Carboniferous Subduction Complex in the Harz Mountains, Germany

TIMOTHY A. ANDERSON Bundesanstalt fuer Bodenforscbung, 3 Hannover-Buchholz, Stille Weg 2, Federal Republic of Germany

ABSTRACT gin of the belt may have originated in the same manner. Key words: historical geology, Harz Mountains, subduction, graywacke. Rocks in the Harz Mountains probably accumulated during an episode of Carboniferous subduction. Oceanic crust moving rela- INTRODUCTION tively southeast was consumed at a Benioff zone dipping southeast. Pelagic and abyssal sediments, graywacke, basalt, keratophyre, and The Silurian to Upper Carboniferous rocks of the Harz Moun- other rocks were emplaced at the leading edge of the overriding tains are an uplifted, 30 by 90 km block of the Variscan structural plate. Present-day northwestward structural imbrication (vergence) belt extending across central Europe (Fig. 1). The block is sur- reflects the original dip of the Benioff zone. As a block typical of the rounded by Mesozoic rocks and extends west-northwest across the northern part of the Variscan belt of Europe, the origin of the Harz border between the Federal Republic of Germany and the German Mountains by subduction suggests that much of the northern mar- Democratic Republic. It is suggested here that an analysis based on

Figure 1. Generalized map of the Harz Mountains showing geologic zones. Basalt, keratophyre, and tuff are shown in solid black.

Geological Society of America Bulletin, v. 86, p. 77-82, 4 figs., January 1975, Doc. no. 50110.

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the process of subduction at olate margins can explain the major Sieber Graywackes), most of these rocks are petrographically simi- geologic features of the Harz Mountains. lar and of similar age. Olistostrome deposits are common in much of the eastern part of GEOLOGIC SETTING the Harz Mountains. The matrix is predominantly shaly and dis- rupted and contains pods, lumps, and lenses of all the other rock Several subdivisions of the Harz Mountains appear within the types exposed in the eastern Harz Mountains (Lutzens, 1972). geologic literature: political (West and East Harz Mountains), his- Slump deposits have been reported from the western Harz Moun- torical and topographic (Upper, Middle, and Lower Harz Moun- tains (Stoppel and Zscheked, 1971). tains), or geologic (a series of zones whose boundaries mostly fol- The Elbingerode complex of the Blankenburg zone and the Iberg low structural or outcrop trends as shown in Fig. 1). block in the Clausthal-Kulm zone are characterized by reef lime- Each geologic zone of the Harz Mountains contains a variety of stone. The reef deposits of the Elbingerode complex overlie 500 to rock types and ages (Fig. 2). Most types occur in more than one 1,000 m of basic tuff, pillow basalt, and keratophyre. The rocks zone, and many different formation and facies names have been beneath the Iberg limestone are not exposed. applied to similar rocks of essentially the same age. Rocks exposed Exposed throughout much of the Harz Mountains are numerous at the surface are generally progressively younger from southeast to bodies of basalt (a translation of the German term "Diabas") and northwest. keratophyre with and without pillows, basic and keratophyric tuff, Thin-bedded, typically dark-colored shale is widespread and related rocks. Siliceous, iron-rich sedimentary rocks are com- throughout the Harz Mountains; the shale is commonly associated monly associated with the pillow lava units (Gundlach and Han- with thin-bedded and (or) lenticular limestone and chert. Locally, nak, 1968; Möhr, 1973). The contacts of many of these volcanic the shale contains graptolites and both the shale and limestone rocks appear to be structural (Lutzens, 1972). Individual bodies are often yield conodonts. Such fossils have led to a re-evaluation of usually stratiform and range in length from centimeters to hun- the age of many of these rocks in the last two decades (Mobus, dreds of meters or more. 1966; Stoppel and Zscheked, 1971). Two major intrusive complexes crop out in the Harz Mountains. Graywacke is extensively exposed. Although several names have By far the larger of the two is the Brocken complex, which consists been applied to graywacke that crops out in different areas of the of the Brocken and Oker Granites and the Harzburg Gabbro. The Harz Mountains (for example, Tanner, Kulm, Selke, Siidharz, and other complex is the Ramberg Granite. Fracture mineralization

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throughout the Harz Mountains is commonly considered to be re- creation of the oceanic crust at a mid-ocean ridge. Within the sub- lated to the intrusion of these two complexes (Mòbus, 1966; Mohr, duction complex, however, the age of the basalt may be difficult to 1973). determine because of lack of fossils and tectonic intercalation with Slaty cleavage is common in the rocks in the Harz Mountains. In younger strata. the Wippra zone (Fig. 1), low-grade regional metamorphism has 4. The age of the youngest turbidite and (or) mélange dates the resulted in the local development of phyllite. Biotite-cordierite time of subduction and generation of the subduction complex. schist (Ecker Gneiss) between the Brocken Granite and Harzburg Either the mélange or turbidite may appear older than they really Gabbro intrusions is commonly considered to be contact- are because of inclusion of older rocks and (or) fossils and tectonic metamorphosed mica schist (Mohr, 1973). intercalation with older strata. The structures in the mountain block strike mainly northeast. A 5. Younger sedimentary rocks will be tectonically inserted be- generally northwestward structural imbrication or vergence results neath older ones during the process of subduction (Fig. 3). The re- from asymmetrical to isoclinal folds inclined or overturned to the sult is an apparently right-side-up sequence with younger strata on northwest and reverse faults and slaty cleavage generally dipping to the bottom. This sequence can indicate the direction of dip of the the southeast. The most important exception is the Acker- Benioff zone and relative direction of plate motion (W. Hamilton, Bruchberg zone where the structural inclination is partly to the 1972, personal commuti.). Structural imbrication produced during southeast. The dominant structures of the Harz Mountains are subduction may well dip in the same direction as the Benioff zone. crossed by systems of faults and fractures striking mainly between 6. Plutonic intrusion and accompanying volcanism and ore-body west-northwest and northwest. The northeast and southeast mar- emplacement occur considerably after the emplacement of the sub- gins of the mountain block are part of one such system. Ore duction complex and above a Benioff zone much deeper than dur- mineralization is common along the fractures. Broad anticlinoria ing the formation of the complex (Oxburgh and Turcotte, 1971; and synclinoria characterize some areas. Most important among Dickinson, 1971). these are the Selke, Siidharz, and Sieber synclines and the "Devo- On the basis of the above assumptions and the sequence and dis- nian anticline." The smaller folds forming these structures usually tribution of rock types in the Harz Mountains, it is possible to de- show the regional northwestward inclination. vise a model for the subduction that could have produced this crus- tal block (Fig. 4).. In general, this model adequately and without DISCUSSION contradiction explains the major geologic features of the area. For this reconstruction, it was also assumed that transform faulting and In order to determine if the Harz Mountains could have origi- relative transport direction were at approximately right angles to nated by subduction at plate margins, certain basic assumptions are the trench axis. From the beginning of Early Carboniferous until necessary: Late Carboniferous time, material that had been scraped off a rela- 1. Mélange formed at the time of subduction commonly typifies tively southeast-moving subducted plate (with oceanic crust) was a subduction complex. It may be composed of any other rocks accreted to and became part of the northwest edge of the overriding present in the complex but, being formed from them, is essentially plate. younger (Hamilton, 1971). From Late Devonian until into Early Carboniferous time, a sub- 2. The stratigraphic sequence continuously emplaced to form a plate must have been subducted to explain outcrop patterns (Tan- subduction complex commonly begins with ultrabasic rocks and ner Graywacke zone, for example) and the presence of similar age basalt that is probably pillowed and intercalated with and im- rocks in several areas of the Harz Mountains (Figs. 4A through mediately overlain by siliceous and (or) calcareous, iron-rich sedi- 4D). A subduction complex formed at both the "leading" and "trail- ments. Over these basal strata are pelagic and (or) abyssal sedi- ing" edges of the subplate because it apparently moved more slowly ments that may include turbidite. The sequence is completed by than the major subducting plate. By the time the subplate was to- clastic sedimentary rocks, turbidite deposits that accumulated in tally consumed in Early Carboniferous time, these two separated the trench at the locus of subduction, and tectonically mixed masses were welded together at the main subduction zone (former mélange consisting of all rock types transported to the subduction trench location in Fig. 4E). zone (Dewey and Bird, 1970; Dickinson, 1971; Hamilton, 1969). The accumulation of material added to the front of the overrid- 3. The age of the basal basalt approximately dates the time of ing plate would have forced at least the upper 20 to 30 km of the subduction zone to migrate toward the northwest. With sufficient migration, the dip of the zone would have become shallow, proba- bly resulting in a sudden shift of the subduction zone to another location (Fig. 4F). The varied inclination of the rocks in the Acker-Bruchberg zone and the volcanic rocks in the Diabase zone are interpreted as the scar of such a shift. Downward movement of the remnant of the formerly subducted plate probably continued for at least a short period of time. "CONTINENTAL- Much of the graywacke, especially the Carboniferous MATERIAL graywacke, probably was deposited by turbidity currents in the (deformed, compacted, trench formed by subduction. Both the flyschlike character of the and/or metamorphosed graywacke and deposition contemporaneous with deformation are sediments I ikm r OCEANIC CRUST well recognized (Kraus, 1967; Krebs, 1968; Lutzens, 1972; (beneath trench Schwan, 1967). sediments) The olistostrome masses of the eastern Harz Mountains could Figure 3. Cross section of part of an active subduction zone (sea-floor have resulted from either tectonic or depositional processes or a trench) and the complex being formed by it (adapted from Scientific Staff, combination of the two. Lutzens (1972) rejected tectonic emplace- DSDP Leg 31, 1973). Youngest rocks in 1 are older than the youngest in 2, ment but considered only nappe tectonics and fold imbrication. those in 2 are older than the youngest in 3, and so on to the youngest rocks Either tectonic or depositional origin are consistent with and in 5 and 6. suggestive of the mélange that characterizes subduction complexes.

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Dewey and Bird (1970, Fig. 15) have shown an idealized subdue- continuously being formed by subduction. The exact sequence may tion sequence with mélange beneath flysch beneath wildflysch. be unclear or irrelevant in ancient deposits. Wildflysch and mélange may be very difficult to distinguish in the Except for part of the Harzburg Gabbro intrusion, the ultrabasic geologic record. Also, immediately after deposition of flysch, at rocks that frequently typify a subduction complex are lacking in least part of it is certain to be incorporated in the mélange that is the Harz Mountains. Either they have not yet been exposed (un-

(a)-earliest Carboniferous

(B)through(G) - Early Carboniferous

(h) and (T)-Late Carboniferous

Figure 4. Diagrammatic model of the subduction that could have produced the Harz Mountains. Map pattern shown is the same as in Figure 1. Mantle rocks are not shown.

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likely in view of the age of the rocks) or they were not made part of Variscan belt are subduction complexes preserving remnants of a the overriding plate during subductior. (Dewey and Bird, 1970, Fig. former oceanic basin, the origin of the Carboniferous coal beds as- 8) or obduction (Coleman, 1971) did not occur during the forma- sociated with the margins of the belt could be related to the final tion of the Harz Mountains. closing and disappearance of this basin. The newly created conti- The pillow lava, tuff, and iron-rich siliceous and calcareous nental crust could also have provided a previously nonexistent plat- sedimentary rocks could well have originated at or near a mid- form for the deposition of Permian evaporite. Also the circum- ocean ridge where this association is common (Davies and Supko, Atlantic land masses could not have formed a single continent 1973; Garrison and others, 1973). The abundant keratophyric (Woodrow and others, 1973) before Late Carboniferous time. In- rocks could have formed by low-grade metamorphism of basalt at stead the Old Red-Catskill continental mass would have been or near the mid-ocean ridge (Miyashiro and others, 1971) or dur- separated from the Variscan areas by an area of oceanic crust. ing subduction. Comparable rock sequences within the Franciscan Dewey and others (1973) expressed the need for determination Formation of California have been similarly interpreted (Page, of plate motion in Europe during the Paleozoic Era. This problem 1972; Hopson and others, 1973). Some of the basalt, keratophyre, may be approached using the information contained in the rocks of and tuff also may be a product of the volcanism that occurred the Harz Mountains. Between the creation of the oceanic crust and above the high-temperature metamorphic zone associated with its subduction, there were about 90 x 106 yr. Assuming a spreading subduction (Dewey and Bird, 1970; Dickinson, 1971). rate of 2.5 cm/yr and continuous spreading at a mid-ocean ridge Thick Middle Devonian shale is commonly assumed to underlie behind the subducted plate, the distance between the ridge and the thick block of basic tuff and other volcanic rocks of the El- subduction zone would have been about 2,250 km. The age differ- bingerode complex (Mobus, 1966; Lutzens, 1972). This assump- ence between the youngest sedimentary rocks in the Wippra zone 6 tion is based on projection from surrounding areas because no and in the Clausthal-Kulm zone is about 17 x 10 yr. Assuming a clear-cut case of such a relation has been found in either outcrops relative closing rate of 2.5 cm/yr, about 675 km of oceanic crust or drill holes. The volcanic complex could be the remains of a sea- would have been subducted to form the subduction complex. The mount or guyot and, therefore, both time-equivalent to and older rocks in the Harz Mountains give no indication about the validity than the surrounding shale. Movement away from the mid-ocean of the assumptions about spreading rate, continuity of spreading, ridge and the subsidence that accompanies such motion (Hess, or closing rate. 1962) adequately explains both the deposition of Upper Devonian reef limestone on top of the volcanic mass and the subsequent de- ACKNOWLEDGMENTS position of apparently deeper water shale and bedded chert. Later the entire mass could have been scraped off the underriding plate, I thank F. Bender, A. Hess, and K. E. Koch for reading an early tectonically mixed, and added to the overriding plate at the subduc- version of this paper and J. F. Dewey for his critical review. tion zone. 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