Effective Elastic Thickness Research Articles

Lithosphere Weakening During Arctic Ocean Opening: Evidence From Effective Elastic Thickness

AbstractEvolution of the Arctic Ocean lithosphere has involved multiple stages of opening with crustal stretching and thinning prior to lithospheric breakup. Mapping lateral variations in lithospheric rheology can help unravel the detailed tectonic history of the Arctic. Here we perform a wavelet analysis of gravity and bathymetry data to map the effective elastic thickness () of the lithosphere in the Arctic. The low overall suggests that large shear stresses and serpentinization weakened the lithosphere at Arctic passive margins during a multistage opening process. Moderately low values along the Gakkel Ridge imply a relatively cold ultraslow‐spreading center compared to typical mid‐ocean ridges.

Estimating the effective elastic thickness of the Arctic lithosphere using the wavelet coherence method: Tectonic implications

Currently, knowledge of structure and tectonic evolution of the Arctic lithosphere remains limited. The effective elastic thickness of the lithosphere (Te) is a parameter that describes the lithospheric integrated strength and reflects its thermal and rheological properties, which helps to reveal intraplate tectonic process. In this study, we present a high-resolution Te model in the Arctic region (north of 67°N), which is highly heterogeneous and strongly correlated with various regional tectonic elements. To verify the recovered Te, its pattern is compared to those obtained in previous studies that used different methods and to available heat flow measurements and seismic velocity model and shows good agreement. High Te values are found in the Greenland shield, North American craton and Siberian craton, which are compatible with results from previous studies of high seismic velocity and low heat flow, suggesting that cold, thick lithosphere remains tectonically undisturbed. Low Te values occur in the Amerasia Basin, Baffin Bay, Eurasia Basin and North Atlantic Ocean where young oceanic lithosphere is created. Moreover, the areas affected by regional volcanism in the Siberian Traps and HALIP are dominated by reductions in Te, which we attribute to thermal rejuvenation triggered by the intrusion of magma. Weak lithosphere also dominates in tectonically active orogens as well, including the Alaska-Chukotka, Novaya Zemlya and Caledonides, and is related to the stress released during collisional deformation. We find that a few deep sedimentary basins are characterized by intermediate Te, which implies mechanical support of the surface sediment loads in a non-isostatic state. Our new Te model provides potentially significant insights into various tectonic and geodynamic problems of the Arctic lithosphere.

Heterogeneous outer‐rise seismicity within the Izu–Bonin subduction zone and its tectonic implications

Combining with available focal mechanisms, we chose 13 events (M ≥ 3.5) for waveform inversion to determine the stress field at the outer‐rise region within the Izu–Bonin subduction zone. Indicated by the distribution of the epicentres, seismic energy release, and stress field within the outer‐rise region, the subduction zone is divided into three segments separated by 29°N and 32.5°N. The outer‐rise seismicity presents an extensional stress field in the upper part (0–40 km) of the subducting oceanic plate. Whereas, the maximum focal depth within the outer‐rise region tends to increase northwards, and the deep part (>40 km) of the subducting slab is gradually governed by a general compressional stress regime that has never been observed before, which implies that the subduction of the old Pacific Plate along the Izu–Bonin trench is characterized by a heterogeneous bending deformation. According to modelling the flexural bending of the oceanic lithosphere, it is observed that effective elastic thickness (Te) of the Izu–Bonin subduction zone decreases from 38 to 28 km along the trench strike, indicating a decrease of plate rigidity northwards. The spatial variations of Te exactly explain the heterogeneous outer‐rise seismicity within the Izu–Bonin subduction zone, which is the energy release of the bending formation of the subducting plate. As shown in the results presented here, the varied flexural deformation of the subducting Pacific Plate is closely correlated with the heterogeneous thermal age, the varied morphology, and the coupling degree of the subducting plate.

Effects of mantle dynamics on estimating effective elastic thickness of the lithosphere

The understanding of the lateral variations in the lithospheric strength can help in identifying the distribution pattern of surface deformation and its response to long-term forces that are related to deep Earth processes. Former studies suggest that Afar and the Ethiopian Plateaus are not in isostatic equilibrium indicating the presence of deep compensation processes. We estimated the effective elastic thickness of the lithosphere (Te) over Ethiopia and its surrounding region using a method that combines the Vening Meinesz-Mortiz and flexural theory of isostasy in a spherical harmonics domain. Our effective elastic thickness model is different from those presented in the previous similar studies, because of considering the effects of mantle dynamics. We incorporated the uplift forces due to mantle plume in additions to surface and sub-surfaces loads, like topographic/bathymetric masses, crustal crystalline and sediments in our method so that it becomes more appropriate for the regions. In addition, laterally-variable models of upper mantle density, Young's modulus and Poisson's ratio have been considered from the CRUST1.0 model. We found a significant improvement of the solution in the Te estimate with consideration of the dynamic effects when we compared it with other studies especially in Afar, the Ethiopian plateaus, and the Main Ethiopian Rift valley. Our results are also consistent with significant lithospheric strength present in cratonic formations, with maximum of Te in Sudan and Tanzanian Cratons. Low Te was found over different tectonically active places such as, Afar and the Main Ethiopian Rift valley.

The mechanical behaviour of the lithosphere beneath the Chad Basin and the Bangui Magnetic Anomaly. Insights from Moho depth and effective elastic thickness estimates

We used high-resolution gravity data to analyse crustal structure beneath the Bangui Magnetic Anomaly (BMA) and the Chad Basin within the North-Central African region. Moho depths and effective elastic thickness values, calculated respectively by a regularized non-linear inversion method and by analysis of the coherence function between gravity and topography, have been interpreted with reference to seismic data, previous gravity studies and regional geology. Our aim is to determine the crustal architecture and to analyse its mechanical behaviour. Estimated Moho depths range from 30 to 43 km in the North-Central African region. Beneath the uplift zones (Adamawa-Yade plateau, Ouaddaï, Tibesti, Hoggar, Tuareg shield) and the Congo Craton, the Moho depths range between 35 and 37 km. These values can be interpreted by tectonic and seismic events recorded along the regional shear zones; but also to the remobilization of the Precambrian basement. Analysis of the coherence function between the Bouguer gravity and the topography shows two different domains of rigidity: the highest values (100 km < Te < 180 km) are observed in Central African Republic, Chad, Niger, Algeria, and Egypt; the lowest values (20 < Te < 100 km) are observed in Cameroon, Nigeria, and Sudan. The lithosphere beneath the Chad Basin has a higher rigidity, similar to that observed in Central African Republic. In Chad and the Central African Republic, there is a positive correlation between the Te and the Moho depths; this indicates that cratonic remnant (Chad craton) beneath the Chad Basin is well preserved and covered by the sediments. The low Te values in Cameroon and the constant value in the Moho depth, are a proof of the reactivation of the Precambrian basement. This reactivation is related to the tectonic, seismic and thermal events occurred during the Eburnean and Pan-African orogenies. The contrast between the Te values in Cameroon compared to others regions (Chad, Central African Republic, Niger, Egypt, Algeria) show the magnitude of these events on the crustal architecture. This being defined by the presence of cratonic cores, the Cameroon Volcanic Line (CVL) and hot spots as well as large regional shear zones, explain the fact that thermo-tectonic and seismic events that occurred at the northern edge of the Congo Craton in Cameroon, are more intense than in the Central African Republic. So according to the geodynamic process in Cameroon and Central African Republic, related to the hot spots along the CVL, thermo-tectonic and seismic events recorded along the regional shear zones (Sanaga Shear Zone and the Central Cameroon Shear Zone) and the Congo Craton, the BMA is associated to two different geotectonic contexts. The sources of the BMA are closely linked to the Congo Craton; they bottom should not exceed −37 km depth. The present results agree with previous studies which allow us to understand the evolution of the Saharan metacraton and the preservation of cratonic remnants (Murzuq, Al-Kufrah, Chad); similarly the estimated Moho depths is an important step that would serve as an input to model the sources of the Bangui Magnetic Anomaly.

Beveling the Colorado Plateau: Early Mesozoic Rift‐Related Flexure Explains Erosion and Anomalous Deposition in the Southern Cordilleran Foreland Basin

AbstractDeposition of the Late Jurassic Morrison Formation in a back‐bulge depozone and formation of the overlying sub‐Cretaceous unconformity above a forebulge mark the birth of the foreland basin system in the central U.S. Cordillera. In the southern U.S. Cordillera, the Morrison Formation is either anomalously thick or absent and the sub‐Cretaceous unconformity cuts out progressively older stratigraphy toward the south on the Colorado Plateau. Based on results of 2D and 3D flexural modeling, we suggest that flexural uplift of the northern rift flank of the Bisbee segment of the Borderland Rift Belt can explain these observations. Structural restoration of the sub‐Cretaceous unconformity indicates a minimum of 1.5 km of uplift and flexural models with an effective elastic thickness of 55 ± 5 km can reproduce the geometry of the unconformity and rift flank. This implies that effective elastic thickness has decreased between the Jurassic and the present, consistent with hypotheses for uplift and modification of the Colorado Plateau lithosphere during the Late Mesozoic to Cenozoic. Modeling results also predict the presence of a rift‐related flexural trough in the Four Corners region of the Colorado Plateau, which may explain above‐average thickness of the Morrison Formation. Constructive interference between a flexural back‐bulge depozone and a flexural rift‐flank trough may help explain anomalously high Late Jurassic subsidence.

Tectonic control on effective elastic thickness over the Northeast Indian Seamount Province and surrounding regions

Tectonic control on effective elastic thickness over the Northeast Indian Seamount Province and surrounding regions

Crust-mantle structure and lithospheric destruction of the oldest craton in the Indian shield

The Singhbhum craton is among the five Archean cratons of Peninsular India that preserves some of the oldest continental nuclei. In this work, we present a new and complete Bouguer gravity map of this craton with insights into its deep crust-mantle structure, lithospheric thickness and density variations beneath this craton. The conspicuous presence of high-order residual gravity low anomalies, together with low estimated densities, suggests voluminous presence of Singhbhum granitic batholiths that built the dominant crustal architecture. The isolated residual gravity highs correspond to the mafic and ultramafic volcanic suites like, Dhanjori, Simlipal and Dalma, while the relatively low gravity anomalies observed over the western volcanic suites like Malangtoli, Jagannathpur and Ongarbira, indicate their relatively felsic nature. The estimated lithospheric thickness of about ~ 130 km below the granitic batholithic region, and about 112 km beneath the Precambrian volcanic terranes, together with low effective elastic thickness (Te,) of only about 31 km, suggest a thin and weak lithosphere. The craton witnessed extensive lithospheric destruction with the removal of nearly 100–150 km of the cratonic root. The decratonization may be linked to subduction during the Paleo-Mesoproterozoic period, together with mantle plumes at different times, suggesting a combined mechanical, thermal and chemical erosion of the cratonic keel.

Stress field estimation by the geoid undulations of the Samos-Kuşadası Bay and implications for seismogenic behavior

The strong earthquake with magnitude 6.9 occurred offshore at the northernmost edge of the Samos Island and was strongly felt in the north Aegean islands and Izmir metropolitan city. In this study, the effective elastic thicknesses of the lithosphere and seismogenic layer thickness were correlated with each other in order to understand the nature of the earthquakes. We determined that the upper and lower depth limits of seismogenic layer are in a range of 5–15 km, meaning that only the upper crust is mostly involved in earthquakes in the study area. The fact that seismogenic layer and effective elastic thicknesses are close to each other indicates that the earthquake potential may be within the seismogenic layer. Following that, we estimate the stress field from the geoid undulations as a proxy of gravity potential energy in order to analyze the amplitude and orientation of the stress vectors and seismogenic behavior implications. The discrete wavelet transform has been carried out to decompose the isostatic residual gravity anomalies into horizontal, vertical and diagonal detail coefficients. The results delineated edges of gravity anomalies that reveal some previously unknown features.

Thermo-Rheological Properties of the Ethiopian Lithosphere and Evidence for Transient Fluid Induced Lower Crustal Seismicity Beneath the Ethiopian Rift

Lower crustal earthquakes at plate boundaries and intraplate settings occur at depth where deformation is normally expected to occur in a ductile manner. Here we use the available earthquake catalogs and compute theoretical predictions for a range of conditions for the occurrence of lower crustal earthquakes beneath the Main Ethiopian Rift (MER) and adjacent north-western (NW) plateau. Yield strength envelops are constructed using information on geothermal gradient, strain rate, and composition constrained by geophysical observations. Our models suggest that away from the MER beneath the NW plateau the depth distribution of earthquakes in the lower crust is best explained by strong mafic lower crustal rheology and hydrostatic fluid pore pressure conditions. In the same region the effective elastic thickness is similar to seismogenic thickness showing that the lower crust has long-term strength and hence can physically support brittle deformation. On the contrary, in the central MER the seismogenic thickness is much larger than the effective elastic layer thickness implying that the lower crust has no long-term strength. Here our models show that both hydrostatic and near-lithostatic fluid pore pressures fail to explain the observed seismicity and instead a combination of near-lithostatic pore fluid pressure and transient high strain rate due to the movement of fluids provide a plausible mechanism for the occurrence of seismicity in the lower crust. Our interpretations are supported by occurrence of swarms of deep earthquakes beneath the MER, as opposed to more continuous background deep seismicity away from the rift. Using time-depth progression of earthquakes, we estimate permeability values of 5.9 × 10−15m2and 1.8 × 10−14m2at lower crustal depth. The range of permeability implies that seismicity can be induced by pore-pressure diffusion, likely from fluids sourced from the mantle that reactivate preexisting faults in the lower crust. Our thermo-rheological models explain the first order differences in lower crustal earthquakes both directly beneath and outboard of the rift valley.

The effective elastic thickness of the lithosphere in the Amerasia Basin, Arctic Ocean

AbstractAs a proxy for the lithospheric strength at the time of loading, the effective elastic thickness of the lithosphere (Te) can aid in understanding the structure and evolution of the lithosphere. Here, we obtain spatial variations in Te of Amerasia Basin and surrounding regions using the fan‐shaped Morlet wavelet method. Our results show that variations in Te generally agree well with tectonic provinces in the region. The eastern part of Makarov Basin has the same lithospheric strength as Alpha and Mendeleev ridges, which may be attributed to the influence of the High Arctic large igneous province. N‐S and E‐W Te variations in Podvodnikov Basin may reflect two phases of its formation. Western and eastern parts of south Canada Basin have prominent low and high Te values, respectively. This study provides the reliable Te results of central Arctic and improves our understanding of the evolution of the Arctic.

What Controls Effective Elastic Thickness of the Lithosphere in the Pacific Ocean?

AbstractThe effective elastic thickness () of the lithosphere is a proxy for mechanical strength and can be used to constrain lithospheric rheology and understand how surface deformation relates to deep Earth processes. Here, we map variations over the Pacific Ocean from the inversion of the admittance between free‐air gravity anomaly and bathymetry data calculated using a continuous wavelet transform, taking both surface and subsurface loads into account. The Pacific lithosphere show ranging between 0 and 80 km with a mean of 13.5 km and a standard deviation of 12.3 km. We find that is generally poorly correlated with plate loading age, crustal age, heat flow and Curie point depth, except for relatively young (&lt;60 Ma) and warm lithospheres. Most oceanic plateaus and seamounts show km, with the lowest values (&lt;5 km) around active spreading centers. The highest estimates (&gt;30 km) are found along subduction zones and around the Hawaiian‐Emperor Seamount Chain (HESC). Taken together, these results support a temperature control on for small loads and warm lithosphere through steady‐state creep processes, but strain hardening operating at large plastic strain and low temperature could explain high associated with large‐amplitude and long‐wavelength loads (subduction zones and the HESC) and should be incorporated in yield strength models of oceanic .

Effective Elastic Thickness Map Reveals Subglacial Structure of East Antarctica

AbstractEast Antarctica is the least well‐exposed Precambrian shield on Earth. What is known, or surmised, of its geological structure comes from extrapolation over large distances and from geophysics. Here, we present a map of effective elastic thickness, Te, computed from Antarctic bedrock topography and both terrestrial and satellite gravity data. Te is directly related to lithospheric strength and can distinguish domains of differing tectonic history. Our map reveals a broad region of high Te in Wilkes Land, interpreted as a craton, while elsewhere a corridor of low Te meanders between areas of higher Te. The low‐Te belt follows the trend of prominent linear magnetic anomalies identified in the Gamburtsev and Dronning Maud Land provinces, two of which have previously been identified as possible Grenville or Pan‐African age sutures.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

Spatial variations of effective elastic thickness of the lithosphere in the Okinawa Trough

The Okinawa Trough is an active back-arc basin of the Ryukyu island arc-trench system caused by the Phillippine Sea plate subduction. Its unique location makes it attractive to study the lithosphere of continental margins, and it is of great significance to understand the dynamic mechanism of the trench-arc-basin convergent plate boundary. In this paper, we developed the spectral correlation method to calculate the effective elastic thickness of the lithosphere in the study area. The results show that the effective elastic thickness of the lithosphere is distributed in the range of 9–30 km and the lithosphere strength gradually decreases accordingly. Comparing the spatial variations of effective elastic thickness with the data sets of tectonic activities, we conclude that: The spatial variations of the effective elastic thickness have the negative correlation with that of the heat flow. The thermal tectonic activities of the trough gradually become intense from north to south. There are close links between the spatial variations of the effective elastic thickness and the distribution earthquakes. Most of earthquakes are distributed in the relatively lower effective elastic thickness areas or steep effective elastic thickness gradient zones, which indicate that the weak lithosphere and areas with steep change of effective elastic thickness are prone to accumulate and then release tectonic stresses causing earthquakes. The total characteristics of tectonic activities of the Okinawa Trough indicate a strong trend from north to south in the deep region, and the opposite in the shallow region.

Lithospheric Density Structure and Effective Elastic Thickness Beneath Himalaya and Tibetan Plateau: Inference From the Integrated Analysis of Gravity, Geoid, and Topographic Data Incorporating Seismic Constraints

AbstractInvestigation of deep crustal and lithospheric structures is essential to understand the nature of geodynamical processes beneath the Himalaya and Tibetan plateau of the India‐Eurasia collision zone. Our density cross sections across the Himalaya‐Eurasia collision zone using integrated 2‐D modeling of gravity, topography, and geoid data incorporating constraints from seismic information supports the above contention. Analysis of gravity, geoid, and elevation data over the interior of the Tibetan plateau predicts complete isostatic compensation, whereas margins of the plateau, having large topographic gradients, show lack of isostatic compensation as the Airy Moho differs from flexural Moho and seismic Moho beneath the Himalaya. Our 2‐D modeled lithospheric cross sections show thick crust (~75 km) and thick lithosphere (~240 km) beneath the Himalayas and southern Tibetan plateau and relatively thin crust (~60 km) and thin lithosphere (~140 km) beneath the northern Tibetan plateau. Therefore, depth of lithosphere‐asthenosphere boundary (LAB) mimics the Moho relief. Thinner crust and thin lithosphere under northern Tibetan plateau suggest the importance of the mantle isostasy where the temperature is anomalously high. This corroborates with the presence of recent potassic volcanism, inefficient Sn propagation, east and southeast oriented global positioning system displacements, and large shear wave splitting anisotropy (&gt;2 s). Excellent correlation between effective elastic thickness and lithospheric thickness predicts hot and deformable lithosphere in the northern Tibet and underthrusting of cold Indian mantle beneath the Himalayas.

Meso‐Cenozoic rheological structure of the Linqing Depression: Implications for cratonic destruction

The evolution of the lithospheric rheological structure can reveal geodynamic processes of cratonic destruction. Using data of palaeo‐heat flow, crustal structure, and rheological parameters, we studied the rheological structure of the Linqing Depression. The results show that the lithosphere of the Linqing Depression experienced two stages of weakening, one in the Cretaceous and the other in the Palaeogene, with lithospheric strengths of approximately 2.09 and 1.67 × 1012 N/m and effective elastic thicknesses (Te) of approximately 24 and 20 km, respectively. The Meso‐Cenozoic rheological structures correspond to the ‘jelly sandwich’, ‘crème brûlée’, and ‘banana split’ models. The transformation of these models is closely related to the thermal regime, stress environment, and mantle water content of the Linqing Depression. The rheological structure evolution of the Linqing Depression is correlated with the cratonic destruction of the North China Craton. The geodynamic mechanism of lithospheric weakening in the late Early Cretaceous is related to the subduction and retreat of the Pacific Plate, and the lithospheric weakening in the late Early Palaeogene was a combined effect of the Indo‐Asian collision and subductions of the Pacific and Philippine Sea plates.

Strength variations of the Australian continent: Effects of temperature, strain rate, and rheological changes

The Australian continent is composed of several geologic provinces, showing a general age progression from Archean in the west to Phanerozoic in the east. The lithospheric heterogeneity and complex tectonic history of this region make it a key area for studying the thermal and rheological structure of the geological provinces and testing the influence of different conditions, such as temperature, rheology, and strain rate on the plate strength. In a previous study, temperature and compositional variations of the Australian upper mantle have been determined based on a joint interpretation of the seismic tomography and gravity data. In this study, we further implement a thermal model of the crust, based on available surface heat flow data from regional and global database. The crustal and upper mantle thermal models show different anomalies distribution, indicating a significant variation of the thermal conditions with depth. The new thermal models are used to estimate strength and effective elastic thickness ( Te ) distribution in the lithosphere. For this aim, we assigned the rheology of the crust based on the seismic velocities provided by the AuSREM model and used the strain rate values obtained from a global mantle flow model, constrained by seismic and gravity data. The maximal strength and Te are found in the West Australian Craton, on account of the low temperatures in the lithospheric mantle. We found that locations of the intraplate earthquakes attend to sharp changes in the lithospheric strength. Comparison of the results with those obtained for uniform rheology and strain rate, indicate that in the Officer basin the variations of the crustal rheology enhance the effect of temperature changes, while in the Yilgarn craton they reduce it. On the other hand, the lower values of the strain rate in the cratons than in the Phanerozoic regions influence the strength/ Te in the opposite way with respect to temperatures. Integrated strength (Pa m) and strength (Pa) variations along three cross-sections. Horizontal and vertical axes of the cross-sections display the distance and depth (both in km), respectively. (a) Integrated strength of the lithosphere. The three black lines show the location of the cross-sections; (b) Integrated strength of the crust. White circles show the earthquakes location, occurred from 1990 to present-day, with moment magnitudes (Mw) >2.5, taken from the USGS seismic catalogue ( https://earthquake.usgs.gov/earthquakes/search/ ); (c) Integrated strength of the mantle lithosphere; (d) Percentage of crustal strength; (e) Strength variations along three cross-sections (A-A′, B-B′, and C-C′, respectively). • A crustal themal and rheological model of the Australian continent is set up. • Crustal thermal anomalies correlate with surface heat flow variations. • Upper mantle thermal anomalies correlate with age of the tectonic features. • The strength distribution is strongly influenced by the temperature variations.

Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

AbstractThis paper investigates the causes of the Late Cretaceous transition from “Sevier” to “Laramide” orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction.

Effective elastic thickness of Zealandia and its implications for lithospheric deformation

Zealandia was once located at the active Gondwana margin and experienced complicated tectonic evolution from continental breakup to subduction and oblique slipping along the plate boundary. Here, we investigate the lithospheric thermal and rheological properties via spatial variations in effective elastic thickness (Te). Using the topography, sediment thickness and gravity data, the fan wavelet coherence method is employed to recover Te values. The effects of dynamic topography and gravity in the subduction zone on the estimated Te are considered and removed. The final results reveal that the Te variation pattern generally coincides well with the tectonic regimes. Relatively high Te values up to 22 km are observed in North Island and the southernmost part of South Island, while central and northern South Island are dominated by smaller Te values (~15 km) similar to those of the surrounding submerged plateaus. We interpret the relatively high Te values as indicating the combined lithospheric strength due to the contacting plates in the subduction zone. To the north along the Tonga-Kermadec subduction zone, the lithosphere surprisingly exhibits anomalously high Te values, ranging from 22 to 36 km, which contradicts the geological evidence of normal faults developed in the incoming Pacific Plate and the magmatism in the overlying Australian Plate that generally contribute to low lithospheric strength. We speculate that the high-Te zone is related to the large bathymetry and gravity anomalies in the bending plate and controlled by non-isostatic factors. Finally, we find widespread low Te values covering the entire arcs and oceanic basins in northern Zealandia, which may indicate that regional lithospheric thinning associated with delamination weakened the integrated strength of the lithosphere.