A second look at the geologic map of China:
the "Sloss approach"
Pieter Vermeesch,
Department of Geological and Environmental Sciences,
Stanford University, Stanford, CA 94305
(email: pvermees@pangea.stanford.edu)
Abstract
A key tool for geologic or tectonic reconstruction is the geologic
map. When one attempts to understand an area as large and complicated
as China, this tool contains more information than is optimal. The
presence of too much detail can obscure important general trends. To
facilitate the understanding of the major tectonic events that took
place in China during the Phanerozoic, the geologic and tectonic maps
of China are simplified and recast in an easily interpretable format.
A methodology is presented that is similar to the one introduced by
L.L.Sloss in the early days of plate tectonics and sequence
stratigraphy. Each tectonic zone of China is represented by one "Sloss
curve", which is a time series representation of the geologic map. The
curve shape reflects the geological response to tectonic
changes. Patterns emerge when the "Sloss curves" are compared and
correlated between the tectonic zones. The compiled "Sloss map" can be
used as a lowpass filter of tectonic events. Two events clearly stand
out across the "Sloss map" of China: the Permo-Triassic North China -
South China collision, and the Cenozoic India - Eurasia collision.
Introduction
The tectonic history of China is a very complicated one,
characterized by amalgamation of numerous microcontinents throughout
the entire Phanerozoic (e.g. Zhang et al., 1984; Hendrix and
Davis, 2001). A simplified tectonic domain map is presented as Fig.1
. In order to understand the chronology of
events, it may be fruitful to look at the geologic record from a
distance. Only by a thorough understanding of the big picture can one
attempt to fully comprehend the more detailed geologic record.
Figure
Figure 1: The tectonic map of China, based on USGS Open File Report 97-470C
(Wandrey and Low, 1998)
With his 1963 paper, L.L. Sloss was one of the fathers of sequence
stratigraphy, more than a decade before this term was introduced by
Vail and Mitchum (1977). On the eve of the plate tectonic revolution,
Sloss described cycles of alternating sedimentation and erosion on a
continental (Sloss, 1960, 1963), and later global (Sloss, 1976)
scale. We now know that these first- and second-order stratigraphic
cycles (Vail and Mitchum, 1977) mainly are the result of the breakup
and assemblage of super-continents, and of the related periodicity in
the rate of ocean spreading and the volume of mid-oceanic ridges
(e.g. Flemming and Roberts, 1973). In order to reconstruct the
depositional cycles, and to correlate them across the continents,
Sloss applied a conceptually simple method, which is similar to the
one that will be used in this paper for the description of the
tectonic history of China. Therefore, I term it the "Sloss method".
L.L. Sloss used isopach maps of the United States, of Canada, and of
the USSR to calculate the preserved areal extent and volume of each
of the stratigraphic units of the maps. Converting these units to
physical time by means of the geological time scale, he obtained
depositional time series, which he found to be remarkably similar for
the three areas studied (Sloss, 1976). Unfortunately, no isopach
atlas exists that covers the entire territory of China. Instead, I
use the digitized geologic and tectonic (Fig.1
) map at a scale of 1:5,000,000
(USGS open file reports 97-470F and 97-470C). Although we have the
disadvantage of poorer data quality, we also have advantages that
were not available to Sloss. Computing power has increased
tremendously since 1976. The tools of GIS and statistical analysis
prove to be very useful for our purposes.
Figure 2: The "Sloss map" of continental China summarizes
both the tectonic and the geologic map. The numbers indicate the
tectonic zones, as specified by Fig.1. Some
tectonic zones were omitted if they were too small for the
calculation of a meaningful "Sloss curve". In each of the "Sloss
curves", the black line represent the percentage of the total area
(on a logarithmic scale) represented by rocks of a certain age (on a
linear scale) in the Phanerozoic. The area shaded grey under the
curve represents the percentage covered by magmatic rocks (see
inset).
The Method
For each of the tectonic zones, the area covered by rocks of
a certain stratigraphic age was calculated as a percentage of the
total area of the tectonic zone. Next, these data were converted
into time series by means of the time scale of Harland et
al.(1990). The time resolution of the geologic map is rather poor
and not uniform, with most units defined at the chronostratigraphic
systems level, while some span as much as an entire era. To solve
this problem, the following strategy was used: given overlapping
stratigraphic units 1 and 2, of duration S1 and S2, respectively,
AS1S2 is the areal contribution of unit 2 to the part of the
"Sloss curve" that overlaps in time with unit 1:
AS1S2 = AS2S2 × |
æ è
|
S1
S2
|
ö ø
|
|
| (1) |
The use of equation 1
is illustrated by Fig.3. We will use it for
calculating the "Sloss map" of China, which is shown in Fig.
2.
Figure
Figure 3: If the total area of
the geologic map that is labeled S1 is A1, and the stratigraphic
subdivision named S2 covers an area A2, the corresponding value of
the "Sloss curve" (black curve, lowermost part of the figure) for
each elementary time step (e.g. S1) is a function of the relative
time overlap, and given by equation 1 of the text.
For the western United States, a GIS version of the geologic map (at
a scale of 1:2,500,000) is available, in a similar format as the
geologic map of China (King and Beikman, 1974). A "Sloss curve" was
calculated for a rectangular area, located between 39oN/100oW
and 49oN/114oW and compared to the curves that were based on
isopach maps for the same area (Sloss, 1976). Thus, the validity of
the above proposed method could be tested. The results of this
exercise are shown on Fig.4. For reasons that are
discussed further below, the fit is not perfect. Still, most
depositional peaks are located at the same times. Taking into
account these remarks, the "Sloss curves" of Figs.
2 and 4 represent
an important simplification and abstraction of the geologic map. We
will use them to compare the different tectonic zones of China,
and reconstruct their history. Such analysis is reliable because all
"Sloss curves" on Fig.2 are based on
the same geologic map, using the same stratigraphic subdivisions. It
would be more dangerous to make inferences based on a single
"Sloss curve", or use the method to compare different data sets,
which is what was done for the construction of
Fig.4. Fig.2 shows
some interesting and tectonically meaningful features that will be
interpreted in the discussion section of this paper.
Figure
Figure 4:
Comparison of the "Sloss curve" of the western United States
(rectangular area between 34oN/100oW and 49oN/114oW), as
calculated from the geologic map (King and Beikman, 1974)(black
curve), with the curves of sedimentary volume published by Sloss
(1976) (gray area) for the same area. Because two different data
sources were used, it was necessary to re-calculate the
"Sloss-curves" for identical time steps. Not surprisingly, the
height of the peaks differs between the two curves in most cases.
This is especially clear during the Paleozoic, which is especially
affected by the "blanketing effect". The important thing to remember
about this figure is that the major peaks and troughs are located at
approximately the same times. Thus, the geologic map can be used as a
proxy for the isopach map.
Limitations and Uncertainties of the Method
The most important problem Sloss (1976) pointed out in his
method was the incompleteness of the geologic record: erosional
processes progressively remove greater fractions of the rock record
with time. Because the analysis that is presented in this paper is
not based on isopach maps, but on the geologic map, these problems
are aggravated and additional problems arise.
- First and foremost, not only erosional processes, but also
sedimentary processes obscure the more distant geological past. It is
possible for a relatively thin cover of horizontally deposited
sediments to completely dominate the geologic map of an area, even in
cases where older formations represent much larger volumes and
thicknesses. This "sedimentary blanketing effect" is illustrated by
Fig.5. The importance of the "blanketing effect"
dramatically decreases with increasing age. Older geological
formations are more likely to have been affected by previous stages
of tectonism, which cause folding and tilting. These processes
improve the quality of outcrop area as a proxy for sedimentary
thickness and volume (Fig.5). Therefore, the
"Sloss-like methodology" should be more reliable for the more distant
past. However, there is a tradeoff with the time resolution, which
generally decreases with age. The Precambrian is only
subdivided into 2 parts: Archean and Proterozoic. For this reason,
the Precambrian has been omitted from the map of China in Fig.
2.
- Sloss included lithological data in his analysis. The GIS
version of the geologic map of China distinguishes between
sedimentary and magmatic rocks, but does not subdivide the
sedimentary rocks. Lithologies obviously can provide important
information regarding the tectonic setting of an area, but they are
less easily represented by numbers that can be objectively
compared. Also Sloss based his conclusions mostly on the numerical
values of volume and preserved area. On
Fig.2, the area under the "Sloss
curves" covered by magmatic rocks is shaded gray.
Figure
Figure 5: Illustration of the sedimentary
"blanketing effect" : steeply tilted layers make smaller outcrops,
even though they can have greater thickness and volume than their
sub-horizontal sedimentary cover ("blanket"). The more deformed a
region is, the better the geologic map, and its corresponding "Sloss
curve", represents the volumetric distribution of the
strata.
Discussion: Tectonic History of China as Illustrated by its
"Sloss Map".
Figure 6; shows how the 64 tectonic zones
of China can be grouped into a smaller number of tectonic
"regions". This grouping is in close agreement with the tectonic
zonation of Zhang et al (1984) and Yin and Nie (1996). This
section will discuss how the "Sloss Map" can provide a
semi-independent confirmation of its validity. On
Fig.6, the tectonic affinities are indicated by
different hatching patterns. The hatching density further divides
the tectonic groups in subgroups that are more convenient to discuss
together. It is important to note that some of the tectonic zones
were assigned to a tectonic group in a rather arbitrary way.
Generally, this is the case for suture zones. For example, the Tian
Shan was considered as part of the "northern tectonic region".
However, we could also have put this range in the Tarim-Qaidam block,
because the Tian Shan contains fragments of both tectonic groups.
Similarly, the Qinling-Dabie zone was assigned to the North China
block, but being the welding zone with the South China block, it
could just as well have been made part of the latter. The only
ßuture zone" for which an exception was made is the vast Songpan
Ganzi fold belt. A special discussion will be dedicated to this
zone, and its relationship with the surrounding tectonic blocks
(section dealing with Central China).
Figure
Figure 6: The tectonic map of China, hatched according to
tectonic affinity. Some tectonic groups are further divided by the
density of the hatching where this facilitates discussion of the
tectonic history in sections of the text.
Northern tectonic region
The Northern tectonic region consists of a number of
microcontinents and early Paleozoic island arcs. These were accreted
to the southern margin of the Siberia-Kazakhstan plate and to the
northern margin of the Tarim-North China block prior to and during
the final collision between these two, which occured diachronously
from the Carboniferous (west), until the Late Permian (east) (Yin and
Nie, 1996). The Northern tectonic region recorded changes in the
regional tectonic stress field, induced by distant accretionary
events. During the Mesozoic and Cenozoic, Paleozoic fold belts were
reactivated several times as complicated systems of intracontinental
mountain ranges and sedimentary basins. Although its tectonic zones
have a similar (Paleozoic) tectonic history, the Northern tectonic
region is so large, that we will continue our discussion by further
dividing it into an eastern and a western part.
Northeast China
Northeast China is part of the Mongolian accretionary fold
belt, which is a collage of Ordovician to Early Permian island arcs,
blueschist-bearing assemblages, Paleozoic ophiolites, and possible
microcontinental blocks (Davis et al., 2001). These tectonic
settings stand out in the "Sloss curves" of Fig 7.
by the great relative importance of magmatic lithologies, which are
shaded gray. During the Late Permian and Early Triassic, the
Mongolian arc terrane was finally welded together with the North
China craton along the Suolon suture (Yin and Nie, 1996; Davis
et al., 2001). On the "Sloss map", this event is represented by a
subsequent absence of deposits during the Triassic, which is probably
due to the compression and mountain building that followed
collision. During the Late Triassic, the controversial
Mongolo-Okhotsk ocean opened to the north of the Mongolian arc
terrane. This ocean started subducting both to the north and to the
south, causing a continued importance of magmatic rocks in the "Sloss
curves" (Figure 7) (Yin and Nie, 1996). During the
Jurassic and the Cretaceous, the Suolon suture was reactivated as the
Yanshan fold and thrust belt, on the northern margin of the North
China block (Davis et al, 1998, 2001). This will be discussed
in more detail later. Important to note here is that one of the
hypotheses for the driving force behind this reactivation is the
Cretaceous closure of the Mongolo-Okhotsk ocean (Zonenshain, 1990;
Yin and Nie, 1996; Davis et al., 1998, 2001).
Figure
Figure 7: Northeast China is a tectonic
collage of accreted late Paleozoic terranes. It is delimited to the
south by the Permo-Triassic Suolon suture, and to the north by the
Cretaceous Mongolo-Okhotsk suture. The importance of subduction
processes during most of the history of this region is reflected by
the large contribution of magmatic rocks to the "Sloss curves".
During the Mesozoic large sedimentary basins formed, whereas the
Suolon suture was reactivated south of it, to form the Yanshan fold
belt. The Paleogene is characterized by a dip in the "Sloss curves",
except for the Yitong graben, which indicates that this feature is
at least that old.
Alternatively, also the collisions that occured on the southern
margin of the Asian continent (notably the Cretaceous accretion of
the Lhasa block), could have been responsible for the Yanshan
compression (Graham et al, 2001). Curiously, this compression
was also associated with basin-forming extension in the Mongolian arc
terranes (Davis et al, 2001; Graham et al, 2001).
Examples of such basins are the Hailar basin and the Songliao basin,
which may have been caused by Pacific back-arc extension, or by
gravitational collapse of the Late Paleozoic orogen (Graham et
al, 2001). This apparent paradox can be reconciled by partitioning
of the Gobi extensional province from the contractional
Yinshan-Yanshan orogenic belt by escape-tectonic strike-slip faults,
such as the East Mongolian Zuunbayan fault, which would be associated
with the coeval collisions on the southern Asian and Mongolo-Okhotsk
margins (Graham et al., 2001). During the Paleogene, all the
"Sloss curves" of Northeast China show a dip, which could be a
distant result of the India-Eurasia collision, or alternatively, be
caused by changes in the Pacific plate subduction regime. The only
"Sloss curve" in Northeast China that does show Paleogene
sedimentation of any significance is the Yitong graben, which
suggests that this feature is at least that old (Tian and Du, 1987).
Northwest China
The pre-Devonian history of Northwest China is poorly
understood. Several microcontinents and island arcs were drifting
around on the subducting Turkestan ocean that separated the
Tarim-North China block from Siberia-Kazakhstan (Heubeck, 2001).
Between the Early and the Middle Devonian, southerly sourced clastic
sediments were deposited along the passive continental margin of
North Tarim (Yin and Nie, 1996). These rocks presently make up the
southern Tian Shan; thus the latter does not really belong to the
"Northern tectonic region". During the Carboniferous, two more
components of the Tian Shan were welded to the Tarim block: (1) the
central Tian Shan block, a microcontinent with Precambrian basement;
and then (2) the northern Tian Shan and Junggar blocks, a
post-Devonian arc terrane (Zhou et al., 2001). The collision of
the Junggar arcs and the Devonian Altai arcs with the Tarim - North
China block in the Carboniferous - Early Permian marked the beginning
of the diachronous closing of the Turkestan ocean, which eventually
led to the formation in the northeast of the Permo-Triassic fold
belt. After its formation in the Carboniferous, the Tian Shan, in a
very similar way to the Yinshan-Yanshan, would be reactivated during
the Late Mesozoic and the Late Cenozoic, as a result of collisional
events that occured far to the south of the range (Dumitru et
al, 2001). Examples of such events are the Jurassic Qiangtang-Tarim
collision and the Cretaceous Qiangtang-Lhasa collision. Furthermore,
the Cenozoic Himalaya orogeny has caused stresses that are
transferred deep into the Asian continent. They have made the modern
Tian Shan the most spectacular of all intracontinental mountain
belts, with elevations of up to 7,400m, at more than 1,000km from the
suture zone (Molnar and Tapponnier, 1975).
Figure
Figure 8:
Very similar to Northeast China, Northwest China also is composed of
a number of microcontinents and island arcs, that were accreted
during the Paleozoic. Magmatic rocks dominate the "Sloss curves"
until the Carboniferous-Permian, which corresponds with the final
closure of the Turkestan ocean and the formation of the ancestral
Tian Shan. In contrast with the Northeast (Fig.7),
there is no post-Paleozoic magmatism. The "Sloss curves" of the fold
belts all show a Triassic dip, a Jurassic high, a Cretaceous dip, and
a Cenozoic high. These reflect the repeated reactivation of this
region due to ongoing collisions on the southern margin of the Asian
continent.
The "Sloss curves" of different portions of Northwest China
(Fig.8) share numerous characteristics with each
other, indicating a common tectonic history since the Carboniferous.
The Late Paleozoic is characterized by voluminous magmatic
lithologies, which represents the subduction dominated setting of
many of the terranes of this area. From the Permian onward,
magmatism ceased, in marked contrast with the tectonic zones of
northeastern China. Indeed, in the northeast the existence of the
Mongolo-Okhotsk ocean, and the proximity to the subducting Pacific
Ocean, continued to generate magmas throughout the Mesozoic and
Cenozoic, which was not the case for northwest China. Another
characteristic of most zones of the "Sloss map" of Northwest China is
a pronounced dip during the Triassic, which was probably caused by
mountain building that followed closure of the Turkestan ocean, with
the addition of the compressional stresses caused by the southerly
Qiangtang-Tarim collision. The Jurassic shows up as a peak in most
"Sloss curves". This could represent post-orogenic subsidence
("collapse"). The Cretaceous is a dip again, corresponding to Late
Mesozoic reactivation of the Tian Shan that is detected by a cluster
of Cretaceous apatite fission track ages (Bullen et al., 2001;
Dumitru et al, 2001). Finally, the Cenozoic "Sloss curve"
shows an increase. This is caused by a strong "blanketing effect",
and reflects the formation of large foreland basins (e.g, Junggar,
Turpan) between the different mountain ranges (e.g., Tian Shan, Bogda
Shan). The structural and stratigraphic relief between these
coexisting tectonic elements can attain several thousand meters over
distances of just a few tens of kilometers.
North China Block
The Tarim block and the North China craton are often
postulated to have behaved as a single tectonic block since at least
the early Paleozoic (e.g. Zhang et al., 1984; Yin and Nie,
1996). Others have suggested that they were separate blocks until
the Permo-Triassic closure of the Turkestan ocean (Zhou and Graham,
1996a; Yang et al., 1997). That the Tarim block and the North
China craton are only connected by a very narrow strip. This
"problem" is solved by restoring ~ 400km of Cenozoic sinistral
displacement along a controversial Altyn Tagh-Alxa-East Mongolia
fault (Yue and Liou, 1999; Yue et al., 2001). In this section,
we will discuss the North China craton sensu stricto, which is
the eastern part of the North China-Tarim block. The North China
craton is bordered to the north by the Permo-Triassic Suolon suture,
which was reactivated during the Jurassic as the Yanshan
intracontinental fold belt (Davis et al., 1998, 2001). To the
southwest, the North China craton is sutured against the Qaidam block
along the Qilian Shan; the suture marks a collisional event of
Devonian age. Due east of the Qilian Shan is the Qinling-Dabie Shan,
which represents the Permo-Triassic collision of the North China
block with the South China block (Yin and Nie, 1996; Zhou and Graham,
1996b). To the southeast, the North China block is offset by the
sinistral Tan Lu fault system.
Although named a "craton" (it has Archean basement), the North China
tectonic group underwent substantial internal deformation over the
course of its Phanerozoic history. The "Sloss curves" of the North
China craton do not tell a simple story. Important information has
been obscured by the "blanketing effect". This is especially the case
for the Cenozoic Bohai rift basin. During the Late Triassic, this
zone roughly corresponded to the "Huabei plateau", which was the
result of continuing convergence after the collision of South China
with North China (Yin and Nie, 1996). Sediments derived from this
topographic high were deposited in the neighboring Shanxi and Ordos
basins, where they are responsible for a Triassic peak in the "Sloss
curves". This is a feature that will be seen more often in the "Sloss
map" of central China (discussed below; Fig.9).
South China block
The South China block is a relatively stable craton (also
called the "Yangtze craton") that has a relatively insensitive "Sloss
curve". In the Jiangsu- and South China fold belts, all stratigraphic
ages are represented approximately to an equal degree, with a small
preference to the Quaternary due to the "blanketing effect". The
"Sloss curves" of the western part of the South China tectonic group
show a distinct peak in the Triassic. This marks the collision of
the South China block with the North China block, as will be
discussed below.
Tarim-Qaidam
As was previously discussed, disagreement exists as to
whether or not the Tarim block and the North China craton were
separate tectonic entities before the Permian. A similar controversy
exists about the Qaidam block. Some have suggested that it was part
of the Tarim before being cut off and displaced by the Altyn Tagh
fault (Zhou and Graham 1996a; some extent also Yin and Nie, 1996),
whereas others think that the two to represent separate tectonic
blocks (e.g. Zhang et al., 1984). The discovery of a Middle
Paleozoic suture zone in the Altyn Tagh favors the latter point of
view (Sobel and Arnaud, 1999). After the Qaidam and Tarim blocks were
welded together, an extensive, continuous, late Paleozoic - early
Mesozoic Kunlun magmatic arc developed along their southern
margin. This arc died during the Triassic collision of the
Tarim-Qaidam block with the Qiangtang block.
The closure of the ocean basin that existed between the Tarim-Qaidam
block and the Qiangtang block, is marked by the abrupt ending of the
(gray-shaded) magmatic area under the "Sloss curve" of the Kunlun
tectonic zone. The same is true for the Altun Shan, where magmatism
ended with the closure of the ocean that existed between Tarim and
Qaidam. Due to the "blanketing effect", little can be said about the
"Sloss curves" of the Qaidam and Tarim basins themselves. In this
case, it would be particularly valuable to have access to isopach
maps such as the ones Sloss (1976) used.
Central China, Songpan Ganzi, and the
Permian-Triassic N-China/S-China collision
The most representative tectonic element for this region is
the Songpan-Ganzi fold belt. This tectonic zone is characterized by
a striking depositional peak in the Triassic
(Fig.9). This peak can be recognized slightly less
spectacularly in the zones immediately to the south of Songpan-Ganzi
(e.g., the Sichuan Basin, Sulong Shan). Sloss (1963, 1976) was not
the first person to recognize that the Triassic was a lull in
continental sedimentation. It followed immediately after the last
(Variscan) stages of the assemblage of the Pangea supercontinent, and
is characterized by very low global sea level (Vail and Mitchum,
1977). Hence, the abundance of Triassic deposits in the tectonic
zones of Central China must indicate an important tectonic
event. This event is the diachronous suturing of the North and South
China blocks (Zhou and Graham, 1996b). North of Songpan-Ganzi (e.g.,
Altun Shan), the Triassic shows up as a low point in the "Sloss
curve". This could either be the result of the worldwide sea level
drop described above or alternatively, be caused by the same events
that caused the Triassic peak in and south of Songpan-Ganzi. Indeed,
if the Triassic orogeny was the result of northward underthrusting of
the South China block under the North China block, it seems likely
that the zones north of the suture zone would be uplifted, whereas
the southern zones would flexurally subside. A modern day analogue to
this would be the association on either side of the Indus-Tsangpo
suture zone of the North Indian Ganges foreland basin, and the
Tibet-Qinghai Plateau.
Figure
Figure 9:
The "Sloss map" of Central China shows a very characteristic peak of
sedimentation and magmatism in the Triassic for the tectonic zones
south of the Songpan Ganzi complex. This peak is caused by the
diachronous collision between the North China and South China blocks
(Zhou and Graham, 1996). The zones north of the suture zone show a
lack of sedimentation at the same time, possibly related to the
uplift and compression caused by continental collision.
Southwestern China
The Qiangtang block drifted off from Gondwana some time
during the Paleozoic (Yin and Harrison, 2000). During the Triassic,
the ocean that separated the Qiangtang terrane from the Asian
continent started subducting beneath it. The northern margin of the
Qiangtang terrane became an active one, which is reflected in an
increase of the magmatic area under its "Sloss curves"
(Fig.10). Almost immediately following the
Permo-Triassic collision of South China with North China, the
Qiangtang-Indochina block was underthrusted by the amalgamated South
China and Qaidam-Tarim blocks along the Jinsha suture (Chang et
al, 1986; Yin and Nie, 1996; Yin and Harrison, 2000). The
Songpan-Ganzi remnant ocean basin was trapped in the triangular space
between the three aforementioned blocks. The Triassic peak in the
"Sloss curves" of the Qiangtang block testify of this event
(Figs.9 and 10). Although
subduction-related magmatism stopped in the Kunlun, it continued in
the Qiangtang block, along with the approach from the south of the
Gondwana-derived Lhasa block. The Lhasa block collided with the
Qiangtang block along the Banggong-Nujiang suture during the
Cretaceous (Allègre et al., 1984; Dewey et al., 1988; Yin
and Nie, 1996; Yin and Harrisson, 2000). Whereas the underthrusted
Qiangtang block was deformed and uplifted (marked by a relative low
in its "Sloss curves"), the underthrusting Lhasa block underwent
flexural subsidence and is generally characterized by a "Sloss peak".
After the collision of the Lhasa block with the Qiangtang block, the
oceanic crust that separated the Lhasa block from the Indian
subcontinent subducted beneath the Lhasa block, which resulted in
extensive magmatic activity and the development of the Gangdese
batholith (Allègre et al., 1984; Yin and Harrison, 2000). At
approximately 45Ma, India finally collided with Asia along the
Indus-Tsangpo suture (Le Pichon et al., 1992). This led to the
intensely studied, but poorly understood Himalaya-Tibet orogeny. The
Tibet-Qinghai Plateau is believed to have resulted mainly from this
final collision, although it has been suggested that a pre-existing
plateau formed after the Kunlun-Qiangtang collision (Murphy et
al, 1997). The present day Tibet-Qinghai Plateau comprises the Lhasa
terrane, the Qiangtang terrane, and to some extent the
Kunlun-Qaidam-Qilian Shan tectonic group. At the same time, the
Himalaya-Tibet orogeny reactivated many of the tectonic zones and
groups that have been discussed in previous paragraphs. This is the
case for the Tian Shan and the Altai, but also for the Altun Shan,
which appear to have acted as a zone of weakness that became the
preferred locus for the continental-scale Altyn Tagh sinistral
strike-slip fault. The Kunlun, Xianshui-He, and Red River faults
roughly follow the Kunlun-Anyemaqen, Jinsha, and Banggong-Nujiang
sutures, respectively (Tapponnier et al., 2001). Thus, fault
reactivation seems to be a major factor in the deformation of the
Chinese tectonic "jigsaw puzzle". The translation of this in terms
of "Sloss curves" is a trivial one: topographic depressions cause
Cenozoic peaks, whereas areas of high relief show a dip in the
Cenozoic "Sloss curve". These signatures can be seen all over China,
and perhaps even affect the Pacific margin (e.g. Jolivet et
al, 1990).
Figure
Figure 10:
The Tibet Plateau consists of three tectonic blocks: the Qiantang,
Lhasa, and India blocks. Qiangtang underthrusted the Kunlun block in
the Late Triassic, which shows up as a peak in its "Sloss curve".
Subsequently, the Lhasa block collided with the Qiangtang block,
after the closure of an oceanic basin that was associated with a
magmatic arc and a substantial of gray under the "Sloss
curve". Finally, the southern margin of the Lhasa block became an
active one (represented by a predominantly gray shaded "Sloss
curve"), until the final collision with India, which caused a dip in
the Cenozoic curves of both the Qiangtang and the Lhasa block.
Conclusions
I have developed a method for representing a two-dimensional
geologic map by a one-dimensional depositional time series, similar
to the way L.L.Sloss represented the geologic maps of North America
and eastern Europe (Sloss, 1976). Going one step further, the
geologic map and the tectonic map of China were united into one
"Sloss map". Despite limitations and assumptions of the method, the
"Sloss map" proves useful for recognizing the most important tectonic
events of Phanerozoic China, and for delimiting tectonic regions,
which can comprise multiple tectonic zones. The Permian-Triassic
North China - South China collision and the Cenozoic India - Eurasia
collision stand out especially clearly. Also, difference between
stable cratons and tectonically more sensitive accretionary fold
belts can easily be recognized.
Major improvements could be made with the incorporation of isopach
maps, rather than ordinary geologic maps. Other welcome changes would
be increased time resolution, and the addition of more lithological
parameters to the geologic map. If all these conditions were
fulfilled, it might be possible to fully automate the interpretation
of the "Sloss map" by means of a statistical correlation analysis. If
extended to other continents, the "Sloss analysis" could prove to be
as useful in tracing and describing the breakup of continents,
as it is with continental accretion, which I have demonstrated
in this paper with China's history of amalgamation. The first places
to examine in the case of China would be Gondwana, and the southern
margin of the Tethys. Indeed, most of the tectonic blocks of
Fig.2 originally drifted off from
continental areas now consituting India and Australia ( Sengör
and Natal'in, 1996). The methodology described in this paper might
prove to be a good first-order way of detecting such associations.
Another application of the "Sloss method" in its present form would
be the study of orogenic terranes. If geologic maps exist in a GIS
form, the "Sloss curve" derived from the geologic map could be used
as a proxy for the isopach map, as was suggested in
Fig.4. Doing this may yield insight in the
evolution of rates of deposition until the time of collision, which
is important for understanding the mechanisms of mountain building.
Acknowledgements
Many thanks to Steve Graham for useful suggestions and encouragement,
and to Stanford University for financial support.
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