Mantle heat flow and geotherms for the main geologic domains in Morocco

 Thermal and deep seismic soundings data are used to study the dependence between the compressional P_n velocity and the surface heat flow or the temperature at the Moho discontinuity in Morocco. This correlation indicates a significant decrease in P_n velocity where high heat flow and Moho temperature are observed. This result is consistent with respect to other regions of the world. Crustal heat generation models and geotherms are constructed for the major Moroccan geological domains extending from the Precambrian units in the south to the Alpine units in the north. The crustal contribution in surface heat flow is on average 35 mWm^–2, with high values of 41–42 mWm^–2 in the western and eastern Meseta where Hercynian granite intrusions could enrich the crust in radioactive heat sources. High mantle heat flow values are obtained beneath the Alboran neogene basin (62 mWm^–2), the Rif (47 mWm^–2), the Middle Atlas (41 mWm^–2), and the south Atlantic margin (40 mWm^–2) where the crust is thinned by an extensional tectonic regime. Despite their similar formation context, the intra-continental belts of the Middle and the High Atlas show different geothermal field components. A lithospheric heating process in the Middle Atlas could be the result of a Plio-Quaternary basaltic volcanism. Finally, the Precambrian basement of the Anti-Atlas like all the West African shield is a stable domain showing the lowest subsurface temperatures. Received: 14 January 1998 / Accepted: 29 June 1999     

On the correlation between heat flow and crustal thickness

In this study the results of a search for correlation between terrestrial heat flow and the thickness of the earth's crust are presented. The correlation is sought for separately in different categories of the tectonic formations considered. These categories are defined by the thickness of the consolidated crust and also the degree of correspondence between the variations of the positions of the upper and lower boundaries of the consolidated crust along particular tectonic units. The calculations show a definite correlation to exist between heat flow and Moho-depths in every category considered. This correlation is positive for tectonic formations with thick crust, and negative for those with normal and thinned crust. It is found that the thinning of the crust with increasing heat flow results in one and the same correlational relationship for three different particular categories of platform structures, which, however, have quite different thermal regimes. This particular correlation appears to be stronger than any of the other correlational relationships obtained in this work. A physical interpretation of this close correlation will be given in a subsequent paper.     

On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses

Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations.     

中国大陆岩石圈厚度分布研究

利用不同物理性质所估计的岩石圈厚度可能具有不同的地球动力学意义。大陆岩石圈等效弹性厚度往往只与岩石圈内部的某些岩层相关,因此它可能不代表一般意义上的岩石圈厚度。地震学岩石圈厚度虽然有较高的精度,但依赖于人为地对岩石圈的定义;并且其具有的短时间尺度效应决定了它与长时间尺度的岩石圈概念不一致。热学岩石圈厚度体现了长时间尺度上的岩石圈热学作用,因此其厚度定义的标准是较合理的。地震-热学岩石圈厚度研究利用地震波速反演得到的温度数据按照热学岩石圈标准来对岩石圈厚度进行研究,具有地震学和热学岩石圈厚度两者的优点,是较合理的对岩石圈厚度的估计。中国大陆地震-热学岩石圈厚度分布有如下特点:(1)中国东部岩石圈较薄,厚度约100 km,其中包括中国东北、中朝克拉通、扬子克拉通东部和华南造山带;(2)青藏高原和塔里木克拉通以南地区的厚度变化较大,厚度约在160~220 km;(3)三大克拉通的岩石圈厚度有较大区别,扬子克拉通的核心最厚达约170 km,塔里木克拉通的核心厚度约140 km,中朝克拉通的厚度约100 km;(4)昆仑秦岭造山带的岩石圈上地幔内部较复杂,可能有大面积的部分熔融;(5)整个大陆岩石圈厚度分布并没有显示出与地壳年龄的线性相关关系,却表现出了与大地构造格局的直接关系。受板块碰撞强烈影响的地区,岩石圈较厚;受大洋俯冲带影响较强的地区,岩石圈较薄。     

Vertical variation of heat flow density in the continental crust

Terrestrial heat flow density is a key parameter in understanding the past, present and future development of our planet. Most phenomena studied in deep crustal geophysics are temperature dependent and therefore reliable assesments of deep temperatures are necessary. Most heat flow measurements have been made in drill holes which are shallow (< 1 km) in comparison to the thicknesses of the crust and lithosphere. The recent findings in deep drilling projects (e.g. the Kola deep hole in Russia and the KTB hole in Germany) have yielded results which suggest that there is a distinct contrast between heat flow densities measured in the uppermost 1 km and values measured at deeper levels. The factors contributing to the vertical variation in the uppermost few kilometres are discussed with special emphasis on palaeoclimatic ground surface temperature changes and groundwater circulation in the bedrock.     

松辽盆地中央坳陷区中—新生代岩石圈厚度演化

岩石圈厚度演化为揭示深部动力学过程提供依据和重要信息。本文通过恢复松辽盆地中央凹陷区典型单井的热历史,利用实测的热物性参数和前人所确定的地壳分层结构,基于地热学相关理论计算了松辽盆地中—新生代主要地质时期的“热”岩石圈厚度。研究表明,松辽盆地中央凹陷区热流演化有两个阶段：1）晚白垩世逐渐升温阶段,此期间热流逐渐增加,最终在白垩纪末期达到最大热流值;2）新生代以来的热流衰退阶段,此阶段构造活动相对稳定,大地热流在此期间逐渐减少,现今热流平均为78.2 mW/m²。松辽盆地岩石圈厚度在早白垩世为42 km左右,白垩世末期降低至36 km左右,现今岩石圈厚度增加至55 km。这主要是由于在侏罗纪—白垩纪时期,中国东部大陆岩石圈整体处于东西拉张的环境中,地壳和岩石圈伸展减薄,导致白垩纪末期岩石圈减薄,此后盆地整体处于拗陷构造发育期,岩石圈再次增厚至现今的55 km。本文的研究成果初步揭示了松辽盆地中—新生代岩石圈演化的过程,为松辽盆地深部动力学过程提供依据,并具有重要的科学意义。

     

冀中坳陷中部现今热岩石圈厚度及地热学意义探讨

热岩石圈厚度是研究盆地的构造演化和板块动力学的重要参数,本文通过实测数据构建地壳分层模型,根据热传导的基本原理,计算了冀中坳陷中部的Moho面温度以及热岩石圈厚度,并探讨其地热学意义。结果表明:冀中坳陷中部的Moho面温度分布在500～600℃,西南侧整体温度较东北部高,热岩石圈厚度介于102～122km,其平面展布特征与华北克拉通热岩石圈厚度西厚东薄的特征相吻合,为华北克拉通受太平洋板块西向俯冲导致东部遭受破坏提供了依据,并且较薄的岩石圈使热流更易传导到地壳浅部,成为了该地区热异常的成因背景。     

藏北岩石圈厚度与减薄机制分析

天然地震S波和大地电磁测深给出了两种不同的藏北岩石圈厚度模型,两种测量结果的地质含义至今还不十分清楚。通过对地表高程与地壳厚度回归关系的研究,以回归直线的斜率和截距作为地壳和岩石圈地幔平均密度取值的约束,并考虑相变因素对软流圈密度的影响,采用均衡理论对藏北岩石圈厚度进行了计算。计算结果表明,在可能的软流圈温度取值范围内藏北岩石圈的平均厚度约为106～120km,地壳增厚前的岩石圈平均厚度约80km。藏北新生代火山作用和岩浆起源-分凝深度分析表明,藏北现今岩石圈厚度主要受金云母脱水深度所控制。增厚前岩石圈地幔底部温度高于橄榄岩湿固相线温度,并受闪石和金云母高压脱水作用的影响。加厚岩石圈地幔因其底部不断发生脱水低程度熔融而进入软流圈小尺度对流体系,使岩石圈加厚过程中伴随有底部的脉动减薄作用。     

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 (T_e). Using the topography, sediment thickness and gravity data, the fan wavelet coherence method is employed to recover T_e values. The effects of dynamic topography and gravity in the subduction zone on the estimated T_e are considered and removed. The final results reveal that the T_e variation pattern generally coincides well with the tectonic regimes. Relatively high T_e 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 T_e values (~15 km) similar to those of the surrounding submerged plateaus. We interpret the relatively high T_e 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 T_e 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-T_e 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 T_e 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.     

New heat flow data from the immediate vicinity of the Kola super-deep borehole: Vertical variation in heat flow confirmed and attributed to advection

We present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12.4-km deep Kola super-deep borehole, NW Russia. Our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure (1.6 km). Our results confirm earlier results on vertical heat flow trends of in the uppermost part of the Kola super-deep hole, and imply that the thermal regime is not in steady-state conductive conditions. In an area of 3-km × 5-km measurements were performed in 1–2-km deep boreholes surrounding the Kola super-deep hole and on core samples from these holes. Temperature logs are available from 36 holes. Core data exists from 23 boreholes with a total length of 11.5 km at a vertical resolution of 10 m. We carried out a very detailed study on thermal conductivity with regard to anisotropy, inhomogeneity and temperature dependence. Tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements. Additionally, we measured specific heat capacity, heat generation rate, density, porosity, and permeability on selected subsets of core samples. Heat flow from 19 boreholes varies between 31 and 45 mW m⁻² with an average value of 38 mW m⁻². In most boreholes the vertical heat flow profiles show a considerable variation with depth. This is consistent with observations in the upper part of the Kola super-deep borehole. We conclude that this variation is not caused by technical operations but reflects a natural process. It is considered to be due to a combination of advective, structural and paleoclimatic effects. Preliminary 3-D numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors: advective heat transfer turns out to have a major influence on the vertical variation of heat flow, although transient changes in surface temperature may also cause a significant variation. Heterogeneity of the rocks in the study area is less important.     

The variability of heat flow both regional and with depth in southern Alberta, Canada: Effect of groundwater flow?

Detailed studies of terrestrial heat flow in southern and central Alberta estimated on the basis of an order of magnitude larger data base than ever used before (33653 bottom-hole temperature data from 18711 wells) and thermal conductivity values based on detailed rock studies and measured rock conductivities show significant regional and local variations and variations with depth. Heat flow values were estimated for each 3 × 3 township/range area (28.8 × 28.8 km). A difference in heat flow exists between Paleozoic and Mesozoic strata. Generally lower heat flow values are observed in the strata above the Paleozoic erosional surface (20–75 mW m⁻²). Much higher values are estimated for the Younger Paleozoic formations, with large local and regional variations between 40 and 100 mW m⁻².Average heat flow values based on heat flow determinations below and above the Paleozoic surface that agree within 20% show an increase from values less than 40 mW m⁻² in southern and southwestern Alberta to values as high as 70 mW m⁻² in central Alberta. The predominance of regional downward groundwater flows in Mesozoic strata seem to be responsible for the generally observed heat flow increase with depth.The results show that the basin heat flow pattern is influenced by water movement and even careful detailed heat flow measurements will not give correct values of background steady-state heat flow within the sedimentary strata.     

南海岩石圈厚度变化特征及其构造意义

地震层析资料表明,南海地区,自红河口向南经南海、苏录海到苏拉威西海,岩石圈速度低,底部横波速度仅4.4km/s,岩石圈厚度在60～80km,为薄岩石圈地区;而软流层的速度也较低,在4.2～4.4km/s,但厚度较大,大于200km。从红河—莺歌海断裂带经南海到苏录海,存在一条北西向宽约200km的上地幔北西向低速带,速度在4.05～4.25km/s(面波速度)。它反映了新生代南海地区上地幔的动力学过程。南海岩石圈厚度变化存在明显差异,南海陆缘,岩石圈厚度在70～80km,而在南海洋盆之下,岩石圈厚度超过100km,岩石圈底部存在高速岩石层,并且洋盆下的岩石圈之厚度比大陆边缘厚,在海盆岩石圈下部的60～80km深度上存在一高速层,纵波速度为8.2～8.3km/s。特别是中央海盆及西北海盆与西南海盆,其下部岩石圈中均存在一高速岩石层,这是非常具有构造意义的。由此笔者提出大陆岩石圈裂离、上地幔因减压而部分熔融所产生的基性岩浆形成南海新生代洋壳的猜想。     

On the forces not moving lithospheric plates

Plume tectonics is examined as an alternative to thermal convection as a mechanism of continental drift and plate movements. The concept of plumes seems difficult to reconcile with the principles of mechanics.     

Heat flow and lithospheric thermal regime in the Northeast German Basin

New values of surface heat flow are reported for 13 deep borehole locations in the Northeast German Basin (NEGB) ranging from 68 to 91 mW m^− 2 with a mean of 77 ± 3 mW m^− 2. The values are derived from continuous temperature logs, measured thermal conductivity, and log-derived radiogenic heat production. The heat-flow values are supposed free of effects from surface palaeoclimatic temperature variations, from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow. Two-D numerical lithospheric thermal models are developed for a 500 km section along the DEKORP-BASIN 9601 deep seismic line across the basin with a north-eastward extension across the Tornquist Zone. A detailed conceptual model of crustal structure and composition, thermal conductivity, and heat production distribution is developed. Different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow. The best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the NEGB. This estimate is corroborated by seismological studies and somewhat less than typical for stabilized Phanerozoic lithosphere. Modelled Moho temperatures in the basin are about 800 °C; heat flow from the mantle is about 35 to 40 mW m^− 2. In the southernmost part of the section, beneath the Harz Mountains, higher Moho temperatures up to 900 to 1000 °C are shown. While the relatively high level of surface heat flow in the NEGB obviously is of longer wave length and related to lithosphere thickness, changes in crustal structure and composition are responsible for short-wave-length anomalies.     

New terrestrial heat flow map of Europe after regional paleoclimatic correction application

Heat flow variations with depth in Europe can be explained by a model of surface temperature changes >10°C. New heat flow map of Europe is based on updated database of uncorrected heat flow values to which paleoclimatic correction is applied across the continent. Correction is depth dependent due to a diffusive thermal transfer of the surface temperature forcing of which glacial–interglacial history has the largest impact. It is obvious that large part of the uncorrected heat flow values in the existing heat flow databases from wells as shallow as few hundreds of meters is underestimated. This explains some very low uncorrected heat flow values 20–30 mW/m² in the shields and shallow basin areas of the craton. Also, heat flow values in other areas including orogenic belts are likely underestimated. Based on the uncorrected and corrected heat flow maps using 5 km × 5 km grid, we have calculated average heat flow values (uncorrected heat flow: 56.0 mW/m²; SD 20.3 mW/m² vs. corrected heat flow: 63.2 mW/m²; SD 19.6 m/Wm²) and heat loss for the continental part. Total heat loss is 928 E09 W for the uncorrected values versus corrected 1050 E09 W.     

华北深部岩石圈存在弱的新元古代热活动的同位素年代学信息：证据及意义

人们普遍认为华北区别于华南的主要特征在于缺少广泛的新元古代岩浆活动,但原因是什么还不清楚。本文汇总了华北四个地区深源岩石包体中有这样同位素年龄的结果,并就它们所反映的华北当时在Rodinia超大陆裂解中心的位置和可能的岩石圈厚度进行了讨论。这些深源岩石包体分别是辽宁复县古生代金伯利岩中的基性麻粒岩、河北涉县碳酸岩化金伯利岩中石榴石辉石岩、河南信阳中生代火山岩中的橄榄岩及河北汉诺坝新生代玄武岩中橄榄岩。所记录的新元古代年龄信息包括:复县基性麻粒岩锆石0.61Ga的下交点年龄,涉县石榴石辉石岩全岩-单矿物的0.76GaSm-Nd内部等时年龄、信阳橄榄岩锆石的新元古代(>0.64Ga)年龄以及汉诺坝橄榄岩硫化物0.9～0.6Ga的Re-Os年龄。与华南广泛发育新元古代岩浆活动不同,华北有限的新元古代年龄信息(包括地表出现的)可能反映着它们当时在Rodinia超大陆的位置有所不同,即华南更靠近于超大陆裂解的中心、而华北可能远离该中心(如边缘)。位置的差别也预示着当时华南岩石圈的可能比较薄、而华北具巨厚的岩石圈。因此,我们认为弱的热事件和巨厚的岩石圈可能是造成华北新元古代热活动不明显的原因。     

Garnet lherzolites from Somerset Island,Canada and aspects of the nature of perturbed geotherms

Garnet lherzolite xenoliths of similar petrography and mineralogy are found in the Elwin Bay, Nanorluk, and Amayersuk kimberlites. The xenoliths are either coarse equant to coarse tabular or porphyroclastic in texture. Compositions of coexisting pyroxenes indicates equilibration at 1000–1270° C at 34–41 kb (Wood-Banno/Wood method) or 865–1200° C at 29–36 kb (Wells/Wood method). No simple correlation exists between textural types and equilibration temperature. A primary spinel-bearing garnet lherzolite has equilibrated at 840° C at 21 kb (Wells/Wood) and provides the only known example of a xenolith with relatively high Cr/Cr+Al which has equilibrated at the spinel to garnet lherzolite transition along the continental geotherm. The pressure and temperature estimates for the xenoliths lie above those of the steady state geotherm and indicate that a perturbed geotherm existed in this region at the time of kimberlite intrusion. The formation of perturbed geotherms is discussed and it is considered that the upper high temperature limbs of inflected geotherms are transient pseudogeotherms generated in response to a thermal aureole about a rising mantle diapir and that the lherzolites which define such a geotherm represent a telescoped section of the mantle and include xenoliths derived from above and below the point of kimberlite liquid segregation. The lower temperature limbs of inflected geotherms are considered to be representative of the steady state geotherm and are sampled by the kimberlite which after segregation from the diapir rises at a much faster rate than the parent diapir and passes through material which is unaffected by the diapir thermal aureole.     

The variation of heat flow density with depth in the prairies basin of western Canada

Bottom-hole temperature values from approximately 36,000 wells in Alberta. Saskatchewan and Manitoba, Canada, have been used to study thermal gradients and heat flow density there. It is found that variations of heat flow density with depth occur throughout the Prairies basin. Differences in heat flow density exist between the Mesozoic + Cenozoic and Paleozoic sediments and are related to the hydrodynamics which is controlled by the topography. The heat flow density through the Mesozoic + Cenozoic of the upper part of the section is less than that in the Paleozoic formations of the lower part of the section in recharge areas, but greater in discharge areas. A zone in which heat flow is approximately constant with depth extends down the central part of the basin between the recharge and discharge areas. Heat flowdensity in this zone lies between 60 mW m^?2 and 80 mW m^?2 and is thought to be representative of the deep crustal heat flow density. It is suggested that temperature variations on the Precambrian basement that are not depth related may be associated with anomalous heat flow regimes in the lower crust.     

大地热流研究揭示的中国地壳成分横向变化

依据大地热流值、地壳厚度以及大陆壳／幔热流比与地下流体氦同位素比值的相关关系，计算出中国主要构造单元地壳生热率。同时，根据Rudnick和Fountain(1995）的数据得到地壳生热率和SiO2质量分数的线性关系，进而利用生热率数据得到地壳SiO2质量分数。此方法得到的中国东部地壳生热率和SiO2质量分数与基于地震波速的成分模型相符。中国大陆地壳生热率和SiO2质量分数横向变化明显，东部地区地壳为中性成分，相对富集强不相容元素；而西北部盆地地壳成分偏于中基性。华北、扬子和塔里木地壳成分差异较大，克拉通内部表现出明显的成分非均匀性，褶皱带地壳一般较克拉通略富长英质组分。     

南海北部陆缘热流变化特征及其影响因素分析

热流调查和构造热演化数值模拟是油气地热研究不可或缺的重要内容。沉积盆地在其演化过程中往往叠加了特殊构造事件。通过热流调查和构造热演化数值计算可以更好地约束这些特殊过程,重建更为真实的构造热演化历史。该文通过对南海北部琼东南盆地和珠江口盆地中段热流变化特征分析和构造热演化数值模拟,探讨了影响其热流变化的主要因素。结果表明,琼东南盆地可分3个热流分区:北部陆架与上陆坡区(50~70mW/m~2)、中央坳陷带深水区(70~85mW/m~2)和盆地东部深水区高热流带(85mW/m~2);珠江口盆地中段从陆架往海盆方向热流呈阶梯式抬高,西江凹陷平均热流为55mW/m~2,番禺低隆起为58mW/m~2,白云凹陷为70mW/m~2,下陆坡区为85mW/m~2;陆坡区高热流不仅与岩石圈强烈减薄相关,而且还受到岩石圈破裂时引起的深部热物质上涌的影响,后者对现今陆坡区还有约20mW/m~2的热流贡献;琼东南盆地东部高热流值则主要受到晚中新世以来的岩体侵位热事件的影响,岩体侵入热事件对现今热流值贡献可达10~25mW/m~2。分析表明,在南海深水盆地开展构造热演化数值计算时,需要考虑沉积过程、海底扩张以及岩浆活动等影响因素。