Showing papers on "Terrane published in 1993"

Paleomagnetism of the Atlantic, Tethys, and Iapetus oceans

Abstract: 1. Prologue 2. Paleopoles and paleomagnetic directions 3. Megaplates, microplates, blocks, terranes, accreted slivers, thrusts and olistostromes 4. Paleomagnetic information - what makes a paleopole valuable? 5. The major continents and Pangea 6. The opening of the Atlantic Ocean 7. The Tethyan blocks 8. The Paleozoic terranes of Iapetus 9. Epilogue 10. Appendices 11. References 12. Index.

The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block, Western Australia

Abstract: Most Archaean gold ores belong to a coherent genetic group of structurally controlled lode-deposits that are characteristically enriched in Au with variable enrichments in Ag, As, W, Sb, Bi, Te, B and Pb, but rarely Cu or Zn, and are surrounded by wallrock alteration haloes enriched in K, LILE and CO2, with variable Na and/or Ca addition. Evidence from the Yilgarn Block of Western Australia, combined with similar evidence from Canada and elsewhere, indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at PT conditions ranging from 180°C at <1 kb to 700°C at 5 kb. Individual deposits, separated by tens to hundreds of kilometres, collectively show transitional variations in structural style of mineralisation, vein textures, and mineralogy of wallrock alteration that relate to the PT conditions of their formation at varying crustal depths. Specific transitions within the total spectrum may be shown also by deposits within gold camps, although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt. Recognition of the crustal continuum of deposits implicates the existence of giant late-Archaean hydrothermal systems with a deep source for the primary ore fluid. A number of deep fluid and solute reservoirs are possible, including the basal segments of greenstone belts, deep-level intrusive granitoids, mid-to lower-crustal granitoidgneisses, mantle lithosphere, or even subducted oceanic lithosphere, given the probable convergent-margin setting of the host greenstone terranes. Individual stable and radiogenic isotope ratios of fluid and solute components implicate fluid evolution from, or equilibrium with, a number of these reservoirs, stressing the potential complexity of pathways for fluid advection to depositional sites. Lead and strontium isotope ratios of ore-associated minerals provide the most persuasive evidence for fluid advection through deep-level intrusive granitoids or granitoid-gneiss crust, whereas preliminary oxygen isotope data show that mixing of deeply sourced fluid and surface waters only occurred at the highest crustal levels recorded by the lode gold deposits.

Tectonics of an ultrahigh-pressure metamorphic terrane: The Dabie Shan/Tongbai Shan Orogen, China

Abstract: Ultrahigh-pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino-Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold-thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino-Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm-Nd, Rb-Sr, and Ar-Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial eNd values indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan-type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100-km-thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh-pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process.

U-Pb ages from SW Poland: evidence for a Caledonian suture zone between Baltica and Gondwana

Abstract: Faunal and palaeomagnetic evidence suggests the existence of a c 3000 km-wide Tornquist Sea between Gondwana and Baltica in Early Ordovician times, which narrowed to −50 –2 Ma) An unmetamorphosed ophiolite gives a 420 −20 –2 Ma age, whilst in two other terranes amphibolite-facies island-arc lavas have fossiliferous Lower Ludlow (424–415 Ma) sequences: all are unconformably overlain by unmetamorphosed Upper Devonian conglomerates A sinistral transpressive regime is observed in regionally extensive mylonite zones Titanite and zircon ages (338 −2 –3 and 339 ± 4 Ma) record Variscan magmatism The data suggest considerable narrowing of the Tornquist Sea during the Ordovician, continuing ocean floor and island arc activity in the Silurian, and final sinistral transpressive closure by the Mid-Devonian

Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area

Abstract: A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west.

Proterozoic Crustal Evolution

Abstract: Volcanic rocks of early Proterozoic greenstone belts, T.E. Smith the Proterozoic ophiolite problem, H.H. Helmstaedt and D.J. Scott Proterozoic rifts, J.C. Green geochemistry and significance of mafic dyke swarms in the Proterozoic, J. Tarney tectonic setting of Proterozoic layered intrusions with special reference to the bushveld complex, G. von Gruenewaldt and R.E. Harmer Proterozoic anorthosite complexes, R.A. Wiebe the role of anorogenic granites in the Proterozoic crustal development of North America, J. Lawford Anderson and J. Morrison Proterozoic granulite terranes, S.L. Harley xenoliths in Proterozoic crust - evidence for reworking of the lower crust, J. ruiz Proterozoic iron formations, C. Klein and N.J. Beukes Proterozoic collisional and accretionary orogens, B.F. Windley Proterozoic terranes and continental accretion in southwestern North America, K.C. Condie isotopic studies of Protozoic crustal growth and evolution, P.J. Patchett.

Collisional successor basins of western China: Impact of tectonic inheritance on sand composition

Abstract: Provenance interpretations from sandstone detrital modes are widely used as criteria for inferring tectonic settings of ancient sedimentary basins. Although global schemes of sandstone petrography generally work quite well, they fail to adequately identify the correct tectonic setting of sands of modern basins of Xinjiang Uygur Autonomous Region, northwest China, because of the complicated tectonic history of sediment-source terranes. Modern sands of the Junggar and Turpan basins are markedly litharenitic because of derivation from upper Paleozoic arc sequences that form the bedrock of the northern Tian Shan and Bogda Shan. In contrast, sands of the Tarim basin are compositionally much more varied, ranging from quartzofeldspathic to litharenitic. Tarim sands reflect derivation from plutonic and thrusted sedimentary and metamorphic terranes exposed in surrounding mountain ranges. Extrabasinal carbonate sedimentary-lithic fragments, intentionally omitted from provenance analyses by some workers, are important provenance indicators in Tarim samples. Interpreted in terms of schemes for global sandstone composition, Junggar and Turpan basin sands suggest an arc-related setting; Tarim sands suggest settings with erosional access to arc terranes, basement (for example, rifts), and sedimentary-metamorphic sequences (for example, foreland basins). Modern basins of Xinjiang, however, do not easily fit any of these tectonic settings, and composition of modern sand reflects relict source terranes rather than current tectonic setting. In fact, Xinjiang basins are difficult to fit into most basin classifications. Modern Xiujiang basins are deformed at their margins by active thrust belts, and to a lesser extent, by strike-slip faults associated with the Cenozoic Himalayan collision, and were built upon Mesozoic retroarc-foreland basins. We term the modern basins of Xinjiang "collisional successor basins" in recognition of their tectonic setting and history. The highly varied composition of Xinjiang basin sands may be typical of collisional successor basins, particularly where the tectonically accreted substrate of the basins consists of numerous and diverse small terranes.

History and modes of Mesozoic accretion in Southeastern Russia

Abstract: The history of Mesozoic accretion and growth of the Asia eastern margin, occupied by Southeastern Russia, includes five main events; two main tectonic regimes were responsible for the growth of the continent. In the Triassic-Jurassic, Early Cretaceous and Late Cretaceous-Paleogene, the subduction of the oceanic lithosphere resulted in the formation of wide accretionary wedges of the Mongol-Okhotsk, Khingan-Okhotsk and Eastern Sikhote-Alin active continental margins, respectively. These stages of the comparatively slow growth of the continent were broken by stages of rapid growth and drastic changes in the shape of the continent, since at these stages large terranes of various tectonic nature collided with active continental margins. At the end of the Early-Middle Jurassic, the Bureya terranes collided with the Mongol-Okhotsk active margin, and at the beginning of the Late Cretaceous there was collision of the Central and Southern Sikhote-Alin terranes with the Khingan-Okhotsk active margin. Collision-related structural styles in all cases are indicative of oblique collision and great strike-slip motions along the main sutures. The peculiarities of the terrane's geological structure show that prior to collision with the Mongol-Okhotsk and Khingan-Okhotsk active margins, they had already accreted to Asia and then migrated along its margins along the strike-slip faults. The Bureya terranes were squeezed out of the compression zone between Siberia and North China. This compression zone originated after the Paleozoic oceans which divided these cratons had closed. The Khanka terranes and Mesozoic accretionary wedge terranes of the Sikhote-Alin shifted along the strike-slip faults subparallel to the Asia Pacific margin. Strike-slip motions resulted in duplication of the primary tectonic zonation.

U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics

Abstract: Based on lithological, structural and geophysi- cal characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones In order to constrain the timing of meta- morphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites In addi- tion, U-Pb ages were obtained for monazites from meta- pelites and for a rutile from an amphibolite These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals: Central Granulite Terrane:

Transcontinental Proterozoic provinces

Abstract: Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Accretion of Archean oceanic plateau fragments in the Abitibi, greenstone belt, Canada

Abstract: The arc-arc collision model is often applied to the evolution of Archean greenstone belts. This model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic-tholeiitic affinities toward calc-alkalic affinities. New geological data from the Malartic composite block of the Abitibi greenstone belt indicate that this volcanic pile, rather than representing a conformable sequence, is composed of four distinct mafic lithotectonic domains that are overlain by a calc-alkalic sequence. The mafic domains are thought to be examples of accreted Archean oceanic plateau material that were deformed during their collage state. The younger calc-alkalic sequence represents extension-related volcanism that erupted through the ac- creted and deformed plateau material, possibly as a consequence of ridge subduction in a regime of oblique convergence. The relations described here may represent a common feature of Archean terranes.

Basement seismicity beneath the Andean precordillera thin-skinned thrust belt and implications for crustal and lithospheric behavior

Abstract: Data from a digitally recording seismic network in San Juan, Argentina, provide the first images of crustal scale basement faults beneath the Precordillera. This seismicity is near the boundary between the Precordillera (a thin-skinned thrust belt) and the Sierras Pampeanas (a region of thick-skinned basement deformation), two seismically active tectonic provinces of the Andean foreland. The seismicity data support models for this region in which crustal thickening, rather than magmatic addition or thermal uplift, plays the dominant mountain building role. The Precordillera seismicity occurs in three segments distributed north to south. The southern segment is an area of diffuse activity extending across the Precordillera and eastward into the Sierras Pampeanas that shows no patterns in map or cross section. The northern and central segments have well-defined dipping planes that define crustal scale faults extending from 5 to 35 km depth. It is clear from the relative fault geometries that the overlying Precordillera is not simply related to the basement activity. The seismicity here may result from reactivation of an ancient suture between the Precordillera and Pampeanas terranes or be occurring in basement of unknown affinity west of the suture. The seismicity provides the first constraints on basement fault geometries, and we present models integrating this information with the surface geology. These basement faults may have been responsible for the 1944 Ms 7.4 earthquake that destroyed the city of San Juan. The imaging of these faults suggests that seismic risk estimates for San Juan made on the basis of surface geologic studies may be too low.

Guerrero terrane of Mexico: Its role in the Southern, Cordillera from new geochemical data

Abstract: The Guerrero terrane makes up most of the western part of Mexico, is one of the largest terranes of the North American Cordillera, and is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of are affinity. Metamorphic rocks that crop out in the western area of the terrane (Arteaga complex) may represent its basement. They are mostly composed of terrigenous sediments (Varales Formation) with minor basaltic pillow lavas, chert, tuff, and limestone. Initial ϵ Nd values (+13) and rare earth element (REE) values for pillow lavas of the Arteaga complex are characteristic of mid-ocean ridge basalts (MORB). In contrast, the Varales Formation sedimentary rocks from the Arteaga complex have negative initial ϵ Nd (-6.2 and -7.2) and are enriched in light REEs. These data indicate that the sediments of the Varales Formation were supplied from an evolved continental crust. The overlying Jurassic(?)-Cretaceous arc-related rocks have initial ϵ Nd (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. These data show that the evolution of the Guerrero terrane had an early pre-Cretaceous(?) stage, which consisted of an oceanic crust receiving sediments from a continental source, and a Cretaceous stage, which was the development of an island arc.The oceanic-continental isotopic signature of the Arteaga complex is different from other western North American Cordilleran terranes (e.g., Alexander, Wrangelfia) that are more completely "oceanic" in affinity. Nevertheless, the extensive Jurassic(?).Cretaceous arc represents additions of juvenile material to the western North American Cordillera.

A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension

Abstract: Plutonic and metamorphic rocks of the southwest part of the Rhodope massif in Greece correspond to ductile lower crust exhumed and deformed along a major detachment during post-thickening extensional tectonics. Extension started during the Oligocene and is responsible for the development of Miocene–Quaternary sedimentary basins. Both brittle and ductile deformations result from gravity collapse of previously thickened lithosphere, as proposed for others large extended terranes. This interpretation disagrees with the previous models which attributed Tertiary ductile deformation to Alpine thrusting and brittle extensional deformation to back arc tectonics above a subduction zone.

A review of the Median Tectonic Zone: Terrane boundaries and terrane amalgamation near the Median Tectonic Line

Abstract: The pre‐Late Cretaceous basement rocks of New Zealand are divided into a Western Province and an Eastern Province along a tectonic boundary called the Median Tectonic Line. The two provinces comprise assemblages of terranes of fundamentally different origin. The Western Province attained continental‐type thickness and structure by the end of the Carboniferous. The Eastern Province developed as a result of convergent margin crustal processes in the Permian‐Cretaceous. The boundary between the two is drawn in one of two positions in Nelson, either east or west of the Drumduan Terrane. The evidence presented so far on the affinities of the Drumduan Terrane is equivocal, and the suggested eastward extension of the Tuhua Orogen is not well supported. To the south, similar difficulties of interpretation occur, and it seems that between the Takaka Terrane in the west and the Brook Street Terrane in the east there is a belt of enigmatic rocks best termed the Median Tectonic Zone. In a broader context, th...

Late Precambrian crustal evolution in NE Sudan: isotopic and geochronologic constraints

Abstract: The continental crust of NE Africa and Arabia was formed or modified during the late Precambrian (ca. 900-550 Ma) Pan-African orogenic cycle. The relationship between juvenile crust of the Arabian-Nubian shield (formed by accretion of ensimatic terranes) and older rocks of the Nile craton to the west (thermally reworked at the same time), is poorly understood. A regional geochronologic and isotopic study, including U-Pb zircon and Rb-Sr ages and Sm-Nd, Rb-Sr, and feldspar Pb isotopic studies indicates that the crust of NE Sudan formed between about 810 and 580 Ma, with an apparent peak in the volume of crust generated at about 700 Ma. Isotopie data indicate that the sources for these melts were juvenile, with initial $^{87}Sr/^{86}Sr = 0.7019$ to 0.7030, $\epsilon_{Nd(T)} = 5.1-7.7$, and with Pb isotopie characteristics indicating unresolvable contributions from older crust. In most cases, neodymium model ages ($T_{DM}$) are very similar to the crystallization ages, and both initial $^{87}Sr/^{86}Sr$ and ...

Jurassic tectonic history of the Otago Schist, New Zealand

Abstract: A regional reanalysis of the Otago Schist shows that ductile mesoscopic structures can be interpreted as elements of a single, progressive, heterogeneous, noncoaxial, Jurassic deformation related to juxtaposition of the Caples and Torlesse terranes. Stretching lineations are parallel, oblique, and perpendicular to the orogen in various parts of Otago, but changes in trend are generally unrelated to the terrane boundary. Structural geometry and rare shear criteria indicate that the Caples terrane overthrust the Torlesse terrane from the south and west, but precise transport directions are not known, being model-dependent. Macroscopic recumbent folds may be present in the schist, but mainly in the form of partially developed nappe structures truncated by zones of retransposed foliation and/or high strain.

Structural continuity of the Ross and Delamerian orogens of Antarctica and Australia along the margin of the paleo-Pacific

Abstract: The Delamerian orogen (southeast Australia) and the Wilson terrane (northern Victoria Land, Antarctica) constitute a formerly continuous lower Paleozoic fold-and-thrust belt developed along the paleo-Pacific margin of eastern Gondwana. Major folds and thrust faults in these regions, rooted in mid-crustal detachment zones, transported Cambrian-Ordovician granites and high-temperature- low-pressure metamorphic rocks divergently toward their respective western craton margins and eastern orogen margins and associated cover sequences. The structural imprints are related to the accretion of lower Paleozoic terranes at the eastern margin of the Delamerian orogen and the Wilson terrane. The continuity of the contemporaneous structure patterns in Australia and Antarctica is evidence for continuous convergent tectonism along the lower Paleozoic-Pacific margin of Gondwana.

Queen Charlotte Area Cenozoic tectonics and volcanism and their association with relative plate motions along the northeastern Pacific Margin

Abstract: Cenozoic relative plate interactions along the Queen Charlotte margin, based on published models for the North America and northeast Pacific plates, are related to the tectonic and igneous history of the region. Plate motion models indicate convergence and subduction prior to the Eocene. Since that time, there has been transcurrent motion with varying small amounts of oblique extension or compression, assuming that the Pacific-America-Farallon triple junction was to the south of the region. A late Miocene or early Pliocene (about 4 Ma) to present period of oblique convergence is well resolved. The present Queen Charlotte fault zone along the west coast of the islands may have initiated more recently. Within the Cenozoic transcurrent regime, a period of oblique extension in the mid-Tertiary (36–20 Ma) in one model is supported by a variety of volcanic and tectonic evidence. A small plate motion change at about 20 Ma may have resulted in the transfer of the Yakutat terrane from the North America plate to the Pacific plate and the resulting motion of the terrane northwestward to its present position along the margin of Alaska. The onset of Tertiary Masset volcanism that is extensive on the Queen Charlotte Islands corresponds within a few million years to the time of major plate reorganization at 43 Ma that has tectonic expression around the entire Pacific basin. The period of most extensive Masset volcanism and plutonism appears to correlate with the model oblique extension in the mid-Tertiary for one model. The geochemistry and physical volcanology of the Masset volcanic s are indicative of an extension regime. The main syntectonic deposition in the Queen Charlotte Basin (Skonun Formation) as inferred from seismic reflection and well data also appears temporally correlated with this time interval of oblique extension. A variety of data, including plate models, dikes, normal faults, basin subsidence, crustal thickness from seismic refraction, and present and paleo-heat flow, indicate mid-Tertiary crustal extension of at least 20% in the Queen Charlotte region, with up to 150% (β up to 2.5) in the main Queen Charlotte Basin. Posttectonic basin subsidence and deposition may correlate with the model time interval of general transcurrent or oblique convergence motion from 20 to 4 Ma. More recent shortening deformation observed in outcrop and seismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 Ma to the present. The latter convergence is associated with underthrusting that formed a trough or trench and an accretionary sedimentary prism off the west coast of the Queen Charlotte Islands and with inferred uplift and erosion of the western part of the islands.

Geometry of the Benioff Zone and state of stress in the overriding plate in central Mexico

Abstract: Analysis of depths and focal mechanisms of 16 small earthquakes in Central Mexico, along with previous data, shows that (a) the subducted Cocos plate becomes subhorizontal between 110 and 275 km from the trench reaching a depth of about 50 km, and (b) the bottom part of the overriding continental plate is in tensional stress regime. Neotectonic structures in Central Mexico and stress orientations (estimated from borehole elongations, cinder cone alignments, and fault-slip analysis) in the Trans Mexican Volcanic Belt, which lies in the northeastern portion of the area under study, also indicate the same stress regime. Absolute motion of the North American plate in the region has a component normal to the trench of 20 mm/yr which, if true, would result in compressional stress in the upper plate if the trench position is fixed in an absolute frame of reference. If we allow for seaward retreat of the trench then the tensional stress in the overriding plate and the observed geometry of the Benioff zone can be explained. This, however, implies that seaward retreat of a trench is possible even for a young (∼15 m.y. old) subducting slab. Alternatively, tectonic erosion of the leading edge of the continent, which has been proposed to explain truncated igneous and metamorphic continental terrane, could give rise to the tensional stress but would not explain the geometry of the Benioff zone.