Paleomagnetism of Basalts from Alborz: Iran Part of Asia in the Cretaceous

Tectonophysics, 68 (1980) 113-129 113 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands PALEOMAGNETISM OF BASALTS FROM ALBORZ: IRAN PART OF ASIA IN THE CRETACEOUS H. W~NSINK and J.C. VAR~KAMP Geological Institute, State University of Utrecht, Utrecht (The Netherlands) (Received March 20, 1979; revised version accepted October 16, 1979) ABSTRACT Wensink, H. and Varekamp, J.C., 1980. Paleomagnetism of basalts from Alborz: Iran part of Asia in the Cretaceous. Tectonophysics, 68 : 113-l 29. Paleomagnetic results are reported from 20 sites within three units of volcanic rocks of Cretaceous age from the Central Albon Mountains, Iran. After application of progressive demonetization either with alternating magnetic fields or with heating, the mean characteristic remanence direction is found to be Ilt = 33.3O, I = 4’7.5O (+s = 7.2’). Both the paleomagnetic analyses and the inspection of polished thin sections of the lavas point to magnetite as the main carrier of remanence. The paleomagnetic pole is located at 61°N, 147.5’E. This is quite near to the positions of the poles of Cretaceous age on the Eurasian apparent polar wander path. It is suggested that in Cretaceous times northern Iran was at its present position with respect to Eurasia. INTRODUCTION The central part of the Alborz Mountains is usually subdivided into seven structural units, viz., from north to south: (1) Caspian Plain; (2) Northern Mesozoic Border Zone; (3) Paleozoic Central Ranges; (4) Central Tertiary Zone; (5,6) Southern Paleozoic-Mesozoic Zone; (7) Southern Frontal Depression (Gansser and Huber, 1962; Stijcklin, 1968). In this orogen the main tectonic phenomena are blocks bounded by faults; the sediments within these blocks are often strongly folded, but no large thrusts are encoun- tered. For our paleomagnetic research we have collected oriented samples from the volcanics of Cretaceous age (Fig. 1) both in the Northern Mesozoic Border Zone (unit 2) and in the Southern Paleozoic-Mesozoic Zone (unit 5). In unit 2, samples were collected from the Chalus Formation in the valley of the river Chalus (Fig. 2); in unit 5, volcanic rocks were sampled from the Gypsum - Melaphyr Formation in the valley of the river Haraz, east of Mount Damavand. The aim of this paleoma~~tic study is to determine the characteristic directions of remanent magnetization of these rocks, and the significance for the paleoposition of northern Iran. 0040-1951/80/0000-0000/$02.25 0 1980 Elsevier Scientific Publishing Company ” --- 5Z0EL Fig. 1. Map of the central part of the Alborz Mountains, Iran, with the sampling localities of the volcanics of the Gypsum-Melaphyr Formation, and the index map of the Chalus area of Fig. 2. CHALUS FM “UPP JURASSIC s = Sampling site 0 I 2 3 -- CENTRAL PART CHALUS AREA Fig. 2. Geological map of the central part of the Chalus area with the sampling localities of the volcanics of the Chalus Formation. (After Cartier, 1971.) 115 GEOLOGICAL PART In the area of the present Alborz Mountains strong epeirogenetic move- ments occurred following the deposition of various kinds of sediments of Triassic age, which resulted in the emersion of the region. The subsequent erosion caused the deposition of detrital rocks of the Shemshak Formation, These rocks consist of grey sandstones, conglomerates and shales occasionally with intercalations of coal. The Shemshak Formation, which has an Early to Middle Jurassic age, covers an extensive area, and in the Alborz has a thickness ranging from about 1000 m in the south to about 2500 m in the north. In the Central Alborz, the overlying sediments of Late Jurassic and Cretaceous age show considerable differences in facies development (Fig. 3). 0 i._..__. it ---r-s i CRETACEOUS 2 t nfi =i LIMESTONE” FORMATION FORMATION t-_“_ r&a-24 SHEMSHAK ’ * ’ ’ ’ Oao hj FM ------ ___. -- __- Fig. 3. Stratigraphic columns of the formations in the central part of the Alborz Moun- tains from Early Jurassic through late Cretaceous age in the Chalus area (left) and in the area east of Mount Damavand (right), with the positions of,the sites. The positions of the sites WKA and WKB from member 1 of the Chalus Formation are not known exactly, they are not indicated in the column. (After Allenbach, 1966; Cartier, 1971; Steiger, 1966.) 116 In the area east of Mount Damavand marly fossiliferous limestones are found on top of the Shemshak Formation. The limestones are about 100 m thick and have a Middle Jurassic age. The succeeding Lar Formation consists oi 400 m of pure, fossiliferous limestones with ammonites, pointing to an Late Jurassic age. In the Chalus valley, about 80 km northwest of Mount Dama- vand, there are no sediments of Middle Jurassic and early Late cJurassic age. In the northern part of the Chalus area, sedimentation started with trans- gression conglomerates and breccias overlain by dolomites with intercalations of red marl, red silts, and reddish sandstones; hereupon follow limestones with biosparites and oijsparites with intercalations of reddish sandy limestones. In the southern part of the Chalus valley, sedimentation begins later, and here the sequence consists of limestones with intercalations of reddish sandy beds. The thickness of the formation decreases from 400 m in the north to 100 m in the south. The mi~rofauna point~s to an age ranging from Late Jurassic to Middle Barremian age (Cartier, 19’71). During Early Cretaceous times there was a regression in the area north- east to southeast of Mount Damavand. After the deposition of the Late Jurassic limestones of the Lar Formation, layers of gypsum were laid down, followed by dark-coloured subaerial lava flows -- mainly basalts I with a total thickness of up to ‘200 m. The t,op layer often consists of a paleosoil with crusts of iron oxyde (Steiger, 1966). The age of this formation, the Gypsum-Melaphyr Formation, is post-Malm and pre-Aptian. Then follow approximately 250 m of light-coloured Orbitolina limestones of the Tiz Kuh Formation, which indicates marine conditions during Aptian times. There are no sediments of Aibian age. Marine conditions prevailed during most of the Late Cretaceous. During Cretaceous times, in the Chalus area different conditions prevailed. In the period post-Malm to pre-Barremian there are neither deposits of gypsum or volcanics. We have seen that during Late Jurassic and Early Cretaceous times limestones with sandy intercalations were deposited. Here- upon follows the Chalus Formation with a very thick sequence of volcanics alternating with limestones. This formation, the type section of which is about 1200 m thick, is subdivided into five members. The successive members have markedly different thicknesses. Member 1 is 320 m thick in the type section, and is built up of many dark-coloured, basic lavas with thin intercalations of tuffite. Member 2 is about 100 m thick, and consists of variegated limestones with sandy and silty intercalations; the Orbitolines indicate a Late Barremian to Aptian age. The next member is composed of basic lava flows and is up to 100 m thick. Member 4 is only 40 m thick. Its lower part consists of limestones with Orbitolines, indicating a Late Barre- mian and Early Aptian age. After a break in the sedimentation, sandy limestones and shales follow; these littoral sediments with macrofossils have a Turonian to Early Senonian age. Member 5 is up to 550 m thick, and is built up of lavas and agglomerates. The Chalus Formation is overlain by thick-bedded limestones and marls with abundant Globotruncana, pointing to a Late Cretaceous age. 117 SAMPLING LOCALITIES For our paleomagnetic research we collected oriented samples from three volcanic units (Table I). The collection consisted mainly of hand samples. In the mountainous area portable drilling equipment could be used sporadi- cally. In the area near the river Haraz east of Mount Damavand, the volcanics of the Gypsum-Melaphyr Formation were sampled at two localities: one from four lava flows in the mountains 2 km southwest of Lut, and the other from three lava flows in the Haraz valley, 10 km north of Baidjan along the road from Tehran to Amol (Figs. 1 and 3). In the Chalus area we sampled both member 1 and member 5 from the Chalus Formation. Samples were collected from member 1 at two localities. The first locality included two sites 17 km south of Chalus, one being a lava flow and the other a tuffite (WKB). The second locality, which was further south, 3 km south of Mianegh, included seven lava flows and one tuffite (WKN). Member 5 was sampled east of Marzanabad at six sites representing six successive lava flows (Figs. 2 and 3). In all, 103 hand samples and 30 cores were collected from 21 lava flows and 2 tuffite layers. PALEOMAGNETIC TREATMENT In the laboratory, cores with a diameter of 25 mm were drilled from the hand samples. All cores were cut into segments 22 mm long. In Table I, we list for each site both the number of hand samples or cores and the number of specimens used in the analyses. For the paleomagnetic research use was made of the standard equipment available. The procedures were as follows. From each site one pilot specimen was subjected to a progressive demagnetization in alternating magnetic fields (af) in 12-15 successive steps up to 200 mT peak (2000 Oe) and occasionally even up to 300 mT peak value (3000 Oe). Eighteen specimens, selected from individual sites, were progres- sively heated up to 574°C in 9 successive steps in an argon-filled furnace. After each step of progressive demagnetization the specimens were mea- sured either on an astatic or a spinner magnetometer.

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