The Geomorphic Mapping of Faults
Most of the evidence for the existence of the Wellington Fault and its associated minor faults is found in the geomorphic features of the surrounding country. The customary tools used in the mapping of faults are lithology and stratigraphy, but in the environment of the very broken, unfossiliferous and uniform greywacke facies of sediments which form the bulk of the Wellington Peninsula, such aids are unavailing and only certain recent deposits contribute to the elucidation of structure. While the use of geomorphology as a tool in the mapping of faults appears at first sight to be a crude device compared with a well defined stratigraphical column there is no question as to its adequacy. There are, for example, few geologists who would fail to recognize the Wellington Fault in Pl. 43, although there may be some who would not appreciate the geomorphic nature of the evidence presented there. An examination of Pl. 43 suggests that a block of resistant rock has been juxtaposed against softer and that is perhaps sufficient to indicate the presence of the fault. That analysis is crude, but by applying and refining the geomorphic tool it is possible to define the Wellington Fault more closely, to determine the amount of throw and sense of lateral displacement on faults, and to recognize small features such as minor faults. Thus in this area where stratigraphic methods have hitherto failed it is possible to map a general fault pattern.Many major faults may be mapped with precision, and it is found that in certain respects geomorphic criteria are superior to stratigraphic.
Photographs of the N.Z. Aerial Mapping Ltd. series (4in to the mile) were used as field sheets, and in the area treated in detail field information was plotted on the based maps (1: 15840).
Shatter belts, dislocated Kaukau Surface remnants and alluvial gravels, crest-line and spur jogs and notches were mapped in the field. Trains of minor features as well as straight stream courses have also been mapped in the field as “lineations”. On aerial photographs innumerable additional lineations which are not obvious on the ground can be observed, and these, along with the field observations, conform to the general pattern of Figs. 1 and 2.
Criteria of Faulting
Miller (1941) working in a similar environment of transcurrent faulting in California was faced with the same problem as the writer, namely, fault recognition in a homogeneous rock terrain lacking cover beds, but possessing a widely developed and dislocated erosion surface. Miller wrote:
“When it comes to a consideration of the number of faults in southern California and the evidence for their existence, geologists are more or less divided into two groups. In one group are those who believe that a fault should not be mapped unless the actual fault can be seen nearly everywhere or unless there is positive stratigraphic evidence for it. Many of these workers are not very familiar with fault problems in crystalline rocks. In another group are those who recognize much more faulting on the basis of still other criteria, particularly in areas of crystalline rocks.” (p. 87.)
Criteria for the location of faults are given by Miller (1941: 89–97) and the subject of tectonic features is discussed by Cotton (1950) In the following pages the critern
of these authors will be discussed in so far as they apply to the Wellington area. Miller's comment (p. 89) that the criteria are of varying value is still true; a single criterion may be conclusive evidence of faulting, another may be of little value by itself.
1. Fault fractures clearly visible.
In the Hutt Valley there is to the writer's knowledge only one instance where offset at a fault plane is visible. This is on Haywards Road, where the Haywards Gravel Formation (Stevens, 1956) has been set against greywacke and a sharp reverse fault produced. In other areas actual fault planes are unrecognizable owing to lack of exposures and prevalence of shattering in the rocks. Miller reported similar problems in southern California (1941: 89).
2. Vertically displaced strata.
This criterion can be applied to a few cases where alluvial gravels straddle the Wellington Fault Zone but elsewhere it has no application because of the lack of cover beds and the homogeneity of the terrain. Further, and so far as any but recent faults are concerned, fault-line erosion adjacent to faults has obliterated most primary fault features, including evidence of offsetting of superficial deposits.
3. Recent fault scarplets.
These structures, which dislocate the present-day erosional or depositional land surface, are comparable to faults cutting unconformities or bedding. In this context “fault scarplet” is preferable to “fault trace”, as “trace” is a general (geometric) term which should refer to the intersection of a fault plane with any kind of surface, including bedding or fault planes as well as the topographic surface. Recent fault scarplets along the line of the Wellington Fault occur at Emerald Hill (Fig. 2) and southern Wellington.
4. Vertically displaced erosion surfaces.
Miller (pp. 90–91) found this criterion to be of the utmost value in the determination of faulting in southern California. He states (p. 90):
“. relatively long, straight scarps, separating such old erosion surfaces sharply at different levels, are almost as good as positive stratigraphic evidence of faulting.”
Identification of dislocated remnants of an old erosion surface (the Kaukau Surface) provides the key to fault identification in the Hutt Valley. These remnants are readily distinguished by their senile topography and the great depth of weathering present on their surfaces, so that positions and throws of many faults are well defined.
In the special circumstances of unfossiliferous greywacke country and Pleistocene faulting, geomorphological mapping of faults is superior to the more customary procedures of fault determination. Conventional methods deal with isolated points in space (i.e, outcrops) and yet allow the joining of such points to represent bedding or fault planes. On the other hand, geomorphological procedures deal with dislocations of a visible landscape surface which can be identified over much of its extent.In the Hutt district the Kaukau Surface is well defined and of considerable area, and dislocation of this surface is revealed by the “stepped” topography on either side of the Wellington Fault and frequently by the presence between “steps” of strong fault lineations.
5. Horizontal displacements.
The greatest disadvantage of mapping faults in terms of surfaces is that the method can only be applied to relatively recent faulting. This does, however, aid the study of faulting processes, for it enables the selection of faults of only one period and also allows the study of rates of earth movement. Further advantages lie in the
fact that transcurrent movements of only a few yards can be readily detected and in so far as the surface of reference, the earth's surface, is everywhere exposed, most of the faults over an area can be examined. Allowing for the restriction to faults of one age, the geomorphic method is the most certain of widely applicable methods of measuring transcurrent movement. In areas adjacent to that studied here, horizontal offsets have been recorded by Cotton (1950, 1951) and Adkin (1954), and it is generally accepted that the Wellington Fault and several other faults in the Wellington area are transcurrent.
6. Shatter Zones.
In the greywacke terrain of the Wellington Peninsula, erosion is controlled and lineations determined by intimate emposed weaknesses rather than by inherent sedimentary variations in the strength of strata. Despite the segregation of argillites and arenites into rhythmic beds in the Wellington greywackes, the succession is on the large scale homogeneous and of uniform lithology and variation of strength across bedding is slight. Thus the controlling effect of bedding on erosion is much reduced and the landscapes may be described as tending towards isotropy to erosive processes. Furthering this tendency is the intense shattering which pervades almost all the greywacke sandstones and renders the hardest bands as weak to attrition as the softest Still further, in the young topography of Wellington Province the steep gradients favour the transport of large rock particles and allow erosion of shattered hard beds as well as soft. Finally it seems that transcurrent faulting imposes intense local shearing stresses on the rocks so that faults are marked by zones of extreme comminution. In a broadly isotropic environment this cataclastic effect is directly reflected by erosional fault-line features such as stream lineations and notches in crests and spurs.
7.Alignment of saddles.
In a full account of geomorphic expressions of faulting, Cotton (1950: 739) maintains:
“… jogs and notches, according to the direction of down-throw, remain in the landscape after erosion has eliminated all geomorphic traces of faulting in adjacent valleys; on resistant terrains they may survive into full or late maturity of dissection, whatever the prefaulting form of the surface or its postfaulting history.”
To the west of the Wellington Fault notched spurs and aligned saddles are very well developed. Such features are fault-line features sensu stricto, for an “etching-out” of a fault pattern by subsequent erosion has occurred.Faults distinguished by such etching characters may be of any age and should not be confused with recent faults which are generally identified as dislocations of topographic surfaces.
8. Stream lineations.
Commonly it is only the river pattern of a greywacke terrain which gives any direct indication of a fault pattern. Valleys bear little relation to the strike of inclined or folded strata and in many places rivers follow zones of obviously fault-shattered rock. The straight stretch of the Tauwharenikau River, Tararua Mountains (N. Z. M.S. 1, N161), provides a good example of deep etching of a shatter zone. Here recent fault traces and fault-controlled saddles have been recognzed to the north-east and south-west of the stream lineation and also in the river valley. Though on a smaller scale, similar stream lineations occur to the west of the Wellington Fault and in particular along the line of the Liverton Fault. In the latter examples shatter belts mapped in the field line up with stream lineations, spur or ridge notches and jogs, and similar features.(Pl. 44, Figs. 1, 2, 3.)
Cotton (1950: 746) has discussed the theory of river guidance by “shatter belts” in these words:
“In New Zealand the fault-line erosional hypothesis has been called upon to explain alignment of some valleys near Wellington with faults and crush zones.… and generally it has been assumed that these faults zones are ancient.”
Fig. 1 (right).—Fault map of the Hutt Valley. The “Orongorongo Shatter Zone” with associated faults is after Grant-Taylor (1949).
Fig. 2 (left).—The Wellington Fault zone north-east from the Upper Hutt Basin. The continuation of the Kaitoke Fault north-east from the Pakuratahi River is after G. J. Lensen.
Cotton points out, however, that some of the fault-valley features, like those along the south-west portion of the Wellington Fault, are primary and of recent age. Allowing for such recent faults, it is still considered that in the area dealt with here the greatest part of the fault pattern is ancient and originated by etching-out along old fault lines. To complicate matters, it seems probable that in many instances recent faulting has followed along earlier-formed faults and so primary and secondary fault features coincide.
9. Zones of excessive jointing with slickensides.
On both sides of the Wellington Fault Zone secondary quartz is often present along slickensided joints. Zones of fractures are the extension away from the fault of the fault shatter zones and where fault-line breccias and pugs have been removed from the centre of fault zones by erosion joints provide useful indicators of the presence and trend of the major fault and suggest its hade.