.New Frontiers in Integrated Solid Earth Sciences Phần 2 ppsx

Perpectives on Integrated Solid Earth Sciences 27

Fig. 18 Schematic section of San Andreas fault zone observa￾tory at depth with different phases of drilling. Background color

shows electric resistivity measured along a profile perpendicu￾lar to the faults strike. The bold black lines at the bottom show

sidetrack coreholes drilled through the active trace of the San

Andreas Fault. The core photo shows a large black serpentine

clast cut by calcite veins embedded in foliated fault gouge (cour￾tesy ICDP, see also http://www.icdp-online.org)

the dome. The detected dacitic dyke system which

feeds both effusive and explosive eruptions was unex￾pectedly cool due to enhanced groundwater circulation

(Sakuma et al., 2008). Even the structure of oceanic

hot spots, a highly debated topic in Earth sciences, has

been tested by scientific drilling. At the most promi￾nent volcanic edifice on the globe, Hawaii, one Mil￾lion years of volcanic built-up is documented in the

pancake-like pile of lava flows of Mauna Kea. This

layered structure allows charting of the complex chan￾neled buoyancy of lowermost mantle materials entrain￾ing as plume upper mantle on its passage to surface

(Stolper et al., 2009).

Almost 180 craters on Earth are known currently

that have been formed by astrophysical chance when

celestial bodies such as asteroids collided with our

planet. Drilling to study cratering processes provides

data not only for modeling the impactor size but also

for modeling the energy release through melting, evap￾oration, ejection and, most importantly, for model￾ing of the environmental consequences of such dra￾matic events. ICDP drilled the 200-km-wide Chicxu￾lub Crater in Mexico (Hecht et al., 2004; Dressler et al.,

2003) and the 60 km Chesapeake Bay Crater in the

Eastern U.S. The latter underwent a complex microbio￾logical evolution initiated by an impact-related thermal

sterilization and subsequent post-impact repopulation

(Gohn et al., 2008). For such large craters fluidization

of target rocks leads to the formation of a central uplift,

whereas the peak of the small, 10 km Bosumtwi Crater

in Ghana (Ferrière et al., 2008) was formed by brittle

deformation processes.

With smart, cost-effective drilling, paleo-climate

and paleo-environmental evolution is being studied

on continuous lake sediments from Lakes Titicaca,

Malawi, Bosumtwi, Qinghai, and Peten Itza. The lat￾ter for example provided new insights into the changes

of precipitation patterns due to variations of the

Intertropical Convergence Zone over Central America

(Hodell et al., 2008). Sediments in the African trop￾ical Lakes Malawi and Bosumtwi shed new light on

a megadrought at about 100 K years before present

with implications for migration of early humans out

of Africa (Scholz et al., 2007).

Several other ICDP-funded projects provided novel

awareness about active processes and geological

resources (Harms et al., 2007), while ongoing and

future exploration can be monitored on the programs

web resource (http://www.icdp-online.org).

Perspectives on Integrated Solid Earth

Sciences

The papers in this IYPE volume provide a review

of recent developments in aspects of integrated solid

Earth sciences that can be considered as frontier

research.

Tesauro et al. (2009a) (this volume) present

EuCRUST-07, a new 3D model of the crust for West￾ern and Central Europe that offers a starting point in

any kind of numerical modelling to remove the crustal

effect beforehand. The digital model (35ºN, 71ºN;

25ºW, 35ºE) consists of three layers: sediments and

two layers of the crystalline crust. The latter are char￾acterized by average P-wave velocities (Vp), which

were defined based on seismic data. The model was

28 S.A.P.L. Cloetingh and J.F.W. Negendank

obtained by assembling together at a uniform 15

×15

grid available results of deep seismic reflection, refrac￾tion and receiver function studies. The Moho depth

variations were reconstructed by these authors by

merging the most robust and recent maps existing

for the European region and compiled using pub￾lished interpretations of seismic profiles. EuCRUST￾07 demonstrates large differences in Moho depth with

previous compilations: over ±10 km in some specific

areas (e.g., the Baltic Shield). The basement is out￾cropping in some parts of Eastern Europe, while in

Western Europe is up to ∼16 km deep, with an aver￾age value of 3–4 km, reflecting the presence of rel￾atively shallow basins. The velocity structure of the

crystalline crust turns to be much more heterogeneous

than demonstrated in previous compilations, average

Vp varying from 6.0 to 6.9 km/s. In comparison to

existing models, the new model shows average crustal

velocity values distributed over a larger and continuous

range. Furthermore, the results of EuCRUST-07 are

used by Tesauro et al. (this volume) to make inferences

on the lithology, which is typical for different parts

of Europe. The new lithology map shows the Eastern

European tectonic provinces represented by a granite￾felsic granulite upper crust and a mafic granulite lower

crust. Differently, the younger Western European tec￾tonic provinces are mostly characterized by an upper

and lower crust of granite-gneiss and dioritic composi￾tion, respectively.

In the companion paper by Tesauro et al. (2009b)

(this volume), a new thermal and rheological model

of the European lithosphere (10◦W–35E; 35 N–60 N)

is implemented based on a combination of recently

obtained geophysical models. To determine tempera￾ture distribution they use a new tomography model,

which is improved by correcting a-priori for the crustal

effect using the digital model of the European crust

(EuCRUST-07). The uppermost mantle under West￾ern Europe is mostly characterized by temperatures in

a range of 900–1,100◦C with the hottest areas corre￾sponding to the basins, which have experienced recent

extension (e.g., Tyrrhenian Sea and Pannonian Basin).

By contrast, the mantle temperatures under Eastern

Europe are about 550–750◦C at the same depth and

the minimum values are found in the north-eastern

part of the study area. EuCRUST-07 and the new ther￾mal model are used to calculate strength distributions

within the European lithosphere. Lateral variations

of lithology and density derived from EuCRUST-07

are used to construct the new model. Following the

approach of Burov and Diament (1995), the litho￾spheric rheology is employed to calculate variations

of the effective elastic thickness of the lithosphere.

According to these estimates, in Western Europe the

lithosphere is more heterogeneous than that in East￾ern Europe. Western Europe with its predominant

crust-mantle decoupling is mostly characterized by

lower values of strength and elastic thickness. The

crustal strength provides a large contribution (∼50%

of the integrated strength for the whole lithosphere)

in most part of the study area (∼60%). The results

reviewed in this paper shed light on the current debate

on the strength partition between crust and mantle

lithosphere.

As pointed out by Burov (2009) (this volume),

simple mechanical considerations show that many

tectonic-scale surface constructions, such as mountain

ranges or rift flanks that exceed certain critical height

(about 3 km in altitude, depending on rheology and

width) should flatten and collapse within a few My as a

result of gravitational spreading that may be enhanced

by flow in the ductile part of the crust. The elevated

topography is also attacked by surface erosion that, in

case of static topography, would lead to its exponen￾tial decay on a time scale of less than 2.5 My. How￾ever, in nature, mountains or rift flanks grow and stay

as localized tectonic features over geologically impor￾tant periods of time (> 10 My). To explain the long￾term persistence and localized growth of, in particu￾lar, mountain belts, a number of workers have empha￾sized the importance of dynamic feedbacks between

surface processes and tectonic evolution. Surface pro￾cesses modify the topography and redistribute tec￾tonically significant volumes of sedimentary material,

which acts as vertical loading over large horizontal dis￾tances. This results in dynamic loading and unload￾ing of the underlying crust and mantle lithosphere,

whereas topographic contrasts are required to set up

erosion and sedimentation processes. As demonstrated

by Burov (2009), tectonics therefore could be a forcing

factor of surface processes and vice versa. He suggests

that the feedbacks between tectonic and surface pro￾cesses are realised via 2 interdependent mechanisms:

(1) slope, curvature and height dependence of the ero￾sion/deposition rates; (2) surface load-dependent sub￾surface processes such as isostatic rebound and lat￾eral ductile flow in the lower or intermediate crustal

channel. Loading/unloading of the surface due to

Perpectives on Integrated Solid Earth Sciences 29

surface processes results in lateral pressure gradients

that, together with low viscosity of the ductile crust,

may permit rapid relocation of the matter both in hor￾izontal and vertical direction (upward/downward flow

in the ductile crust). In his paper, Burov (2009) pro￾vides an overview of a number of coupled models of

surface and tectonic processes, with a particular focus

on 3 representative cases: (1) slow convergence and

erosion rates (Western Alps), (2) intermediate rates

(Tien Shan, Central Asia), and (3) fast convergence and

erosion rates (Himalaya, Central Asia).

Roure et al. (2009) (this volume) point out that

thanks to a continuous effort for unravelling geologi￾cal records since the early days of coal and petroleum

exploration and water management, the architecture

and chrono-litho-stratigraphy of most sedimentary

basins has been accurately described by means of con￾ventional geological and geophysical studies, using

both surface (outcrops) and subsurface (exploration

wells and industry seismic reflection profiles) data.

However, the understanding of the early development

and long term evolution of sedimentary basins usu￾ally requires the integration of additional data on

the deep Earth, as well as kinematic-sedimentological

and thermo-mechanical modelling approaches cou￾pling both surface and deep processes.

In the last twenty years, major national and interna￾tional efforts, frequently linking academy and indus￾try, have been devoted to the recording of deep seismic

profiles in many intracratonic sedimentary basins and

offshore passive margins, thus providing a direct con￾trol on the structural configuration of the basement and

the architecture of the crust. At the same time, needs

for documenting also the current thickness of the man￾tle lithosphere and the fate of subducted lithospheric

slabs have led to the development of more academic

and new tomographic techniques. When put together,

these various techniques now provide a direct access

to the bulk 3D architecture of sedimentary basins, crys￾talline basement and Moho, as well as underlying man￾tle lithosphere.

Inherited structures, anisotropies in the composition

of the sediments, crust and underlying mantle as well

as thermicity and phase transitions are now taken into

account when predicting the localization of deforma￾tion in the lithosphere during compression and exten￾sion episodes, and reconstructing the geodynamic evo￾lution of rift basins, passive margins and foreland fold￾and-thrust belts.

Source to sink studies also provide accurate esti￾mates of sedimentary budget at basin-scale. Extensive

use of low temperature chrono-thermometers and cou￾pled kinematic, sedimentological and thermal models

allow a precise control on the amount and timing of

erosion and unroofing of source areas, but also the

reconstruction of the sedimentary burial, strata archi￾tecture and litho-facies distribution in the sink areas.

Coupling deep mantle processes with erosion and

climate constitutes a new challenge for understand￾ing the present topography, morphology and long term

evolution of continents, especially in such sensitive

areas as the near shore coastal plains, low lands and

intra-mountain valleys which may be subject to devas￾tating flooding and landslides.

In addition to the search for hydrocarbon resources

and geothermal energy, other societal needs such as

CO2 storage and underground water management will

benefit from upgraded basin modelling techniques.

New 2D and 3D basin modelling tools are progres￾sively developed, coupling in different ways deep

thermo-mechanical processes of the mantle (astheno￾sphere and lithosphere), geomechanics of the upper

crust and sediments (compaction, pressure-solution

and fracturing of seals and reservoirs), basin-scale fluid

and sediment transfers (development of overpressures,

hydrocarbon generation and migration). As pointed out

by Roure et al. (2009), further challenges related to

CO2 storage will soon require the integration of fluid￾rock interactions (reactive transport) in basin and reser￾voir models, in order to cope with the changes induced

by diagenesis in the overall mechanical properties, and

the continuous changes in fluid flow induced by com￾paction, fracturing and cementation.

As pointed out by Mooney and White (2009) (this

volume), seismology has greatly advanced in the past

century. Starting with the invention of the pen-and￾paper seismograph in the 1880s and the advent of

plate tectonics theory in the 1960s, scientists have

been made progress in understanding, forecasting and

preparing for earthquakes and their effects. Tectonic

plate theory explains the occurrence of earthquakes

as two or more plates meeting one another at plate

boundaries where they may collide, rift apart, or drag

against each other. These authors point out that diffuse

plate boundaries, unlike convergent, divergent and lat￾eral boundaries, are not completely defined and spread

over a large area thereby spreading seismic hazards

over a broad region. Intraplate earthquakes occur far

30 S.A.P.L. Cloetingh and J.F.W. Negendank

away from any plate boundary, cause a great loss of

life and cannot be explained by classical plate tec￾tonics. However, classical plate tectonics is evolving,

and now there are more theories behind earthquake

generation dealing not only with the Earth’s crust but

also the hot, viscous lower lithosphere. These authors

draw attention to the notion that in addition to damag￾ing buildings and infrastructure and taking lives, earth￾quakes may also trigger other earthquakes due to stress

changes once seismic energy is released.

Bohnhoff et al. (2009) (this volume) draw attention

to an important discovery in crustal mechanics that

the Earth’s crust is commonly stressed close to fail￾ure, even in tectonically quiet areas. As a result, small

natural or man-made perturbations to the local stress

field may trigger earthquakes. To understand these pro￾cesses, Passive Seismic Monitoring (PSM) with seis￾mometer arrays is a widely used technique that has

been successfully applied to study seismicity at differ￾ent magnitude levels ranging from acoustic emissions

generated in the laboratory under controlled condi￾tions, to seismicity induced by hydraulic stimulations

in geological reservoirs, and up to great earthquakes

occurring along plate boundaries. In all these environ￾ments the appropriate deployment of seismic sensors,

i.e., directly on the rock sample, at the Earth’s sur￾face or in boreholes close to the seismic sources allows

for the detection and location of brittle failure pro￾cesses at sufficiently low magnitude-detection thresh￾old and with adequate spatial resolution for further

analysis. One principal aim is to develop an improved

understanding of the physical processes occurring at

the seismic source and their relationship to the host

geologic environment. In their paper, Bohnhoff et al.

(2009) (this volume) review selected case studies and

future directions of PSM efforts across a wide range of

scales and environments. These include induced fail￾ure within small rock samples, hydrocarbon reservoirs,

and natural seismicity at convergent and transform

plate boundaries. They demonstrate that each exam￾ple represents a milestone with regard to bridging the

gap between laboratory-scale experiments under con￾trolled boundary conditions and large-scale field stud￾ies. The common motivation for all studies is to refine

the understanding of how earthquakes nucleate, how

they proceed and how they interact in space and time.

This is of special relevance at the larger end of the mag￾nitude scale, i.e., for large devastating earthquakes due

to their severe socio-economic impact.

As pointed out by Rubinstein et al. (2009) (this vol￾ume), the recent discovery of non-volcanic tremor in

Japan and the coincidence of tremor with slow-slip in

Cascadia have made Earth scientists re-evaluate mod￾els for the physical processes in subduction zones and

on faults in general. Subduction zones have been stud￾ied very closely since the discovery of slow-slip and

tremor. This has led to the discovery of a number

of related phenomena including very low frequency

earthquakes. All of these events fall into what some

have called a new class of events that are governed

under a different frictional regime than simple brittle

failure. While this model is appealing to many, con￾sensus as to exactly what process generates tremor

has yet to be reached. As demonstrated by Rubinstein

et al., tremor and related events also provide a win￾dow into the deep roots of subduction zones, a poorly

understood region that is largely devoid of seismicity.

Given that such fundamental questions remain about

non-volcanic tremor, slow-slip, and the region in which

they occur, these authors expect that this will be a fruit￾ful field for a long time to come.

The paper by Tibaldi et al. (2009) (this volume)

examines recent data demonstrating that volcanism

also occurs in compressional tectonic settings (reverse

and strike-slip faulting), rather than the traditional

view that volcanism requires an extensional state of

stress in the crust. Data describing the tectonic set￾ting, structural analysis, analogue modelling, petrol￾ogy, and geochemistry are integrated to provide a

comprehensive presentation. An increasing amount of

field data describes stratovolcanoes in areas of coeval

reverse faulting, and stratovolcanoes, shield volca￾noes and monogenic edifices along strike-slip faults,

whereas calderas are associated with pull-apart struc￾tures in transcurrent regimes. Physically-scaled ana￾logue experiments simulate the propagation of magma

in these settings and taken together with data from sub￾volcanic magma bodies provide insight into the magma

paths followed from the crust to the surface. In sev￾eral transcurrent tectonic plate boundary regions, vol￾canoes are aligned along both the strike-slip faults

and along fractures normal to the local least princi￾pal stress. As pointed out by these authors, at sub￾duction zones, intra-arc tectonics is frequently charac￾terised by contraction or transpression. In intra-plate

tectonic settings, volcanism can develop in conjunc￾tion with reverse faults or strike slip faults. In most

of these cases, magma appears to reach the surface