The development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional-scale thermal regimes. These are: the total radiogenic heat supply within the thickened crust, the supply of heat from the mantle, the thermal conductivity of the medium and the length and time scales of erosion of the continental crust. The orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening, during which little temperature change occurs in individual rocks, followed by a lengthier phase of erosion, at the end of which the crust is at its original thickness. The principal features of pressure-temperature-time (PTt) paths followed by rocks in this environment are a period of thermal relaxation, during which the temperature rises towards the higher geotherm that would be supported by the thickened crust, followed by a period of cooling as the rock approaches the cold land surface. The temperature increase that occurs is governed by the degree of thickening of the crust, its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary. For much of the parameter range considered, the heating phase encompasses a considerable portion of the exhumation (decompression) part of the PTt path. In addition to the detailed calculation of PTt paths we present an idealized model of the thickening and exhumation process, which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path. An important feature of these PTt paths is that most of them lie within 50 °C of the maximum temperature attained for one third or more of the total duration of their burial and uplift, and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience. Considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm. A companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes. © 1984 Oxford University Press.

/pdf/pressure-temperature-time-paths-of-regional-metamorphism-i-3lc2isq1vh.pdf

Pressure-Temperature-Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust

The continental crust has ultimately been extracted from the mantle and knowledge of its composition is therefore fundamental to an understanding of the chemical evolution of the Earth. Attempts to model its average chemistry are complicated by marked vertical and lateral chemical and lithological heterogeneity1–3 and by problems of sampling the deeper levels of the crust. We have adopted a different, empirical, approach to try and take account of geophysical and isotopic constraints by considering the major and trace element chemistry of components which are typical of various crustal levels and ages. Our estimate differs from that of the ‘andesitic’ model in being more siliceous and having a more fractionated rare earth element pattern. Its higher Th, U and K contents generate a higher heat production of 0.95 µW m−3 compared with previous estimates of 0.75–0.91 µW m−3.

Empirical approach to estimating the composition of the continental crust

About 3.4 Ga ago voluminous calc-alkaline magmas represented by the granitoid gneisses of southern India were emplaced into a now poorly preserved non-continental crust. Unstable ensialic basins, initiated at about 3.0 Ga, were filled with volcanic and sedimentary rocks up to about 2.6 Ga. This basement-cover sequence was deformed at the close of the Archaean, first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments. Structures produced by this episode were refolded and dislocated by large N-S strike-slip shear belts, to impart intense, steep planar fabrics to large volumes of the crust. Fluids rich in CO₂, possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism, purged the deep crust of H₂O to form granulites. Upward migration of hot H₂O-rich fluids encouraged partial melting at intermediate crustal levels. These late-Archaean tectonic and thermal events began soon after the close of basin f...

Precambrian Tectonics and Crustal Evolution in South India

Summary A set of preferred geotherms for the continental crust is calculated using assumptions of steady state conductive heat transfer. The calculations use observed heat flow as the principal constraint, temperature and pressure dependent thermal conductivity functions, and a radiogenic heat generation profile that is consistent with a generalized petrological model of the lithosphere. Properties of these geotherms include: (1) a nearly constant gradient through the upper crust because the decrease in thermal conductivity at higher temperatures for felsic rocks counteracts the decreasing heat flow caused by crustal radioactivity; (2) divergence of the geotherms amounting to temperature differences of more than 500 K between rifts and shields at Moho depths, and (3) convergence of geotherms below 250 km in the asthenosphere as convective heat transfer dominates. Emphasis is also placed on understanding the sensitivity of calculated temperatures to measureable properties. Computed geotherms are overall most sensitive to surface heat flow or reduced heat flow, and uncertainties in this quantity will ultimately limit the accuracy of temperature estimations for the deeper crust and lithosphere. Otherwise, the sensitivity to parameters such as heat production, thermal conductivity and its temperature dependence throughout the lithosphere and crustal thickness depends on depth and whether the region is characterized by high or low heat flow. Upper mantle heat production, pressure coefficients of thermal conductivity, and the position of the upper/lower crust boundary are relatively insensitive parameters for lower crustal temperature calculations. Dynamic events accompanying crust forming processes lead to significant perturbations of the steady state static temperature field described above. Pressure-temperature pairs from granulite terrains plot near high temperature limits for those steady state geotherms and imply either a transient P-T-t path or a magmatic heat input from the mantle accompanying crustal thickening.

Thermal gradients in the continental crust

The geothermal resource assessment presented is a refinement and updating of USGS Circular 726. Nonproprietary information available in June 1978 is used to assess geothermal energy in the ground and, when possible, to evaluate the fraction that might be recovered at the surface. Five categories of geothermal energy are discussed: conduction-dominated regimes, igneous-related geothermal systems, high-temperature (> 150/sup 0/C) and intermediate-temperature (90 to 150/sup 0/C) hydrothermal convection systems, low-temperature (< 90/sup 0/C) geothermal waters, and geopressured-geothermal energy (both thermal energy and energy from dissolved methane). Assessment data are presented on three colored maps prepared in cooperation with the National Oceanic and Atmospheric Administration. Separate abstracts were prepared for papers on these five categories.

/pdf/assessment-of-geothermal-resources-of-the-united-states-1978-1gwcdydag5.pdf

Assessment of geothermal resources of the United States: 1978

Recent heat flow measurements at 10 separate localities in southern New Mexico and West Texas range from 0.9 to 3.6 HFU (1 HFU = 1 × 10−6 cal/cm2 s). Radiogenic heat production determinations at eight of these localities range from 2.2 to 4.9 HGU (1 HGU = 1 × 10−13 cal/cm3 s). When the heat flow measurements are reduced by the heat from bedrock radioactivity, the Rio Grande rift and immediately adjacent areas are characterized by very high reduced flux (≥1.8 HFU), which contrasts strongly with 1.2 to 1.5-HFU values in bordering portions of the Basin and Range province and a 0.8-HFU value assumed for the southern Great Plains. Analyses of 2000 gravity observations in the Basin and Range province and the Great Plains reveal that a positive gravity ridge of about +30 mGal occurs along the Rio Grande rift in the Franklin and San Andres mountains and the Tularosa Valley. This relative gravity high can be caused by crustal thinning and/or shallow sills and dikes of mafic rocks in the crust under the rift. Gravity anomalies and a thin crust are compatible with a low-density upper mantle beneath most of the Basin and Range province. The new heat flow and radioactivity data indicate that the Basin and Range-Great Plains heat flow transition is marked by unusually high observed and reduced flux in the neighborhood of the Rio Grande rift. The combined thermal and gravity observations may be explained by shallow crustal intrusions, a hot abnormal upper mantle, and relative crustal thinning beneath the rift.

Heat flow and gravity interpretation across the Rio Grande Rift in southern New Mexico and west Texas

A continental crustal model and its geothermal implications

Crustal Structure and Evolution of the Southern Rio Grande Rift

Preliminary transient heat flow model of the Rio Grande rift in southern New Mexico

K, U, and Th distribution between dry and wet facies of a syenitic intrusion and the role of fluid content

Popular Tasks used by Researchers

How can I help with your research?

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

We encountered an error while loading the panel. Please try refreshing the page.

Something went wrong

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

Anyone with the link can view this chat. You can switch it back to private anytime.

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

Edward R. Decker

Author Tools

Chat about Author