Earth and Planetary Sctence l.etter~'. 75 (1985) 93. 10() 93 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands [41 Magnitude of crustal extension across the northern Basin and Range province: constraints from paleomagnetism Nicholas L. Bogen t,, and Richard A. Schweickert 2 I Department of Geological Sciences, Unioersit.v of Michigan, A nn Arhor. MI 48109 (U.S.A. ) e Mackay SchoolofMines. Unioersi(v of Net;aria at Reno, Reno. NV89557 (U.S.A.) Received December 8. 1984; revised version received March 30. 1985 The magnitude of crustal extension across the northern Basin and Range province is a matter of longstanding controversy; estimates range from 10 to 3009~,. Recently published estimates of extension across the southern Basin and Range province (36°N) are in the range of 80-100%. Thus, the larger values suggested for the northern part of the province (40°N) seem to require substantial counterclockwise rotation of the Sierra Nevada during 'l-ertiary extension. Paleomagnetic data from the range, however, limit rotation to 4 + 10 ° at the 95~ confidence level. These limits. combined with estimates of extension near the Garlock fault, allow severe constraints to be placed on the magnitude of extension across more northerly parts of the province. We conclude that the maximum extension at 40°N is about 50%. and that values of 39 5- 12q~ (188 + 43 km) are likely. I. Introduction ber of north-striking, planar normal faults. In such models, reasonable dip-slip displacements on the The continental crust beneath the Basin and faults yield values of extension across the entire Range province (Fig. 1) has undergone a large province of 10-20% [4-7]. Hamilton and Myers amount of extension during Cenozoic time and [8] pointed out that listric normal faults with the extension is continuing at present. The magnitude same throw as planar faults would produce greater of extension is not well known, however, particu- extension and they speculated that as much as larly in the northern part of the province; esti- 300% extension might have occurred across the mates range widely from 10 to 300%. More precise northern Basin and Range. More recently, estimates arc required to understand the structural Wernicke [9] and Allmendinger ct al. [10] have evolution of the Basin and Range province and, in demonstrated the existence of low-angle normal general, the uplift, subsidence and rifting of ex- faults in the northern Basin and Range, but neither tended continental lithosphere. In contrast, the the number of these faults nor their displacements magnitude of shortening strains across large re- are well known. To determine the magnitude of gions of the crust have been known for some time extension across the Basin and Range from crustal in at least a few areas, for example, the Idaho- structure alone, it would be necessary to know the Wyoming and southern Canadian overthrust belts location, dip and displacement of all of the normal [1,2]. Yet, few similar data have been available for faults. regions characterized by extensional strains, de- Recently, Wernicke et al. [11] showed that there spite the importance of such strains in studies of has been a minimum of 65% (140 km) of east-west continental basin and plateau formation [3]. extension across the southern part of the Basin Early estimates of the magnitude of crustal and Range, just north of the Garlock fault (Fig. 1). extension across the northern part of the Basin They compared the distances between various geo- and Range province were based largely on simple logic features north of the fault with the distances models of crustal structure involving a large num- between correlative features to the south where little or no extension has occurred since the end of * New address: Woodv,'ard-(.'lyde Consultants, 201 Wil- Miocene time. Their result is the first estimate of Iowbrook Boulevard. p.o. Box 290. Wayne. NJ 07470. U.S.A. extension in the region that is not tied to a model 0t)12-821X/85/$03.30 :' 1985 Elsevier Science Publishers B.V. 94 \ Sierra Nevada has rotated counterclockwise. From this test, we are able to constrain the magnitude of crustal extension across the northern Basin and SRP Range province. A similar method relating block .i : , rotation to extension has been applied by Le Pichon and Angelier [15] to estimate the extension across the Aegean Sea, although they did not use paleomagnetic data to constrain the accompanying °+ n_ovc, rotation of the Aegean arc. Frei et al. [16] also have studied the rotation of the Sierra Nevada in conjunction with rotations of other crustal blocks to the northwest, including the Klamath Moun- tains and Oregon Coast Range. Their principal concern was the paleogeographic and tectonic evolution of the continental margin during Ceno- zoic time but also included the westward move- ment of the Sierra Nevada block. In contrast, the main purpose of this study is to determine some of the large-scale structural properties of extending continental crust and therefore, our methods, as- sumptions and data are different from and inde- pendent of theirs. Yet, we note that our main conclusion regarding the magnitude of extension Fig. 1. Generalized map of the Basin and Range province. bounded by the Sierra Nevada (SN). Snake River plain (SRP), across the northern part of the Basin and Range Colorado plateau (CP) and Garlock fault (GF). A rotation of province is not substantially different and the two the Sierra Nevada bkv,:k about a pole shown here at its south- approaches are mutually st, pportive, in large part. ern end (the location is not critical) of 23 ° counterclockwise would cause 185 km more east-west extension at 40°N than at 2. Paleomagnetism 36°N. Paleomagnctic data, however, allow countercltx:kwise rotation up to 6 ° . which would produce at most 48 km more east-west extension at 40°N than at 36°N. Paleomagnetic data have been collected from five formations in the western Sierra Nevada metamorphic belt (Fig. 2). The formations range of the crustal structure. in age from Mississippian to Late Jurassic: four of The width of the Basin and Range province the units are volcanic and one is a mafic dike increases from about 325 km at 36°N to about 680 swarm. In the far-northern Sierra Nevada, the km at 40°N (Fig. 1). This observation led Ham- Mississipian Taylor and Permian Reeve Forma- ilton and Myers [8] to suggest that extension in the lions record northwesterly, normal magnetizations north has been far greater than in the south. They based on plots of stepwise demagnetization data noted that such differential extension would re- [17]. Linear trajectories on vector-endpoint di- quire the northern Sierra Nevada to have moved agrams indicate that a single component is pre- westward more than the southern part of the range, sent. Formation mean directions and paleomag- resulting in a significafft counterclockwise rotation nctic pole positions are listed in Table 1. Similar (Fig. 1). Their suggestion seems reasonable in light magnetization directions are present in three of paleomagnetic evidence of rotations in other Jurassic formations in the central part of the west- regions of far-western North America during Ter- ern belt, including the Lower Jurassic Penon tiary time, although these rotations are generally Blanco Formation, the Callovian-Oxfordian Log- considered to have occurred in a clockwise direc- town Ridge Formation and the Oxfordian Sonora tion [12--14]. In the following, we use dike swarm (Table 1, [18]). These formation means paleomagnetic data from the Sierra Nevada and were also determined by regression analysis of Colorado plateau to test the hypothesis that the vector end-point diagrams, largely from thermal- 95 ---'T £,o 119" Sonora dikes bear no marked cleavage or foliation I in contrast to the other units which yielded only a Me/ones Fault i single component of magnetization. Mr" Tcyior ~ Reeve fm Si~es Demagnetization studies of the Reeve and Taylor Formations in alternating fields (AF) indi- cate that the remanence is of low coercivity with ' T-:::--- : ,4) nof Area f,; stable end-point directions obtained between 15 -12 and 30 mT [17]. AF demagnetization did not Z J2 ._ break down the remanence of most samples of the M-;- three Jurassic units, however, and thus, thermal 0 5 ~m demagnetization was employed. Negative fold tests from both the northern and central Sierra and a conglomerate test of pillow z_ \ breccia from the Logtown Ridge Formation indi- cate that the northwesterly magnetization is sec- \. ondary [17,18]. There is no firm upper limit on the \ R~dge Frn Sties age of magnetization yet, all of the units were folded and metamorphosed to low-greenschist -38°~onorf3 DJkes "~04 / #.& facies during the Late Jurassic Nevadan orogeny, .CSFT the youngest structural-metamorphic event to have Penon L3fanco occurred in the region and the only such event to Frn S;fes Sonora Fault have affected all of the units studied [19]. We ~eiones Fautt suggest, as did tIannah and Verosub [17]. that the secondary magnetizations formed during the -!'°. Nevadan metamorphic culmination, after folding but during final development of the foliation. This Fig. 2. Paleomagnetic sampling sites in the western metamor- point is discussed further below. phic belt of the Sierra Nevada, California. Taylor and Reeve The Nevadan orogeny is one of the most tightly Formation sites are from [17] and the remainder are from [18]. bracketed deformational-metamorphic events Mz and Pz indicate Mesozoic and Paleozoic rocks, respec- tively. known. Folded strata are as young as early Kim- meridgian and folds and cleavages are cut by plutons as old as late Kimmeridgian [19,20]. The demagnetization data. metamorphic culmination is late tectonic and Thermal demagnetism studies revealed a broad therefore is of late Kimmeridgian age, although it range of unblocking temperatures for each of the may have extended into the early Tithonian Age as five formations.