Variations In Lithospheric Strength Research Articles

Three-dimensional inverse modelling of the thermal structure and implications for lithospheric strength in Denmark and adjacent areas of Northwest Europe

SUMMARY The subsurface thermal structure of Denmark and adjacent areas and lateral variations in lithospheric strength are investigated using a 3-D finite element solution of the steady-state heat equation and the equations of motion of a continuum. The optimum thermal model is determined by least-squares inversion of surface heat flow with prior information on the distribution of crustal radiogenic heat production rate, sediment and crustal thermal conductivity values, and the heat flow from the asthenosphere into the base of the lithosphere. The mechanical model invokes elastic, plastic and viscous deformation, depending on rock type, temperature, confining pressure and deviatoric stress. The thermal structure derived by the inversion is non-unique. A Monte Carlo-based resolution analysis shows that data and parameter uncertainties result in standard deviations in the Moho temperature of 75‐160 uC. The standard deviation increases almost linearly with Moho depth. All admissible thermal structures are characterized by laterally relatively homogeneous temperatures, and it is a robust conclusion that the lowest Moho temperatures occur in areas where the Moho is the shallowest, and the highest temperatures in areas where it is the deepest. Therefore, in the absence of lateral variations of the rheology of the lithospheric layers, and particularly for the upper mantle, the strength of the lithosphere in Denmark and the bordering areas is greatest in the basins where the Moho is the shallowest. This shows that in transition zones from cratons to younger sedimentary basins, lateral variations in lithospheric rheological properties are significant and should be considered in any attempt to understand the geodynamics of such areas.

The effect of plate stresses and shallow mantle temperatures on tectonics of northwestern Europe

Northwestern Europe is tectonically more active, in terms of seismicity, vertical motions and volcanism, than would be expected from its location far from any plate boundaries. In the context of the Netherlands Earth System Dynamics Initiative, we investigated the implications of two recent modeling efforts, of Eurasian plate forces and European mantle structure, for our understanding of the dynamics of these intraplate tectonics. We find that: (1) a simple balance between ridge push and collision forces along the southern European boundary does not seem sufficient to explain the observed direction of maximum horizontal compression in northwestern Europe. Our stress model, which imposes dynamical equilibrium on the full Eurasian plate, predicts that collision forces along the African–European boundary are relatively weak and have only a minor effect on the stress field in northwestern Europe; (2) seismic velocity anomalies in the shallow mantle imply 100–300°C variations in temperature under northwestern Europe. This regional mantle structure probably plays a significant role in the high level of intraplate tectonic activity and the regional variations in stress and tectonic style. For most tectonically active areas in Europe, observed surface heat flow anomalies coincide with anomalies in mantle velocity. Low velocity anomalies under northwestern Europe coincide with areas of recent volcanism and uplift, but are offset from the regions of maximum surface heat flow. This suggests that the thermal regime of the central European lithosphere is not in a steady state, probably due to changing mantle conditions. The effect of strong variations in lithospheric strength, predicted from the modeled thermal gradients in the shallow mantle, and of dynamic stresses induced by proposed active mantle upwellings may account for (some of) the differences between the observed and modeled stress field and will be investigated in future stress models.

Segmentation in gravity and magnetic anomalies along the U.S. East Coast passive margin: Implications for incipient structure of the oceanic lithosphere

Segmentation is a characteristic feature of seafloor spreading along the global mid‐ocean ridge system. While segmentation of active spreading centers has been the focus of much recent research, the process by which a rifted continental margin develops into a segmented mid‐ocean ridge is still poorly understood. In this study we investigate the segmentation character of the U.S. East Coast margin through a modeling study of the margin basement structure, magnetics, and gravity anomalies. The East Coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust, presumably formed during rifting. The East Coast Magnetic Anomaly (ECMA), a distinct total field magnetic high running offshore along the margin, is commonly located seaward of the thickest sections of the high‐velocity crust and displays segmentation on length scales (100–120 km) similar to the segmentation observed at the Mid‐Atlantic Ridge (MAR). Isostatic gravity anomalies were calculated by removing from free‐air gravity the predicted effects of seafloor, sediments, and a crust‐mantle model assuming local isostatic compensation. The resultant residuals show a corridor of high anomaly running along the margin, situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines. Reduction to the pole (R‐T‐P) of the total field magnetic anomaly shows that after the removal of skewness from the ECMA, the location of the isostatic gravity high is closely correlated to the ECMA. The isostatic gravity high is also segmented but in two distinct wave bands: 100–150 km and 300–500 km. The short‐wavelength (100–150 km) segmentation in the R‐T‐P magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle Bouguer anomaly observed along the present‐day MAR. The 300–500 km segmentation in the along‐margin isostatic gravity anomaly is similar in length scale to both intermediate‐wavelength tectonic segmentation observed in the South Atlantic and variations in lithospheric strength observed along the African margin. Furthermore, two of the intermediate‐wavelength (300–500 km) isostatic gravity lows correspond to the early traces of the Kane and Atlantis fracture zones, suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the North Atlantic. We hypothesize that the direct cause of the intermediate‐wavelength segmentation may be along‐margin variations in both the amount of underplated igneous crust and the strength of the lithosphere, although the relative importance of these two effects remains unresolved. Our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting.

Glacio-seismotectonics: ice sheets, crustal deformation and seismicity

The last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity. This growth has been driven largely by the emergence of postglacial rebound models (PGM) constrained by new field observations that incorporate increasingly realistic rheological, mechanical, and glacial parameters. In this paper, we highlight some of these recent field-based investigations and new PGMs, and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation. The emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern Canada and Fennoscandia. However, many of these models neglect important parameters, such as topography, lateral variations in lithospheric strength and tectonic strain built up during glaciation. In glaciated mountainous terrains, glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism. Such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields. In general, the short-term response to ice fluctuations is similar to the Earth's response to fluctuations in water reservoirs with the subsequent increase or decrease in seismicity which depends on the pre-existing stress state. When the ice fluctuations occur on the spatial scale, and magnitude, of the Late Pleistocene glaciation and deglaciation, however, the viscoelastic response of the Earth (especially the mantle) causes significant changes in crustal deformation and earthquake activity that is spatially extensive and temporally complex. The regions of greatest ice thickness and the regions marginal to the Late Pleistocene ice sheets indicate the most dramatic evidence of earthquake faulting. The mantle response to these large ice mass fluctuations and the change in mass between the oceans and land caused, and continues to cause, measureable crustal deformation at hundreds of kilometres from the ice margins. For both tectonically active and cratonic regions, palaeoseismic investigations of Late Pleistocene and Holocene faults are an important tool in evaluating earthquake hazard. The predicted response of the Earth over this time, however, is very dependent on the model assumed. For most regions, far more carefully designed field observations are needed to constrain the existing rheological and ice models.

Geodynamics of the Tarim Basin and the Tian Shan in central Asia

The India‐Asia collision has caused crustal thickening in Tibet by at least a factor of 2. In the last 20–30 Myr of this collision the Tian Shan mountain range has also been reactivated. The Tarim Basin, however, shows little internal deformation. We describe a series of numerical experiments which constrain the effective lithospheric strength parameters of the Tarim Basin and the Tian Shan in the context of a thin viscous sheet model. We use a finite element representation of the thin viscous sheet model to approximate the deformation field in the India‐Asia collision and compare model crustal thickness distributions with that inferred from a local isostatic model of topography. The experiments show that a strong Tarim Basin, while undergoing little internal deformation, transfers strain to the Tian Shan, producing significant crustal thickening in the Tian Shan region. Based only on diagnostic parameters such as the maximum thickening in the Tian Shan and the minimum in the Tarim Basin, the principal features of the topography can be approximately reproduced using models in which either the Tarim Basin is strong, or the Tian Shan is weak, or both. For a rheological model in which the stress versus strain‐rate exponent is n = 3 the strength coefficient for the strong Tarim Basin model, VTarim is between about 1.7 and 2. For the weak Tian Shan model the relative strength coefficient VTian is between about 0.65 and 0.75. If both strong Tarim and weak Tian Shan are included, there is a trade‐off between the required values of VTarim and VTian and the shape of the predicted crustal thickness profiles better matches the observed profiles. The steep topographic slope on the southern margin of the Tarim Basin requires that it is anomalously strong, while the rapid decrease of topographic height to the north of the Tian Shan requires that it is anomalously weak. Similar conclusions are obtained with a rheological model based on n = 10. Simplified rheological models of the lithosphere show that the variations in lithospheric strength may be explained by changes to the Moho temperature of the order of 10° to 30°C.

Mapping the descent of Indian and Eurasian plates beneath the Tibetan Plateau from gravity anomalies

The collision of India with Asia has produced a complicated continental‐continental plate boundary involving folding and faulting of variable trends and styles within and along the margins of the Tibetan Plateau. Numerous lines of evidence, including the development of two scales of folding in Tibet, suggest that the lowermost crust is behaving in a ductile fashion. This weak lower crust might then decouple the fundamental plate tectonic motions in the uppermost mantle from the complex pattern of surface faulting. In this study, we use Bouguer gravity anomalies to map out the geometry of Indian and Eurasian plate interactions in the mantle beneath the plateau based on both the inferred geometry of the Moho and lateral variations in lithospheric strength determined from mechanical modeling. In our preferred model, the lithosphere beneath Tibet consists of two distinct units: (1) the underthrust (to the north) Indian plate, which sutures with the Eurasian plate in the upper mantle below the Yarlung‐Zangpo Suture or the Gangdese igneous belt, 200–400 km north of the Main Boundary Thrust, and (2) the underthrust (to the south) Eurasian plate. A subducting slab of Indian upper mantle extends about 200 km into the asthenosphere north from the mantle suture and exerts a bending moment of about 3.5 × 1017 N on the Indian plate. Thus the mantle lithosphere appears to be behaving in the simple fashion of converging oceanic plates, while the more buoyant continental crust deforms under high gravitational potential in a complex pattern controlled by its lateral and vertical strength heterogeneity.

Lithospheric structure and compensation mechanisms of the Galápagos Archipelago

Volcanic islands of the Galápagos Archipelago are the most recent subaerial expression of the Galápagos hotspot. These islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young (<10 m.y.) oceanic lithosphere just south of the active Galápagos Spreading Center. The 91°W fracture zone crosses the platform and creates an estimated 5‐m.y. age discontinuity in the lithosphere. Major tectonic features of the Galápagos include an unusually broad distribution of volcanic centers, pronounced structural trends such as the NW‐SE Wolf‐Darwin Lineament (WDL), and a steep escarpment along the western and southern margins of the archipelago. We use shipboard gravity and bathymetry data along with Geosat geoid data to explain the tectonic and structural evolution of the Galápagos region. We model the gravity anomalies using a variety of compensation models, including Airy isostasy, continuous elastic flexure of the lithosphere, and an elastic plate with embedded weaknesses, and we infer significant lithospheric strength variations across the archipelago. The outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of ∼12 km. This area includes the large shield volcanoes of Fenandina and Isabela Islands, where the lithosphere regionally supports these volcanic loads. The central platform is weaker, with an elastic thickness of 6 km or less, and close to Airy isostasy. The greatest depths to the Moho are located beneath eastern Isabela Island and the central platform. Thinner lithosphere in this region may account for the broad distribution of volcanoes, the extended period of eruption of the central volcanoes, and their reduced size. The transition from strong to weak lithosphere along the southern escarpment appears to be abrupt, within the resolution of our models, and can be best represented by a free end or faultlike discontinuity. Also, modeling the WDL as a lithospheric fault increases the match to the observed gravity anomalies. The primary feature is the weak central platform, whose existence is a logical convergence of several factors: the 91°W transform/age offset, the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping, and the formation of the central platform at a spreading ridge.

Morphology and seismic stratigraphy of the inner continental shelf off Nova Scotia, Canada: Evidence for a −65 m lowstand between 11,650 and 11,250 C14 yr B.P.

Nova Scotia's position near the margin of large Pleistocene ice sheets makes the Scotian Shelf region critical for evaluation of glaciation models and sea-level change. The sea floor of the inner Scotian Shelf was mapped using multibeam bathymetry, a combination of conventional seismic and sonar techniques, and sampling. Multibeam bathymetry provides an areal image of the sea floor. Combining this unique image with sub-bottom seismic imaging, the relationships between sea floor topography and the underlying strata were explored.The inner continental shelf of Nova Scotia can be subdivided into five major terrain “zones”. These are the: (1) Truncation Zone, (2) Morainal Zone, (3) Outcrop Zone, (4) Basin Zone and (5) Scotian Shelf End-Moraine Complex. The Scotian Shelf End-Moraine Complex and Basin Zone are glacial-depositional zones at the seaward edge of the inner shelf. Landward of these zones is a region of high relief bedrock, with ridges and valleys largely devoid of surficial sediments extending from 80 to 120 m water depth (Outcrop Zone). This zone is interpreted as a relict bedrock surface, preserved under frozen-bed glacier conditions. To the east of the Outcrop Zone, in similar water depths, are unmodified till ridges overlying bedrock (Morainal Zone). The Truncation Zone is a region of the inner shelf from −90 m to the present shoreline characterized by muted acoustic topography and planar erosional surfaces truncating bedrock and surficial sediments. The Truncation Zone is subdivided into the Valley Subzone, the Transition Subzone, the Platform Subzone and the Estuarine Subzone. The Valley Subzone is typified by sediment-infilled valleys occurring in 75 to 90 m water depths. These valleys contain seismic facies with a ponded style of deposition planed off at the sea floor or by erosional unconformities near the sea floor. The Transition Subzone is marked by a relatively steep “ramp” and terraces. The ramp at the type section extends from 75 to 65 m water depth. The surface of the ramp can be either an erosional unconformity or a depositional surface formed by clinoform beds. The −65 m former shoreline is interpreted as the top of the Transition Subzone ramp surface. At one ramp locality, clinoform beds form part of a progradational sequence called the Sambro Delta. A mussel valve fragment (Mytilus edulis) was obtained from a core in the foresets and radiocarbon dated. An age of 11,650 ± 110 yr B.P. was obtained, adjusted for isotopic fractionation. Above −65 m the sea floor forms a low relief, gently sloping erosional surface (Platform Subzone). Estuarine deposits have been found above −50 m.The new sea-level curve constructed with the dated −65 m shoreline and recently published data is at variance with calculated RSL curves based on geophysical models, primarily in the amplitudes and rates of RSL change. Reasons for the discrepancies may be late-melting ice and regional variations in lithospheric strength and thickness.

Lithospheric strength variations as a control on new plate boundaries: examples from the northern Red Sea region

The complex plate boundary between Arabia and Africa at the northern end of the Red Sea includes the Gulf of Suez rift and the Gulf of Aqaba—Dead Sea transform. Geologic evidence indicates that during the earliest phase of rifting the Red Sea propagated NNW towards the Mediterranean Sea creating the Gulf of Suez. Subsequently, the majority of the relative movement between the plates shifted eastward to the Dead Sea transform. We propose that an increase in the strength of the lithosphere across the Mediterranean continental margin acted as a barrier to the propagation of the rift. A new plate boundary, the Dead Sea transform formed along a zone of minimum strength. We present an analysis of lithospheric strength variations across the Mediterranean continental margin. The main factors controlling these variations are the geotherm, crustal thickness and composition, and sediment thickness. The analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone. This strength profile also favors the creation of thin continental slivers such as the Levant west of the Dead Sea transform and the continental promontory containing Socotra Island at the mouth of the Gulf of Aden. Calculations of strength variations based on changes of crustal thickness, geotherm and sediment thickness can be extended to other geologic settings as well. They can explain the location of rerifting events at intracratonic basins, of backarc basins and of major continental strike-slip zones.