests a period of relaxed compression and EW-directed tension that preceded the formation of more general neutral stress in the region as represented by the documented trend of perhaps younger radial dike centers shown in Figure 12. The NNW-trending alignment of breccia masses at Los Bronces (Toro et al., 2012), and earlier late Miocene highly elongate NNW-trending stocks (Deckart et al., 2010), may equally be used to argue for a period of arc-perpendicular tension.
A long-standing general concept of Andean transverse structures controlling the location of porphyry deposits (e.g., Richards, 2003) can be examined by comparison to the distribution of dikes. In particular, if these postulated crustal scale structures are sufficient to control the positions of batholiths, plutons, and stocks, then they should likewise place a control on the location of dikes. In fact, one would expect to find multiple generations of dikes of varying compositions following these zones of structural weakness. This present compilation finds the opposite to be true- none of the apparent transverse linears coincide with dike swarms or individual dikes following the supposed through-going fracture or fault. Even at the deposit scale, when dikes are reported, in many instances they do not align with any known transverse structure. At Zafranal and Tintaya, Peru (Rivera et al., 2010; Chelle-Michou et al., 2014), the dikes are parallel or sub-parallel to the arc axis. At Potrerillos, Chile, the dikes are arc-perpendicular, lacking any relationship to a transverse arc-oblique or conjugate structure.
The Antamina endoskarn and late Miocene porphyry-related deposit in northern Perú shows remarkable dike-related controls where thick NE-striking arc-perpendicular dike complex make a deviation from what is typically an NW-striking arc-parallel pattern of dikes and elongate stocks extending discontinuously and stepped southwards through Huanzala-Pachapaqui-Hilaron. Love (2004) asserted Antamina is situated on an NE-trending transverse structure. However, review of SRTM90 DEM data and Landsat images found no support for this interpretation. The orientation of the Antamina stock is arc-perpendicular, not in the transverse orientation in the sense of Richards (2003). Fold hinge lines to the east of the deposit track through with no offset from any transverse faults. Instead, the NE-striking dike-like stock of Antamina may better be explained as a dilational jog along longitudinal faults. A regional transverse structure through this area would likewise have to influence the Cordillera Blanca batholith to the west, something that there is no evidence for.
At numerous places along the South American margin where “transverse” oriented (in this study termed conjugate directed structures) dikes, they are not occurring with any known porphyry deposit or regional lineament. In another example, at Los Bronces, Chile, the structural trends of aligned breccia pipes and elongation direction of late Miocene stocks follow an arc-parallel direction and no arc transverse control is required. Even the mega-porphyry of Chuquicamata has a very elongate and deformed character aligned with the Andean parallel West Fissure fault, showing no evidence for a transverse structural control. Furthermore, the Andean transverse linears in several locations record a different trend than the alignment of sub-arc perpendicular linears defined by volcanic vents. Other linears in the volcanic arc of southern Chile have been linked to underlying structures (Katz, 1971; Lara et al. 2006; Cembrano and Lara, 2009; Sanchez et al. 2013). Reiterating these inferred structural controls on porphyry locations, such as in Sillitoe (2010), has led to worldwide emulation of this linear locating explanation. Unfortunately, this approach, little supported with detailed structural studies, tends to close off the question of explaining the control on porphyry location rather than providing new insights. Given the economic impetuous of porphyry copper mines yielding some the most tightly controlled geology for geochronology and the greatest manmade exposure and 3D data from drilling, a concise regional synthesis of dike patterns found at South American porphyries is overdue.
CONCLUSIONS
Dike intrusion accompanies all types of magmatism from deep level plutons to shallow volcanic edifices showing a range of geometric controls and compositions. The predominance of arc-parallel diking (~50% of the population) along the margin may indicate the major paleo-stress control was configured by the plate’s free edge instead of the induced stresses from subduction traction beneath the plate. By similar induction, that 62% of the dike population comes from basaltic andesite to mafic rocks suggests magmatism during extension to deliver unevolved melts to the subsurface. In contrast, the tractive forces or plate coupling is generally responsible for the phases of compression, and under these conditions the less abundant conjugate, oblique, and arc-perpendicular dikes develop, likely under then conditions driven by transpression and transtension (Dewey and Lamb, 1992) with the intermediate sigma 2 principal stress direction being vertically oriented.
Scale restrictions in quadrangle mapping and lack of appropriate resolution aerial imagery limit reporting of dikes, as does perhaps a general under appreciation of both their importance and abundance. Most studies that report on dikes do so only at a very local scale. Age determinations on dikes remain extremely under sampled. Given the importance of dike sets in major mineral districts, the various geological surveys of South American countries should expend greater effort in recording dikes during geological mapping. This study is the first regional-scale summary of dikes in western South America that hopefully will begin a dialogue and incentive to further improve our knowledge on dikes belonging to all classes.
Acknowledgements
A formal review by Jose Cembrano provided additional clarifications on the structural geology and brought to the author’s attention some supporting references that have been cited in the text. David Greene is thanked for making suggestions on study area context.
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