scholarly journals Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

2021 ◽  
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Upper Mantle ◽  
Upper Crust ◽  
Crustal Layer ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
Analogue Models ◽  
History Of ◽  
Lower Crustal ◽  
Degree Of Coupling ◽  
Over Time

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remains poorly understood. Here we present a series of 3D brittle-viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface with the formation of graben parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

scholarly journals Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

Solid Earth ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 1473-1495
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Upper Mantle ◽  
Upper Crust ◽  
Crustal Layer ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
Analogue Models ◽  
History Of ◽  
Lower Crustal ◽  
Degree Of Coupling ◽  
Over Time

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remain poorly understood. Here we present a series of 3D brittle–viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface, with the formation of grabens parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

scholarly journals The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)

2012 ◽  
Vol 150 (1) ◽  
pp. 153-172 ◽  
Author(s):  
PAOLA MANZOTTI ◽  
MICHELE ZUCALI
Keyword(s):  
Early Stage ◽  
Granulite Facies ◽  
Subsequent Stage ◽  
Italian Alps ◽  
Northern Margin ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
History Of ◽  
Nw Italy ◽  
High Temperature Metamorphism

AbstractThe Valpelline unit is a large slice of continental crust constituting the Austroalpine Dent Blanche nappe (NW Italy). The pre-Alpine evolution of this unit holds important clues about the Palaeozoic crustal structure at the northern margin of the Adria continent, about the history of rifting in the Alpine region, and thus about the thermomechanical conditions that preceded the Alpine convergent evolution. Several stages of the deformation history and of partial re-equilibration were identified, combining meso- and micro-structural analyses with thermobarometry. Reconstructed pre-Alpine P–T–t–d paths demonstrate that the Valpelline unit experienced an early stage at pressures between 4.5 and 6.5 kbar followed by migmatite formation. A subsequent stage reached amphibolite to granulite facies conditions. This stage was associated with the development of the most penetrative fabrics affecting all of the Valpelline lithotypes. The pre-Alpine evolution ended with a weak deformation associated with a local mineral-chemical re-equilibration under greenschist facies conditions at ≈ 4 kbar and T < 450°C. A Permo-Mesozoic lithospheric extension is thought to be responsible for asthenosphere upwelling, thereby causing high temperature metamorphism at medium pressure and widespread partial melting, which led to upper crustal magmatic activity.

Crustal velocity structure in the eastern Insular and southernmost Coast belts, Canadian Cordillera

1993 ◽  
Vol 30 (5) ◽  
pp. 1014-1027 ◽  
Author(s):  
B. C. Zelt ◽  
R. M. Ellis ◽  
R. M. Clowes
Keyword(s):  
High Velocity ◽  
Velocity Structure ◽  
Canadian Cordillera ◽  
Western Coast ◽  
Upper Crust ◽  
Crustal Layer ◽  
Near Surface ◽  
Crustal Velocity ◽  
Crustal Velocity Structure ◽  
Lower Crustal

Seismic refraction data recorded along a 330 km cross-strike profile through the eastern Insular and southernmost Coast belts of the Canadian Cordillera are interpreted using an iterative combination of traveltime inversion and amplitude forward modelling. The resultant model is characterized by large lateral variations in velocity. The most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the Harrison fault, which likely represents the transition from crust of the Insular superterrane to that of the Intermontane superterrane. This interpretation is consistent with some present geological models that place the possible (probable) location of the suture between the two superterranes less than 20 km east of the Harrison fault. Velocities at the base of the upper crust average 6.4 and 6.2 km/s west and east of the fault, respectively. Mid-crustal velocities average 6.6–6.9 km/s to the west and 6.35–6.45 km/s to the east of the fault. Lower crustal velocities also decrease slightly to the east. Other features of the velocity model include (i) a thin near-surface layer with velocities between 2.5 and 6.1 km/s; (ii) upper crustal thickness of 12.5 km, thinning to 8 km at the eastern boundary of the Western Coast Belt (WCB); (iii) high velocity (6.6–6.9 km/s) mid-crustal layer west of the Harrison fault extending to 21 km depth; (iv) high-velocity (6.75–7.1 km/s) lower crustal layer; (v) low-velocity gradient upper mantle with depth to Moho at 34–37 km beneath most of the Coast Belt, decreasing to 30 km beneath the eastern Insular Belt, a depth much less than previous estimates. The inferred crustal velocity structure beneath the WCB is consistent with the three-layer electrical conductivity structure for this area derived from magnetotelluric surveys. The association of high resistivities with the upper crust suggests that the upper 8–12 km represents the massive cover of plutonic rocks which characterizes the WCB. Middle and lower crustal velocities beneath the WCB are consistent with Wrangellian velocities found beneath Vancouver Island, suggesting Wrangellia may extend at depth eastward as far as the Harrison fault.

Crustal structure in Northwestern Ontario: Regional magnetic anomalies

1969 ◽  
Vol 6 (1) ◽  
pp. 101-107 ◽  
Author(s):  
Peter H. McGrath ◽  
Donald H. Hall
Keyword(s):  
Crustal Structure ◽  
Upper Mantle ◽  
Magnetic Anomalies ◽  
Two Dimensional ◽  
Crustal Layer ◽  
Smoothing Operator ◽  
Northwestern Ontario ◽  
Map Series ◽  
Lower Crustal ◽  
Seismic Discontinuity

A regional aeromagnetic map, portraying the regional magnetic anomaly system in Northwestern Ontario west of longitude 92 °W and south of latitude 55 °N and extending westward into Manitoba to longitude 97 °W (with an additional block bounded by latitudes 54° N and 56 °N and longitudes 97° W and 102 °W) is presented. The map was prepared by multiple application of a two-dimensional smoothing operator applied to data digitized at 3 km intervals from the 1-inch-to-1-mile aeromagnetic map series published by the Geological Survey of Canada. Comparison was made with previous maps overlapping on portions of the area, which had been made by various techniques, including Fourier analysis, fitting of 6th-order polynomials, and photographic reduction. The general features of the anomaly system were found to be similar for all of these techniques. The regional anomaly system is found to be related in some cases to the thickness of the upper crustal layer (defined as lying above the Intermediate seismic discontinuity) and to structure within it, but not to the lower crustal layer or to the upper mantle.

Crustal structure of the Canadian polar margin: results of the 1985 seismic refraction survey

1989 ◽  
Vol 26 (5) ◽  
pp. 853-866 ◽  
Author(s):  
I. Asudeh ◽  
D. A. Forsyth ◽  
R. Stephenson ◽  
A. Embry ◽  
H. R. Jackson ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Refraction ◽  
Thick Layer ◽  
Parallel Line ◽  
Inner Shelf ◽  
Lower Paleozoic ◽  
Crustal Layer ◽  
Interval Velocity ◽  
Upper Paleozoic ◽  
Lower Crustal

The 1985 refraction survey based on Ice Island covered a northern transition zone along the Canadian polar margin north of Axel Heiberg Island. The refraction survey included a 60 km line along the inner shelf, a 180 km parallel line along the outer shelf, and a 60 km connecting line. Shotpoints offset from the line ends recorded upper mantle observations to a distance of 240 km.Along the inner shelf, the upper 700 m, with an interval velocity of 3.7 km/s, is interpreted as Tertiary–Cretaceous strata. The underlying 4 km thick layer has a starting velocity of 5 km/s and a gradient of 0.2 s−1. It is thought to consist of mainly deformed lower Paleozoic strata capped by upper Paleozoic – Triassic clastics and carbonates and (or) Cretaceous volcanics. Sequentially, the lower unit, with a starting velocity of 5.8 km/s, most likely consists of Proterozoic – lower Paleozoic rocks.Beneath the offshore line, up to 5 km of strata with a starting velocity of 2.2 km/s and a gradient of 0.5 s−1 probably represents Tertiary–Cretaceous elastics. The underlying material with a starting velocity of 4.5 km/s and a gradient of 0.1 s−1 is interpreted as a sedimentary succession of either Cretaceous–Tertiary elastics or upper Paleozoic to Cretaceous strata. Beneath this section, a probable Proterozoic – lower Paleozoic lower crustal layer with a starting velocity of 6.2 km/s extends to about 25 km. Apparent upper mantle velocities in the 8.0–8.2 km/s range are observed.Beneath the transitional onshore–offshore line, a Neogene sedimentary basin is interpreted as being floored by faulted blocks of probably deformed Proterozoic to lower Paleozoic rocks on the landward side and possibly Cretaceous to lower Tertiary rocks on the seaward side.

scholarly journals Competition between 3D structural inheritance and kinematics during rifting: insights from analogue models

2021 ◽  
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Experimental Studies ◽  
Surface Expression ◽  
Crustal Layer ◽  
Structural Inheritance ◽  
Analogue Models ◽  
Localize Deformation ◽  
Internal And External Factors ◽  
Lower Crustal ◽  
The Impact ◽  
Plate Kinematics

The competition between the impact of inherited weaknesses and plate kinematics determines the location and style of deformation during rifting, yet the relative impacts of these “internal” and “external” factors remain poorly understood, especially in 3D. In this study we used brittle-viscous analogue models to assess how multiphase rifting, i.e., changes in plate divergence rate or direction, and the distribution of weaknesses in the competent mantle and crust influence rift evolution. We find that the combined reactivation of mantle and crustal weaknesses without kinematic changes creates complex rift structures. Divergence rates affects the strength of the weak lower crustal layer and hence the degree of mantle-crustal coupling. In this context slow rifting decreases coupling, so that crustal weaknesses can easily localize deformation and dominate surface structures, whereas fast rifting increases coupling so that deformation related to mantle weaknesses can have a dominant surface expression. Through a change from slow to fast rifting mantle-related deformation can overprint previous structures that formed along (differently oriented) crustal weaknesses. Conversely, a change from fast to slow rifting may shift deformation from mantle-controlled towards crust-controlled. When changing divergence directions, structures from the first rifting phase may control where subsequent deformation occurs, but only when they are well developed. Alternatively, they are ignored during subsequent rifting. We furthermore place our results in a larger framework of brittle-viscous rift modelling results from previous experimental studies, showing the importance of genral lithospheric layering, divergence rate, the type of deformation in the mantle, and finally upper crustal structural inheritance. The interaction between these parameters can lead to a large variety of deformation styles that may often lead to comparable end products. Therefore, detailed investigation of faulting and to an equal extent basin depocenter distribution over time is required to properly determine the evolution of complex rift systems. These insights provide a strong incentive to revisit various natural examples.

Tectonic history of the Dent Blanche

2017 ◽  
Vol 9 (2.1) ◽  
pp. 1-73 ◽  
Author(s):  
Paola Manzotti ◽  
Michel Ballèvrei
Keyword(s):  
Tectonic History ◽  
History Of

The Path of Standard Albanian Language Formation

2018 ◽  
Vol 5 (2) ◽  
pp. 106-115
Author(s):  
Sindorela Doli Kryeziu
Keyword(s):  
Single Unit ◽  
Cultural Factors ◽  
Language History ◽  
Whole Process ◽  
History Of ◽  
Social And Cultural Factors ◽  
Cultural Body ◽  
Over Time ◽  
Language Literature

Abstract In our paper we will talk about the whole process of standardization of the Albanian language, where it has gone through a long historical route, for almost a century.When talking about standard Albanian language history and according to Albanian language literature, it is often thought that the Albanian language was standardized in the Albanian Language Orthography Congress, held in Tirana in 1972, or after the publication of the Orthographic Rules (which was a project at that time) of 1967 and the decisions of the Linguistic Conference, a conference of great importance that took place in Pristina, in 1968. All of these have influenced chronologically during a very difficult historical journey, until the standardization of the Albanian language.Considering a slightly wider and more complex view than what is often presented in Albanian language literature, we will try to describe the path (history) of the standard Albanian formation under the influence of many historical, political, social and cultural factors that are known in the history of the Albanian people. These factors have contributed to the formation of a common state, which would have, over time, a common standard language.It is fair to think that "all activity in the development of writing and the Albanian language, in the field of standardization and linguistic planning, should be seen as a single unit of Albanian culture, of course with frequent manifestations of specific polycentric organization, either because of divisions within the cultural body itself, or because of the external imposition"(Rexhep Ismajli," In Language and for Language ", Dukagjini, Peja, 1998, pp. 15-18.)

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