THE STRUCTURE OF THE EARTH'S CRUST OF THE HANOI TECTONIC TROUGH ACCORDING TO GEOPHYSICAL DATA AND THE PROBLEM OF ITS RIFTOGENESIS
HA LIONG TIN 1 and O. K. LITVINENKO:
i Dept. qf Geology, Mining and Geological Institute, Hanoi, SR V Dept. of Geology, Moscow State University, Moscow, USSR
(Received December 7, 1984; accepted September 15, 1985)
ABSTRACT
Ha Lion Tin and Litvinenko, O. K., 1986. The structure of the earth's crust of the Hanoi trough according to geophysical data and the problem of its riftogenesis. In: V. (~errnfik (Ed.), Geophysical Fields, Their Nature and Geological Interpretation. Journal of Geodynamics, 5: 179-203.
The Hanoi depression, situated in the northern part of Vietnam, is an interesting but poorly studied structure.
The integrated interpretation of geophysical data was aimed at constructing three models along a profile traversing the south-eastern part of this tectonic trough. To construct these models, we had to analyze the velocity, resistivity and density of rocks in several wells located in the area of the trough. This allows us to express the regularities of their change with depth in the form of exponential and linear functions, to find simultaneously the functional relations between these physical properties, and to com- pute the root-mean-square error of approximation.
The obtained dependences of the changes of physical properties with depth and their interrelations have been used to construct the first, second and third gravity models from the data of the common- depth-point, correlation refraction, and magnetotelluric sounding methods, respectively. A direct problem of gravity exploration has been computed independently of these models, according to the programme resolving it for the gradient media. By excluding the gravitational influence of these models, the authors have obtained residual gravity anomalies whose minima were independently integrated for each model.
This interpretation has resulted in the detection of seven blocks, bounded by faults, in the basement of the trough. The upper parts of some of these faults had been previously detected from geologic and seismic data. These faults confine the submerged blocks, composed either of basement rocks (crushed and characterized by decreased values of density and magnetic susceptibility) or sedimentary rocks wilh
somewhat increased values of density (compared to the upper part of the section 7). A block, composed of rocks with a density of 2.55 g/cm 3 (excessive density Aa ~ -0 .15 g/cm 3 relative to the basement rocks) at depths from 5.5 to 9.0 km and of rocks with the density of 2.65 g/cm 3 (Aa = -0.05 g/cm 3) at depths from 9 km to 14.0 km, has been detected in the central part of the trough. If the anomalous density is less than -0 .05 g/cm 3, then the depth of this block significantly exceeds 14 km and may reach 20--25 kin. As is shown by the computation of the earth's crustal thickness, it is decreasing in the central part of this depression and does not exceed 25 kin.
The results of interpreting the gravity data, data from the literature on the relationship between den-
sity and heat generation, and data on the age of this structure, have been used to assess the value of the
heat flow which can be expected in this depression. According to these calculations, in the stable part of the trough (on its wing) it is possible to expect a heat flow with a total value of 55 65 mW/m 2, whereas
in its central unstable part, the heat flow is supposed to be generated (apart from the stationary process accounting for the value of 31 mW/m 2) by a nonstationary process. This additional nonstationary source
of heat, located in the mantle at a depth of 25 125 km, generates a heat flow of 47 mW/m 2. The integrated geophysical model developed testifies to the origin of the Hanoi depression as a rift (ils rif-
togenesis).
INTRODUCTION
The majority of rift systems cross the oceans and in several regions of the earth, their branches penetrate into continents. Large grabens on the con- tinents have been termed rift zones or rifts. These zones are characterized by a number of geologic and geophysical peculiarities.
The continental rift zones coincide with the strike of folded or rupture structures of the Proterozoic basement. There are three main but mutually contradictory concepts of rift formation, holding that rifts have been formed as a result of: (1) upbulging and collapse of an arch; (2) contraction or (3) extension of the earth's crust (Milanovsky, 1976). The latter results structurally in the formation of deep but narrow (from several kilometres to several tens of kilometres), often step-shaped grabens whose bot tom is dis- sected by faults and extension fractures.
Topographically, these structures are represented by linear depressions. Asymmetric rises are framed on one or both sides by rifts. Transverse (transform) faults are especially typical of oceanic rifts. The epigeosyn- clinical orogeny often terminated in the Late Cenozoic, Mesozoic, and Late Paleozoic or was combined with rifting. Volcanism is characteristic of the rift zones. Volcanism of the calc-alkali series (basal, medium and acid intrusives) is typical of epiorogenic rift zones situated on the flanks of oceanic rift belts.
The geophysical characteristics of continental rift zones are as follows: a decrease in the earth's crustal thickness; anomalously low values of the physical properties of rocks composing the upper mantle below the Mohorovicic boundary (M); increased values of heat flow; linearly stretched gravity field minima, and a characteristic anomalous magnetic field.
Let us consider some of these characteristics. An anomalous layer, up to several kilometres thick, with abnormally low velocities of compressional waves has been detected under all continental rifts by the method of explosion seismology. Krylov (1977) compared the values of the com- pressional-wave velocity, resistivity and density of rocks in the Baikal rift zone, Iceland and Kenya rifts, and the Basin and Range Province (western
CRUST OF THE HANOI T R O U G H 181
USA) with those calculated for models, based on several hypotheses, on the status of the upper-mantle substance. He considered the hypotheses of heating and partial melting, serpentinization and eclogitization, as well as the hypothesis of the crustal-mantle mixture formation. His resultant con- clusion holds that the M-boundary in the rift zones coincides with the roof of the layer which is evidently composed of partially melted upper-mantle rocks, with Vc= 7.3-7.8-km/s and density a = 3.1-3.2 g/cm 3. Consequently, the earth's crustal thickness in rifts decreases to 12-15 km (since the anomalous layer belongs to the upper mantle), compared to an average thickness of the continental crust, equalling 40 km.
The analysis of geologic and especially of the geophysical characteristics of the Hanoi tectonic trough allows us to suppose its possible riftogenic origin. Investigation of the deep structure of this trough and, specifically of its riftogenesis, is of paramount significance in selecting a set of geophysical methods to be employed in prospecting for oil and gas fields in the southern part of the Hanoi trough and in the Gulf of Tong King.
1. CHARACTERISTICS OF THE T R O U G H ' S A N O M A L O U S
POTENTIAL FIELDS
The Hanoi tectonic trough is situated in Northern Vietnam (Fig. 1). There are several viewpoints as to its tectonic structure, incorporated in one conclusion holding that it is located in the regional north-east striking suture zone separating the North Vietnam folded zone from the south- western margin of the South China platform. In accordance with the con- cepts of global plate tectonics, the trough was formed on the boundary of' two lithospheric plates (Moody, 1975).
The gravity field of the trough is negative, with values of zig amounting to several dozens of milligals. The total deep minimum (down to 50 mgal) is typical of deep troughs composed of rocks of great thickness. Gravitational step-type anomalies associated with the north-west striking faults are dis- tinct in the gravity field. The horizontal gradients in the zones of this type of gravity steps reach substantial values of about 3-6 mgal/km. A chain of relatively positive and negative anomalies stretched linearly in the north-. western direction, and with an amplitude of several milligals, extends along the strike of the trough.
Several zones located across the strike of the trough are of special interest for understanding its riftogenesis. The gravity field isolines in these zones change their direction and the amplitudes of local anomalies decrease. This type of gravity field is characteristic of transverse faults dissecting linearly stretched structures. Gravity and magnetic fields of this type in the oceanic rift zones are caused by transform faults. Three zones with characteristic
182 TIN AND LITVINENKO
22 °
20 °
106 °
1 1 104 ° east of G r e e n w i c h ~k. %, C b ~ ,
t %
x'-,. x~ ~-,....__ ~ ( .r--Cg x bdb
• oo 0- o0
" SZx",, N , < ' O ~ h o ~ - ~o// Vinh eac Bo "~70£'~-~"~X " % ~ (Gull of Tongking)
./J
1 0 8 °
2 2 ° N
L E G E N D
20 ° [ ~ 3
0 3 0 6 0 L I I
km -~ 4
Fig. 1. Map of the region under investigation, showing the position of the estimated profile. 1. The trough's boundary according to geologic data. 2. Faults observed and suggested. 3. The estimated profile. 4. One of the zones of supposed transverse faults of transform type.
indications of transverse faults are traced in the Hanoi trough. The most distinct zone is situated in the south-eastern part of the trough. The strike and location of transverse faults are indicated by the field of residual anomalies Agres (more intensive than Ag). Figure 2 presents one of such zones whose field is freed from the regional background impact by the method of variations.
Agres(0, 0) = Ag(0, 0) Z 16 Ag(O, r)
16 ' (1)
where Agres(0, 0) is a residual anomaly value at the estimated point (0, 0); Ag(0, 0) is the anomalous field value in Bouguer reduction at the same point; Ag(0, r) are the values of Ag on the contours of a square with an inscribed radius r = 2 km (the length of the radius is chosen experimentally). Because of the insufficiently high accuracy of the Ag and AT fields which were at the authors' disposition, it is difficult to affÉrm unambiguously that the transverse faults observed in the Hanoi Trough are transform ones (the shift along the fault plane is evidently not recorded because of an insuf- ficiently high accuracy of the surveys). The occurrence of transverse faults in the trough is, however, indisputable. Their characteristic geophysical
CRUST OF THE HANOI T R O U G H 183
indications in the north-western zone are not so pronounced as in the south-eastern zone, which suggests the growing intensity of transverse faults in the south-eastern part of the trough.
The AT anomalies in the south-western and north-eastern parts of the flanks of the trough, caused by effusives of basal and acidic composition, respectively, were detected by aeromagnetic survey. The intensity of anomalies in the central part of the Hanoi Trough declines. It is interesting to note that a positive AT anomaly with an amplitude of 20 nT, evidently caused by rock intrusions along the deep fault, corresponds to the most pronounced Ag minimum. The depth to the upper fringe of these rock masses, exciting magnetic anomalies, is estimated at about 15 km.
2. THE D E P E N D E N C E OF VALUES OF PHYSICAL PROPERTIES OF ROCKS ON THEIR
D E P T H AND ANALYTIC RELATIONS BETWEEN THESE PROPERTIES
The values of the physical properties of rocks composing the structure under consideration were obtained by measuring the density, electric resistivity and velocity of samples taken from natural exposures in the marginal parts of the trough and core samples from the wells located on its territory. Since systematic investigation of these parameters if insufficient, the changes in the physical properties of rocks with depth and in the horizontal plane have been poorly studied. Table 1 gives the values of the physical properties of rocks from a combined geologic-geophysical column constructed by Nguen Hiep by generalizing the data obtained in the wells drilled on the region of the trough. It can be seen in Table 1 and Fig. 3 that density ~ and velocity Vc values increase with depth. The value of specific electric resistivity fluctuates around a certain constant value down to a depth of about 1.8 km and increases with depth. An increase in a and V~ depth ~ may be expressed analytically through an experimental or linear function:
y = A~ B (211
y = A~ + B (3t
where y denotes either density or velocity. Specific electric resistivity within the depth range 0 <~ ~ ~< 1.8 km may be considered as independent of depth, whereas below ¢ > 1.8 km, it increases linearly with depth (2).
In order to construct a regional model from a set of geophysical data, let us suggest the existence of an analytical relation a = f ( V ) and a = f ( p ) between physical parameters. In this case, velocity or resistivity will be represented as an argument in expressions (2) or (3). The computation of these analytical relations between the parameters is of special significance in
regions with poorly studied physical properties of rocks. Determinations of coefficients A and B in (2) and (3) were made by the least-squares method, with a simultaneous computat ion of the mean-square error 6 according to the well-known formula. Every dependence ~r =f(4) , V = f(4), and p = f(~), as well as relations between parameters a = f ( V ) and a = f ( p ) , were calculated according to both formulas (2) and (3). To construct a model, we selected that one which gave a smaller error. The functions given below approximate the changes of parameters with depth and the relations between physical properties:
cr = 2.30~ °-°54 + 0.01 g/cm 3
V = 2.13~ °243 _ 0.04 km/z
p = const = 5 ohm.m + 2.6 ohm.m (for 0 ~< 4 ~< 1.8 km)
p = (2.54 - 42.2) + 2.4 ohm.m (for ~ > 1.8 km)
a = 1.87 V 0243 "}- 0.03 g/cm 3
o" = 2.26p °'°m -t- 0.22 g/cm 3
(4)
(5)
(6)
(7)
(8)
( 9 )
As is seen from (4) to (9), the relations a = f(4), V =f (~) and a = f(V) contain small errors and, therefore, the values of density calculated from these analytical formulas can be used to construct an integrated model, whereas relations p = (4) and a = f (p ) contain substantial errors; therefore, a gravity model developed from the data of electrical prospecting should be used rather cautiously.
3. I N T E G R A T E D M O D E L OF THE UP P E R PART OF THE EARTH'S CRUST
Three gravity models were constructed independently along the profile traversing the south-eastern part of the trough (Fig. 1).
To compute these models, we had to solve the direct problem of gravity exploration for a gradient medium (Litvinenko and Ha Liong, 1982). The sum of prisms was substituted for the initial section in the models under considerations. The direct problem of gravity exploration V~ for the media where density (o-) changes with depth is solved according to the linear dependence:
a ( ~ ) = A ~ + B (10)
Fig. 2. Determination of the location of a transform fault from residual anomalies Ague ~. The contour
interval is 1 mgal.
186 TIN AND LITVINENKO
TABLE 1
The values of density, specific electric resistivity and velocity of compressional waves taken from a
generalized geologic-geophysical column
Group System Thickness, m Density, g/cm 3 Specific electric Velocity, km/s
where (~I, ~2, ~1, ~2) are the coordinates of a prism faced in the plane xoz (Fig. 4); (x, z) are the coordinates of the points for calculating the vertical derivative of gravity potential Vz; f is the gravity constant, and A and B are the coefficients in (10), (12) and (14). Integral (15) is calculated numerically.
Litvinenko and Ha Liong (1982) gave expressions Vz also for a vertical parallelopiped for cases (10) and (14), but since only linearly stretched anomalies of Ag are observed in the region under investigation, expressions ( 11 ), (13) and (15) were made use of. These calculations were computerized.
Let us consider each model in succession. The first model was developed from the data of the common-depth-point
o
I I I I I I
z
Fig. 4. A vertical prism. The value of V z (x, z) is calculated in point A(x, y), according to the depen-
dence of density change a = f(~), a = f(¢).
CRUST OF THE HANOI TROUGH 189
method which shed light on the structure of the sedimentary rocks of the trough down to a depth of about 5.5 km. To calculate Vz in this section according to (15), the dependences of density on depth (4) and of density on velocity (8) were used. The lower boundary of this section was sub- stituted by a stepwise function. Figure 5 presents a calculated curve Vz and residual anomalies Agree, obtained by subtracting Vz from the anomalies of gravity zig. Three minima, which manifested themselves on the zigre., curve, were used to calculate the depth, dimensions and anomalous density of these prisms, supposing that each minimum was caused by a prism-shaped, anomalous body (Fig. 5).
The second model was developed according to a seismic section construc- ted from the data obtained by the correlation refraction method. The authors constructed this version on the assumption that the average velocity in every layer is constant, where it is possible to observe three refracting
E so
>~ 40
ao
20
~ z k o k,,
S-"~ lo 20 ao 4o 5o eo zo k , , N j . . . . 2 ; . . . . . . . ' . . . . . . . . ' . . . . . . . . . _ t . . . . . . ~ . . . . . L 7 . . . . . : _ L . ~ . . . .
Fig. 5. Gravity model of the Hanoi Trough developed from the data obtained by the common-depth- point method at ~(~)= 2.30 ~0.054. I, II, III are the local minima of xlg~e,~. Legend: 1. Reflecting boundaries and faults detected from the data obtained by the CDP method. 2. Gravity model. 3. Prism-shaped blocks, detected from the data of gravitational exploration, filled with rocks of a den- sity of 2.5 g/cm 3 or 2.55 g/cm 3.
4. A block in the form of two prisms, with the densities of the upper and lower ones equalling 2.55 g/cm a and 2.65 g/cm 3, respectively.
JOG 5;2 6
190 T I N A N D L I T V I N E N K O
boundaries. The boundary velocity along one of them changes from 4.2 km/s to 5.4 km/s. Let us assume that both the boundary velocity and density in this layer change in horizontal direction (along the axis) roughly according to the laws governing the change of these properties along the vertical axis (. On this assumption, we calculated from formulas (4) and (8) the variable density in one of the layers of this three-layer model and the total gravity effect Vz of the model where the velocity is constant in two layers and changing in horizontal direction in one layer. The value of V, for this layer was calculated from (13). The minima on the residual curve (Fig. 6) were interpreted similarly to the first model.
The third model was based on a section constructed by Nguen Dyk Tien from the data obtained by magnetotelluric sounding. The layered-blocky gradient model (Fig. 7) was calculated from the resistivity values obtained by this author and from formulas (4), (6), (7) and (9). The local minima
Fig. 6. Gravity mode l of the Hano i Trough constructed from the data of the correlat ion refraction
method at a constant density in the first and third strata and a changing density in the hor izonta l direc-
tion in the second stratum. I. II. III are the local m i n i m a of z/gre ~. Legend:
1. Refracting hor izons and faults detected from the data obtained by the correlat ion refraction method. 2. Gravity model .
3. The prism-shaped blocks detected from the data of gravitat ional exploration. T w o blocks are filled
with rocks of a density of 2.50 - 2.55 g/cm3; the density of rocks in the third block is 2.55 + 2.60 g/cm ~.
C R U S T O F T H E H A N O I T R O U G H 191
~ 60 E
so
> 40
o E 3e
2o
~ ' ~ ~1~ I 1 - ~ I . / \
i i t
111 f ~ ~ ~ g r
/ \ /
~ , ~ / "
~ ~ - / " - - ~VV~
, o
S w
. . T _J
,o 20 ,o *o ,o 6o ,o ,.. \ )
6=231
l
F i g . 7. Gravity model of the Hanoi Trough developed from the data obtained by magnetotellurk sounding. I , I I , I I I are the local minima of zlgr~ ~.
Legend: 1. Initial layered-blocky model. 2. Prism-shaped blocks detected from the data of gravitational exploration. Two blocks are filled w i t h
rocks of a density of 2 . 3 8 - 2 . 5 0 g / c m 3 ; the density of rocks in the third block is 2 . 5 0 g / c m 3.
distinguished on the residual c u r v e Agre s were interpreted similarly to the preceding models.
Figures 5, 6 and 7 show that blocks forming three grabens were detected in each model. The results obtained in the different models differ by: (1) the displacement of the location of the grabens, relative to each other, by several kilometres; (2) the density of composing rocks, varying in the dif- ferent models from 0.17 g/cm 3 to 0.05 g/cm 3, and (3) the difference in the thickness of the rocks composing the grabens, ranging from 3.0 km to 5.0 km. Consequently, in regional respect, the results of interpretations according to independent geophysical models give closely related charac- teristics of depth, density and location of grabens in the section, thus reflecting the general features of the structure of the trough and making it possible to construct generalized section for it (Fig. 8).
Let us emphasize some peculiar leatures of the structure of the trough resulting from the interpretation. The basement is split by faults bounding
192 T I N A N D L I T V I N E N K O
3O
1020 / ~ ' ~ ' ~ ~ ~ ~ ~'" "-" ~ ~ ~ A g c
I- m '~ 190 ~ 1~0 / 1 4 0
O) = ~ k / 0 6kin E O c
Song Hong Ha V i n M i n Song Lo S -W 190 River t00 fault River fault 140 N - £
4 2.7 o - ~ f l . . : .: .:.: .:- ~ , 1 + ~ -~.~ vj ---." "t"1 I l l+i lL,
~_ A R 2 P R ~ ? i ! 2 : ~ ! : ? ? i i 2 ? , ~ 5
Fig. 8. Generalized geologic-geophysical section, whose location is shown in Fig. 1.
Legend:
1. Sediments. 2. Reflecting boundaries and faults detected by the CDP method. 3. Faults detected by gravitational exploration. 4. A basement block composed of rocks having smaller density than that of the basement. 5. Zones characterized by the change of magnetic properties of the basement. 6. The crystalline basement in the south-western flank of the trough. 7. Paleozoic rocks composing the north-eastern flank of the trough. 8. A well.
24 t 6~ 8 E
g;
E O c
zones filled either with strongly metamorphosed basement rocks (crushed, and characterized by decreased values of density and magnetic suscep- tibility) or sedimentary rocks with somewhat increased values of density (compared to the upper part of the section. A block confined by deep faults, the upper parts of which were known from geologic and seismic data, was detected in the central part of the trough (Fig. 1, 5, 6, 7). The lower part of this block is composed of rocks with a decreased density (reaching 2.55 g/cm 3 at a depth of 10 km) as compared to the basement rocks. It follows from the estimation of anomalous density L/o-= - 0 . 0 5 g/cm 3 that the lower
CRUST OF THE HANOI T R O U G H 193
boundary of this graben-like structure is at about 14 km (Fig. 8). If the anomalous density is less than -0 .05 g/cm 3, then the depth of this zone substantially exceeds 15 km and may reach 20-25 kin.
Consequently, the above-stated peculiarities of the structure of the Hanoi Trough, established from the qualitative and quantitative interpretations of geophysical data, give grounds for suggesting that the Hanoi Trough is apparently a rift at a late stage of development. This is evidenced on its territory by the developed grabens bounded by faults 15 and more km deep, by transverse faults, and by the type of gravity and magnetic fields.
4. SCHEMATIC MODEL OF THE CRUST OF' THE HANOI TROUGH
Since deep seismic soundings have not been made in the region under investigation, great significance is attributed to the preliminary evidence on the crustal structure of the trough obtained from gravimetric data.
Based on conclusions arrived at in the preceding section, let us develop a schematic model of the earth's crust, attempting to see whether it con- tradicts the already available Ag field or not. Since selection of a multilayer section according to ,gg does not give an unambiguous solution, let us make use of the already known data on the average densities and crustal thicknesses in the rift zones (Krylov, 1977), and regard as constant the data on the thickness and density of the upper part of the section established through interpretation (Fig. 8). Let us assign the average densities of 2.7 and 2.9 g/cm 3, respectively, to the "granite" and lower strata, and the den- sity of o- = 3.2 g/cm 3 to the rift zone below the Moho boundary. Let us also assume that the thickness of the "granite" stratum on the wings of the rift equals 20 km and assign the same thickness to the underlying stratum (the average thickness of the continental crust is known to equal 40 km). Now let us solve the inverse problem of gravitational exploration by the suc- cessive approximation method and select the crustal thickness in the rift zone at every approximation. As seen in Fig. 9, as an estimate, the 4th approximation already gives a satisfactory accuracy of the coincidence of the calculated and anomalous curves.
The calculated curve V z in the rift zone corresponds to the crustal thickness of 25 km shown in Fig. 9, which includes the sedimentary stratum.
5. GRAVITY AND GEOTHERMAL MODEL OF THE CRUST OF THE TROUGH
Geothermal investigations involving the measurement and analysis of temperature T and heat flow q are of great importance in forecasting the presence of oil and gas fields. If a sedimentary basin has all the geological factors necessary for the accumulation of hydrocarbons, then, as has been
Fig. 9. Model of the earth's crust of the Hanoi Depression (Ag (n~ is the n-th approximation of Ag). The
predicted maximum values of heat flow q in the stable zone on the trough's flank and in the unstable
central part of this trough were estimated from the corresponding values of heat generation given in
Fig. 10.
shown by Klemme (1975), high values of heat flow contribute to the for- mation of hydrocarbon deposits exceeding the average ones. In his paper, Klemme refers to the data of Philippi (1965), who established that starting with the increase at T >~ 125°C, the velocity of chemical reactions increases sharply at temperature gradients of 4.8 _ °C/100 m + 1.6 4- ~C/100 m at a depth interval of about 2 .0+6 .5 km (at a maximum gradient (4.8~C), a sharp increase in the velocity of chemical reactions starts at a depth of 2.0 kin).
It has been established that there is an exponential relationship between temperature and generation of hydrocarbons (velocity of chemical reac- tions), i.e., an increase in temperature promotes the release of fluids from non-parent rocks. An analysis of the relationship between the tectonic struc- ture of basins, the thermal regime, and the distribution of oil and gas fields in the world, has shown that the best conditions for oil and gas generation and accumulation exist in more mobile basins, i.e., in those with relatively high values of heat flow and situated in the marginal parts of the continents
CRUST OF THE H A N O I T R O U G H 195
(Klemme, 1975). Although the authors had no data on the heat flow in the Hanoi Trough, some estimations of the type of assessments are given below. Using the results of Ag interpretation and data from the literature on the relationship between density ~ (g/cm 3) and heat generation Q (/~W/m 3), let us assess the values of the expected heat flow in the two sites of the struc- ture under consideration (one on the rift's wing and another in its central zone). As follows from the interpretation of gravity data (Fig. 9), the "granite" stratum in the first site is 20 km thick and its density is 2.7 g/cm3.. whereas the "metamorphous" stratum has the same thickness and a density of 2.9 g/cm 3. The value of heat generation (#W/m 3) in the "granite" and "metamorphous" strata is within the limits of 1.4~<Q~<2.1 and 0.4 ~< Q ~< 1.5, respectively (Kutas, 1978). To select a narrower interval of Q, let us make use of the experimentally obtained dependence between the value of heat generation Q and rock density (Kutas, 1978). According to this dependence, rocks with densities of 2.7 g/cm 3 and 2.9 g/cm 3 have values of Q equalling 1.4-1.7/~W/m 3 and 0.6-0.7 pW/m 3, respectively. Let us attribute these values to the "granite" and "metamorphous" strata and calculate the possible value of heat flow q on the surface:
i
q = ~ Q i A Z i (16)
where Qi is heat generation in the i-th stratum with a thickness of AZ~. For- mula (16) is applicable to the stationary heat processes. Based on (16), the total heat flow from the crust of the site situated in the stable zone at the flank of the trough (Fig. 10) is estimated at q = 40 + 48 mW/m 2. According to the assessments made by Kutas (1978), the heat flow from the mantle q,,, in the stable continental zones, to which the type of the earth's crust under consideration also belongs, amounts to 15-17mW/m2; hence, the total value of heat flow in the zone being considered can be expected to equal 55-65 mW/m 2.
This estimated value may be checked independently with the use of the experimentally established relationship between the age of the structure and the mean value of heat flow in continental structures (Fig. 11). These data show that heat flows of q ~ 4 5 - 5 5 mW/m 2 are typical of structures of Precambrian age (~ ~ 600.106 years), whereas increased values of heat flow ( q ~ 5 4 - 6 6 mW/m 2) are characteristic of structures of Paleozoic age (PC) (230-350). 106 years.
Thus, there is a good agreement between the heat flow estimates obtained from (16) and those based on the age of the stable zones (flanks) of the Hanoi Trough.
Now, let us make a similar assessment for the central unstable part of the Hanoi Trough (Fig. 10). The mean value of heat generation Q in a sedimen-
196 TIN AND LITVINENKO
S T A B L E ZONE AT THE DEPRESSION~S F L A N G E
Heat g e n e r a t i o n , Q t~w/m 3
4 . +
v2. E C3
4O
0.4 1,0 ~. 7 7.0
_ _ ]
L E G E N D
U N S T A B L E C E N T R A L PART OF DEPRESSION
Po £
3o a
4(:
Heat 9 e n e r a ~ i o n , Q ~w/m 3
- - I I i i 1"21 ~ 1 I b i I I , I
' I 12~
1 1 5 0.4[--I
Fig. 10. The thickness of strata on the trough's flank and central part with the corresponding values of heat generation. Legend:
1. Sediments; 2. "Granite" stratum; 3. Stratum of"metamorphous" composition. 4. Part of the upper mantle. 5. Maximum values of the possible change in heat generation Q. 6. Changes in the mean values of Q obtained from an experimental curve showing the relationship Q = f(a) between heat generation Q and density ~r. 7. Mean values of Q, / IW/m 3.
tary rock mass equals 1.25pW/m 3 (Kutas, 1978). Assuming that heat generation from the upper mantle in this zone is normal for the upper man- tle (Q=0.4 / tW/m3) , it is possible to use formula (16). If we suppose that this zone is stable x} then the heat flow value ~ in the central zone of the trough is estimated, for the sedimentary, "granite" and "metamorphous" strata, and the upper part of the "normal" mantle, as follows:
Ct = qc + qm ~ 31 + 6 = 37 mW/m 2 (17)
The q value obtained is, however, too small and contradicts the following data.
Firstly, the value of q in young structures of Neogene age to which, according to geologic evidence, the Hanoi Trough belongs, is known to exceed 80 mW/m 2 and higher (Fig. 11).
Secondly, the heat flow from the mantle in the unstable zones of con- tinents increases to 25 -40mW /m 2 and more. Heat generation of mantle rocks qm in these zones increases to 8.0/~W/m 3, the mean value being about 4 #W/m 3. Assuming that Q ~ 4 / tW/m 3 is characteristic of tectonic unstable zones, we obtain from (16):
q = q c + q m = 9 1 mW/m 2 (18)
x) In view of the age of the trough it is possible to suggest that its central part is unstable. To show this, we shall make some assessments on the assumption that it is stable like the marginal parts of the trough.
C R U S T O F T H E H A N O I T R O U G H 197
This value of heat flow is typical of young rifts (the mean value of heat flow in rift structures amounts to 90-100 roW/m2).
Let us make another assessment of the heat-flow value, using the known relationship between gravity and stationary heat fields. For instance, Sim- mons (1967), and Zorin and Lysak (1972) have shown that the mathematical apparatus of the theory of gravity fields is suitable for describ- ing the observed surface anomalies of temperature gradient and heat flow with the following admissions: V2T = 0, To = const, ).-- const, Q = const, a = const.
For a half-space, this relationship is expressed (Simmons, 1967) as
q = ~ f V z (19)
where q is the heat flow, Vz is the derivative of the gravity potential, Q is the heat generation in a unit of volume, f is a gravitational constant, and is density.
Since the calculation is made according to an anomalous field Ag, the heat-flow anomalies Aq will be assessed not from (19) but from
AQa A q = ~ A g (20)
2xfA~,
The values of average anomalous density Ao-a and average anomalous heat generation AQa are calculated from
~ i A Z i A a a - (21j
~i AZi (7" i - - O" 0
~ ' i A Z i AQ.- t22
AZi ~ i Qi-Q0
The values of the thickness of the AZ,-th stratum, with corresponding values of density and heat generation Q~ of the earth's crust and upper man- tle strata of the Hanoi Trough gravity model, are given in Table 2.
The Aa.~ and AQa values for this gravity and geothermal model, derived from (21) and (22) and the data given in Table2, are as follows: Ao" a ~ - -0 .5 g / cm 3 and ~ 2.0 ~> AQa/> ~0.8 #W/m 3. For the anomaly of the gravity field in the trough, Ag ~ --43 mgal (Fig. 8) and the obtained values of Ao" a and AQa, the heat-flow anomaly value calculated from (20), is several but not several dozens of mW/m 2, which explicitly contradicts the estimate based on the Neogene age of the Hanoi Trough (q ~90 mW/m 2)
198 TIN AND L I T V I N E N K O
c~
E
~oo
o 80
60 ~D
I 40
0 0 0
0 100 200 300 400 500
T i m e "c'.1o 6 years
NI' IKIJ. ITI el CIDISlOI¢ I Kz{Mz { Pz
Fig. 11. The dependence of the value of heat flow q on the age of structures (After Kutas, 1978).
(Fig. 11) and proves once again that the heat field of the model being con- sidered is nonstationary.
It is known that heat-flow anomalies in tectonically active zones are narrowly localized and their amplitude decreases with time. These anomalies are produced by a short-term heat release in the bodies of the right geometrical shape for which, in particular cases, there is a solution of the nonstationary equation of heat conductivity (Lykov, 1967). If a short- term source of heat (impulse), with energy Q2, located on the line (x~, z~), is introduced into an unconfined solid body at a t ime-moment r, then a non- stationary heat field T(x, z, r) appears in this body. In this case, the non- stationary equation of heat conductivity is solved as follows (Lykov, 1967):
where Q2 is the source power per unit of length (J/m), a is the coefficient of thermal diffusivity (m2/s), c is specific heat capacity (J/kg.degree), O" is den- sity (kg/m 3) and ~ is time, (s). Assuming that half-space z > 0, and taking into account that the relation between the thermal parameters and density has the form of
Q a ( z - z i ) I ( x - x , ) 2 + ( z - z , ) 2 ] q 4nat 2 exp (26) 4az
If a heat source has the form of an infinite (along y) rectangle, then from (26) we obtain:
l fj2f~zf, Q2(z -z , ) l _ ,~ . . . . ,)2+(z Zl,2,/4ardx, dz, d r (27) q---4-~ ~ , -1 ar 2
where - 1 ~<xl ~ 1 and hi ~<z~h2, T-time rl < r < r 2 . Let us estimate q from (27) with the help of the theorem on the mean value at the point x = z = O, where the heat flow has its maximum:
Ql(h2 - - h Z ) ( ' r 2 - r l ) 1 (22'/4ar) q ~ (28)
4rta rl r2
where h~ < 2 < h 2 and rl < g < r 2 . Starting from the gravity model (Fig. 9), let us assign the following
dimensions to the heat-releasing body: l = _ 3 0 k m , h l = 2 5 k m and h 2 = 1 2 5 k m , then ~ = 7 5 k m . Heat generation by the rocks of the anomalous upper mantle equals, on the average, 4.0 ~W/m 3, and the coef- ficient of heat conductivity a = 8.5 10 -7 m2/s. As the Hanoi Trough belongs among young structures of Neogene age, r2 = 26" 106__ I" 106 years. If the error in the age of this structure is taken as the lower limit r~, then r--- 12.5" 106years. For these values, and satisfying the gravity model,
q ~ 47 mW/m 2. (29)
Thus, the value of heat flow from the crust qc and upper mantle qm, when the latter is generated by a rectangular source at depths of 25--125 km dur- ing a time-period close to the Neogene age, is
q = qc + qm ~ 78 mW/m 2 (301
Consequently, in constructing a geothermal model for the Hanoi Trough (within the framework of its geophysical model), we have to admit heat- flow generation, apart from the stationary process giving qc ~ 31 mW/m 2, by an additional prism-shaped source of heat located in the upper mantle. It is only with this admission that one can construct a non-contradictory geothermal and gravity model of this trough, proving the instability of its central part, which will be in good agreement with the known values of heat flow in young structures of Neogene age. The geothermal model developed (Figs. 9, 10) confirms this structure's genesis as a rift. Factual data on the temperature and heat flow distribution in the Hanoi Trough and their inter-
CRUST OF THE HANOI TROUGH 201
pretation would make it possible to construct a more detailed geothermal model, which would be of great importance for assessing the prospects for exploration for oil and gas in this region. For instance, relatively high values of geothermal gradients were recorded in the Dnieper-Donetsk and Bukhara rift basins. In South-East Asia, the basins of intermediate type are situated behind the volcanic arc of Indonesia and extend over the regions of the Central Sumatra, South Sumatra and Sunda basins (Java Sea). In the Sunda Basin, geothermal gradients ranging from very high to very low values are recorded in Central Sumatra (in the environs of the supergiant Minas Field, where geothermal gradients of 7.3_+ °C/100 m are not uncom- mon).
An analysis of gigantic fields has demonstrated that the dependence between the value of the geothermal gradient and the volume of hydrocar- bon deposits is most pronounced in zones with crust of intermediate type, i.e., in the regions where the continental crust changes into oceanic crust. Two-thirds of the world's giant hydrocarbon deposits are concentrated in these basins (Moody, 1975).
CONCLUSIONS
1. A comparison of the typical peculiarities of continental rifts with the main features of the geological structure of the Hanoi tectonic trough, situated in northern Vietnam, has shown that its geological features suggest its riftogenesis.
2. An analysis of geophysical data shows that the Hanoi Trough is characterized by a number of features typical of continental rifts such as: the type of gravity and magnetic fields; the existence of north-west striking grabens bounded by faults 15 and more kilometres deep: decreased values of the density of rocks composing the central graben, the presence of magnetic-anomaly-exciting rock masses occurring in the central graben at depths of 15 and more kilometres; very con- spicuous (upon appropriate processing of the gravity field) transform- type transverse faults whose intensity is apparently increasing in the south-east direction towards the Gulf of Tong King.
3. According to the results of geophysical data interpretations, which shed light on the trough's structure down to a depth of 10-15 km, a schematic model of the crust of the trough was constructed which showed that its thickness in the central part of the trough apparently decreases to 25 km, including 15 km-thick sedimentary, 5 km-thick "granite", and 5 km-thick "metamorphous" layers. The Moho surface is evidently underlain by rocks with anomalously low (for the upper mantle) density.
202
4.
TIN AND LITVINENKO
Heat-flow values which may be expected in the area of such a rif- togenic structure as the Hanoi Trough were estimated. It was found that the mean values of the heat flow to be expected on the trough's flank's amount to about 55-65 mW/m 2, whereas in the central part, the heat flow increases, with possibly 80-90 mW/m 2 being generated, apart from the sources located in the earth's crust, by an additional nonstationary source in the upper mantle. Only this admission enables one to construct non-contradictory geothermal and gravity models which are in agreement with the known values of heat flow in young structures of Neogene age. The importance of geothermal investigations for predicting oil and gas fields on the territory of Northern Vietnam and in the Gulf of Tong King is emphasized.
ACKNOWLEDGMENTS
The authors would like to express their thanks to Dr. V. (~erm~k for valuable comments on and attention to this paper and to V. I. Sverdlov for the help rendered in preparing the manuscript.
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