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At least 469 records · Page 26Linked to original sources
USGS1981

A lithologic-tectonic framework for the metallogenic provinces of California

The lithologic-tectonic framework of California developed principally during Mesozoic time when various terranes of oceanic crust and island-arc crust were accreted to older sialic crust, resulting in westward growth of the continent. Emplacement of great batholithic masses of granitoid rocks cutting all these crustal types also took place during the Mesozoic period. The discrete tectonostratigraphic terranes that resulted from these events and subsequent Tertiary and Quaternary volcanic events are characterized by specific types of metallic mineral deposits or, in some terranes, by the virtual absence of deposits. Lead-silver-zinc replacement-type deposits are common in the Paleozoic carbonate terrane in the eastern part of the state and occur sporadically elsewhere in the miogeoclinal and cratonal terranes but are absent from the oceanic and island-arc terranes. The vast majority of contact metasomatic tungsten deposits, including all the large ones, are in pendants of miogeoclinal rocks in the Sierra Nevada batholith, but the important Atolia deposits reside in granitoid rocks that invade oceanic terrane. Molybdenum distribution closely follows that of tungsten. All the large contact metasomatic iron deposits in California are in craton and miogeoclinal terranes, but sparse small deposits of this type also occur in island-arc terranes of the northern Sierra Nevada and eastern Klamath Mountains. Lode gold deposits, although widely scattered, show a marked preference for oceanic and island-arc terranes that have been invaded by granitoid plutons. All the major deposits, including late Tertiary bonanza deposits such as Bodie, are in such terranes. It appears that magmatic processes were responsible for mobilizing and transporting the gold, but the metal was perhaps derived from the eugeosynclinal rocks, notably the mafic volcanics. Most mercury deposits are found in the Coast Ranges, where they commonly occur in silica-carbonate rock, an alteration product of serpentinite. The deposits appear to be spatially related to the Coast Range thrust, and the source of the mercury may have been sedimentary rocks of the underlying Franciscan assemblage. Epigenetic mineralization occurred at several different times during the Mesozoic, and again during Miocene and Pliocene time. The timing of mineralization events and the distribution of various deposit types indicate that no broad-scale zoning of epigenetic deposits exists around the Sierra Nevada batholith.Syngenetic deposits are represented mainly by massive sulfides, chert-associated manganese, and chromite. The massive sulfide deposits, with one exception, are restricted to island-arc terranes, and nearly all of these deposits are in silicic volcanic rocks. They are interpreted to be syngenetic with the enclosing rocks, although some redistribution of metals may have occurred after the original deposition. The deposits occur in volcanic sequences of at least five different ages ranging from Early Devonian to Late Jurassic or Early Cretaceous and, along with their enclosing rocks, were probably formed at some distance from their present sites. Chert-associated manganese deposits occur mainly in exotic blocks of oceanic crust in melange and probably formed in fairly deep ocean environments. Chromite is confined to ultramafic rock, much of which occupies suture zones separating various accreted terranes.

Economic Geology↗
USGS1985

Paragenesis and tectonic significance of base and precious metal occurrences along the San Andreas fault at Point Delgada, California

Studies of sulfide vein mineralization at Point Delgada, California, suggest that the mineralizing process was intimately associated with the San Andreas fault system and/or plate-margin tectonism. The deposits occur along north-south to northeast-southwest-trending tension faults that intersect and cross a major northwest-trending fault previously considered to be a major trace of the San Andreas fault. Galena and sphalerite are concentrated near the intersection of the principal mineralized vein with the fault, suggesting that brecciated rocks at this intersection acted as a major conduit for hydrothermal circulation. Fluid inclusion data show that the sulfide mineralization occurred at a depth of at least 400 m, probably at a temperature of about 250 degrees C. Mineralization fluids were in the salinity range of 10.5 to 5.2 equivalent weight percent NaCl. Sodium and potassium were depleted from the mineralizing fluids in the early stages of mineralization, and traces of argentian tetrahedrite and cassiterite in the fluid inclusions probably account for some of the anomalous values of silver and tin in galena and sphalerite. The age of mineralization, based on K-Ar dating of adularia, is 13.8 + or - 0.4 m.y. This age suggests that the purported trace of the San Andreas fault crossed by the sulfide veins has been largely inactive since middle Miocene time. Lead isotope and geochemical data suggest that base and precious metals in the sulfide veins were derived largely from arc-related sedimentary rocks. The heat source for hydrothermal circulation may have been associated with intrusion of asthenosphere into a triangular slab window opened southeast of the northward-propagating Mendocino triple junction shortly before 13.8 m.y. ago.

Economic Geology↗
USGS2003

Tectonic evolution of the Proterozoic Colorado Province, Southern Rocky Mountains: A summary and appraisal

The Colorado province is a major component of a >1000-km-wide belt of Paleoproterozoic ocean-arc rocks that occupies the southwestern United States. Known as the Transcontinental Proterozoic provinces, this belt of largely juvenile rocks was added to the southern margin of the North American craton during the interval 1.8-1.70 Ga by convergent tectonism along the Cheyenne belt. A growing body of data suggests that these rocks were deposited, at least locally, on older rocks of earliest Proterozoic and Archean ages, probably correlative with the Trans-Hudson and Penokean orogens. The volcano-plutonic and associated sedimentary rocks of the Colorado province record two major, regional orogenies: (1) an older, protracted thermotectonic episode (1.78-1.70 Ga), named the Colorado orogeny, which involved mainly amphibolite-facies metamorphism during piecemeal assembly of various ocean-arc terranes; and (2) a younger, Mesoproterozoic intra-continental orogeny, named the Berthoud orogeny, which involved associated regional heating and A-type plutonism chiefly during the interval 1.45-1.40 Ga. The term "Colorado orogeny" is proposed for the regional Paleoproterozoic dynamothermal deformation. To distinguish differences in the geodynamics and ages of deformation and facilitate comparisons from place to place, type localities are proposed for separate phases or events of the Colorado orogeny. The central Front Range is suggested as a type area for the Berthoud orogeny, because the character and orientation of structures there are readily distinguished from those of the older Colorado orogeny.

Colorado↗
USGS2006

Cenozoic tectonic evolution of the basin and range province in northwestern Nevada

A regional synthesis of new and existing geologic and thermochronologic data document late Cretaceous - early Cenozoic regional erosion, Oligocene - Miocene volcanism, and subsequent late Miocene extension of the Basin and Range Province in northwestern Nevada and northeastern California. Across an ???220-km-wide region between the Santa Rosa and Warner Ranges, conformable sequences of 35 to 15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ???23 km of total east-west extension (???12%). Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11 to 12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. Extension in the Santa Rosa Range to the east took place during the same interval, although the post-6 Ma part of this history is less well constrained. Less complete constraints from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued up to the present. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but may represent westward migration of Basin and Range extension during the Pliocene. Compared to the many parts of the Basin and Range in central and southern Nevada, extension in northwestern Nevada began more recently, is of lesser total magnitude, and was accommodated entirely by high-angle normal faults. Fission-track data document Late Cretaceous unroofing of Cretaceous (115 - 100 Ma) granitic basement rocks in northwestern Nevada, followed by a long period of relative tectonic quiescence that persisted through Oligocene and Miocene volcanism until the onset of Basin and Range extension at ???12 Ma. The low magnitude of extension (12%) and early Tertiary stability suggest that the modern ???31 km thick crust in northwestern Nevada was only slightly thicker (???35 km) prior to extension at 12 Ma, and was no thicker than ???38 km in the Late Cretaceous. This stands in contrast to other parts of the Basin and Range, where the crust was thickened to at least 45 to 50 km by Cretaceous thrusting and subsequently thinned to ???30 km by large magnitude (>50%) extension.

American Journal of Science↗
USGS1983

Discovery of a Late Triassic basin north of Boston and some implications as to post-Paleozoic tectonics in northeastern Massachusetts (USA)

As deduced from various data the Triassic-Jurassic Middleton basin-trends NE and is probably approx 5.7km in length and no >0.5km in width. Two sets of dikes which have been K/Ar dated as Late Triassic-Early Jurassic have the same NNE strikes as strike-slip faults. There are indications that a tectonic event of some importance may have occurred in Middle Mesozoic time. -from Author

Massachusetts↗
USGS1984

Evidence for tectonic emplacement of ultramafic and associated rocks in the pre-Silurian eugeoclinal belt of western New England: Vestiges of an ancient accretionary wedge

In northern Vermont, detailed 1:10,000 mapping of the Hazens Notch, Ottauquechee, Stowe, and Moretown formations in the 60 km 2 Jay area has shown that metasedimentary rocks and serpentinites are highly faulted to produce a tectonic stratigraphy in which serpentinites and talc-carbonate rocks occur as slivers along faults that separate contrasting lithic assemblages. Remnants of the ophiolite sequences of Quebec. In western Massachusetts, the ultramafic-bearing section of the Rowe Schist and the western part of the Moretown Formation are largely equivalent to the rocks in northern Vermont. Mapping at 1:24,000 scale and compilation work for the new bedrock map of Massachusetts have shown that the Rowe Schist consists of numerous lenses of greenschist, carbonaceous schist, gray schist, amphibolite, and serpentinite. Although the geometric arrangement of the lenses and serpentinites is strikingly similar to that seen in the Jay area, higher grade Taconian and Acadian metamorphism has partially obscured the fault fabrics seen in Vermont.

Massachusetts, Vermont↗
USGS2017

Investigating the landscape of Arroyo Seco—Decoding the past—A teaching guide to climate-controlled landscape evolution in a tectonically active region

Introduction Arroyo Seco is a river that flows eastward out of the Santa Lucia Range in Monterey County, California. The Santa Lucia Range is considered part of the central California Coast Range. Arroyo Seco flows out of the Santa Lucia Range into the Salinas River valley, near the town of Greenfield, where it joins the Salinas River. The Salinas River flows north into Monterey Bay about 40 miles from where it merges with Arroyo Seco. In the mountain range, Arroyo Seco has cut or eroded a broad and deep valley. This valley preserves a geologic story in the landscape that is influenced by both fault-controlled mountain building (tectonics) and sea level fluctuations (regional climate). Broad flat surfaces called river terraces, once eroded by Arroyo Seco, can be observed along the modern drainage. In the valley, terraces are also preserved like climbing stairs up to 1,800 feet above Arroyo Seco today. These terraces mark where Arroyo Seco once flowed.The terraces were formed by the river because no matter how high they are, the terraces are covered by gravel deposits exactly like those that can be observed in the river today. The Santa Lucia Range, Arroyo Seco, and the Salinas River valley must have looked very different when the highest and oldest terraces were forming. The Santa Lucia Range may have been lower, the Arroyo Seco may have been steeper and wider, and the Salinas River valley may have been much smaller. Arroyo Seco, like all rivers, is always changing. Some-times rivers flow very straight, and sometimes they are curvy. Sometimes rivers are cutting down or eroding the landscape, and sometimes they are not eroding but depositing material. Sometimes rivers are neither eroding nor transporting material. The influences that change the behavior of Arroyo Seco are mountain uplift caused by fault moment and sea level changes driven by regional climate change. When a stream is affected by one or both of these influences, the stream accommodates the change by eroding, depositing, and (or) changing its shape. In the vicinity of Arroyo Seco, the geologically young faulting history is relatively well understood. Geologists have some sense of the most recent faulting event and of the faulting in the recent geologic past. The timing of regional climate changes is also well accepted. In this area, warm climate cycles tend to cause the sea level to rise, and cool climate cycles tend to cause the sea level to fall. If we understand the way the terraces form and their ages in Arroyo Seco, we can draw conclusions about whether faulting and (or) climate contributed to their formation. This publication serves as a descriptive companion to the formal geologic map of Arroyo Seco (Taylor and Sweetkind, 2014) and is intended for use by nonscientists and students. Included is a discussion of the processes that controlled the evolution of the drainage and the formation of the terraces in Arroyo Seco. The reader is guided to well-exposed landscape features in an easily accessible environment that will help nonscientists gain an understanding of how features on a geologic map are interpreted in terms of earth processes.

California↗
USGS2002

Spatial digital database for the tectonic map of southeast Arizona

A spatial database was created for Drewes' (1980) tectonic map of southeast Arizona: this database supercedes Drewes and others (2001, ver. 1.0). Staff and a contractor at the U.S. Geological Survey in Tucson, Arizona completed an interim digital geologic map database for the east part of the map in 2001, made revisions to the previously released digital data for the west part of the map (Drewes and others, 2001, ver. 1.0), merged data files for the east and west parts, and added additional data not previously captured. Digital base map data files (such as topography, roads, towns, rivers and lakes) are not included: they may be obtained from a variety of commercial and government sources. This digital geospatial database is one of many being created by the U.S. Geological Survey as an ongoing effort to provide geologic information in a geographic information system (GIS) for use in spatial analysis. The resulting digital geologic map database can be queried in many ways to produce a variety of geologic maps and derivative products. Because Drewes' (1980) map sheets include additional text and graphics that were not included in this report, scanned images of his maps (i1109_e.jpg, i1109_w.jpg) are included as a courtesy to the reader. This database should not be used or displayed at any scale larger than 1:125,000 (for example, 1:100,000 or 1:24,000). The digital geologic map plot files (i1109_e.pdf and i1109_w.pdf) that are provided herein are representations of the database (see Appendix A). The map area is located in southeastern Arizona (fig. 1). This report describes the map units (from Drewes, 1980), the methods used to convert the geologic map data into a digital format, the ArcInfo GIS file structures and relationships, and explains how to download the digital files from the U.S. Geological Survey public access World Wide Web site on the Internet. The manuscript and digital data review by Helen Kayser (Information Systems Support, Inc.) is greatly appreciated.

Arizona↗
USGS2007

Tectonic history of mid-Miocene to present southern Victoria Land Basin, inferred from seismic stratigraphy in McMurdo Sound, Antarctica

New and existing seismic reflection data in southern McMurdo Sound have been used to investigate Neogene tectonic history of the Terror Rift adjacent to the Transantarctic Mountains and along the western margin of the West Antarctic Rift System. Seismic data image a young rifting episode that is largely unsampled by CRP and CIROS drill holes. Data reveal up to 3.5 km of post middle Miocene strata deposited in this part of the NNW- SSE trending Terror Rift basin. Mapped fault trends in the southern Terror Rift parallel the axis of the basin and are prominent in a 40 km wide zone north of Ross Island. Displacement on individual faults in this zone can exceed 100 m and faults collectively accommodate approximately 10-15 km of middle Miocene to recent extension.

Open-File Report↗
USGS2007

The Ross Orogen and Lachlan Fold Belt in Marie Byrd Land, Northern Victoria Land and New Zealand: implication for the tectonic setting of the Lachlan Fold Belt in Antarctica

Correlation of the Cambrian Delamerian Orogen of Australia and Ross Orogen of the Transantarctic Mountains widely accepted but the extension of the adjacent Lachlan Orogen into Antarctica is controversial. Outside the main Ross-Delamerian belt, evidence of this orogeny is preserved at Mt Murphy in Marie Byrd Land and the in Takaka Terrane of New Zealand. In all pre-break- configurations of the SW Pacific, these two areas are far removed from the Ross-Delamerian belt. Evidence from conglomerates in the Takaka Terrane, however, shows that in Late Cambrian times it was adjacent to the Ross Orogen. This indicates major tectonic displacements within Gondwana after the Cambrian and before break-up. The Lachlan Orogen formed in an extensional belt in a supra-subduction zone setting and the Cambrian rocks of Marie Byrd Land and New Zealand are interpreted as parts of a rifted continental ribbon on the outboard side of the Lachlan belt.

Open-File Report↗
USGS2007

Active tectonics on Deception Island (West-Antarctica): A new approach by using the fractal anisotropy of lineaments, fault slip measurements and the caldera collapse shape

The tectonic field on Deception Island (South Shetlands, West Antarctica) is determined from structural and fractal analyses. Three different analyses are applied to the study of the strain and stress fields in the area: (1) field measurements of faults (strain analysis), (2) fractal geometry of the spatial distribution of lineaments and (3) the caldera shape (stress analyses). In this work, the identified strain field is extensional with the maximum horizontal shortening trending NE-SW and NW-SE. The fractal technique applied to the spatial distribution of lineaments indicates a stress field with SHMAX oriented NE-SW. The elliptical caldera of Deception Island, determined from field mapping, satellite imagery, vents and fissure eruptions, has an elongate shape and a stress field with SHMAX trending NE-SW.

Open-File Report↗
USGS2014

The 1946 Unimak Tsunami Earthquake Area: revised tectonic structure in reprocessed seismic images and a suspect near field tsunami source

In 1946 at Unimak Pass, Alaska, a tsunami destroyed the lighthouse at Scotch Cap, Unimak Island, took 159 lives on the Hawaiian Islands, damaged island coastal facilities across the south Pacific, and destroyed a hut in Antarctica. The tsunami magnitude of 9.3 is comparable to the magnitude 9.1 tsunami that devastated the Tohoku coast of Japan in 2011. Both causative earthquake epicenters occurred in shallow reaches of the subduction zone. Contractile tectonism along the Alaska margin presumably generated the far-field tsunami by producing a seafloor elevation change. However, the Scotch Cap lighthouse was destroyed by a near-field tsunami that was probably generated by a coeval large undersea landslide, yet bathymetric surveys showed no fresh large landslide scar. We investigated this problem by reprocessing five seismic lines, presented here as high-resolution graphic images, both uninterpreted and interpreted, and available for the reader to download. In addition, the processed seismic data for each line are available for download as seismic industry-standard SEG-Y files. One line, processed through prestack depth migration, crosses a 10 × 15 kilometer and 800-meter-high hill presumed previously to be basement, but that instead is composed of stratified rock superimposed on the slope sediment. This image and multibeam bathymetry illustrate a slide block that could have sourced the 1946 near-field tsunami because it is positioned within a distance determined by the time between earthquake shaking and the tsunami arrival at Scotch Cap and is consistent with the local extent of high runup of 42 meters along the adjacent Alaskan coast. The Unimak/Scotch Cap margin is structurally similar to the 2011 Tohoku tsunamigenic margin where a large landslide at the trench, coeval with the Tohoku earthquake, has been documented. Further study can improve our understanding of tsunami sources along Alaska’s erosional margins.

Alaska↗
USGS1975

Land subsidence and tectonism, Raft River Valley, Idaho

A comparison of 1974 leveling data with elevations established 40 years earlier reveals two types of vertical ground movement which have occurred in Raft River Valley, Idaho: (1) regional differential movement of about 0.22 ft (6.4 cm), apparently due to tectonism, and (2) extensive land subsidence of as much as 2.61 ft (0.80 m) caused by withdrawal of ground water. Data are too sparse to calculate the magnitude or areal extent of subsidence; however, tentative lines of equal subsidence suggest that the area affected by subsidence probably exceeds 100 mi 2 (260 km 2 ). In order to estimate historic subsidence or subsidence potential in Raft River Valley serious consideration should be given to a field program of basic-data collection. Leveling along a few carefully selected lines of existing control and the installation and operation of extensometer water-level recorders in areas of continuing water-level decline would provide useful data for evaluating past and estimating future subsidence.

Idaho↗
USGS1979

Bibliography of selected reports related to studies on earthquake tectonics and risk, 1975-1978

The reports and abstracts included in this bibliography have been selected arbitrarily on the basis of their relation to U.S. Geological Survey studies on earthquake tectonics and risk. U.S. Geological Survey scientists involved in other aspects of earthquake studies have also written many dozens of reports during the same time frame, but such reports are precluded from this collection. The bibliography contains a subject index, constructed generally from key words found in the titles, and a junior-author index. Code numbers printed at the left of each reference entry have no significance other than that of making it easy to follow the notations in the subject and junior-author indexes. The reader may notice that some of these code numbers are missing from the ordinary running list; others have been supplemented by A's or B's. This has happened because either references assigned to numbers have been dropped or references originally omitted by mistake have been added. The references have been typed in accepted U.S. Geological Survey format, which of late means that abbreviations are not used, except for minor notations such as "v." for volume or "p." for page.

Open-File Report↗
USGS1980

Biostratigraphic results of dart-coring in the western Gulf of Alaska, and their tectonic implications

Age determinations are reported for microfossils from 56 dart-cores collected in 1978 from the Kodiak shelf. The ages suggest that rocks cropping out along Albatross Bank, at the shelf edge, are as old as middle or late Miocene, and that the anticline forming Albatross Bank, previously defined with multi- and single-channel seismic records, plunges northeastward in the sampled area. Foraminiferal paleobathymetry further suggests that m.ajor tectonic activity has occurred on the Kodiak shelf since the early or middle Pliocene, including two episodes of vertical movement--first subsidence on the order of 2000 m, then uplift of at least 3000 m. Microfossil ages and paleobathymetric determinations indicate an average uplift rate of 1000- 3000 m/m.y. for the seaward edge of the Kodiak shelf during the Quaternary.

Alaska↗
USGS1981

Cooperative tectonic studies from seismic network in central Italy

The purpose of the subject contract work was to explore the possibility of tectonic research-work based on seismic data from a local network carried out as a cooperative effort between the Institute Nazionale di Geofisica (ING), Rome, and Columbia University (L-DGO), New York. This effort has been rather successful, and we hope to be able to continue this collaboration in the future.

Open-File Report↗
USGS1982

Paleomagnetic evidence bearing on Tertiary tectonics of the Tihamat Asir coastal plain, southwestern Saudi Arabia

Paleomagnetic directions determined for an upper Oligocene to lower Miocene dike swarm and from two lower Miocene layered gabbros in the Tihamat Asir coastal plain of southwestern Saudi Arabia are used to test several hypotheses concerning the tectonics of rifting along the eastern margin of the Red Sea. The dikes and gabbros were emplaced during the initial phases of Red Sea rifting and may mark the transition between continental and oceanic crust. Although these rocks have been hydrothermally altered to varying degrees, reliable remanent directions after alternating-field demagnetization were obtained for 23 dikes and for gabbros at Jabal at Tirf and Wadi Liyyah. Twelve of the dikes are reversely magnetized. After the directions of the reversely magnetized dikes are inverted 180?, the mean direction calculated for the normal dikes is approximately 24? more downward than that calculated for the reversed dikes. This result is similar to that found for the As Sarat volcanic field, 100 km to the north, and may be due to a displaced dipole source for the field. The unrotated mean remanent direction for the dikes (inverting reversed dike directions 180?) is D (declination) = 353.2? and I (inclination) = 6.8? with a95 (radius of the cone of 95 percent confidence) = 8.9? whereas directions from the Jabal at Tirf and Wadi Liyyah gabbros lie at D = 176.2?, I = -1.6? (a95 = 7 1 ?) and D = 17.1?, I = 16.3? (a 95 = 8.7?), respectively. Comparing these results with the results from the As Sarat volcanic field, all the paleomagnetic evidence supports a model for approximately 20 ? of westward tilting of the Wadi Damad and Wadi Jizan areas after the emplacement of the Jabal at Tiff gabbro. The Wadi Liyyah area may have been tilted even more toward the Red Sea. The paleomagnetic directions from three widely separated localities in the Jabal at Tirf gabbro are not significantly different, a fact which indicates that the body cooled in approximately its present bowl shape. Evidence suggests that the ratio of normal to reversed dikes may change significantly along a 6-km-long traverse normal to the trend of the dike swarm, possibly reflecting migration of .a spreading axis.

Open-File Report↗
USGS1983

Late Proterozoic island-arc complexes and tectonic belts in the southern part of the Arabian Shield, Kingdom of Saudi Arabia

Two main subdivisions of layered rocks are recognized in the southern Arabian Shield south of lat 22? N. These are an older ensimatic-arc complex, which formed 1100-800 m.y. ago, and a younger marginal-arc complex, which formed 800-690 m.y. ago. The older ensimatic-arc complex, located in the southwestern part of the Shield, includes graywacke and mafic to intermediate volcanic rocks of the essentially contemporaneous Baish, Bahah, and Jiddah groups. Although the younger arc complex is also dominantly ensimatic in character, it is also partly superimposed over the older ensimaticarc complex. The superimposed portions of the younger arc complex are represented by the Ablah, Samran, and possibly the Ararat groups. The ensimatic portion of the younger arc group is represented by the Halaban group, which was deposited to the east and northeast of the older ensimatic-arc complex. The Halaban group includes andesitic and dacitic volcanic rocks and associated clastic sedimentary rocks. The layered rocks of both arc complexes are intruded by dioritic (quartz diorite, tonalite, trondhjemite) plutonic rocks. The southern Shield is also subdivided into a number of structurally bounded, north-trending tectonic belts. Within the older ensimatic complex, three belts are recognized. From west to east, these are the Lith, Bidah, and Tayyah belts. Within these three belts, progressive facies changes indicate a gradation from deep-water facies in the south to shallow-water or-terrestrial facies in the north. The distribution of dioritic batholiths, as well as the distribution of layered-rock facies, suggests a northwest-trending axis for the older ensimatic-arc complex. The younger arc complex is present within six belts, the Makkah source papers. In Fleck and others (1980), the term 'quartz diorite' includes both tonalite and quartz diorite as defined in the International Union of Geological Sciences (IUGS) system of plutonic rock classification (Streckeisen, 1973). Initial 87Sr/86Sr ratios are not included in the appendix, but all rocks more than 660 m.y. old have initial ratios in the range 0.7021-0.7035, with only two greater than 0.7030. Thus, nothing in the Rb-Sr data suggests involvement of an older continental crust during the evolution of the southern Shield. A lead isotope study of ore minerals and potassium feldspars of the Arabian Shield by Stacey and others (1980) also suggests that no older (Archean to early Proterozoic) evolved continental-type crust underlies the southern Shield. An early summary of mapping (Schmidt and others, 1973) suggests that older sialic basement underlies the late Proterozoic layered rocks in the southern Shield. However, subsequent-mapping and the isotopic studies cited above have established that all of these rocks are of late Proterozoic age and that all rocks of the southern Shield that are more than 660 m.y. old have ensimatic or mantle isotopic characteristics. Figure 2 shows, with only two exceptions, that rocks more than 800 m.y. old are present west of the boundary separating the Tayyah and Khadra belts. The exceptions are two poorly controlled Rb-Sr ages obtained by Fleck (1980) on two quartz diorite plutons in the Malahah region (appendix 1, localities 26 and 27). Preliminary uranium-thorium zircon data of Stacey now suggest that one of these quartz diorite plutons (locality 26) has an age of approximately 640 m.y. Therefore, we prefer to discount the two dates of Fleck until further information is available. As noted earlier and as described below, most of the rocks of the southern Arabian Shield have characteristics typical of those formed in the island-arc environment by subduction-related processes. We shall refer to the group of rocks in the western part of the southern Shield, which formed from 1100 to 800 m.y. ago, as the 'older ensimatic-arc complex' and those in the eastern and northwestern parts, which formed from 800 to 690 m.y. ago, as the 'younger marginal-arc compl

Open-File Report↗