Studying the variation of crustal temperature and vertical specific heat flow is important for the interpretation of the recent thermal regime. The variation is caused by different physical processes and is important for assessing the long-term behavior of thermal power and temperatures of geothermal energy. Often a constant vertical specific heat flow q is taken as basal boundary condition for 3D heat and fluid flow models. We use a 3D model to perform sensitivity analyses with respect to different physical processes and study their effects on the longterm variation of specific heat flow and temperatures. Using 3D transient models we quantify contribution of different geologic and tectonic process acting over years on transient thermal regime. In particular, we study the effects of extension, erosion or sedimentation, water flow, paleoclimate and deep magmatic processes such as the upwelling of the Eifel plume. These processes are quantified with respect to their thermal response period and there contribution to the present specific heat flow.

First we describe the geology of the study area, then the physical processes and their associated effects on transient specific heat flow follow. These are quantified using 1D, 2D and 3D models to determine the thermal response time and to account for heterogeneity in thermal and hydraulic conductivity. The generic of 2D crosssection models are conductive and do not account for topography. The 3D model is based on measurements, maps, and deep balanced profiles.

To use a methaphor, if we drop a stone into the water, a little later the effect is gone, but if we have a tsunami the effect is much larger and longer. Similarly, if a sudden temperature change occurs at a surface of a infinite half layer, the thermal diffusivity (“thermal velocity”) into the earth is very slow; changes of a million years ago, which remained a large period (1000 years) can still have an effect on the present deep borehole temperatures. Contrary, temperature changes with a short time period, like day and night, have only a very shallow effect. To show this, a responce time [1]. The red circle indicates the study area curve is presented from each geologic phenomena.

Our study area extends across the borders of Belgium, the Netherlands and Germany. It is bordered to the North by the Brabant Massif, to the South by the Eifel-Venn Massif, to the East by the Rur Graben and to the West by the Maas. The area forms the front of the Variscan orogenic belt in Northwestern Europe, which experienced erosion during the Permion to Triassic periods. At present the Eifel-Venn Massif, as part of the Rhenish Massif, forms an elevated plateau up to 700 m elevation. The study area lies at the Variscan front between the river Maas and the Eifel-Venn Massif (red circle in Figure 1).

To give an insight in the geological history of the formation of Eifel and foreland Stratigraphical sequences, a chronological reconstruction of geological processes as synthesis of current ideas is presented.

In the Silurian (Figure 2), 420 Ma B.P., Avalonia moved to the North, resulting in a collision between Baltica, Avalonia and Laurentia, known as the Caledonian orogenese. The crust is compressed, so large mountains are formed, and Baltica slab is subducted underneath the Avalonian crust. This slab is quickly heated, resulting in upwelling of hot mantle magma. The Caledonial orogen; now known as the Brabant Massif, is a large anticlinal structure with a W-E direction; with Cambrian rocks in the center and Ordovician to Silurian rocks at the flanks. The total thickness of these rocks is about 7000 m [2]. Old Silurian Vulcanic belts lie at the Southern border of the Brabant Massif and at Visé, 20 km S-W of Aachen. The volcano’s have dacitic (63% - 68% of SiO₂) and rhyolitic (68% - 78% of SiO₂) deposits and therefore belonged to the most explosive category [3,4].

The volcans might be caused by a granitic batholite. This body is interpreted as the product of the melting of siliceous crust induced by continent-continent collision, and Precambrium rocks (72% of Gneis). Geophysical interpretations of gravity anomalies suggest a granite intrusion at a depth of 1 km to 5 km. A study near of the granite intrusion shows pegmatites enriched in gold,

wolframite and tin and at a larger distance sulphides with copper, zinc, lead, silver, molybdenum.

The Bilzen magnetic high in the Northwest may reflect either the pre-Silurian basement of the Brabant Massif, or an old intrusion of possibly (pre) Variscan age [5].

During the Devonian (Figure 3) to Carboniferous (416 - 359 Ma B.P.), the huge mountains of the Brabant Massif are eroded and uplifted. Their sediments filled the existing southern ocean (see Figure 3). The sediment thickness of Devonian to Carboniferous age increases in southern direction [6]. See reconstruction profile of the Brabant Massif in southeasterly direction (Figure 4).

During the process of erosion and sedimentation there were small sea level variations [7]. The part of the Brabant Massif is only covered with middle Devonian Frasnian deposits [7]. In the early Devonian and the late Devonian, it was above sea level.

At a later stage, during the Carboniferous Figure 5 Westfalian (315 Ma. B.P.), the Gondwana continent in the South moves northward forwards to Avalonia. This

causes a shortening of the crust and a faster subduction of the basin with accordingly large deposits.

Large swamps are formed, with a luxuriant vegetation at the equator. Note that the Brabant Massif was totally covered with carboniferous sediments. Studies indicate that one meter of present coal thickness corresponds to 25 m of organic deposited material. Dead plants were covered relative fast by sediments, consequently there was too little oxygen to degrade them. The water and formed gas could not escape due to the fast subduction and due to the low permeability of peat and shale. This results in a very thick Carboniferous swamp with verymud water and high gas pressure and temperature. The latter is caused by the very low thermal conductivity of gas, water and peat: low thermal conductivity causes the temperature gradient to increase. These large swamps correspond to today’s large coal mining regions.

During the late Carboniferous, Gondwana moved to the North resulting in an arc-continental collision of Gondwana with Avalonia, known as the Variscan orogeneses. [9] studied this orogeneses numerically using different models. All models indicate that the crustal root formation, delineation, and hot asthenosperic heat advection might have occured within 1 Ma - 10 Ma period.

Due to this collision large mountains were formed again Figure 4. In the study area—Eifel-Maas, Brabant Massif—more phases of this orogenic stage can be distinguished.

Firstly, deeper parallel overstrusted faults developed between the Namurian and the Westfalian B periods. Secondly, a large sheet overthrust, eroding the Carboniferous sediments and forming a sheet overlaying the older formations. A reason for travelling this long distance might be the large, over-pressured Westfalian swamp; high temperature and high fluid pressures provide small resistance to external stresses. When a dense sheet loads on a low density swamp with high fluid pore pressure, the warm water and gas is squeezed out, resulting in the large fluid flow at the front of the Variscan reported by [10,11].

The overthrust sheet partly removes the original Westfalian and Namurian sediments deposited them as huge mountain over the Brabant Massif in the Northeast. High vitrinite reflections [5] at the northeastern side of the Brabant Massif support this scenario. Models performed at the Netherlands suggest a cover of at least 2700 m [8] in post-Westfalien C age. During the Permian period, 280 Ma B.P., all continents together formed the so-called super continent Pangea with a very low sea level. The area which currently lies at the North Sea, but at that time laying near the equator was drying, which formed large and thick Salt layers.

Between the Permian and the Triassic period (251 Ma B.P.) 90% of all marine species and 70% of all terrestrial species died.

During the Triassic and Jurassic period (251 Ma - 145 Ma B.P.) the large mountains (i.e. the Brabant Massif in today’s Belgium), erodes filling the oceans with sediments. In the study area, no Triassic or Jurassic sediments were deposited.

During the late Cretaceous, the Supercontinent Pangea split up into the present American and European plates separated by the Atlantic Ocean. At that time the sea level was about 100 m to 300 m higher than the present sea level. Hydrothermal precipitation formed sulphide ore deposits during the early inversion of the Rhine Graben in the Cretaceous. The deposits occured mostly in the Dinantian chalk, but also in the under Cretaceous limestones, consisting of zinc and lead. The Rur Graben was reactivated and several SW-NE striking faults were formed in an area presently marked by a prominent gravity low.

In the Tertiary 45 Ma B.P., volcanic eruptions occurred in the Eifel mountains. Two new volcanic fields evolved during the Quaternary last 600 ky with the last eruptions only 11 ky - 12 ky B.P. At the same time the Rhenish Shield experienced strong uplift of the (up to 250 m in 600 ky). This uplift is an isostatic response of the lithosphere to a deepseated buoyant hot body. This hot body was detected by seismic tomography [12] and is known as the known Eifel mantle plume.

The volcanic deposits are dominated by potassiumrich, mafic (Mg-Fe rich) minerals, poor in silica. Larger peridotite xenoliths occur, which are typical for intraplate, continental settings. Many xenoliths originate from lower crustal and upper mantle sources. The Western Eifel volcanic field is about 600 km² in area and contains around 240 individual volcanoes. The Eifel volcanic region is associated with the Rhine Graben, a continental rift zone. The Rhine Graben became active during the Eocene, and fault movements intensified during the Miocene. Since the Miocene, the tectonic activity of the Rhine Graben and other rift zones of western Europe have continued at a lower level minor fault movements, with occasional small earthquakes, and Quaternary volcanism in Eifel.

A lot of study has already been done in the area. So I present you the available information of these studies, and discuss their relevance to model.