Method for carrying out paleogeography reconstruction by using spatio-temporal age distribution characteristics of clastic zircon

Abstract

The invention provides a method for carrying out paleogeography reconstruction by using detrital zircon age space-time distribution characteristics. The method comprises the following steps: obtaining detrital zircon age data; sorting and analyzing the obtained fragment zircon age data to obtain age distribution characteristics of the group of data; grouping according to regional or stratigraphic times, and determining a main age distribution interval; obtaining age data and proportion of a single sample in each main component; drawing spatial distribution of main components of all samples in the age interval; and according to the obtained detrital zircon age space distribution diagram, in combination with the zircon age characteristics of the potential source area, detrital material source system analysis is carried out, and paleogeography reconstruction is realized, so that the spatial and temporal distribution characteristics of detrital zircon age data can be reflected more accurately, the evolution rule of detrital material sources in space along with time is effectively described, and the reconstruction efficiency is improved. The paleogeography pattern is reconstructed, and a reliable basis is provided for research of important geological events.

Description

Technical Field

[0001] The present invention relates to the technical field of paleogeographic reconstruction, and specifically to a method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages. Background Art

[0002] Paleogeographic reconstruction has important scientific significance and research value for in-depth understanding of global or regional tectonic evolution. It can restore the past landforms and water system changes of the surface system, and provide good temporal and spatial constraints for research work such as geology, paleoclimatology, paleoecology, and paleoceanography. With the reporting of a large amount of detrital zircon age data, it has created the possibility for us to reshape the sediment path system, which provides a new method for reconstructing paleogeography. Paleogeographic reconstruction can provide a reliable basis for the study of important geological events. For example, for the strike-slip displacement of a large strike-slip fault zone, if the paleogeographic pattern before the strike-slip action can be reconstructed, the strike-slip displacement can be well constrained.

[0003] However, in the prior art, the displacement of strike-slip faults is generally determined based on the offset distance of the markers on both sides of the fault. This method may be difficult to effectively constrain the total strike-slip displacement of large strike-slip fault zones with long-term strike-slip. The main reasons include: the lack of effective markers due to late structural transformation and erosion; most large fault zones have several parallel branch strike-slip faults, and the use of markers may only be able to determine the displacement of a branch strike-slip fault, and cannot constrain the total strike-slip displacement of the fault zone. Summary of the invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon age data, so as to solve the problems raised in the above-mentioned background technology. The present invention can more accurately reflect the spatiotemporal distribution characteristics of detrital zircon age data, effectively characterize the evolution of detrital sources over time and the evolution law in space, reconstruct the paleogeographic pattern, and provide a reliable basis for the study of important geological events (such as paleogeographic changes caused by orogeny, effective determination of slip displacement on large strike-slip fault zones, changes in detrital sources caused by water system evolution, etc.).

[0005] In order to achieve the above object, the present invention is implemented by the following technical scheme: a method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages, comprising the following steps:

[0006] Step 1: Obtain detrital zircon age data. The methods include collecting reported data and conducting detrital zircon U-Pb dating tests.

[0007] Step 2: Organize the obtained detrital zircon age data and analyze them using probability density plot (PDP) analysis method to obtain the age distribution characteristics of this group of data, and determine the main age distribution range according to the region or stratigraphic age (for example: according to the KDE diagram, a group of 24 samples with a total of 2000 age data are divided into 5 main age ranges: 200-300Ma, 400-550Ma, 750-980Ma, 1750-1890Ma, 2470-2580Ma).

[0008] Step 3: Obtain the age data of a single sample in each principal component, and the proportion of the number of ages in each age interval to the total age of the sample (for example: there are 100 age data of a single sample, of which 25 and 15 are at 200-300Ma and 750-980Ma, respectively. Then the age proportions of the two age intervals of 200-300Ma and 700-900Ma are 25% and 15%, respectively).

[0009] Step 4: For each age interval, the age ratio of a single sample is used as the Z axis of the zircon U-Pb age spatial distribution map, where the X and Y values ​​are the latitude and longitude of the sample sampling location. After calculating the ratio of each component of all individual samples, Kriging interpolation is performed on the X, Y, and Z coordinates to plot the spatial distribution of the main components of all samples in this age interval.

[0010] Step 5: Based on the obtained spatial distribution map of detrital zircon ages and combined with the zircon age characteristics of potential provenance areas, conduct a detrital provenance system analysis to achieve paleogeographic reconstruction.

[0011] Further, in step 1, for data with zircon age > 1000Ma, n( 207 Pb) / n( 206 Pb) age, while for data <1000Ma, n( 206 Pb) / n( 238 U) Age. The harmony degree is between 90% and 110% for valid data.

[0012] Furthermore, the zircon age concordance diagram, kernel density estimation diagram, multidimensional calibration diagram and probability density diagram were completed using Isoplot and IsoplotR analysis and processing software.

[0013] Furthermore, the calculation formula of the kernel density estimation map in step 2 is as follows:

[0014]

[0015] Where: h represents bandwidth, also known as window width or smoothing parameter; k(x) is the kernel function, xi is the average age of the particles. h (x) is a mixture density function consisting of n density estimates with the same kernel function.

[0016] Furthermore, the bandwidth h is selected by using a bandwidth optimization algorithm, a local adaptive KDE (LA-KDE) and a deconvolution technique, wherein in the LA-KDE model the local bandwidth is inversely proportional to the data density in the local sample space, the high-density region is smoothly reduced, and the low-density region is smoothly increased.

[0017] Furthermore, a diffusion adaptive bandwidth model is used to estimate the optimal bandwidth and evenly apply it to the entire sample space to solve the PDP age peak distortion phenomenon.

[0018] Furthermore, in step three, the U-Pb age of detrital zircon is regarded as the sum of one or more discrete age components, and then the component is defined as a set of discontinuous zircon ages centered on the average value, each age component representing some zircon formation events.

[0019] Furthermore, the mean and standard deviation of each peak interval were calculated, and the age interval of each principal component was defined as the mean ± twice the standard deviation. These principal components were obtained in the zircon age of the overall sample, and the approximate proportion of each principal component present in a single sample was calculated. This proportion was used as the Z-axis of the zircon U-Pb age spatial distribution diagram, where the X and Y values ​​are the longitude and latitude of the sample sampling location. After calculating the proportion of each component of all individual samples, Kriging interpolation was performed on the X, Y and Z coordinates to plot the spatial distribution of the principal components of all samples.

[0020] Beneficial effects of the present invention:

[0021] 1. Method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon age For the analysis of detrital zircon age data, the KDE diagram is used as an alternative to PDP, which discards the uncertainty associated with data acquisition and introduces a specific bandwidth, which can more accurately reflect the distribution characteristics of detrital zircon age data. At the same time, the bandwidth optimization algorithm, local adaptive KDE and deconvolution technology are used to optimize the calculation method of the distribution characteristics of detrital zircon age data, thereby effectively solving the problem of PDP age peak distortion.

[0022] 2. Utilize the spatiotemporal distribution characteristics of detrital zircon ages to more intuitively establish a "source-sink" system, provide direct evidence for paleogeographic reconstruction, and further provide a reliable basis for the study of important geological events, such as paleogeographic changes caused by orogeny, effective determination of fault slip displacement, and changes in debris provenance caused by water system evolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The spatial distribution diagram of detrital zircon U-Pb ages in the Yangtze Block and the Songpan-Ganzi Block in the embodiment of the present invention;

[0024] Figure 2 This is a KDE diagram of detrital zircon ages for the Lower-Middle Triassic of the Songpan-Ganzi terrane in an embodiment of the present invention;

[0025] Figure 3 This is the KDE diagram of detrital zircon ages of the Lower-Middle Triassic in the western margin of the Yangtze in the embodiment of the present invention;

[0026] Figure 4 The age interval division of the main components of all Lower-Middle Triassic detrital zircon ages in the Yangtze Block and Songpan-Ganzi Terrane in the embodiment of the present invention;

[0027] Figure 5 The paleogeographic pattern (a) and tectonic transformation model (b) of the Early-Middle Triassic in the western margin of the Yangtze River in the embodiment of the present invention. DETAILED DESCRIPTION

[0028] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0029] See also Figures 1 to 5 The present invention provides the following technical solution: a method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages, specifically including the following contents:

[0030] Example 1

[0031] (1) In this example, U-Pb dating was performed on four detrital zircon samples. Samples XYF1601, XYF1602, and YLG1601 are located in the Songpan-Ganzi terrane; sample HSW1602 is located on the western margin of the Yangtze.

[0032] (2) Fresh rock samples collected in the field were crushed, screened and separated to obtain detrital zircon particles for zircon target preparation and LA-ICP-MS zircon U-Pb determination. For zircon age data > 1000 Ma, n( 207 Pb) / n( 206 Pb) age, while for data <1000Ma, n( 206 Pb) / n( 238 U) age. The concordance between 90% and 110% is considered valid data. Zircon age concordance diagram, kernel density estimation diagram (KDE) and probability density plot (PDP) were completed using Isoplot and IsoplotR analysis and processing software.

[0033] (3) For the analysis of detrital zircon age data, KDE diagram, as an alternative to PDP, has unique advantages. It is a non-parametric method for estimating probability density functions, which discards the uncertainty associated with data acquisition and introduces a specific bandwidth, which can more accurately reflect the distribution characteristics of age data. The calculation formula is as follows:

[0034]

[0035] Where: h represents bandwidth, also known as window width or smoothing parameter; k(x) is the kernel function, x i is the average age of the particles. h (x) is a mixture density function composed of n density estimates with the same kernel function. The kernel function k(x) can be selected in a variety of forms, including trigonometric functions or Gaussian functions. In the KDE method, the selection of bandwidth h is crucial. If h is too large, the age map will be over-smoothed, resulting in reduced resolution and possible loss of key information; if h is too small, the age map will be too rough and show too many unnecessary components.

[0036] In order to select the bandwidth h, scholars have developed a series of algorithms to optimize it, including bandwidth optimization algorithm, local adaptive KDE (LA-KDE) and deconvolution technology. In the LA-KDE model, the local bandwidth is inversely proportional to the data density in the local sample space, the high-density area is smoothly reduced, and the low-density area is smoothly increased. The proposed diffusion-based adaptive bandwidth model can estimate the optimal bandwidth and apply it evenly to the entire sample space. The optimized KDE can effectively solve the problem of PDP age peak distortion.

[0037] (4) In this embodiment, four Early-Middle Triassic clastic rock samples were collected and the U-Pb ages of detrital zircons were obtained. In addition, 29 Lower-Middle Triassic detrital zircon chronology data from the vicinity of the Xianshuihe-Xiaojiang fault zone published by previous researchers were collected. According to the spatial interpolation method, the U-Pb ages of these detrital zircons are regarded as the sum of one or more discrete age components. In this embodiment, a component is defined as a set of discontinuous zircon ages centered on the average value. Each age component represents some zircon formation events. Using Isoplot software, the detrital zircon ages of all samples are plotted into a probability density map (PDP).

[0038] The mean and standard deviation of each peak interval were calculated, and the age interval of each principal component was defined as the mean ± twice the standard deviation. These principal components were obtained from the zircon age of the overall sample, and the proportion of each principal component present in a single sample was calculated. Based on the age ratio of a principal component corresponding to all samples, combined with the longitude and latitude coordinates of these samples, the spatial distribution map of the proportion of zircon U-Pb age in the age interval of the principal component can be drawn by Kriging interpolation using Surfer software. A major advantage of using the Kriging function is that the sum of all weights is one, which helps to eliminate bias associated with multiple sample parts. Kriging also allows the calculation of empty parts of the grid to create a smooth, continuous surface. For each principal component, a corresponding spatial distribution map of zircon U-Pb age is created ( Figure 1 ).

[0039] Example 2

[0040] (1) This example is based on the determination of the U-Pb age of detrital zircons in the Lower-Middle Triassic in the vicinity of the Xianshuihe-Xiaojiang fault zone by previous researchers, and the collection of samples. The detrital zircon kernel density estimation (KDE) map was drawn using Isoplot software. The results show that the distribution of detrital zircon ages in the Middle-Lower Triassic in the Songpan-Ganzi terrane and the Yangtze block is quite different. From the figure ( Figure 2 , 3 ) It can be clearly found that there are almost no detrital zircon ages above 1200 Ma in the samples of the Yangtze Block.

[0041] (2) There are 19 detrital zircon samples from the Lower-Middle Triassic of the Songpan-Ganzi terrane, with three main age peaks, namely 230-290Ma, 400-450Ma, and 1650-1890Ma. Due to the large range of the Songpan-Ganzi terrane, the sources of materials in different sub-areas may also be different. The age of detrital zircons of 230-290Ma is widely distributed, which is consistent with the age of the island arc granites in the East Kunlun orogenic belt. Therefore, the zircons in this age range come from the source area of ​​the East Kunlun orogenic belt. The detrital zircons of 400-450Ma are related to the magmatism of the Caledonian period. Since there are large-scale Paleozoic granites in the Qinling magmatic rocks, and the ages of the granites in the North Qinling are mainly 421±27Ma and 343±7Ma, it is speculated that the sediments of this period may come from the North Qinling. 1650-1890Ma coincides with the time limit of the Luliang Movement. Considering the geographical location, the Qiangtang terrane is most likely one of the provenance areas of the Triassic Songpan-Ganzi terrane. Therefore, the clastic materials of the Songpan-Ganzi terrane in the Early-Middle Triassic mainly came from the Qiangtang terrane, Kunlun block, Qinling orogenic belt and Qilian orogenic belt, and none (or very little) came from the Kangdian paleo-land.

[0042] The analysis of detrital zircon U-Pb chronology data in the vicinity of the Xianshuihe-Xiaojiang fault zone shows that the samples of the Yangtze Block (i.e., samples on the east side of the fault zone) generally have strong affinity, except for samples YY02 and Z 5-29-2, which are far away from other samples. The detrital zircon ages of these two samples are almost concentrated in the age range of 240-290Ma, which is consistent with the reported eruption time of the Emeishan Large Igneous Province, reflecting the episodic magmatic activity before and after the formation of the main eruption event of the Emeishan Large Igneous Province. The samples of the Yangtze Block mainly come from the Emeishan Large Igneous Province, Kangdian Ancient Land, and the Neoproterozoic Rodinia Supercontinent Breakup Event.

[0043] (3) According to the spatial distribution of detrital zircon U-Pb ages, it can be intuitively found that the detrital zircon age distribution of the Songpan-Ganzi terrane and the western margin of the Yangtze on both sides of the Xianshuihe-Xiaojiang fault zone is significantly different, which reveals that they come from different provenance areas. This obvious age difference on both sides of the Xianshuihe-Xiaojiang fault zone suggests that the Songpan-Ganzi terrane of the Qinghai-Tibet Plateau may have undergone a large-scale extrusion southward along the Xianshuihe-Xiaojiang fault zone.

[0044] Example 3

[0045] This embodiment also provides a measurement of the displacement of a strike-slip fault, specifically as follows:

[0046] For the strike-slip displacement of the Xianshuihe-Xiaojiang fault zone since the Indosinian period, this embodiment proposes a new strike-slip displacement determination method based on the regional geological evolution characteristics, that is, by reconstructing the paleogeographic pattern of the Early-Middle Triassic, the strike-slip displacement of the Xianshuihe-Xiaojiang fault zone since the Indosinian period is effectively constrained. The main reason for using this method to constrain the displacement is that the region has the following geological evolution characteristics:

[0047] (1) The Kangdian paleocontinent cut by the Xianshuihe-Xiaojiang fault zone continued to rise at the end of the Late Permian. After the eruption of the Emeishan large igneous province, it covered the Kangdian paleocontinent, forming the paleogeographic erosion highland on the western margin of the Yangtze block. Based on the above analysis, it is preliminarily determined that the Early-Middle Triassic sedimentation on the western margin of the Yangtze block was mainly affected by the Kangdian paleocontinent, and the paleogeographic pattern was relatively simple - an erosion highland provided debris sources for the surrounding margins.

[0048] (2) The Early to Middle Triassic strata on both sides of the Xianshuihe-Xiaojiang fault zone are well exposed, which can provide a good basis for reconstructing the paleogeographic pattern of the area and can be used as markers to effectively measure the strike-slip displacement of the fault.

[0049] (3) The detrital zircon age data for the Lower-Middle Triassic in the western margin of the Yangtze and Songpan-Ganzi terranes are relatively abundant, which provides a basis for the development of the spatial distribution characteristics of detrital zircon ages and can well constrain the provenance characteristics of the debris on both sides of the Xianshuihe-Xiaojiang fault zone.

[0050] In summary, based on the Early-Middle Triassic paleogeographic reconstruction ( Figure 5 a), Kangdian paleoland, as the only denuded highland on the western margin of the Yangtze in the Early-Middle Triassic, provided a large amount of clastic sources to its east, but the Lower-Middle Triassic to its west did not (or only received a small amount) of sources from Kangdian paleoland. Combined with the tectonic evolution of the region, it is believed that the left-slip movement of the Xianshuihe-Xiaojiang fault zone since the Indosinian period caused the Early-Middle Triassic strata to its west to move from the interior of the Songpan-Ganzi basin to the present position ( Figure 5 b) Based on the spatial distribution characteristics of the Late Triassic paleogeography on both sides of the Xianshuihe-Xiaojiang fault zone and the distance of its faulting, it can be inferred that the strike-slip displacement may exceed 200 km.

[0051] The basic principles, main features and advantages of the present invention are described above. For those skilled in the art, the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0052] In addition, although this specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages, characterized in that: The following steps are involved: Step 1: Obtain detrital zircon age data, including collecting reported data and conducting detrital zircon U-Pb dating tests; Step 2: Organize the obtained detrital zircon age data, analyze the detrital zircon age data using probability density plot (PDP) analysis method, obtain the age distribution characteristics of this group of data, and determine the main age distribution range according to the region or stratigraphic age; Step 3: Obtain the age data of a single sample in each principal component, as well as the proportion of the age number in each age interval to the total age of the sample; Step 4: For each age interval, the age ratio of a single sample is used as the Z axis of the zircon U-Pb age spatial distribution map, where the X and Y values ​​are the latitude and longitude of the sample sampling location. After calculating the ratio of each component of all single samples, the X, Y and Z coordinates are kriging interpolated to draw the spatial distribution of the main components of all samples in the age interval; Step 5: Based on the obtained spatial distribution map of detrital zircon ages and combined with the zircon age characteristics of potential source areas, conduct a detrital provenance system analysis in order to achieve paleogeographic reconstruction.

2. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 1, characterized in that: In step 1, for data with zircon age > 1000 Ma, n( 207 Pb) / n( 206 Pb) age, while for data <1000Ma, n( 206 Pb) / n( 238 U) Age, harmony between 90%-110% is valid data.

3. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 2, characterized in that: Zircon age concordance diagram, kernel density estimation diagram, multidimensional calibration diagram and probability density diagram were completed using Isoplot and IsoplotR analysis and processing software.

4. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 1, characterized in that: The calculation formula of the kernel density estimation map in step 2 is as follows: Where: h represents bandwidth, also known as window width or smoothing parameter; k(x) is the kernel function, x i is the average age of the particles, f h (x) is a mixture density function consisting of n density estimates with the same kernel function.

5. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 4, characterized in that: The bandwidth h is selected by using a bandwidth optimization algorithm, a local adaptive KDE (LA-KDE) and a deconvolution technique, wherein in the LA-KDE model, the local bandwidth is inversely proportional to the data density in the local sample space, the high-density region is smoothly reduced, and the low-density region is smoothly increased.

6. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 5, characterized in that: A diffusion adaptive bandwidth model is used to estimate the optimal bandwidth and evenly apply it to the entire sample space to solve the PDP age peak distortion phenomenon.

7. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 1, characterized in that: In step 4, the U-Pb age of detrital zircon is regarded as the sum of one or more discrete age components, and then the component is defined as a set of discontinuous zircon ages centered on the average value, each age component representing some zircon formation events.

8. The method for paleogeographic reconstruction using the spatiotemporal distribution characteristics of detrital zircon ages according to claim 7, characterized in that: Calculate the mean and standard deviation of each peak interval, define the age interval of each principal component as the mean ± twice the standard deviation, obtain these principal components in the zircon age of the overall sample, and calculate the approximate proportion of each principal component present in a single sample. This proportion is used as the Z axis of the zircon U-Pb age spatial distribution diagram, where the X and Y values ​​are the longitude and latitude of the sample sampling location. After calculating the proportion of each component of all individual samples, perform Kriging interpolation on the X, Y and Z coordinates to draw the spatial distribution of the principal components of all samples. The calculation formula for the node value at point X, Y (Zxy) is: Where Z i is a given weight W i The value of the sample position.

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