Problems and Solutions in Structural Geology and Tectonics

• Publisher: Elsevier
• Year: 2019
• ISBN:
  9780128140499
• Topic: Ciencias físicas
• Subtopic: Geología y ciencias de la Tierra

Problems and Solutions in Structural Geology and Tectonics, Volume 5, in the series Developments in Structural Geology and Tectonics, presents students, researchers and practitioners with an all-new set of problems and solutions that structural geologists and tectonics researchers commonly face. Topics covered include ductile deformation (such as strain analyses), brittle deformation (such as rock fracturing), brittle-ductile deformation, collisional and shortening tectonics, thrust-related exercises, rift and extensional tectonics, strike slip tectonics, and cross-section balancing exercises. The book provides a how-to guide for students of structural geology and geologists working in the oil, gas and mining industries.

• Provides practical solutions to industry-related issues, such as well bore stability
• Allows for self-study and includes background information and explanation of research and industry jargon
• Includes full color diagrams to explain 3D issues

Part I

Integrating Observation and Interpretation To Understand Tectonics, Past and Present

Chapter 1

Cross-Section Construction and Balancing: Examples From the Spanish Pyrenees

Berta Lopez-Mir CASP, Cambridge, United Kingdom

Abstract

Cross-sections are used by geologists to represent the structure of the Earth's subsurface. To construct a cross-section, observations from the Earth's surface and other available information are projected onto an underground vertical plane to interpret how data relate to each other. The main challenge is transforming the scattered data into a reasonable geological geometry. Interpretations are not unique, so geologists need validation rules to assess their results. A cross-section is considered valid if: (1) it fits the available data and geological knowledge (admissible cross-section); (2) it is retro-deformable (restored cross-section); and (3) the volume of material between the deformed and the restored state remains constant (balanced cross-section). This chapter explains the general methodology to construct a cross-section from a geological map and the standard techniques to balance it (bed length, area, and area-excess). For each method, a practical problem is presented. Problems are real examples from the Spanish Pyrenees.

Keywords

Cross-section construction; Balanced cross-sections; Balancing methods; Line-length restoration; Area-length restoration; Area-excess restoration

1 Introduction

Cross-sections are vertical slices through the Earth used to predict the structure of the ground subsurface. Besides the scientific interest of illustrating what you would see in a journey inside the Earth, geologists need cross-sections to find, evaluate, and extract hydrocarbons, minerals, groundwater, and other resources; as well as to store industrial wastes like CO₂ and radioactive material. In general, cross-sections are constructed by projecting observations from a geological map onto an underground vertical plane, to interpret how these observations relate to each other (e.g., Davis et al., 2012; Fossen, 2016). Cross-sections can also be built or complemented by the interpretation of subsurface data (e.g., seismic reflection and refraction, magnetotellurics, gravity, or other geophysical data).

Accordingly, cross-sections rely on data interpretation. Interpretations are only approximations to reality, some of which are wrong (if they are inconsistent with data or violate a geological principle) and some of which are sound, but none is “right.” The acquisition of new data, the learning of new conceptual models, and/or scientific progress might change our interpretations. The more data and knowledge we have, the more constrained our interpretations will be, but outputs are not unique. This is known as uncertainty and is an inherent element on the interpretation of any geological data (Bond, 2015). Understanding uncertainty is important because the success of underground extractive activities or storage relies on those interpretations, and any mistake could lead to significant economic or environmental loss.

Structural geologists need validation rules to assess their interpretations (e.g., Groshong, 2006; Rowland et al., 2007). The fundamental requirements for a cross-section to be valid are:

1. (1) It honors the available data and is consistent with known geological concepts (admissible cross-section).
2. (2) It is retro-deformable. This means it can be recomposed into a plausible predeformational geometry (restored cross-section).
3. (3) There is no gain or loss of material between the deformed and the restored states (balanced cross-section). This means that if we compare the deformed and the predeformational restored strata geometries, there are no large gaps or overlaps. For this assumption to be correct, cross-sections must be parallel to the tectonic transport direction.

According to this, an admissible, retro-deformable, and balanced cross-section can be considered as a possible sound interpretation. This does not mean that it is correct, but our solution is reasonable and consistent with the data we have. Correspondingly, an admissible cross-section that does not balance is not plausible, and we need to readdress our interpretations or find an explanation for loss and/or gain of material (e.g., material transport in and out of the section ...