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'{{Short description|Rift system in North Island, New Zealand}} {{Infobox fault | name = Hauraki Rift | other_name = | named_for = [[Hauraki Gulf]] | named_by = | year_def = | image = | image_alt = | caption = | map_image = Rift.Zones.North.Island.NZ.jpg | map_caption = Intra-arc continental rifts [[North Island]], [[New Zealand]]. Purple approximately delineates the active Hauraki Rift, red the active modern [[Taupō Rift]] and yellow the inactive old Taupō Rift. | map_alt = Intra-arc continental rifts North Island, New Zealand | pushpin_map = | pushpin_map_width = | pushpin_map_caption = | pushpin_map_alt = | pushpin_relief = 1 <!-- location --> | country = [[New Zealand]] | region = [[North Island]] | state = | cities = | coordinates = {{coords|37|15|0|S|175|28|0|E}} (extends 37°30′S to at least 36°30′S) | elevation = | elevation_ft = | elevation_ref = | topdepth_m = | topdepth_ft = | topdepth_ref = | range = | part_of = | segments = Multiple | length = {{convert|250|km|mi|abbr=on}}<ref name="Persaud et al."/> | width = {{convert|25|km|mi|abbr=on}} | depth = | strike = | dip = | dip_angle = | displacement = {{convert|0.46|mm|in|abbr=on}}/yr<ref name="Persaud et al."/> | plate = [[Indo-Australian Plate|Indo-Australian]] | status = [[Active fault|Active]] | earthquakes = Mainly tectonic | type = Intra-arc continental rift | movement = Possibly up to {{M|w|7.2-7.4|link=y}}<ref name="Persaud et al."/> | rockunit = | age = [[Miocene]]-[[Holocene]] | orogeny = | volcanic_arc/belt = Coromandel volcanoes | embed = [https://data.gns.cri.nz/geology/ New Zealand geology database (includes faults)] }} The '''Hauraki Rift''' is an active NeS-to NWeSE-striking [[rift valley]] system in the [[North Island]] of [[New Zealand]] that has produced the [[Firth of Thames]] and the [[Hauraki Plains]]. It is approximately {{convert|25|km|mi}} wide and {{convert|250|km|mi}} long. ==Geology== [[File:auckland.arp.750pix.jpg|thumb|left|A true-colour image acquired by NASA's Terra satellite, on 23 October 2002. The Hauraki Rift extends north from the Hauraki Plains at the base of the large bay (the [[Firth of Thames]]) just to the lower right of centre. The sides of the rift valley containing the intra-rift fault are defined by the dark green forest outlines of the [[Coromandel Range|Coromandel]] and [[Kaimai Range]]s to the east, and the [[Hunua Ranges]] to the west.]] The rift valley in the north appears to be delimited in the west by the line of the Firth of Thames Fault and to the east by the [[Hauraki Fault]].<ref name="Persaud et al.">{{cite journal |last1=Persaud |first1=Mira |last2=Villamor |first2=Pilar |last3=Berryman |first3=Kelvin|last4=Ries |first4=William |last5=Cousins |first5=J. |last6=Litchfield |first6=N. |last7=Alloway |first7=Brent |date =2 Jan 2016 |title=The Kerepehi Fault, Hauraki Rift, North Island, New Zealand: active fault characterisation and hazard |volume=59 |issue=1 |doi=10.1080/00288306.2015.1127826 |journal=New Zealand Journal of Geology and Geophysics |pages=117–135 |bibcode=2016NZJGG..59..117P |s2cid=130085657 |url=https://www.researchgate.net/publication/301907071 }} </ref> The mountains of the [[Coromandel Range|Coromandel]] (which include [[andesite|andesitic]] and [[Basalt|basaltic]] back arc volcanoes) and [[Kaimai Range]] to the east and the [[Hunua Ranges|Hunua Range]] to the west give it the appearance of a full graben although it has also been described as a simple structure of 2 to 3 [[half-graben]]s<ref name="Hochstein1993">{{cite journal| last1=Hochstein |first1=Manfred P.|last2=Ballance |first2=Peter F. |year=1993 |pages=295–305 |title=Hauraki Rift: A young, active, intra-continental rift in a back-arc setting |volume=2 |journal=South Pacific Sedimentary Basins |url=https://www.researchgate.net/publication/252322254 }}</ref> so the term Hauraki Graben for the low lying portions would be incorrect. However the apparent line in the south of the Firth of Thames Fault no longer follows the apparent horst structure to the west and some believe the line of the [[Waikato River]] after it leaves the old [[Taupō Rift]] gives a better guide to the western line of the rift as structures have been heavily modified in the last 26,000 years in the south, and the Waikato River historically used the graben in its route to the sea. The active intra-rift [[Kerepehi Fault]] in the southern portion of the Hauraki Rift is complex and has the potential for large earthquakes.<ref name="Persaud et al."/> ===Structure=== Initially between 10 and 5 million years ago the rift developed in a back arc region to the now extinct Coromandel volcanoes. It has a basement of mesozoic [[greywacke]] and [[argillite]] and at least two [[andesite|andesitic]] [[volcano]]es erupted within it.<ref name="Hochstein1993"/> To the north the rift is filled with {{convert|2.5|–|3|km}} thick Tertiary and Quaternary terrestrial sediments and beyond [[Waiheke Island]] it opens up into an oceanic basin.<ref name="Fromont2017">{{cite web|title=A seismic investigation of the evolution and structure of the Kerepehi Fault, in the context of the Hauraki Rift, New Zealand |url=https://researchspace.auckland.ac.nz/handle/2292/34795|first1=Ashleigh|last1=Fromont |year=2017}}</ref> Early work suggested the northern boundary might come ashore in Northland, near [[Whangārei]] and that the southern boundary while modified by events after the formation of the [[Mangakino caldera complex]] continued to be active. Work using geolocation on the Hauraki Rift reveals a widening rate of {{convert|0.9|mm|in|abbr=on}}/year although some of the raw data suggests that at the [[Te Poi]] end it might be up to {{convert|1.5|mm|in|abbr=on}}/year which would be consistent with this.<ref name="risk Hauraki Rift">{{cite web|title=Structure and Activity of the Hauraki Rift |url=https://www.eqc.govt.nz/assets/Publications-Resources/Funded-Research-papers/2350-Structure-and-activity-Hauraki-Rift.pdf | first1=Jennifer |last1=Eccles |first2=Sigrun |last2=Hreinsdottir |first3=Julie |last3=Rowland |year=2019}}</ref> However, in this region it is now dominated by the much more recent [[Taupō Rift]] (2 million years old at most) associated with the [[Taupō Volcanic Zone]] (TVZ) to which it is off-set by 60 degrees. TVZ eruptive output of 240,000 years ago, being Mamaku ignimbrite from the [[Rotorua caldera]] has filled in the far southern extremes. ===Geothermal activity=== Thermal springs along the rift edges and center are found throughout the region with a heat output of the order of 6.5MW in total.<ref>{{cite journal| title=Geophysical study of the Hauraki Depression, North Island, New Zealand |url=https://www.researchgate.net/publication/254286143 |year=1979 |journal=New Zealand Journal of Geology and Geophysics |volume= 22 |issue= 1 | pages=1–19 |doi=10.1080/00288306.1979.10422550 |first1= Manfred P. |last1= Hochstein |first2=I.M. |last2=Nixon|bibcode=1979NZJGG..22....1H |doi-access=free }}</ref> ===Other Context=== At the far side of the [[Coromandel Peninsula]] there is a parallel structure of Late [[Miocene]]/Quaternary origin extending north, that has been called the East Coromandel Rift. To the south east is the [[Tauranga Volcanic Centre]] which is likely related to activity in the old [[Taupō Volcanic Zone]] and the postulated [[Omanawa Caldera]] would have been at the intersection of the two rifts about 2 million years ago. ==References== {{Reflist}} {{Seismic faults of New Zealand}} [[Category:Rifts and grabens|+]] [[Category:Thames-Coromandel District]] [[Category:Firth of Thames]] [[Category:Hauraki District]] [[Category:Geology of New Zealand]] [[Category:Geologic formations of New Zealand]] [[Category:Seismic zones of New Zealand]]'