Early Stages Of Extension Research Articles

Tectonic implications of rapid cooling of lower plate rocks from the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Lower plate rocks exposed in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, underwent a pronounced increase in cooling rate 7–12 m.y. after the onset of extensional tectonic denudation at ca. 27 Ma. A detailed 40 Ar/ 39 Ar study of lower plate rocks exposed near Planet Peak, in the southwest part of the core complex, indicates that the highest-level mylonitic rocks exposed in the complex cooled below 350 °C prior to 20 ± 1 Ma, at an average rate of ≤15 °C/m.y. Thereafter, the cooling rate of these rocks increased progressively, peaking at 280–80 °C/m.y. prior to initial exposure of the core between 15.1 and 13.3 Ma. Published K-Ar, 40 Ar/ 39 Ar, and fission-track ages for the most recently unroofed lower plate rocks, exposed at the northeastern end of the core complex, are consistent with a similar cooling history, although the abrupt increase in cooling rate occurred as many as 5 m.y. later in this area. Because the increase in the cooling rate appears to have occurred progressively later toward the northeast, it is unlikely to reflect changes in slip rate. It appears to have coincided with cooling below 350–300 °C, and thus is interpreted to reflect steepening of the detachment system above the brittle-ductile transition. Thermochronological data from lower plate rocks in the Planet Peak area, together with constraints on their initial exposure at the surface, are consistent with an increase in the dip of the detachment from le;5° at midcrustal depths (where it was represented by a shear zone) to At temperatures below 200–150 °C, lower plate rocks at both ends of the core complex probably cooled at rates >130 °C/m.y., which can not be reconciled with the average dip (le;25°) and slip rate (6–12 mm/yr) for the detachment fault, given the likely geothermal gradient (∼25 °C/km) during the early stages of extension. This implies that cooling did not keep pace with denudation at upper crustal levels. The very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault (possibly >100 °C), as hot lower plate rocks were transported to shallow crustal levels. Rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate, consistent with models that link the shallow-level (

Evolution of the Mariana Convergent Plate Margin System

The Mariana convergent plate margin system of the western Pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology. The system's relative geologic simplicity and the well‐exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics. Its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins. The Mariana convergent margin's long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the Philippine Sea plate. The trenchward edge of the overriding plate has a relatively sparse sediment cover. Rocks outcropping on the trench's inner slope are typical of the early formed suprasubduction zone's lithosphere and have been subjected to various processes related to its tectonic history. Pervasive forearc faulting has exposed crust and upper mantle lithosphere. Many large serpentinized peridotite seamounts are within 100 km of the trench axis. From these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids. Ocean drilling recovered suprasubduction zone lava sequences erupted since the Eocene that suggest that the forearc region remains volcanologically dynamic. Seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution. Large portions of uplifted southern forearc are exposed at the larger islands. Active volcanoes at the base of the eastern boundary fault of the Mariana Trough vary in size and composition along strike and record regional differences in source composition. Their locations along strike of the arc are controlled in part by cross‐arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin. The western boundary is the West Mariana Ridge, the western portion of the volcanic arc active prior to formation of the Mariana Trough. The trough evolved in a two‐stage extension process of rifting and subsequent seafloor spreading. The back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid‐ocean‐type spreading center south of 20°N. The change from initial rifting to true seafloor spreading is also evident across the Mariana Trough from rifted topography near the West Mariana Ridge to spreading ridges in the central to eastern basin south of 20°N. This morphologic change indicates an early stage of extension with basin‐and‐range‐type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries. The spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys. The center of extension intersects the active volcanic front differently at either end of the Mariana Trough. In the north, extension is by rifting of arc lithosphere where it intersects the arc. In the south a major strike‐slip fault extends from the trench axis across the forearc, through the volcanic arc, and into the back arc basin. Arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center. The complexity of interrelated tectonism and magmatism in this convergent margin is daunting, but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents. Comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments.

Velocity description of deformation. Paper 3: the effects of temperature dependent rheology on extensional basin architecture

The results are presented from two-dimensional, transient numerical models which incorporate temperature, rheology, sedimentation and isostasy for finite rift durations. These models are used to address the role of fault growth and resultant basin geometry in the presence of rheological heterogeneity, generated during continental lithospheric extension. The transition between brittle and plastic rheologies, within the yield strength envelope, is used as a constraint on fault development and is found to be sensitive to the temperature perturbations initiated by the rifting process. Numerical results show that during the early stages of extension, cooling across the fault surface by hanging wall advection induces hardening directly below the fault tip. As a consequence the fault surface becomes detached from the brittle/plastic transition in the upper crust. As rifting continues, major basin-forming faults must grow downwards under the tectonic stress field to maintain contact with the transition surface. This increases the surface area of the fault, allowing the basin to cool further, resulting in an increase in flanl uplift and a deepening of the basin. For longer rift durations, thermal advection from lower lithospheric thinning heats up the basin, cancelling out the earlier cooling effect. The architecture of the rift and its sedimentary infill are therefore controlled by the dynamic response of the lithosphere to varying strength controls. The strength of the lithosphere itself is strongly dependent on temperature, pore pressure and strain rate. This type of thermal feedback process is greatest in areas of rapid extension, where greater discontinuities are set up by the rifting process.

The mechanics of continental extension in western North America: implications for the magmatic and structural evolution of the Great Basin

A finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the Early Tertiary North American Cordillera. Crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the Great Basin during extension. Marginal highlands, corresponding to the Sierra Nevada and Colorado Plateau, develop near the edges of the weakened region, where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain. The great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the Great Basin, favoring the formation of metamorphic core complexes during the early stages of extension. Ductile flow becomes inhibited as the crust thins and cools, possibly contributing to the change in the style of extension from low-angle detachment faulting to high-angle normal faulting during the Middle Miocene. As the lithosphere cools, the strength of the uppermost mantle increases. After about 10 m.y. the upper mantle becomes stronger beneath the Great Basin than beneath adjacent unextended regions. Strain then begins to migrate outward from the interior of the Great Basin onto the marginal highlands, and their interior edges begin to collapse and are incorporated into the widening Great Basin. After 40 m.y. of extension the highest rates of strain are localized at the edges of the Great Basin, in positions corresponding to the Walker Lane tectonic belt of western Nevada and the Colorado Plateau transition zone. The weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle. Portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions, creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies. Rapid thinning of the model lithosphere during the first 10 m.y. of extension results in nearly adiabatic decompression of the lithospheric mantle. Partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present. This mechanism may account for widespread silicic magmatism in the Great Basin during the Middle Tertiary. As the region undergoing extension widens, the rate of lithospheric thinning decreases and the mantle beneath the Great Basin begins to cool. This leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15–20 m.y. after the onset of extension. The appearance of predominantly basaltic and bimodal volcanic suites in the Great Basin during the last 5–10 Ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350°C (approximately 70°C warmer than typical mantle potential temperature).

An integrated model for seismogenesis in the intermountain seismic belt

Abstract The idea that all late Quaternary surface faults in the Intermountain Seismic Belt (ISB) are steeply dipping and penetrate from the surface to depths of 12 to 15 km may be an over-simplification of the actual tectonic-seismogenic process in the ISB/eastern Basin and Range transition zone. Studies of late Quaternary faulting in southwestern Wyoming and northcentral Utah, 125 km east of the Wasatch fault zone, indicate evolving tectonic/structural relationships exist with time and location. The Hebgen Lake and Borah Peak earthquakes represent a geologically mature stage of tectonic/seismogenic development manifested by fault-bounded mountain blocks and recurrent late Quaternary fault movements. Normally reactivated thrust faults and listric normal faults represent an early to intermediate stage of deformation in preexisting thrust-faulted terrains. The degree to which preexisting structures unrelated to the modern stress field affect tectonic development is complex, and a variety of structural relations may exist with time and location. An incipient seismogenic fault in the early stages of extension may exist only as a subdecollement, “blind” structure. As extension continues, “blind” seismogenic faults propagate upward eventually rupturing the surface as 45° to 60° planar structures. Normally reactivated thrust faults and listric normal faults, characteristic of early to intermediate extension, are tectonically beheaded and become inactive with propagation of planar seismogenic faults to the surface. The seismic potential of normally reactivated thrust faults and listric normal faults is open to debate. Whether the seismic potential of low-angle faults can be assessed from analysis of surface rupture parameters alone is a vitally important problem in seismic hazard assessment.

A-type granite and the Red Sea opening

Miocene-Oligocene A-type granite intrudes the eastern side of the Red Sea margin within the zone of extension from Jiddah, Saudi Arabia south to Yemen. The intrusions developed in the early stages of continental extension as Arabia began to move slowly away from Africa (around 30–20 Ma). Within the narrow zone of extension silicic magmas formed dikes, sills, small plutons and extrusive equivalents. In the Jabal Tirf area of Saudi Arabia these rocks occur in an elongate zone consisting of late Precambrian basement to the east, which is gradually invaded by mafic dikes. The number of dikes increases westward until an igneous complex is produced parallel to the present Red Sea axis. The Jabal Tirf igneous complex consists of diabase and rhyolite-granophyre sills (20–24 Ma). Although these are intrusine intrusive rocks their textures indicate shallow depths of intrusion (< 1 km). To the south, in the Yemen, contemporaneous with alkali basaltic eruptions (26–30 Ma) and later silicic eruptions, small plutons, dikes, and stocks of alkali granite invaded thick (1500 m) volcanic series, at various levels and times. Erosion within the uplifted margin of Yemen suggests that the maximum depth of intrusion was less than 1–2 km. Granophyric intrusions (20–30 Ma) within mafic dike swarms similar to the Jabal Tirf complex are present along the western edge of the Yemen volcanic plateau, marking a north-south zone of continental extension. The alkali granites of Yemen consist primarily of perthitic feldspar and quartz with some minor alkali amphiboles and acmite. These granites represent water-poor, hypersolvus magmas generated from parent alkali basalt magmas. The granophyric, two-feldspar granites associated with the mafic dike swarms and layered gabbros formed by fractional crystallization from tholeiitic basalt parent developed in the early stages of extension. Initial 87Sr/ 86Sr ratios of these rocks and their bulk chemistry indicate that production of peralkaline and metaluminous granitic magmas involved both fractiónation and partial melting as they ascended through the late Precambrian crust of the Arabian plate.

Basaltic volcanism, mantle plumes, and the mechanics of rifting: The Paraná flood basalt province of South America

Research Article| March 01, 1992 Basaltic volcanism, mantle plumes, and the mechanics of rifting: The Paraná flood basalt province of South America Dennis L. Harry; Dennis L. Harry 1Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar Dale S. Sawyer Dale S. Sawyer 1Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar Geology (1992) 20 (3): 207–210. https://doi.org/10.1130/0091-7613(1992)020<0207:BVMPAT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Dennis L. Harry, Dale S. Sawyer; Basaltic volcanism, mantle plumes, and the mechanics of rifting: The Paraná flood basalt province of South America. Geology 1992;; 20 (3): 207–210. doi: https://doi.org/10.1130/0091-7613(1992)020<0207:BVMPAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Dynamic modeling of continental extension between South America and Africa shows that the mechanics of rifting played an important role in determining the pattern of volcanism within the Paraná and Etendeka flood-basalt provinces on the Brazilian and Namibian margins. The key feature of the model is the development of a horizontal pressure gradient in the lower crust during the early stages of extension, which provided a mechanism for transporting magma generated beneath the incipient sea-floor spreading axis into the Paraná province, 100-200 km distant. The horizontal pressure gradient developed as a consequence of the dynamic interaction of preexisting weaknesses in the middle crust and upper mantle during rifting. The model accounts for the large quantity of basalt, the asymmetric distribution of basalt on the conjugate margins, and the northward migration of the eruptive center with time. The rapidity of magma genesis is in agreement with models of decompression melting during rifting. The model indicates that although elevated asthenosphere temperatures associated with the Tristan plume account for the volume of melt generated, the mechanics of rifting control the location and style of emplacement. The model suggests that extension in the region began ca. 150-155 Ma, and lasted about 25 m.y. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mantle contribution to the evolution of Middle Tertiary silicic magmatism during early stages of extension: the Egan Range volcanic complex, east-central Nevada

The Egan Range volcanic complex lies 30 km northwest of Ely, on the edge of a highly extended domain in east-central Nevada. It consists mainly of lavas with subordinate tuffs and sedimentary rocks. The rocks are divided into three stratigraphic and lithologic groups that correlated with widespread middle Tertiary volcanic rocks associated with early stages of extension in the region. Volcanic rocks of the early group are predominantly two-pyroxene dacite and andesite lavas, all of which contain quenched, mafic inclusions and have compositions indicating they were derived by mixing between a contaminated mantle melt and a rhyodacitic crustal component. Rocks of the middle group are relatively homogeneous biotite, hornblende dacite and rhyodacite lavas. Elevated compatible and incompatible element concentrations and straight-line correlations of compositional data in the early and middle groups support a simple mixing model. Minor fractionation of clinopyroxene is required to explain some low Cr concentrations. Major element variations of the late group can be successfully modeled by crystal fractionation of observed phenocrysts accompanied by moderate assimilation of a crustal component to account for elevated Rb, Th, U, and light rare earth element concentrations. Rocks of all three groups appear to be related to a common primary magma type, the composition of which can be modeled from the mafic inclusions in the early group. Low Ni and Mg contents in the inclusions indicate that olivine was fractionated prior to their participation in mixing of early group magmas. Based on estimated volumes of volcanic rocks in the Egan Range volcanic complex and in the region, and on the petrologic models for each group, a significant amount of basalt must have been added to the crust during this middle Tertiary magmatic episode.

Thermal history of crystalline nappes of the Maria Fold and Thrust Belt, west central Arizona

The thermal history of the crust in west central Arizona, as documented by 40Ar/39Ar step‐heating analysis of mineral suites from crystalline nappes of the Maria fold and thrust belt, is characterized by three distinct stages for Late Cretaceous and Tertiary time. Regional heating of the crust occurred during basement‐involved folding and thrust faulting in middle Late Cretaceous time (∼80–90 Ma) and represents the earliest phase recorded by the 40Ar/39Ar data. The second stage was dominated by uplift and slow cooling (5–10°C/m.y.) of the crust throughout latest Cretaceous and early Tertiary lime. Tectonic denudation in late Oligocene to early Miocene time resulted in rapid uplift and cooling of midcrastal rocks in the footwall of the Whipple‐Buckskin‐Rawhide detachment system, marking the final thermal signature in the region. Evidence for variable argon loss in the hornblende spectra implies exposure of differing structural levels within the thrust belt and/or significant thermal heterogeneity in the crust during Late Cretaceous orogenesis. Partially outgassed hornblende in the northern Granite Wash Mountains reflects temperatures of ∼450°C, whereas complete resetting of hornblende at Mesquite Mountain suggests that this area attained temperatures in excess of 500°C. In contrast, data from the northern Plomosa Mountains indicate that this area did not attain temperatures sufficient to completely degas biotite or K‐feldspar (&gt;350°C) during Late Cretaceous time. The range of early Tertiary muscovite and biotite ages in the region reflects slow cooling, presumably associated with uplift and erosion following crustal thickening. K‐feldspars yield saddle‐shaped age spectra with minimum apparent ages ranging from 22–28 Ma, and reflect quenching due to rapid exhumation of midcruslal to upper crustal rocks in the detachment terrain. Collectively, the data are consistent with either of two models for the thermal evolution of the crust in middle to late Tertiary time. Either (1) lower plate rocks were still sufficiently hot following Late Cretaceous heating that argon clocks were set simply due to rapid cooling or (2) the area experienced a renewed thermal input associated with extension, causing argon loss from previously cool rocks. Based on consideration of diffusion domain behavior in K‐feldspars, we infer that a renewed thermal pulse was associated with the early stages of extension and was related to the cause for crustal weakening.

Thermal modeling of extensional tectonics: Application to pressure‐temperature‐time histories of metamorphic rocks

One‐ and two‐dimensional finite difference models are used to generate theoretical pressure‐temperature‐time (PTt) paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere (“pure shear”) and via movement of crustal blocks along rooted large‐scale low‐angle normal faults (“simple shear”). The temperature‐time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies, although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear, and experience a greater amount of posttectonic cooling. The depth‐temperature paths of rocks uplifted from deep crustal levels (30 km depth) via pure shear extension are often characterized by a period of nearly isothermal (high dP/dT) uplift during the early stages of extension. A modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces PTt paths characterized by a more protracted period of nearly isothermal uplift. In contrast, simple shear uplift along low‐angle normal faults produces almost linear depth‐temperature paths with moderate dP/dT, and isothermal uplift is not observed. For a given initial geotherm, the single factor controlling the PTt paths of rocks unroofed below gently dipping normal faults that dip less than 30° is the rate at which unroofing takes place (unroofing rate is defined as the rate of uplift relative to the surface), and the syntectonic PTt paths are insensitive to fault dip or horizontal displacement rate. Current geothermometric and geobarometric techniques yield typical uncertainties in real pressure‐temperature and temperature‐time data of ±1 kbar and ±50°C. Thus it will be difficult to distinguish between these various extensional mechanisms on the basis of PTt data derived from a single sample or from a single structural level, although use of data from samples collected from a broad range of structural levels may improve resolution. Ultimately, the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to ±0.5 kbar and ±25°C.

Tension fractures and extensional tectonics

A characteristic pattern of normal faults that we have studied in several regions illustrates successive geometries relating to increasing amounts of extension from 10–50% (Gulf of Suez, Egypt) to 50–100% (Western Gulf of California, Mexico) up to 200% (Southern Basin and Range, USA). Tension fractures formed at an early stage of extension have an essential role. The resulting structure is described as a multiple tilted "packs of cards" model in which rotational deformation within the major blocks is accommodated by normal faulting on previously formed tension fractures, thus defining smaller blocks of second and third generation.

Societies and Academies

Societies and Academies