TECTONOPHYSICS
ELSEVIER
Tectonophysics
240 (1994) 145- 157
Thermal effects of normal faulting during rifted basin formation,
2. The Lugano-Val Grande normal fault
and the role of pre-existing thermal anomalies
G. Bertotti, M. ter Voorde
Institute
ofEarth
Sciences, Vrij~~lJnii~er.siteit, de Boelelaan
Received
9 December
1993; revised version
1085, 10X1-HVAmsterdam,
accepted
24 February
The Netherlunds
1994
Abstract
We investigate
the thermal
consequences
of rift-related
normal faulting and compare
the results with a
well-studied
natural example, the Lugano-Val
Grande normal fault (Southern
Alps). Only limited heating of the
crust is caused by lithospheric
thinning. In the simple but realistic situation where heat conduction
is substantially
faster than heat advection, no major thermal disturbance
is associated with the downward movement of the hanging
wall.
Radiometric
ages and fault rocks associated with the Lugano-Val
Grande normal fault demonstrate
that cooling
rather than heating affected the crust during normal faulting. This pattern is not compatible
with such a simple
numerical
model and is explained by a waning thermal anomaly induced by a magmatic intrusion immediately
preceding or overlapping with the first stages of normal faulting. The magmatic body must have been emplaced at
depths greater than 15-18 km, and probably started to cool in the Carnian, i.e. few million years before the onset of
normal faulting along the Lugano-Val
Grande fault.
1. Introduction
A number of different phenomena
which can
affect in one way or the other the thermal field,
take place during normal faulting and rift-basin
formation such as lithospheric
thinning, the blanketing effect caused by newly deposited
sediments, cooling induced
by the descent
of the
hanging wall with respect to the footwall, uplift of
the footwall and, possibly also of the hanging
wall. There is also an increasingly
widespread
recognition
that magmatic intrusions
play an important role in extensional
tectonics (cf. Skogseid
et al., 1992; Lister and Baldwin,
1993). These
0040.1951/94/$07.00
0 1994 Elsevier
SSDI 0040-1951(94)00119-T
Science
processes may compete with each other and with
the general tendency
of the isotherms
to relax
and thus for anomalies
to vanish. In such situations, the resultant
thermal changes of a given
crustal segment (cooling vs. heating) can be determined only by a quantitative
assessment of the
competing
trends. It is, for instance,
commonly
believed that crustal normal faulting profoundly
affects the thermal field. The hanging wall of a
normal fault is considered
to remain substantially
cold and thus to cool the footwall during its
downward movement.
This reflects the more or
less implicit assumption
that mass movements
arc
always faster than the re-equilibration
of iso-
B.V. All rights reserved
146
G. Bertottr. M. tcr Voorrie / Tectonophysia
therms (e.g., Voorhoeve and Houseman, 1988).
To our knowledge, these assumptions have never
been quantitatively tested. Following the numerical models described by ter Voorde and Bertotti
(19941 we attempt to quantify some of the mentioned processes and their interplay. We concentrate in particular on the mutual effects of lithospheric thinning, of downward movement of the
hanging wall, and of magmatic intrusions.
In this paper we firstly apply the numerical
model for extensional faulting presented by ter
Voorde and Bertotti (1994) to the Lugano-Val
Grande crustal normal fault and the Monte Gen-
240
C1994) 14.5-157
eroso basin. This Mesozoic extensional basin
(Bernoulli, 1964; Bertotti, 1991) is particularly
well suited because the sedimentary successions
of the footwall and of the hanging wall are preserved, and because the shape and the deformation history of the fault can be reconstructed with
a good degree of confidence. As a result, we have
a good kinematic control over the vertical and
horizontal movements of the footwall and of the
hanging wall, both with respect to each other and
to the sea-level. The reconstruction can be carried out independently from any assumption on
the geothermal gradient before and during defor-
Post-orogenicsediments
uJFv=~-)
iysdllomberdo
ldoksio&&occatelk,Fms.
(Lowerliassic)
72%”
Verrwano Lomb., Permian vokanics
&VariscanW
AlpinethrUsts
M. Ceccisyncline
Mesozoic normal faults
other fads
Fig. 1. (a) Schematic geological map of the lake Lugano-Lake Como area. (b) Cartoon illustrating haw the present-day
configuration was achieved through Alpine folding of the east-dipping Lugano-Val Grande normal fault. N-vergent Alpine thrusts
are here omitted. DOZ = Dervio-Olgiasca zone, MMZ = Monte Muggio zone, ML = Monte Legnone, LF’G = Lugano-Val
Grande fault.
147
G. Bertotti, M. ter Voorde / Tectonophysics 240 (19Y41 145-157
mation.
It will be apparent
that, in our case,
rifting was not able to cause any important disturbance of the pre-existing
thermal field.
Secondly we show that several field observations suggest a pattern
of cooling during the
activity of the Lugano-Val
Grande
fault which
obviously is not compatible
with the predictions
of the numerical
model. We shall argue that an
important
thermal anomaly shortly preceding
or
overlapping
with the first stages of normal faulting can explain the observed pattern. In the specific case of the Southern
Alps, the anomaly is
probably related to a magmatic event.
The scale covered in this study is that of several tens of kilometres.
We are therefore observing domains
smaller than those usually considered by modelling
studies on lithospheric
extension (e.g., Buck et al., 1988; Issler et al., 1989).
The scale used is particularly
interesting
because
it is there that the domains of field geologists and
modellers may overlap (Cloetingh
et al., 1993).
hanging
foot-wall
wall
Rhaetlan of the
Lugano swell
Moltraslo LImestone
(Hettang
El
q
Cmx
)
Dolomia Pnnclpale
Rhaetian Fms
(Nonanl
Dolomla Pnnclpale &
Zorzino Fm (Norm)
PRE
RIFT
ml
Variscan basement &
Permian volcanics
upper Permian to Carnlan
ROCKS
FAULT
~72
2. The Monte Generoso basin and the Lugano-Val
Grande normal fault
RIFl
SYN
lower greenschist mylonites
m
ultramylonites
I/
Fig. 2. Palinspastic
reconstruction
of the M. Generoso
at the end of normal faulting (Pliensbachian).
marks the approximate
The
M.
Generoso
sedimentary
basin
(Bernoulli,
1964) developed during the continental rifting which affected the area of the later
Southern
Alps in Mesozoic
times as a consequence of movements between Adria and Europe
leading to the formation
of the Jurassic Ligurian-Piemont
ocean
(e.g.,
Laubscher
and
Bernoulli,
1977). The M. Generoso
basin developed on the hanging wall of a major, east-dipping
normal
fault,
the Lugano-Val
Grande
fault
(Bernoulli,
1964; Bertotti,
1991). The northern
part of the basin and of its substratum
inclusive
of the Lugano-Val
Grande
fault have been
steepened
around a WNW-ESE-striking
fold axis
during Alpine shortening
and are presently
exposed (Fig. 1). A mostly undisturbed,
ca. 15 km
thick section of the Jurassic upper to middle crust
is outcropping
in the Lake Lugano-Lake
Como
region and is limited to the north by the Musso
line (Bertotti et al., 1993b).
The Mesozoic geometry
of the Lugano-Val
Grandc normal fault and of the overlying basin
basin
The shaded bar
position of the northern
Lake Coma
section during the Jurassic.
has been reconstructed
in detail on the base of
the sediment thickness and of informations
gained
from the study of the fault rocks associated with
the fault (Bertotti,
1991). The fault (Fig. 2) had a
primary dip of 50-70” in the uppermost
S-7 km.
Below this depth, it flattened but still preserved a
gentle eastward dip. The fault zone is a few tens
of metres thick in the upper few km and then
widens towards deeper levels reaching a maximal
thickness
of 800-1000
m. Comparing
the scdimentary successions
of the hanging wall and ot
the footwall, a total vertical displacement
of some
8000 m can be estimated; the horizontal displacement was of about 20 km (Table 1). The syn-rift
sedimentary
cover of the footwall records a subsidence of at least 800 m (but most probably less
than 3000 m) during normal faulting. No major
episode of subaerial
erosion is recorded
in the
succession.
Normal
faulting
along the Lugano
148
G. Bertotti, M. ter Voorde / Tectonophysics
Table 1
Kinematic values adopted for the model; absolute values for
time intervals are taken from Haq et al. (1987)
Time interval
(Ma)
Horizontal
displacement
(km)
Displacement rate
fkm/Myr)
223-210
210-186
223-186
14.0
6.0
20
1.08
0.25
0.54
normal fault, similarly to the rest of the Lombardian basin started in the Norian and ended probably before the Toarcian (Bertotti et al., 1993a).
According to the time scale of Haq et al. (1987)
this corresponds to a duration of 37 Myr. This is
not the total duration of the South-Alpine rift
which continued until the Middle Jurassic when
the break-up occurred several tens of km further
to the west (Bertotti et al., 1993a).
3. A numerical model for the M. Generoso basin:
the “norma1” situation
The model described in ter Voorde and
Bertotti (1994) has an upper part where deformation is localized along a fault. In the lower part,
deformation is diffuse and has a “pure shear”
geometry. To model the Generoso basin we have
fixed the boundary between the two layers at a
depth of 30 km (Fig. 3). In the upper part, defor-
distance
240 (1994) 145-157
Table 2
Thermal parameters adopted in the model
T (surface)
T (base lith.)
Initial lithospheric thickness
Diffusivity (sediments)
Diffusivity (basement)
Heat production
Thickness heat producing layer
Specific heat
25°C
-1330°C
125 km
0.75 X 10eh m’ s -’
1.0X 10ehm” s -’
2.3 X lVh W me3
15 km
1100 J kg-’ Km’
mation is concentrated along the listric Lugano
fault. Its geometry is well constrained ~from field
data down to a palaeodepth of around 15 km
(Fig. 2). Below the 30 km transition depth, deformation is assumed to be diffuse at the scale of
the model. The amount and rate of normal faulting are derived from backward palinspastic reconstruction of the Liassic profile (Fig. 2) and are
listed in Table 1. The other parameters used in
the model are given in Table 2.
Some factors have been neglected in our
model, such as the flow of fluids in the sedimentary basin, in the pre-rift sequences as well as
along the fault zones. The results of the model,
therefore, should be taken only as indicative of
trends and of order of magnitude of changes and
not strictly as absolute values.
The results of our numerical model show that
the distortion of the isotherms during normal
faulting is quite limited (Fig. 4). The overall ge-
(km)
Fig. 3. Geometry of the model adopted to describe the evolution of the M. Generoso basin. The numbered points indicate the
positions for which the synthetic T-t curves shown in Figs. 5 and 9 have been constructed.
C;. Bertotti,
M. ter Voorde / Tectonophysics
ometry of the disturbance suggests that this is
associated with lithospheric thinning. The maximum uplift experienced by the isotherms in the
crust is in the order of 1.5 km. This is despite the
fact that the adopted configuration, with subcrustal thinning at the centre of the model, tends
to maximize the heating. In the case of a “simple
shear” geometry, with a normal fault laterally
transferring the l~thospheric thinning away from
the site of maximum crustal thinning, the effect
of heating would be even weaker.
Another interesting result is that no distortion
of the isotherms is predicted by the model as a
consequence of the downward movement of the
hanging wall. This implies that, for the rate of
extension established for the Generoso basin
( _ 0.5 km/ Myr), the thermal re-equilibration is
faster than the movement of masses and, therefore, that the hanging wall is progressively heated
as it reaches greater depths. This also holds for
the sedimental
basin where the isotherms pat-
240 (1994)
tern is similar to that in the surrounding units (cf.
ter Voorde and Bertotti, 1994).
The thermal changes associated with rifting
are clearly visible in the synthetic T-t curves
which have been constructed for some selected
points in the model (Fig. 5). Points from the
footwall which undergo no subsidence during
normal faulting show a temperature
increase
which is purely due to Iith~)spheric thinning. The
temperature changes are quite small, in the order
of a few tens of degrees even for the lower crust.
Points from the hanging wall have supcrimposcd
on this background effect the thermal consequence of their downward movement. This is due
to the fact that no major disturbance of the
isotherms is caused by normal faulting. and that
the rocks are progressively heated as they move
downwards.
In contrast to what might be intuitively expected, the model results (Figs. 4 and 51 show no
substantiai cooling following cessation of exten-
‘u&m--
al
t = 223 Ma .
50
100
Onset of normal faulting.
149
145-1.57
,
,
c)
t = 186Ma
d)
t=176Ma
-
50
100
km
50
100
km
km
___.
50
b)
Fig. 4. Modellcd
Grande
loo
km
t=206Ma
thermal
evolution
fault. The numbered
constructed.
of a crustal segment affected
points indicate
by a crustal normal
the positions for which the synthetic
T-t
fault with the geometry
cures
of the Lugano-Val
shown in Figs. 5 and 9 have been
150
200
150
Time (Ma)
200
150
Time (Ma)
Fig. 5. Synthetic temperature-time
curves for representative points of the South-Alpine crust as predicted by the simple
extensional model (the position of the points is given in Figs. 3 and 4). The shaded bar indicates the duration of normal faulting.
sion. This can be explained by the blanketing
effect of the sediments filling the basin, which
have lower conductivity than the basement rocks.
The sedimentary basin acts as a partial seal,
limiting the escape of heat through the upper
surface of the model and temperatures in the first
lo-15 km beneath the basin are consequently
(slightly) higher than predicted by the regional
geotherm. Taking into account the presence of
fluids in the sediments, the conductivity of the
basin fill would increase and thereby diminish the
importance of the sealing effect.
4.1. Evidence for cooling a’uring rifhg
The results presented above are seer&r&y not
in accordance with two sets of data from the
area: (a) fault rocks along the Lugano-Val
Grande normal fault show a well documented
pattern of decreasing temperature during fault
activity (Bertotti, 1991; Bertotti et al., 1993bI: (b)
radiometric dating with various isotopic systems
has demonstrated cooling in the footwall of the
Lugano-Val Grande normal fault during fault
activity (Mottana et al., 1985).
Based on qualitative arguments, both data sets
were interpreted by Bertotti (1991) as evidence
for cooling induced by the downgoing hanging
wall. In light of the model&g results, this interpretation has to be abandoned.
The fauit rocks associated with the LuganoVal Grande normal fault have been described in
Bertotti (1991) and Bertotti et al. (1993b). so that
only a brief summary will be given here. The
investigation of the fault rocks has shown that
temperatures decreased during fatit activity. This
pattern is particularly clear in the easternmost
parts of the fault zone, which were in the deepest
position during the Mesozoic. A section east of
Lake Coma (Fig. 6) nicely shows this pattern. The
section was at a distance of some 25 km east of
the emergence of the wo-Val
Grande fault
at the surface, and had a subvertical position at a
depth of 11-12 km during the Jurassic (Figs. 1
and 2). Deformation along this segment of the
Lugano-Val
Grande fault zone began under
lower greenschist conditions. Myionites were
formed in which deformation was controlled by
ISI
G. Beriotti, M. ter Voorde / Tectonophysics 240 (I9941 145-157
the Lugano-Val
Grande fault. The mylonites and
ultramylonites
of the Val Grande fault zone are
not annealed,
suggesting that no heating has occurred after deformation.
Some radiometric
age measurements
have been
carried out on the metamorphic
rocks of the area
mainly concentrated
along a north-south
section
along Lake Como (Hanson et al., 1966: Bocchio
et al., 1981; Mottana et al., 1985) (see Fig. 2 for
its palinspastic
position
at the end of normal
faulting). The very few Rb-Sr ages available from
the literature
(Hanson et al., 1966) are too scattered to allow any kind of statement.
The rest of
the data is represented
by conventional
K-Ar
ages (Fig. 7). Samples from the hanging wall, i.e.
from the upper kilometres
of the crust, preserve
their Hercynian
radiometric
signature.
Samples
from the footwall, i.e. from levels deeper than 12
km. show ages falling shortly before or during
extensional
movements
along the Lugano-Val
Grande
fault (Fig. 7). Despite all uncertainties
quartz dynamic recrystallization,
plagioclase
deformed in a brittle manner,
and biotite and garnet were not stable. With persisting
movement
along the fault, quartz dynamic recrystallization
became
less efficient
and deformation
was accommodated
by some form of grain-size-sensitive
creep and/or
by grain boundary
sliding of small
albite, epidote, sericite, chlorite and quartz grains
mainly derived from the break-down
of plagioclase, garnet, biotite and white mica. This led to
the formation
of a few hundred
metres thick
band of ultramylonites
which overprinted
the
southern
(uppermost
in Mesozoic
coordinates)
part of the mylonites. Deformation
continued
under even lower temperatures
and cataclasis
affected the southernmost
t = uppermost)
tens of
metres of the fault zone. All of these fault rocks
show similar stretching
lineations
and sense of
shear and were therefore
interpreted
to be associated with the same deformation
event, i.e. the
Late Triassic to Early Jurassic movements
along
Val
M Croce di Muggio
M Legnone
di
Dervio-Olgiasca
Fig. h. Sketch from photograph
following elements
zone
of the Alpine-steepened
ultramylonites
can be seen: the foot-wall
and cataclasites
to Middle
Muggiasco
Corn0
formations
and structures underlying
the position of the section in Jurassic times). From left to right (from deeper to shallower
Permian
’
s
N
of the Lugano-Val
Grande
the M. Generoso
hasin (see Fig. 2 for
crustal levels in Jurassic coordinates)
normal fault which is gradually
overprinted
associated to the fault zone. South of the line, the hanging wall is formed by basement
Triassic sedimentary
cover.
the
by mylonites.
rocks and hv
G. Bertotti, M. ter Voorde / Tectonophysics 240 (1994) 145-1.57
151
associated with traditional K-Ar dating, we feel
confident that the data indicate a cooling of middle crustal rocks below the closing temperatures
for micas sometime slightly before or during the
first stages of normal faulting.
4.2. Cooling caused by a transient heat source
The two data sets described above clearly point
to decreasing temperatures
shortly before and
during fault activity. Since numerical modelling
shows that normal faulting cannot produce a substantial distortion of the isotherms, a different
explanation has to be found. Cooling caused by
regional uplift of the footwall (and possibly also
of the hanging-wall), as is common in the “core
complex” type of extension, has to be excluded
because the well-documented sedimentary cover
of the footwall shows no evidence of major uplift
(Bernoulli, 1964). We therefore propose that the
cooling pattern documented not only along the
fault zone, but also several kilometres away from
it, was caused by the decay of an intrusion-related thermal anomaly which developed before
the onset of normal faulting.
350
300
250
Two episodes of crustal magmatism are known
in the late Palaeozoic to Late Jurassic history of
the central-western Southern Alps. The older occurred between the Late Carboniferous and the
Middle Permian and is characterized by widespread magmatism at all crustal levels (e.g., Handy
and Zingg, 1991; Bonin et al., 1993). The initial
stages of the associated thermal event overlap
with the latest phases of the Variscan orogeny,
while the early to Middle Permian activity took
place
during
continental-scale
wrenching
(Arthaud and Matte, 1977; Ziegler, 1988). Magmatism in the Southern Alps probably ended
sometime before the deposition of the Late Permian Verrucano conglomerates (Bertotti et al.,
1993a).
A second period of anomalous thermal regime
took place in the Middle Triassic (Ferrara and
Innocenti, 19741, when a volcanic belt developed
in the subsurface of the present-day PO Plain
(Brusca et al., 1981). Tuffs and localized lava
flows are found in the carbonate successions of
the central Southern Alps (Jadoul and Rossi,
1982). The volcanic edifices were eroded during
200
150
WOW
Fig. 7. K-Ar radiometric ages from the Lake Coma area. Solid symbols are muscovite ages, open ones are biotite agesTriangles
are data from Hanson et al. (1966) recalcolated with the new constants by Mottana et al. (1985); circles are data frort~ Mottana et
al. (1985). HW indicates samples from the hanging wall of the Lugano-Val Grande normal fault.
G. Bertottl, hf. trr Voordr / T~~onophysics
the Carnian and their erosiona products are
found in the Val Sabbia Sandstone of the Southern Alps (Garzanti, 19853. intrusions emplaced
around 230 Ma are also reported from the lvrea
zone by Cebauer (19931. Further indirect evidence is offered by a swarm of pegmatites found
in the northern part of Lake Coma immediat~Iy
south of the Musso line (Fig. 1) (Repossi, 1914;
El Tahlawi, 1965). New Rb-Sr radiometric dating
on muscovite of the pegmatites provide ages of
so
to0
a)
t = 237 Ma - Intrusion emplaced
bt
t = 230 Ma
cj
105
t = 223 Ma . Onset of normal fautting
SO
226 rt 2 Ma (Sanders et al,, submitted). Since the
Rb-Sr closure temperature of muscovite (Blanckenburg et al., 19891 is quite ciosc to the consolidation t~mpcrature of granitic pegmatites like
those found in Piona CCerny, 19821, we assume
that the Rb-Sr age is only slightly younger than
the emplacement age. The petrography and the
fIuid inclusions of the pegmatites are indicative of
an anatectic origin (Sanders et al., submitted).
Similar ages (225 F 13 Ma) have been obtained
so
km
loo
km
ei t = 1%
8
Ye”--
Fig. 8.
100 km
d) t=206Ma
- intrusion starts to fool
50
153
240 (1904) 145-157
50
loo
Ma - End of normal fauiting
1
km
i
50
km
i_-..-__,i
IO0
km
f.I t=176Ma
Thermal evolutionof the M. Generoso crustal segment as predicted by the model including the initial thermal anomaly.
j
154
G. Bertotti, M. ter Voorde / Tectonophysics 240 (1994) 145-157
for the emplacement of syenite pegmatites in the
Ivrea Zone (Stlhle et al., 1990). All these observations suggest the existence of a strong anomaly
at middle to lower crustal levels during the Ladinian to Early Carnian (according to the time
scale of Haq et al., 19871, i.e. shortly before the
onset of normal faulting which occurred in the
Norian (Bertotti et al., 1993a).
4.3. A numerical
anomaly
model
including
the thermal
On the basis of the geological observations
suggesting magmatic activity in the Middle Triassic, we modelled a thermal anomaly caused by an
intrusion at the base of the crust with a temperature of 1OOOV(Fig. 8). This estimate corresponds
to the temperature of a gabbroic melt which is
the most likely magma type for an intrusion at
the crust-mantle
boundary. Sensitivity studies
show that 100°C lower or higher intrusion temperatures only cause very minor changes of the
thermal evolution ( < 10% at peak temperatures).
In our model, the intrusion was emplaced at 237
Ma (Anisian-Ladinian
boundary; i.e. the age of
the oldest volcanic intercalations in the sedimentary successions) and remained at constant tem-
perature until 230 Ma (Early Carnian; the age of
the last evidence of active volcanism) when it
started to cool. The residence time of the intrusion is therefore estimated at 7 Myr. Numerical
experiments have shown that a 7 Myr time span is
long enough for the achievement of a nearly
complete thermal steady state. A doubling of the
residence time, causes an increase in peak temperatures of less than 20°C. The shape and dimensions of the intrusion are obviously speculative since the upper to middle crustal section of
Lake Como is detached from its Mesozoic substratum. The kinematic evolution after the onset
of normal faulting (at 223 Ma) is the same as for
the model with no intrusion (Table 1).
The effect of the thermal anomaly is clearly
visible in the first evolutionary stages (Fig. 8).
After a few million years, following the vanishing
of the thermal anomaly, the thermal evolution is
controlled by lithospheric thinning and the general pattern is similar to that of the model with
no intrusion.
T-t curves for selected points show the same
trend in more detail (Fig. 9). The intrusion has a
profound though short-lived influence on the
geothermal field. The magnitude of the temperature increase is obviously inversely proportional
hanging wall
footwall
s0
p-
75-
F
3-
,sl-
150
200
Time (Ma)
150
200
Time (Ma)
Fig. 9. Synthetic T-t curves for the physical points shown in Fig. 3. The shaded bar to the left marks the emplacement
de-activation of the intrusion. The one to the right indicates the onset and end of normal faulting.
and
G. Bertotti,
M. ter Voorde /Tectonophysm
to the distance from the intrusion.
For points in
the middle crust the heating can be of 150-250°C
which is in many cases enough to reopen radiometric systems. For the footwall points, the effect
of the thermal anomaly clearly overshadows
that
of lithospheric
thinning,
and the general pattern
following the rapid initial heating is one of cooling throughout
the history. Most of this cooling is
already achieved
a few million years after the
emplacement
of the anomaly. Radiometric
ages
from these points therefore
date the cooling of
the intrusion
rather than the normal faulting itself. Points in the hanging wall undergo first a
short-lived
phase of heating caused by the intrusion, followed by partial cooling. With the onset
of normal faulting, the points in the hanging wall
are heated because of their increasing depth. For
all the points analyzed in the hanging wall, the
heating caused by the downward movement
is at
least comparable
to that associated with the thermal anomaly. Absolute
ages will therefore
date
the cooling following lithospheric
thinning rather
than the effects of the intrusive body.
5. Discussion
In our first model (Figs. 4 and 5), no extra heat
was introduced.
In this case, the thermal changes
are caused by lithospheric
thinning;
the magnitude of these changes is, however, quite limited.
Material points of the footwall show a temperature increase of a few tens of degrees. Material
points in the hanging wall show a stronger heating of up to 100°C which is, however, related
primarily
to their increased
depth and not to
lithospheric
thinning.
In the second model, we demonstrate
that the
cooling pattern recorded in the fault rocks of the
Lugano-Val
Grande fault zone and in the absolute ages pattern can be adequately
explained by
the effects of a large magmatic
intrusion.
With
this configuration,
the points in the footwall only
record the heating and subsequent
cooling caused
by the thermal anomaly.
Points in the hanging
wall show the effect of the anomaly but no major
cooling takes place because
of the downward
movement.
240 (1994)
145-157
IS.5
Our results can help to better characterize
the
thermal
anomaly which preceded
the onset of
normal faulting and for which we cannot have
any direct evidence. Despite the absence of direct
constraints
on the shape, volume and type of the
magmatic intrusion,
the numerical
model applied
in this study shows that the associated
thermal
anomaly had to be very significant
and that this
cannot be caused by few, small, localized intrusions; a large underplated
body is probably
needed to generate the required thermal disturbance. Our model shows that cooling following
the de-activation
of a heat source is generally a
fast process (e.g., Furlong et al., 1991). The presence of such an intrusion
could help to explain
some of the tectono-stratigraphic
features
observed in the sedimentary
cover, such as the
westward thinning of the Middle Triassic section
from the Bergamasc Alps towards Lake Lugano
(Brack and Rieber, 1993).
Our data and modelling
show that the intrusion was emplaced shortly before or overlapping
with the onset of extension.
This pattern is not
uncommon.
Recent
investigations
in the Austroalpine
nappes
of Valmalenco
have documented
the existence of a large gabbroic body
(Fedoz Gabbro) emplaced
roughly at the crustmantle boundary
in the first stages of extension
(Trommsdorff
et al., 1993). Similar temporal relationships have been put forward by Hendrie et al.
(1993) for the North Sea “triple junction”.
There
seems to be a systematic
pattern but a genetic
link between
intrusion
and subsequent
normal
faulting is not yet proven.
6. Conclusions
The main results from our numerical
modelling are that lithospheric
thinning
causes only
minor temperature
changes and that crustal normal faulting alone is not able to produce important changes in the geothermal
field. Magmatic
intrusion
is a far more efficient way to cream
thermal
anomalies
and thereby
influence
the
thermal evolution of a crustal segment.
That normal faults do not play a substantial
role in affecting the geothermal
field, has been
recently demonstrated for the Simplon normal
fault by Grasemann and Mancktelow (1993). They
showed that, for this particular case. cooling was
caused by a general exhumation of hanging wall
and footwall but at different rates. producing a
normal-fault-relative
displacement between the
two sides. This explanation is obviously not feasible for the Lugano-Val
Grande normal fault
where no major uplift has taken place.
We therefore make a general case of the conclusions reached above for the Lugano-Val
Grande normal fault and M. Generoso basin, in
saying that normal faulting in itself is not able to
cause important perturbations
of the thermal
field. Other processes have to be invoked in order to explain the frequently observed cooling
pattern. Among the most likely candidates are
magmatism-related thermal anomalies preceding
normal faulting and syn-faulting footwall exhumation. An interplay between the two is obviously also possible (John and Foster. 1993: Lister
and Baldwin, 1993). These processes could possibly be related to extensional faulting but in any
case the relation is only an indirect one since, as
shown by the M. Generoso basin case, not all
crustal normal faults arc associated with footwall
exhumation.
The model presented in this paper underlines
the importance of estimating the age of the onset
of extension independently from cooling ages.
Furthermore,
the model gives important constraints for their interpretation.
If the ages are
slightly older or contemporaneous
with the first
stages of extension, then they record phenomena
(like magmatism, footwall exhumation or an interplay of the two) not necessarily directly related
to normal faulting. In the case where the ages are
clearly younger than the end of normal faulting,
they should trace the post-extensional cooling. It
is questionable, however, if the rifting-related
thermal anomaly in the middle and upper crust is
sufficient to cause a re-opening of the radiometric clocks.
Bernoulli. S. Cloetingh, G.K. Manatschal, N.
Mancktelow and 0. Miintener for their criticisms
and comments. R. Gabrielsen. M. Fernandez and
D. Waltham were very helpful with their reviews.
This work was supported by AWON and by the
Integrated Basin Studies project, part of the Joule
11 research program funded by the Commission
of European Communities (Nr. JOU2-CT 9201 IO). IBS contribution No 9. Publication No.
941002 of the Netherlands Research School of
Sedimentary Geology.
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