bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural styles in the frontal zones of the southern Apennines, Italy: An example from the Molise District
Authors: Cello Giuseppe , Tortorici Luigi , Martini Natale , Paltrinieri Werter
Published: February 2008

Structural interpretation of available geological and geophysical data from the Molise area (southern Italy) demonstrates that the frontal zones of this sector of the Apennines are characterized by a threefold zoneography defined by distinct structural styles. Resolved geometry of both pellicular and deep‐seated structures and their relationships, constrain possible reconstructions for the undeformed, pre‐Miocene, southeastern margin of the Afro‐Adriatic continent and its neogene deformation history. Identification of peculiar structural associations in the foreland‐foredeep complex and reinterpretation of relevant stratigraphic data suggest that deformation, in this sector of the Apennines, has already reached the most external (Apulian) domains and is possibly migrating, at present, toward the currently active Adriatic foredeep of the Molisan thrust system.
Seismic identification of basement reflectors: The Bagdad Reflection Sequence in the Basin and Range Province—Colorado Plateau Transition Zone, Arizona
Authors: Goodwin Erik B. , Thompson George A. , Okaya David A.
Published: February 2008

Although seismic reflection profiling has contributed greatly to our knowledge of the crust and upper mantle, it is often difficult to unambiguously identify the geologic origin of reflections. This is particularly true of intrabasement reflectors whose geometry and origin we need to know in order to understand tectonic processes operating in the crust. We present synthetic seismogram modeling of recently collected P and S wave seismic reflection data which suggests that the intrabasement Bagdad reflectors in west central Arizona are intrusive mafic sheets. The identification is based on the determination of the phase (positive polarity) and amplitudes of the reflected arrivals. If the Bagdad reflectors are Cenozoic age mafic sheets, they may be the product of rapid intrusion into an area of weak extension where the principal stresses are approximately equal.
40Ar/39Ar age constraints on deformation and metamorphism in the main central thrust zone and Tibetan slab, eastern Nepal Himalaya
Authors: Hubbard Mary S. , Harrison T. Mark
Published: February 2008

In eastern Nepal, structural and petrologic observations suggest that movement on the Main Central Thrust (MCT), the development of an inverted metamorphic sequence, and leucogranite plutonism were genetically related. Samples from across the MCT zone and its hanging wall, the Tibetan Slab, were collected from four locations in the Everest region for 40Ar/39Ar studies: (1) the MCT zone, (2) the lower Tibetan Slab, (3) the upper Tibetan Slab away from intrusive rocks, and (4) the upper Tibetan Slab adjacent to and including leucogranitic material. Within the MCT zone, muscovite and hornblende yield cooling ages of 12.0±0.2 Ma and 20.9±0.2 Ma, respectively. K‐feldspar gives a minimum age of 8.0±0.2 Ma (closure temperature (TC) =220±15°C). In the lower Tibetan Slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7.7±0.4 Ma and 7.5±0.6 Ma and a K‐feldspar minimum age of 6.4±0.8 Ma (TC = 225±20°C). An adjacent gneiss yields a 9.1±0.2 Ma biotite isochron and a K‐feldspar minimum age of 3.6±0.2 Ma (TC = 210±50°C). In the upper Tibetan Slab, samples collected >200 m from visible intrusives yield a biotite isochron age of 20.2±0.2 Ma and a complex hornblende age spectrum suggestive of excess argon. At the fourth location an augen gneiss, a pegmatite, and a tourmaline‐bearing leucogranite, all in mutual contact, yield indistinguishable mineral ages. The average biotite and muscovite isochron ages for these samples are 17.0±1.4 and 16.6±0.2 Ma, respectively. The minimum age for K‐feldspar from the leucogranite is 15.5±1.8 Ma. As thermobarometric data from the MCT zone in this area suggest synmetamorphic deformation at temperatures of ∼500°–550°C, the MCT hornblende (TC = ∼500°C) dates this event at ∼21 Ma. Diffusion experiments on hornblendes from the MCT zone provide data which support a maximum duration of peak metamorphic temperatures of ≤2 Ma. Biotite ages as old as ∼20 Ma from the upper Tibetan Slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the MCT zone. Very young ages in a ductile shear zone in the lower Tibetan Slab suggest that there has been deformation within the Tibetan Slab that postdates major movement on the MCT.
Subduction of a Late Cretaceous Seamount of the Louisville Ridge at the Tonga Trench: A model of normal and accelerated tectonic erosion
Authors: Ballance Peter F. , Scholl David W. , Vallier Tracy L. , Stevenson Andrew J. , Ryan Holly , Herzer Richard H.
Published: February 2008

The hotspot‐generated Louisville Ridge is a 4000‐km chain of seamounts (typically 2–2.5 km high and 10–40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26°S) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2–3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1–2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestrichtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount The principal effects of hotspot‐ridge collision with a sediment‐starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell, (3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km³ of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc‐derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the ∼37,000 km³ of material which fills subducting grabens each million years is arc‐derived volcanic and pelagic sediment.
Structural evolution of the Ardmore Basin, Oklahoma: Progressive deformation in the foreland of the Ouachita Collision
Authors: Granath James W.
Published: February 2008

Synthesis of oil field studies, seismic reflection data, and surface geology has resulted in a reconstruction of the Pennsylvanian evolution of the structural style of fault systems bordering and within the Ardmore Basin in south central Oklahoma. Faults bounding the margins of the basin were part of a broader left‐lateral shear belt that affected southern Oklahoma during the early Pennsylvanian. The mid‐Pennsylvanian and later zone of deformation contracted in southern Oklahoma to concentrate on the Washita Valley‐Eola Robberson fault systems along the northern edge of the basin, and on the Criner Uplift‐Healdton‐Stephens County fault systems along the southern and western side of the basin. Deformation on the floor of the basin was amplified, with left‐lateral strike‐slip faults slicing the basin into a system of rhombohedral blocks. Deformation continued at least into Virgil time (late Pennsylvanian). A two‐dimensional displacement field derived for the middle to late Pennsylvanian deformation reveals that a strong component of transpression affected both the basin‐bounding faults and, by reason of the geometry of their connections to the west, the Wichita Mountain front as well. Broadly spread left‐lateral shear evolved into crustal scale transpression during the Pennsylvanian Period. That progressive contraction of deformation and the change in style correlate with mid‐Pennsylvanian approach and passage of the Ouachita collision along the Ouachita embayment (Thomas, 1983) on the southern margin of the North American craton. Inasmuch as the Ardmore Basin was located at the sharp internal corner of the embay ment, the coincidence suggests that the style of evolution records (1) early far‐field influence of the approaching Ouachita collision during early Pennsylvanian, (2) passage of the suture during mid‐Pennsylvanian, and (3) concentration of foreland deformation at the corner of the embayment as the Arkoma and Fort Worth flexural basins evolved to the south and east during late Pennsylvanian.
Taconic Plate kinematics as revealed by foredeep stratigraphy, Appalachian Orogen
Authors: Bradley D. C.
Published: February 2008

Destruction of the Ordovician passive margin of eastern North America is recorded by an upward deepening succession of carbonates, shales, and flysch. A compilation of the age of shelf drowning (carbonate‐to‐shale transition) reveals the degree to which orogeny was diachronous both across and along strike. Shelf drowning occurred first at the northern end of the orogen in Newfoundland, then at the southern end of the orogen in Georgia, and finally in Quebec. Diachronism is attributed to oblique collision between an irregular passive margin, that had a deep embayment in Quebec, and at least one east dipping subduction complex. The rate of plate convergence during collision is estimated at 1 to 2 cm/yr, and the minimum width of the ocean that closed is estimated at 500 to 900 km. Far‐traveled deepwater sequences in the thrust belt contain anomalously old Taconic flysch, related to early arrival of the continental slope/rise at a west advancing trench then located far to the east. The drowning isochron map provides a new basis for estimating tectonic transport distances of four of these allochthons (about 165 to 450 km), results not readily obtained by conventional structural analysis.
Patterns of continental extension along the conjugate margins of the central and North ATLANTIC Oceans and Labrador Sea
Authors: Dunbar John A. , Sawyer Dale S.
Published: February 2008

The anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading. We estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and North Atlantic Oceans and Labrador Sea from the total tectonic subsidence of the margins. In the central Atlantic basin the integrated continental extension varies from 225 km for the break between the southern Appalachian segment of the North American margin and Africa to over 600 km for the break between the Blake Plateau region of North America and the Cape Verde Terrace region of Africa. The original width of the extended zone varies from 240 to 770 km. In the North Atlantic and Labrador basins the integrated extension varies from 60 km for the break between northern Labrador and Greenland to over 600 km for the break between Newfoundland and Europe. The original width of the extended zones in these basins varies from 100 km to over 1460 km. In general, continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain. As a result, seafloor spreading tends to begin first along segments which follow the structural grain. Extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain. Failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain. This phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain. We conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain.
Extension, detachments, and sedimentation in continental rifts (with particular reference to East Africa)
Authors: Morley C. K.
Published: February 2008

Plots of extension versus maximum synrift sediment thickness for rifts display a wide range of values. Some rifts display almost equal amounts of vertical displacement and horizontal extension, while others display considerably more extension than vertical displacement. The former appear to be associated with deep depths to detachment and/or steeply dipping boundary faults, while the latter tend to correspond with shallower depths to detachment and/or more gently dipping boundary faults. Boundary fault shape is inferred to exert an important control on relative amounts of extension and maximum sediment thickness and in sedimentation style. The deep anoxic lakes of the Western Branch, East African Rift system are associated with relatively steeply (40°–70°) dipping faults and depths to detachment ranging between 15 and 30 km. The more coarse clastic‐dominated Miocene half grabens in Northern Kenya are associated with relatively gently dipping boundary faults (50°–20°) and shallower depths to detachment (less than 15 km).
Structural and magmatic responses to spreading ridge subduction: An example from southwest Japan
Authors: Hibbard J. P. , Karig D. E.
Published: February 2008

Subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries. The late Oligocene‐early Miocene rocks of the Shimanto accretionary prism at Cape Muroto, Shikoku Island, record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms. These accreted strata have been imprinted with an unusual late stage event that involved (1) regional cusp‐like flexing of structural trends in the prism, (2) near‐trench magmatism, and (3) pervasive faulting. This event overlaps in time with the opening of the Shikoku back arc basin (circa 26–14 Ma); the Shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of Cape Muroto. The regional flexing of structural trends in the prism affects rocks as old as Cretaceous, although the flexure is most tightly developed in the late Oligocene‐early Miocene portion of the prism, where it is steeply plunging. Kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature. At Cape Muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid‐ocean ridge basalts (MORB) from the Shikoku back arc basin. Preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development. These data, combined with plate reconstructions for the area at 15 Ma, indicate that indentation of the prism most likely resulted from early Miocene orthogonal subduction of the topographically elevated Shikoku Basin spreading ridge in the Muroto area; MORB magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction.
Evidence of an imbricate crustal thrust belt in the southern British Variscides: Contributions of southwestern approaches traverse (SWAT) deep seismic reflection profiling recorded through the English Channel and the Celtic Sea
Authors: Le Gall Bernard
Published: February 2008

The Southwestern Approaches Traverse (SWAT) seismic reflection profiles recorded through the Celtic Sea and the English Channel provide unexpected data concerning, notably, the deep structure of the Irish and south British Variscan crust. One of the most significant results is the recognition of prominent deep southerly dipping reflectors, regarded as large‐scale foreland directed Variscan thrusts which constitute, in the southern innermost zones, a crustal stacking wedge. The resulting overthickened crust may have induced the petrogenesis of the Cornubian granitic batholith by an anatectic melting process. The roots of this crustal duplex are not imaged on the SWAT lines because of their subsequent assimilation by the more recent layering of the present‐day lower crust. Upward, the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone, not revealed by the SWAT profiles, and which underlies the thick Devonian allochthonous high‐strained units of Cornwall. Northward, this shallow northerly verging shear zone is overstepped by the Tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as Caledonian features. Southward, the imbricate crustal thrust unit is overridden by the Lizard ophiolitic suture, which appears as a 8‐km‐thick gently southerly dipping sheared zone, crosscuttting the entire crust and rooting deep beneath the present‐day Moho. Its hanging wall is constituted by the Channel Cadomian block, characterized by an almost seismically featureless upper crust which is not involved in the main Variscan thrust stacking. Northward, the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat‐lying decollement level. This frontal ramp emerges along a broadly 110°N trending discontinuous thrust front, located above the northern downfaulted margins of the Munster (Ireland) and Pembroke (Wales) Devonian basins. The SWAT deep seismic data relate the overall Variscan crustal shortening undergone by the south British domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous Channel block and the northern autochthonous foreland. This crustal shortening occurred during a Carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust‐floored basin (Lizard).
Rapid intra‐arc rifting in Miocene northeast Japan
Authors: Yamaji Atsushi
Published: February 2008

Although life‐times of major continental rifts are tens of millions of years, intra‐arc rifting in Miocene NE Japan lasted for only about 3 m.y. before back arc spreading began. The rate of initial subsidence was over 1 km/m.y., much faster than that in major continental rifts. Thus this intra‐arc rift evolved 10 times as fast as major intracontinental rifts. The large temperature dependence of lithosphere rheology and the hot thermal regime of island arc lithosphere account for the rapid rifting. Less than 1 m.y. before the rifting‐spreading transition, the inner NE Japan arc subsided from shallow marine depths to middle or lower bathyal depths (1.0–2.5 km). The average total basement subsidence was 2–3 km during the rifting. The rapid submergence occurred during one third of the duration of rifting but accounts for two thirds of the total subsidence during rifting. Namely, the initial subsidence rate accelerated toward the time of breakup of the arc crust. At the same time, the region along the present Pacific coast remained only shallow marine or terrestrial throughout, presumably influenced by global eustacy.
Paleomagnetically determined rotations in the western Sicilian fold and thrust belt
Authors: Channell J. E. T. , Oldow J. S. , Catalano R. , D'Argenio B.
Published: February 2008

New paleomagnetic data from the west Sicilian fold and thrust belt indicate large‐scale clockwise rotations during tectonic transport. For the platform and seamount paleogeographic units of the fold and thrust belt (Panormide and Trapanese units), there is a general consistency in the magnitude of rotation along strike, suggesting the existence of large coherent structural units. For the Panormide platform, clockwise rotations in the range of 90° to 140° relative to the underlying Iblean parautochthon are recorded along 125 km of outcrop strike. In the case of the Trapanese unit, clockwise rotations in the range of 47° to 70° relative to Iblei are recorded along about 90 km of outcrop strike. Intervening basinal paleogeographic units (Imerese and Sicanian units) show clockwise rotations of similar magnitude; although few reliable data are available from these units. Palinspastic reconstruction of the thrust belt, incorporating the rotations and arguments for the present orientation of the margins of the paleogeographic units, implies that the Panormide platform and Trapanese seamounts were originally elongated more or less north‐south relative to the present orientation of Iblei. The two units apparently coalesced to the south and were discontinuous to the north. They and their intervening basins were part of a passive continental margin which occupied a large part of the area that was subsequently rifted to form the Tyrrhenian Sea.
Tectonic evolution of the Troodos Ophiolite within the Tethyan Framework
Authors: Dilek Yildirim , Thy Peter , Moores Eldridge M. , Ramsden Todd W.
Published: February 2008

A new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the Troodos ophiolite, a well‐preserved remnant of Neotethyan oceanic crust. Grabens and normal faults within the sheeted dike complex and the extrusive sequence of the Troodos ophiolite resemble those of oceanic spreading centers. Diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along‐ and across‐strike variation in volcanic and tectonic activity during development of oceanic crust. Coupled with the existence of the Arakapas transform fault to the south, these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault. The arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue, however, for generation of part of the ophiolite within a subduction zone environment. Regional reconstructions suggest that the Mesozoic Neotethys may have evolved as a marginal basin both to the Afro‐Arabian continent and the Paleotethyan ocean over an active or recently active south dipping subduction zone. The Troodos ophiolite and other eastern Mediterranean ophiolites, whose magma compositions were affected by the subducted Paleotethyan slab, may have formed along east‐west trending spreading centers separated by north‐south trending transform faults within this marginal basin. A rapid change in relative plate motion in late Cretaceous time between Eurasia and Afro‐Arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the Neotethyan realm and in initiation of north dipping subduction zone(s) beneath the Troodos and other ophiolites in the region. The apparent forearc setting of the Troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation.
Evidence for extensional deformation during a collisional orogeny, Rombak Window, north Norway
Authors: Cashman Patricia H.
Published: February 2008

Deformation along the northern edge of the Rombak Window documents an unusual extensional event in the down‐going plate during the evolution of a collisional orogen. The extension occurred along a series of ductile deformation zones that strike slightly east of north and dip moderately west; both mesoscopic and microscopic kinematic indicators demonstrate normal motion on these zones. Two adjacent ductile deformation zones with normal offset are major features, extending at least 6 km along strike (representing a structural depth of 1.1–1.6 km) without changing orientation. The remainder are fairly superficial features, dying out at depths of 100 to 200 m; they are thought to represent local adjustments to offset on the major zones. Fault zones with normal offset along strike at the southern edge of the Rombak Window are thought to be a continuation of the major zones, although the faults in the southern part of the window formed at shallower depths. A prominent north‐trending structural lineament, and isolated patches of Dividal Group sedimentary rocks, in the central part of the window may represent the trace of this extensional feature, connecting the ductile deformation zones in the north to the normal faults in the south. The extension was preceded by emplacement of the Bjørnfjell thrust complex, containing granitic basement and sedimentary cover rocks of Baltic affinity. Both of these events occurred under greenschist facies (biotite grade) conditions. The extension was followed by emplacement of the Treldal thrust complex, containing rocks of Baltic affinity, and by higher nappe sheets containing a variety of rock types generally considered to be exotic with respect to the Baltic craton. Three models which would create the required extension of the subducting Baltic craton, preceded and followed by large‐scale compressional deformation, are being considered: (1) extension of the upper surface of the Baltic craton during flexing prior to subduction, (2) extension due to the flexing of a crystalline thrust sheet over a ramp in an underlying detachment, (3) extension due to an unrecognized deformational event or due to relaxation and/or isostatic adjustment between the emplacement of Bjørnfjell complex and Treldal complex rocks in this area. The author prefers the second model, based on independent evidence for allochthoneity of the crystalline rocks in the Rombak Window, and on analogy with the Nasafjell Window, which occupies a similar structural position.
Observing tilts in midcrustal rocks by paleomagnetism: Examples from southeast British Columbia
Authors: Marquis Guy , Irving E.
Published: February 2008

Metamorphic core complexes that originated in the middle crust have been exposed by uplift and extension. Indicators of paleohorizontal planes are few or absent, thus it is difficult to determine the degree of tilting that might have accompanied this process. However, if aberrant paleomagnetic directions (paleodirections) are observed in core complexes or their associated intrusive rocks, and if large‐scale rotation and translation can be excluded, then these paleodirections can be used to calculate the sense and magnitude of tilting. We describe paleodirections from mid‐crustal rocks in southeastern British Columbia, where uplift and extension occurred in Eocene time about a central culmination aligned north‐south. Several large extensional faults dip both to the west and to the east of this central culmination. The Eocene Granby pluton (49.3°N, 118.5°W), situated to the west of the main culmination, has a mean direction of magnetization (D°, I°) 309°, 41° (α63° = 7° the standard error) and paleopole at 44°N, 138°E (A63° = 7°). To the east of the main culmination, a 20 km long section of altered Eocene Syringa dikes and associated metadiorite (49.3°N, 117.7°W) has a mean direction of 56°, 47° (α63° = 3°) and paleopole at 56°N, 319°E (A63° = 3°). The paleopoles are very different from one another, and from those calculated from contemporaneous (53 to 49 Ma) rocks of the North American craton to the east. They also differ from those observed from coeval bedded volcanic rocks to the west of the sampling area. Hence, these large aberrancies cannot be caused by translations. We suggest that they are caused by tilting which occurred during the extension process. The tilts are in opposite senses on either side of the central culmination. They are estimated to be down 38°±8° at 99°E for the Granby Pluton, and down 41°±4° at 270°W for the dikes and metadiorite. These tilts are consistent with the Wernicke model for simple extensional shear.
Early Proterozoic Collisional Orogeny along the Northern Thelon Tectonic Zone, Northwest Territories, Canada: Evidence from the foreland
Authors: Tirrul Rein , Grotzinger John P.
Published: February 2008

The subsidence history of Kilohigok Basin (1.9 Ga), N.W.T., Canada reflects the opening and closing of an ocean basin along the present Thelon Tectonic Zone. The basal sedimentary sequence represents a passive‐margin shelf that faced SE toward the orogen. Following passive‐margin subsidence, tectonic loading occurred resulting in rapid subsidence of the outer shelf margin and simultaneous uplift and erosion of the platform interior; a flexural moat formed subparallel to the Thelon Tectonic Zone. The shelf was subsequently drowned and successively buried by deep‐water flysch, shallow‐marine shelf sediments, and fluvial molasse, forming a foreland basin tapered to the NW. Direct structural evidence for the tectonic load is provided by the leading edge of a thin‐skinned thrust‐fold belt exposed in oblique cross‐section within the SE extremity of the basin. Upright folds at high structural levels can be traced downward to recumbent, tight folds and thrusts near the basement/cover contact documenting increasing shear strain with depth. Overall northwest vergence and about 50% shortening is indicated by fold‐thrust geometry and finite stretching lineations, above a brittle/ductile decollement that roots within the Thelon Tectonic Zone.
Temperature‐time history of subducted continental crust, Mount Olympos Region, Greece
Authors: Schermer Elizabeth R. , Lux Daniel R. , Burchfiel B. Clark
Published: February 2008

In the Mt. Olympos region of northeastern Greece, continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions, and thrust over carbonate platform strata during Alpine orogenesis. Subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism, its relationship to deformation, and the thermal history of the subducted terrane. Alpine low‐grade metamorphic assemblages occur at four structural levels. Three thrust sheets composed of Paleozoic granitic basement and Mesozoic metasedimentary cover were thrust over Mesozoic carbonate rocks and Eocene flysch; thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from NE to SW. 40Ar/39Ar spectra from hornblende, white mica, and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events: (1) 293–302 Ma crystallization of granites, with cooling from >550°C to <325°C by 284 Ma; (2) 98–100 Ma greenschist to blueschist‐greenschist transition facies metamorphism (T∼350–500°C) and imbrication of continental thrust sheets; (3) 53–61 Ma blueschist facies metamorphism and deformation of the basement and continental margin units at T<350–400°C; (4) 36–40 Ma thrusting of blueschists over the carbonate platform, and metamorphism at T∼200–350°C. Only the Eocene and younger events affected the lower two structural packages. A fifth event, indicated by diffusive loss profiles in microcline spectra, reflects the beginning of uplift and cooling to T<100–150°C at 16–23 Ma, associated with normal faulting which continued until Quaternary time. Incomplete resetting of mica ages in all units constrains the temperature of metamorphism during continental subduction to T≤350°C, the closure temperature for Ar in muscovite. The diffusive loss profiles in micas and K‐feldspars enable us to “see through” the younger events to older events in the high‐T parts of the release spectra. Micas grown during earlier metamorphic events lost relatively small amounts of Ar during subsequent high pressure‐low temperature metamorphism. Release spectra from phengites grown during Eocene metamorphism and deformation record the ages of the Ar‐loss events. Alpine deformation in northern Greece occurred over a long time span (∼90 Ma), and involved subduction and episodic imbrication of continental basement before, during, and after the collision of the Apulian and Eurasian plates. Syn‐subduction uplift and cooling probably combined with intermittently higher cooling rates during extensional events to preserve the blueschist facies mineral assemblages as they were exhumed from depths of >20 km. Extension in the Olympos region was synchronous with extension in the Mesohellenic trough and the Aegean back‐arc, and concurrent with westward‐progressing shortening in the external Hellenides.
Kinematics of Pre‐Miocene ductile deformation in the Santa Catalina Core Complex and adjacent regions
Authors: Guérin Gwénaël , Brun Jean‐Pierre , Van Den Driessche Jean
Published: February 2008

This paper describes structures within mylonites and metamorphic rocks from the Santa Catalina and Tortolita‐Suizo metamorphic core complexes, southeastern Arizona, United States. In these areas, mylonitic rocks constitute an easterly low‐dipping slab more than 4 km thick. Mapping of principal strain directions and analysis of shear sense have been carried out through the whole pile of mylonites. It is shown that although the stretching lineation appears to have a uniform mean SW‐NE trend, three successive shearing events are involved. The first shearing event is toward the west and is accompanied in part by the emplacement of the Leatherwood Quartz Diorite dated at 64 Ma. The second shearing event is toward the NE and is accompanied by the emplacement of the Wilderness Granitic Suite dated at 49 Ma. The third shearing event is localized and directed toward the SW. The top‐to‐west shearing and top‐to‐NE shearing are possibly related to Laramide thrusting. The last ductile shearing is related to post‐Eocene extension.
Reflection‐diffraction seismic pattern at crustal suture zones
Authors: Sadowiak P. , Wever T.
Published: February 2008

Seismic reflection profiling across strong dip (more than 25°) suture zones between different crustal terranes or provinces reveals a distinct seismic reflectivity pattern of dipping reflections and associated diffractions. Suture zones in areas dominated by thin‐skinned tectonics do not show this pattern. Profiles from southern Germany (Deutsches Kontinentales Reflexionsseismisches Programm), Great Britain (British Institutions Reflection Profiling Syndicate) and North America (Consortium for Continental Reflection Profiling and U.S. Geological Survey) crossing major sutures are examined with respect to dipping reflections in the upper crust and accompanying diffractions in the lower crust. The strong resemblance of seismic patterns suggests a similar origin for these structures.
Speculative reconstruction of the Mojave‐Snow Lake Fault: IMPLICATIONS FOR Paleozoic and Mesozoic orogenesis in the western United States
Authors: Schweickert Richard A. , Lahren Mary M.
Published: February 2008

Available evidence suggests that the Mojave‐Snow Lake fault, a major Early Cretaceous dextral fault with up to 400 km displacement, formerly existed in what is now the axial part of the Sierra Nevada batholith. Regional relations suggest that the fault formerly continued in western Nevada, the Mojave Desert, and in rocks now exposed in the Salinian block. Restoration of 400 km of Early Cretaceous dextral displacement on the Mojave‐Snow Lake fault leads to speculative but testable reconstructions of Mesozoic and Paleozoic tectonic features of California and Nevada. Insights derived from these reconstructions include the following: (1) Two regionally extensive Jurassic thrust belts of opposed vergence appear to have intersected in the northern Sierra Nevada. The eastern, east‐vergent thrust belt, which includes the Fencemaker and Liming thrusts in western Nevada and several east‐vergent thrusts in the northern Sierra, may be a contractional belt along which an Early Jurassic interarc or marginal basin was closed in Middle to Late Jurassic time. The western, west‐vergent thrust belt is part of the classic Nevadan orogen of the western Klamath Mountains and the western Sierra metamorphic belt, in which collisions and/or thrusting of various island arc terranes occurred during Middle and Late Jurassic time. (2) The early Mesozoic continental arc included paired belts of Triassic‐Jurassic blueschist and melange, and igneous rocks including Late Triassic and younger plutons and volcanic rocks. This conclusion is not new, but the reconstruction provides a slightly different geometry than previously thought. (3) Sonomia, the late Paleozoic island‐arc terrane of the western U. S., may have formed a relatively narrow belt extending from northwestern Nevada to the latitude of the western Mojave Desert. The geologic development of the Paleozoic basement complex of Sonomia was very different from that of the Cordilleran miogeocline in Death Valley and the Mojave Desert, requiring that a continuation of the Sonoma suture belt (or Golconda thrust) must have extended southward through the Sierra Nevada batholith to the Mojave region. (4) A heterogeneous belt of Permian to Triassic or Triassic deformation in the Mojave and eastern Sierra Nevada, together with the Golconda allochthon in Nevada, may represent the protracted response to the collision of Sonomia with the western margin of North America. The contractional deformation continued during the evolution of the early Mesozoic continental arc. (5) Several predictions can be made about the nature of concealed basement within the Sierra Nevada batholith and within western Nevada. The reconstruction suggests that elements of the Antler and Sonoma orogenic belts should underlie areas of the batholith and western Nevada, as a result of northward displacement along the Mojave‐Snow Lake fault from the western Mojave and southern Sierra Nevada.
Biographical sketch of Dr. John Locke
Authors: Bauer L. A.
Published: February 2008

The city of Cincinnati has the honor of having taken a leading part in the inauguration in this country of scientific work in astronomy, meteorology, and terrestrial magnetism—in astronomy, through the labors of Mitchell, the founder of the Cincinnati Observatory; in meteorology, through the vigorous impulse given to the science by Professor Cleveland Abbe, when he was director of the Cincinnati Observatory; and in terrestrial magnetism, through the enthusiasm and zeal of Dr. John Locke.

I propose to speak here especially of the work of Dr. Locke, who was one of the foremost of the pioneers in terrestrial magnetism in this country. Through the kindness of his son, Colonel Joseph Morris Locke, who resides in Cincinnati, and whose acquaintance I have had the privilege of making, I am enabled to give the excellent portrait of his father herewith reproduced.
Note sur quelques anciennes déclinaisons
Authors: de Moidrey J.
Published: February 2008

La très intéressante collection de données magnétiques, publiée par le Dr. van Bemmelen,1 me fait croire que plusieurs observations anciennes sont moins connues que je ne le pensais, puisque on ne les trouve citées ni par Hansteen, ni par le Dr. van Bemmelen. Bien qu'aucune ne soit manuscrite, il peut y avoir intérêt à les réunir.

I. REMARQUES SUR LA LISTE DU DR. VAN BEMMELEN.
On Earth‐currents and magnetic variations
Authors: Steiner L.
Published: February 2008

The relation between earth‐currents and magnetic variations is very far from being thoroughly known. The main question, whether the magnetic variations are entirely, or, at least, partly due to the electro‐magnetic action of earth‐currents, or whether the earth‐currents, in turn, are induced by the magnetic variations, is not yet definitely solved. Many investigators are inclined to accept the former view, relying mainly upon observations made at times of magnetic storms. The few data at our disposal concerning the daily variations on calm days has not yet settled the question.
Earthquake of April 30, 1919, as recorded on the magnetogram at Watheroo Observatory
Authors: Wallis W. F.
Published: February 2008

On examining the magnetogram for April 30, 1919, the day on which an earthquake and tidal wave swept Pan‐gai, a town in one of the islands of the Tonga group, we found a distant earthquake record, the elements of which are as follows:
Regarding atmospheric electricity and its relation with solar activity
Authors: Bauer Louis A.
Published: February 2008

The following quotation forms the concluding paragraph of Dr. K. Kähler's note, entitled “Das luftelektrische Potentialgefälle in Potsdam 1904 bis 1923,” which appeared in the February 1925 number of the “Meteorologische Zeitschrift,” pages 69‐71, this note being an excerpt from Dr. Kähler's contribution in the “Ergebnisse der Meteorologischen Beobachtungen in Potsdam in den Jahren 1921, 1922 und 1923, Berlin, 1924”:

“Potentialgefälle und Sonnenflecken.—Aus diesem Grunde, eignet sich—darin hat L. A. Bauer recht—Potsdam wenig dafür, grössere säkulare Schwankungen wie den Sonnenfleckeneinfluss festzustellen. Das kann sehr wohl an den “Severe climatic disturbances” (Nature 112, 204, vgl. auch Terr. Mag. 29, 24) liegen. Wenn aber Bauer in demselben Zusammenhang von instrumentellen Änderungen in Potsdam spricht, so trifft das nicht zu. Das Instrumentarium ist unverändert and gut geblieben. Auch die Herabsetzung des Reduktionsfactors von 1.0 auf 0.8 hat keinen Einfluss, weil die Korrektion an alle 20 Jahre gleichmässig angebracht wurde. Der einzige Umstand, der auf kurze Zeit eine säkulare Änderung vrerschleiert haben könnte, ist das Zuwachsen der Messstelle in den Kriegsjahren» Aber auch die jetzt korrigierten Zahlen lassen keinen Parallelismus mit dem Sonnenfleckenzyklus erkennen, wie schon Markgraf1 hervorhebt. Markgraf zeigt an gleicher Stelle, dass auch die Leitfähigkeit in Potsdam, die doch von der Änderung des Reduktionsfaktors ganz unabhängig ist, in den Jahren 1912 bis 1921 keinen entgegengesetzten Gang zu den Sonnenflecken durchmacht ; sie schwankt vielmehr in den l0 Jahren um einen festen Mittelwert. Chree weist übrigens darauf hin, dass gerade diese 10 Jahre, die Bauer vorzugsweise seinen Schlüssen zugrunde legt, wenig für diesen Zweck geeignet waren. So trat in Kew der Sonnenfleckeneinfluss in dieser Zeit in allen meteorologischen Elementen weniger als sonst zutage. Für Kew nimmt Chree, wenn überhaupt, nur einen geringen Einfluss auf das Potentialgefälle an. Auch andere Stationen, beispielweise Heluan in Ägypten, fallen ebenso wie Potsdam heraus. Überdies kommt· Bauer in seiner letzten Arbeit (T.M., 29, p. 23, 1924) zu dem Schluss, das die Wirkung der Sonnenflecken auf das Potentialgefälle in den Messperioden vor 1901, vor allem um 1855 and 1889 herum, gerade umgekehrt gewesen sein muss wie in den letzten beiden Sonnenfleckenzyklen. Es wird also wohl einer ferneren Zukunft vorbehalten bleiben, diese sicherlich sehr interessante und wichtige Frage zu klären.”
Measurement of resistivity of large masses of undisturbed earth
Authors: Gish O. H. , Rooney W. J.
Published: February 2008

The method and equipment for measuring the resistivity of large masses of undisturbed earth, primarily in connection with earth‐current potential measurements at the Carnegie Institution of Washington, magnetic observatories at Watheroo (Western Australia), and at Huancayo (Peru), are described. Measurements, made at four sites in the vicinity of Washington, D.C., during the development of the method, are reported.

Method and procedure are described in Part I. Four contacts to earth are provided at equal intervals along a straight line at the surface. A measured current is sent through the earth between the extreme points and the resulting difference of potential between the intermediate points is observed. These data, together with the length of interval between electrodes or contact points, yield the average resistivity of a body of earth whose linear dimensions are of the same order as that of the electrode interval.

Part II summarizes the results of series of measurements with electrode intervals ranging from 5 feet (1.5 meters) to 1,800 feet (540 meters). These give some knowledge of the variation of resistivity with depth and determine approximately the depths of ground‐water and of changes in the geological sub‐structure.
Inversion de l'inclinaison magnÉtique terrestre aux âges geologiques
Authors: Mercanton P. L.
Published: February 2008

Les laves volcaniques en se refroidissant dans le champ magnétique terrestre y prennent une aimantation rémanente, en général faible, mais très stable et bien représentative de ce champ en direction et sens. Un échantillon de la roche, prélevé après repérage géographique in situ et analysé magnétiquement, nous renseignera donc sur la distribution du champ magnétisant à l'endroit et à l'époque de sa consolidation. C'est, en effet, après que le magma s'est figé et que le refroidissement en a abaissé la température à un certain degré, vraisemblablement voisin, (sinon identique) du “point de Curie” de la magnetite (580°) que la dite aimantation s'établit. Remarque essentielle: à cette température la roche est déjà fort éloignée de l'état de fusion pâteuse et ne peut plus fluer notablement; ceci exclut donc la possibilité d'un changement de situation de la lave par écoulement après aimantation prise.
Construction of magnetic charts
Authors: McFarland W. N.
Published: February 2008

This article is a review of the methods which have been used by the United States Coast and Geodetic Survey in the construction of the isomagnetic charts which it has published, with comment on the adaptibility of the different methods.
The influence of rain on the atmospheric‐electric field
Authors: Telang A. Venkata Rao
Published: February 2008

Photographic records of the atmospheric‐electric field variations at times of rain are examined along with pluviograms covering these periods. It is found that the normal effect of rain is to abruptly reverse the normal positive field in the early stages, while in a few cases it results in a sudden reduction without change of sign. This abrupt change disappears speedily when the disturbing shower is of short duration. An analysis of the results of G. C. Simpson at Simla (1908–1909) in regard to the charge brought down by rain is given. It is shown that in the earliest stages of charged rain, in more than 85 per cent of the cases, the net charge brought down by rain is positive. It is then shown that the effect of rain on the field is due to the transfer of charge by the rain from the upper air. Calculation shows that the charge transferred by the rain is of the order necessary to produce the observed changes of potential. The explanation covers the cases of heavy rain attended with frequent reversals and very high values of the field.